[From the U.S. Government Printing Office, www.gpo.gov]


                                                                                                             FY'91                        Task 11

                                                                                                                    Final Product
                                                                                                                 VA Coastd Resources Mgt. Twgram



                   DEVELOPMENT OF A BUFFER


                                      ZONE EVALUATION


                                    MODEL/PROCEDURE



                                                                         by



                                                          Theo A. Dillaha






                                                           November 1992



                                Department of Agricultural Engineering
                     Virginia Polytechnic Institute and State University
                                                w
                                             Blacksburg, VA 24061-0303









                                    FINAL REPORT
              @DEVELOPMENT OF A BUFFER ZONE
                 EVALUATION MODEL/PROCEDURE






                                           by



                                    Theo A. Dillaha
                                  Associate Professor
                        Department of Agricultural Engineering
                 Virginia Polytechnic Institute and State University
                               Blacksburg, VA 24061-0303
                                 Phone: (703) 231-6813











           This project was funded, in part, by the Virginia Council on the
           Environment's Coastal Resources Management Program through Grant
           #NA170ZO359-01   of   the   National   Oceanic   and    Atmospheric
           Administration, Of f ice of Ocean and Coastal Resource Management,
           under the Coastal Zone Management Act of 1972 as ammended.

           This project was funded, in part, by the Virginia Department of
           Conservation and Recreation, Division of Soil and water
           Conservation through Contract #C199-50311-92-03.






 V                                   November 1992










                                      TABLE OF CONTENTS



                                                                                   Page

           Introduction    . . . . . . . . . . . . . . . . . . . . . . . . .         1


           Literature Review    . . . . . . . . . . . . . . . . . . . . . . .        3
              Sediment Transport in Vegetative Buffers        . . . . . . . . . .    4
              Nutrient Transport in Vegetative Buffers . . i         . . . . . . .   5
              Vegetative Buffer Research in Virginia        . . . . . . . . . . .    7
              Vegetative Buffer Models and Design Procedures         . . . . . . .   9
                 Kentucky Filter Strip Model      . . . . . . . . . . . . . . .      9
                 CREAMS Model   . . . . . . . . . . . . . . . . . . . . . .         10
                 GRAPH Model    . . . . . . . . . . . . . . . . . . . . . .         10
                 Phillips Model   . . . . . . . . . . . . . . . . . . . . .         11
                    Phillips Hydraulic Model      . . . . . . . . . . . . . .       11
                    Phillips Detention Model      . . . . . . . . . . . . . .       13
                 Other Design Approaches      . . . . . . . . . . . . . . . .       14
              State and Federal Vegetative Buffer Programs         . . . . . . .    14
                 Chesapeake Bay Preservation Act       . . . . . . . . . . . .      14
                 Conservation Reserve Program     . . . . . . . . . . . . . .       15
                 National Vegetative Filter Strip      Conservation
                    Practice Standard    . . . . . . . .                            16
              Sediment and Nutrient Loadings      due  to E;od'in*g*Banis*  . . .   17
              Literature Review Summary       . . . . . . . . . . . . . . . .       18

           Model Development    . . . . . . . . . . . . . . . . . . . . . .         20
              Phillips Hydraulic Model      . . . . . . . . . . . . . . . . .       20
              Phillips Detention Model      . . . . . . . . . . . . . . . . .       21
              Shoreline Erosion Model       . . . . . . . . . . . . . . . . .       22
                 Shoreline Erosion     . . . . . . . . . . . . . . . . . . .        22
                 Upland Erosion   . . . . . . . . . . . . . . . . . . . . .         23
                 Impact of Shoreline Stabilization        . . . . . . . . . . .     24

           Buffer Evaluation Procedure      . . . . . . . . . . . . . . . . .       26


           Data for Buffer Evaluation Procedure        . . . . . .          * *  *  28
              Data Requirements for the Hydraulic and Detenti@n*M@dels           .  28
                 Saturated Hydraulic Conductivity, K        . . . . . . . . . .     28
                 Buffer Length, L    . . . . . . . . . . . . .     * 0 . 0 9   0 .  29
                 Upslope Slope-Length Contributing Runoff       to the
                    Buffer, L   . . . . . . . . . . . . . . . . . . . . . .         29
                 sin 6, s     . . . . . . . . . . . . . . . . . . . . . . .         29
                 Manning's Roughness Coefficient, n       . . . . . . . . . . .     30
                 Fraction of Surface Runoff Crossing Buffer as Sheet
                    Flow, C   . . . . . . . . . . . . . . . . . . . . . . .         31
                 Soil Moisture Storage Capacity, M        . . . . . . . . . . .     32
                 Vegetative Uptake or Net Productivity Factor, V          . . . .   32










													Page

             Data Requirements for Shoreline Stabilization Model           . . .   33
                Bank or Shoreline Erosion Rates       . . . . . . . . . . . .      33
                Bank Height    . . . . . . . . . . . . . . . . . . . . . .         33
                Bulk Density of Bank Soil       . . . . . . . . . . . . . . .      34
                Concentration of Nitrogen and Phosphorus in the
                   Bank Material . . ................................  . . . . .   34
                Sediment Loading to the Buffer From the Upland
                   Contributing Area    . . . . . . . . . . . . . . . . . .        34
                Nitrogen and Phosphorus Loading to       the Buffer    . . . . .   34

          Practical Application     . . . . . . . . . . . . . . . . . . . .        35
             Example Problem I      . . . . . . . . . . . . . . . . . . . .        35
                Problem Statement      . . . . . .                                 35
                Data         . . . . . . .                                         35
                Solution    . . . . . . . . .                              35
              Example Problem II      . . . . . ..................................37
                Problem Statement      . .  .  . .................................37
                Data  . . . . . . . . . . . . . . . . . . . . . . . . . .          37
                Solution . ......................................................37
                   Sediment and Nutrient    Losses  Due   to  Shoreline  Erosion   38
                   Sediment and Nutrient    Transport Through the Buffer           38

          Summary and Conclusions      . . . . . .                . . . . . . .    40

          References    . . . . .            . . . . . . .        . . . . . . .    41
 










                                     LIST OF TABLES



                                                                              page

         Table 1.      Estimation of hydraulic conductivity values
                       from Soil Survey permeability estimates       . . . .   28

         Table 11.     Hydraulic  conductivity as a function of soil
                       texture    . . . . . . . . . . . . . . . . . . . .      29


         Table III.    Estimates  of Sin 0 for various buffer
                       slopes in  percent and degrees     . . . . . . . . .    30

         Table IV.     Estimates  of Mannings roughness coefficient
                       as a function of landuse and condition for
                       shallow sheet flow conditions (Engman, 1986)            31

         Table V.      Net primary productivity (adapted from:
                       Leith, 1975)   . . . . . . . . . . . . . . . . . .      33











                                 EXECUTIVE SUMMARY


          A procedure is presented for evaluating the impacts of proposed
          vegetative buffer modifications on buffer effectiveness.         The
          procedure is based on the hydraulic and detention models developed
          by Phillips for evaluating buffer effectiveness.          Phillips's
          original models were modified to correct several limitations
          encountered.    The modified models consider the effects of
          concentrated flow and vegetative uptake on buffer performance.

          The proposed model is relative simple in concept and application
          and is suitable for use by planners. All of the data required by
          the model can be collected on site or can be estimated from the
          literature. Labratory analysis of soil and bank samples,,however,
          will greatly improve model reliability with respect to nutrient
          losses. In areas with shoreline erosion, the procedure also allows
          the benefits of shoreline control to be considered.








































                                          iv










                   DEVELOPMENT OF A BUFFER ZONE EVALUATION
                                  MODEL/PROCEDURE


                                   Theo A. Dillaha
                                 Associate Professor
                       Department of Agricultural Engineering
                 Virginia Polytechnic Institute and State University
                              Blacksburg, VA 24061-0303



                                     INTRODUCTION

          The original purpose of this project was to develop a qualitative
          technique for evaluating vegetative buffer effectiveness with
          respect to sediment and nutrient removal.       The technique was
          intended for use by planners in evaluating the relative
          effectiveness of various buffer zone modification schemes. After
          f ield testing of the proposed procedure, it was agreed that the
          project objectives would be expanded to account for the effects of
          vegetative buffer modification due to shoreline stabilization
          practices. This was necessary because installation of shoreline
          stabilization systems may reduce vegetative buffer effectiveness by
          reducing vegetative buffer length, increasing vegetative buffer
          slope, and disturbing buffer vegetation. However, the benefits of
          shoreline stabilization for reduced sediment and nutrient losses
          due to control of shoreline erosion can more than compensate for
          reduced vegetative buffer effectiveness in most cases.

          The resulting assessment process allows impact predictions to be
          made for site specific conditions of individual vegetative buffer
          alterations as well as relative effectiveness comparisons between
          vegetative buffers.      The vegetative buffer zone evaluation
          procedure is designed to use soil survey data and other parameters
          including slope and surface roughness. The site evaluation methods
          will consider both surface runoff and subsurface flow within the
          buffer.

          Specific objectives of the project were to:

          1.   Select a vegetative buffer assessment methodology best suited
               for soils and shoreline conditions in Virginia.

          2.   Modify the method as required to improve the method's
               suitability for Virginia conditions.

          3.   Determine the availability of physical parameters necessary to
               apply the selected buffer zone evaluation methodology. Define
               ranges of parameters suitable for application in Virginia's
               coastal zone.


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         4.   Test the proposed methodology on specific sites in Virginia.

         5.   Prepare a f inal report and guide detailing the development,
              use and limitations of the proposed methodology.












































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                                     LITERATURE REVIEW

           Vegetative buffers (also referred to as vegetative filter strips,
           grass filter strips, buffer strips, vegetative buffers, riparian
           buffer zones, filter strips, etc.) are bands of planted or
           indigenous vegetation, situated between pollutant source areas and
           receiving waters.      They are presumed to remove sediment and
           chemicals f rom runof f and ground water interacting with the buf f er.
           Pollutant removal in vegetative buffers is accomplished by a
           variety of physical, chemical, and biological processes.           These
           processes are poorly understood and there is considerable
           uncertainty as to the effectiveness of vegetative buffers in
           removing pollutants from surface runoff and ground water.
           Currently, there are no standards or widely accepted methods for
           evaluating vegetative buffer effectiveness. Consequently, it is
           difficult if not impossible to determine how effective vegetative
           buffers are in protecting water quality.

           Numerous short-term studies have found that vegetative buffers are
           initially very effective in removing sediment and sediment-bound
           pollutants f rom. surf ace runof f under shallow sheet f low conditions.
           The long-term (more than one-year) effectiveness of vegetative
           buffer for pollutant removal, however, has not been investigated
           very extensively. Riparian buffer zone design procedures, proposed
           over the past 10-years, have been research oriented and based on
           short-term experimental studies. These studies did not consider
           the long-term effects of sediment and nutrient accumulation in
           vegetative buffers.       These studies and design methods also
           generally ignore the effects of concentrated flow conditions on
           vegetative buffer performance. This is unfortunate, as most flow
           in the real world will enter vegetative buffers as concentrated
           flow rather than the shallow sheet flow used in model development
           (Dillaha et al., 1989).      Consequently, those equations that do
           exist, generally overestimate vegetative buffer effectiveness with
           respect to sediment and nutrient removal because they do not
           consider the effects of concentrated flow and the accumulation of
           sediment and nutrients in vegetative buffers over time.

           The major pollutant removal mechanisms associated with vegetative
           buffers involve changes in flow hydraulics that enhance the
           opportunity for the infiltration of runoff and pollutants into the
           soil profile, deposition of total suspended solids (TSS),
           filtration of suspended sediment by vegetation, adsorption on soil
           and plant surfaces, and absorption of soluble pollutants by plants.
           For these mechanisms to be effective, it is essential that runoff
           pass slowly through the vegetative buffer to provide sufficient
           contact time for the removal mechanisms to function.

           Infiltration is one of the most significant removal mechanisms
           affecting vegetative buffer, performance. Infiltration is important
           since many pollutants associated with surface runoff enter the soil
           profile in the buffer area with infiltrating water. Once in the

                                              3









          soil profile, many pollutants, particularly N and P, are removed by
          a combination of physical, chemical, and biological processes.
          Infiltration is also important because it decreases the amount of
         .surface runoff, thus reducing the ability of runoff to transport
          pollutants.    Since infiltration is one of the more easily
          quantifiable   mechanisms   affecting buffer performance, many
          vegetative buffers have been designed to allow all runoff from
          design storms to infiltrate into the buffer (Midwest Plan Service,
          1985). This approach results in large land requirements because it
          ignores other removal mechanisms.

          vegetative buffers also purify runoff through the process of
          deposition.     Because vegetative buffers usually offer high
          resistance to shallow overland flow, they decrease the velocity of
          overland flow immediately upslope and within the buffer, causing
          significant reductions in sediment transport capacity.       If the
          transport capacity is less than the incoming suspended solids load,
          then the excess suspended solids may be deposited and trapped
          within the buffer. Sediment-bound pollutants will also be removed
          during the deposition process.

          The filtration of solid particles by vegetation during overland
          flow and the absorption and adsorption processes are not as well
          understood   as   the   infiltration   and  deposition    processes.
          Filtration is probably most significant for the larger soil
          particles, aggregates, and organic particles while adsorption is
          thought to be a significant factor with respect to the removal of
          dissolved pollutants.   The major questions concerning adsorption
          and absorption involve their long-term effectiveness as nutrients
          accumulate in the buffer (Lee et al., 1989; Dillaha et al., 1989).

          Sediment Transport in Vegetative Buffers

          Historically, the design of vegetative buffers has been based
          almost entirely upon local custom.     Wilson (1967) presented the
          results of a sediment trapping study in grass buffers which gave
          optimum distances required to trap sand, silt, and clay in flood
          waters on flat slopes.     He concluded that grass buffer length,
          sediment load, flow rate, slope, grass height and density, and
          degree of vegetative submergence all affect sediment removal.
          Neibling and Alberts (1979) used a rainfall simulator on
          experimental field plots with a slope of 7% to show that 0.6, 1.2,
          2.4, and 4.9 m long grass buffers all reduced total sediment
          discharge by over 90% from a 6.1 m long bare soil area. Discharge
          rates for the clay size fraction were reduced by 37, 78, 82, and
          83%, for the 0.6, 1.2, 2.4, and 4.9 m grass buffers, respectively.
          Significant deposition of solids was observed to occur just upslope
          of the leading edge of the grass buffer and 91% of the incoming
          sediment load was removed within the first 0.6 m of the grass
          buffer. Sediment discharge.of clay sized particles (<0.002 mm) was
          reduced 37% by the 0.6 m strip.     No equations were presented to
          estimate the influence of parameters on sediment yield.

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          The most comprehensive research to date on sediment transport in
          vegetative buffers has been conducted by a group of researchers at
          the University of Kentucky (Barfield et al., 1977; 1979; Kao and
          Barfield, 1978; Tollner et al., 1976; 1978; 1982; Hayes et al.,
          1979a,b; 1982). Tollner et al. (1976) presented design equations
          derived from experimental studies relating the fraction of sediment
          trapped in simulated vegetative media to the mean flow velocity,
          flow depth, particle fall velocity, filter length, and the spacing
          hydraulic radius (a parameter similar to the hydraulic radius in
          open channel flow that is used to account for the effect of media
          spacing on flow hydraulics).     The Kentucky researchers reported
          high trapping efficiencies as long as the vegetative media was not
          submerged, but trapping efficiency decreased dramatically at higher
          runoff rates that inundated the media. The Kentucky researchers,
          like Neibling and Alberts (1979), observed that much of the
          sediment deposited just upslope of the filter and within the first
          meter of the filter, until the upper portions of the filter were
          buried in sediment. Subsequent flow of sediment into the filter
          resulted in the advance of a wedge-shaped deposit of sediment down
          through the filter.    The Kentucky resea  'rchers did not consider
          nutrient trapping or the    long-term effectiveness of vegetative
Ilk       buffers.

          Hayes and Hairston (1983)  used field data to evaluate the Kentucky
          model for multiple storm   events in Mississippi. Eroded material
          from fallow cropland subject to natural rainfall was used as a
          sediment source. 'Kentucky 311 (Festuca arundinacea) tall fescue
0         trimmed to 10 cm was used and the model predictions agreed well
          with the measured sediment discharge values.

          Kao et al. (1975) proposed a vegetative buffer arrangement in which
          grass strips were alternated with strips of bare ground to solve,
          the problems associated with sediment inundation of the filter and
0         the killing of vegetation. Kao indicated that with the proper
          vegetative buffer area to source area ratio, most of the trapped
          sediment would be retained in the bare area just upslope of the
          buffer. The trapping of sediment upslope of the buffer maintained
          high buffer efficiencies and enabled periodic removal of deposited
          sediment without damaging the buffer.      Kao's results were based
0         upon laboratory studies with artificial media and were not tested
          in the field.

          Nutrient Transport in Vegetative Buffers

          Nutrient movement through vegetative buffers has been investigated.
0         by several researchers but no comprehensive design methods have
          been presented. Doyle et al. (1977) applied dairy manure to 7 x 5
          m fescue plots on a Chester silt loam (fine-loamy, mixed, thermic,
          Typic Hapludult) soil with a slope of 10%.        Dissolved nutrient
          concentrations were measured after passing through 0.5,      1.5, and
          4.0 m of fescue buffer strips. Dissolved P was reduced by   9, 8, and
0         62% after passage through 0.5, 1.5, and 4.0 m buffers,

                                            5








          respectively.   Nitrate (NOA losses decreased by 0, 57, and 68%,
          respectively, but ammonia (NH3) concentrations increased with
          increasing filter length presumably due to the release of NH3 from
          decomposing organic N, which was trapped in the filter previously.

          Young et al. (1980) used a rainfall simulator to study the ability
          of vegetative buffers to control pollution from feedlot runoff.
          Field plots were constructed on a 4% slope with the upper 13.7 m in
          an active feedlot and the lower 27.4 m planted in either corn (Zea
          mays), oats (Auena sativa), orchardgrass, (Dactylis glomerata) or
          a sorghum-sudangrass (Sorghum vulgare-Sorghum sudanensis) mixture.
          Water was applied to the plots to simulate a 25-year, 24-hour
          duration storm. Total runoff, sediment, P and N were reduced by
          81, 66, 88, and 87%, respectively, by the orchardgrass and by 61,
          82, 81, and 84%, respectively, with the sorghum- sudangrass
          mixture.   The authors concluded that vegetative buffers were a
          promising treatment alternative.

          Thompson et al. (1978) studied the effectiveness of orchardgrass
          buffers on a sandy loam soil in reducing nutrient loss from the
          application of dairy manure to frozen or snow-covered orchardgrass
          plots. Fresh dairy manure was applied to 24 m orchardgrass plots
          and runoff quality determined after traveling through 12 and 30 m
          of additional orchardgrass during natural runoff events. Total P,
          total Kjeldahl nitrogen (TKN), and N losses were reduced by an
          average of 55, 46, 41, and 45%, respectively, after passing through
          12 m of filter.    A 36 m filter resulted in P,     N03, TKN, and N
          reductions of 61, 62, 57, and 69%, respectively.             Nutrient
          concentrations in the runoff from the 36 m filters approached that
          from control plots to which no manure had been added. Bingham et
          al. (1978) applied poultry manure to 13 m long fescue grass plots
          on an eroded Cecil clay loam (clayey, kaolinitic, thermic Typic
          Hapludult) with 6-8% slopes.      Buffer length/waste area length
          ratios of about 1.0 were reported to reduce pollutant loads to near
          background concentrations.    Total P, TKN, N03, and total-N were
          reduced 25, 6, 28, and 28%, respectively.

          Edwards et al. (1983) monitored storm runoff for 3 years from a
          paved feedlot. Storm runoff was measured and sampled as it left
          the feedlot, after passing through a shallow concrete settling
          basin, and after passing through two consecutive 30.5 m, long fescue
          buffers. Runoff, TSS, P, and N were reduced by -2, 50, 49, and 48%,
          respectively, after passing through the first buffer and by an
          additional -6, 45, 52, and 49%, respectively, after passing through
          the second buffer. Total runoff from the buffers was greater than
          the incoming runoff because rainfall rates during runoff events
          exceeded the infiltration capacity of the buffers. This rainfall
          excess coupled with the added area of the buffers resulted in
          increased runoff. Removal efficiencies would have been higher if
          the settling basin located. upslope of the buffer had not removed
          54, 41,, and 35% of the TSS, P, and N, respectively. Most of these
          solids and nutrients would have been removed in the buffers because

                                           6









          they were either settleable solids or nutrients bound to settleable
          solids.

          Patterson et al. (1977) applied liquid dairy waste through a gated
          pipe to a fescue buffer on Hosmer silt loam (fine-silty, mixed,
          mesic Fragiudalf) on a 3.4% slope. After applying dairy waste to
          the 35 m vegetative buffer for one year, pollutant reductions
          averaged 42,   38,  7,  and 71%   f or BOD., NH3.   P04, and TSS,
          respectively.  Nitrate loss from  the filter was greater than NO,
          loading to the buffer, presumably due to mineralization of organic
          N and nitrification of   NH3 that had been trapped in the buffer
          previously.   Paterson et al. (1977) also noted problems with
          maintaining a good grass cover on the buffer area.              They
          recommended that several buffer areas should be use and rotated on
          a weekly basis to maintain good grass cover.

          Magette et al. (1989) used a rainfall simulator on field plots to
          study the effectiveness of 4.6 and 9.2 m long grass buffers in
          removing nutrients and sediment from agricultural runoff. Nutrient
          removal appeared to decrease as the number of runoff events
          increased. Gross sediment, N, and P losses in surface runoff were
          reduced by 52, -15, and 6%, respectively, by the 4.6 m buffers, and
          75, 35, and 20%, respectively, by the 9.2 m buffers. The buffers
          were reported to be much more effective in removing sediment than
          nutrients. Buffer effectiveness was also reported to decrease with
          time and with decreasing buffer to source area ratio.

          Vegetative Buffer Research in Virginia

          Researchers at Virginia Tech (Dillaha et al., 1986; 1987; 1988;
          1989; and Lee et al., 1989) used a rainfall simulator to evaluate
          the effectiveness of grass buffers for the removal of sediment, N,
          and P from cropland runoff.     Simulated rainfall was applied to
          nine experimental field plots on an eroded Groseclose silt loam
          soil (clayey, mixed, mesic Typic Hapludalt) with a 5.5 by 18.3 m
          bare cropland source area and either a 0, 4.6, or 9.1 m long grass
          buffer (5.5 m wide) located at the lower end of each plot.
          Fertilizer was applied to the,plots at rates of 222 kg/ha of liquid
          N and 112 kg/ha of P,O, and K2*0. Water samples were collected from
          the base of each plot and analyzed for sediment and nutrient
          content. One set of plots was constructed so that flow through the
          filters was concentrated rather than shallow and uniform. The 9.1
          and 4.6 m grass buffers with shallow uniform flow removed 87 and
          75% of the incoming suspended solids, 69 and 57% of the incoming P,
          and 72 and 61% of the incoming N, respectively.            Dissolved
          nutrients in the buffer effluent were sometimes greater than the
          incoming dissolved nutrient load, presumably due to lower removal
          efficiencies for dissolved nutrients and the release of nutrients
          previously trapped in the buffers. Plots with concentrated flow
          were much less effective than the shallow uniform flow plots, with
          percentage reductions in sediment and nutrient loadings averaging
          23 to 37% less for sediment, 46 to 53% less for N, and 43 to 46%

                                           7











          less for P.

          The effectiveness of existing vegetative buffers in the
          Commonwealth of Virginia was qualitatively evaluated by visiting
          and observing vegetative buffers on 18 farms in Virginia (Dillaha
          et al., 1986). Buffers were evaluated by talking with landowners
          and soil conservationists and walking the length of the buffers to
          evaluate potential problems. All the vegetative buffers surveyed
          were composed of grasses and other low growing vegetation and were
          used in combination with cropland.        Almost all the buffers were
          installed for water quality improvement in conjunction with
          Virginia's Chesapeake Bay Program. Buffers were rarely used before
          1983 on cropland in Virginia because they were not a recognized
          conservation practice eligible for state or federal cost-sharing
          money. Buffer performance was generally judged to fall into two
          categories depending upon the topography of the site.           In hilly
          areas, buffers were judged to be ineffective for removing sediment
          and nutrients from surface runoff because drainage usually
          concentrated in natural drainageways within the fields before
          reaching the buffers. Flow across the buffers during larger runoff
          producing storms (the most significant in terms of water quality)
          was therefore primarily concentrated and the buffers were locally
          inundated and ineffective.      This assessment was confirmed by the
          fact that little sediment was observed to have accumulated in the
          majority of the buffers observed. Buffers in these areas, while
          not effective for trapping sediment and nutrients, were judged to
          be beneficial because they provided effective cover in areas
          immediately adjacent to streams that are often susceptible to
          severe localized channel and gully erosion. They also provide a
          narrow buffer between cropland and streams that may reduce the
          aerial drift of fertilizers and pesticides to streams during
          application.

          In flatter areas, such as in the Coastal Plain, buffers appeared to
          be more effective.      Slopes were more uniform, and significant
          portions of stormwater runoff entered the buffers as shallow
          uniform flow. This observation was supported by the presence of
          significant sediment accumulations in many of the Coastal Plain
          buffers surveyed.      Several one to three year old buffers were
          observed that had trapped so much sediment that they were higher
          than the fields they were protecting.          In these cases, runoff
          tended to flow parallel to the buffers until a low point was
          reached where it flowed across as concentrated flow.             In this
          situation, the buffers acted more like a terrace than a vegetative
          buffer. Flow parallel to the buffers also was observed on several
          farms where moldboard plowing was practiced.             When soil was
          turn-plowed away from the buffers, a shallow ditch was formed
          parallel to the field.      If this ditch was not removed later by
          careful disking, runoff again concentrated and flowed parallel to
          the buffer until it reached a low point and crossed as channel
          flow.     Conclusions drawn from the plot studies and on-site
          assessments of vegetative buffer effectiveness included (Dillaha et

                                              8









          al.  1989)

          1.   Vegetative buffers are effective for the removal of sediment
               and other suspended solids from cropland runoff only if flow
               is shallow and uniform and if the buffers have not been
               previously inundated with sediment.
          2.   The effectiveness of vegetative buffers for sediment removal
               appears to decrease with time as sediment accumulates within
               buffers.  This may or may not be a problem in "real world"
               buffers because vegetation may be able to grow through
               sediment accumulations.
          3.   Total N and P in runoff are not removed by vegetative buffers
               as effectively as sediment. Presumably, much of the N and P
               in cropland runoff is dissolved or associated with very fine
               sediment which vegetative buffers can not remove efficiently.
          4.   Shorter vegetative buffers (<10 m) are not effective in
               removing dissolved N and P from agricultural runoff.
               Dissolved P and N losses from the experimental vegetative
               buffer plots studied were often higher than the inflow,
               presumably due to the release of P and N trapped in the grass
               buffers previously.
IN        5.   Buffer strips characterized by concentrated or deeper channel
               type flow are much less effective for sediment, N, and P
               removal than vegetative buffers with shallow uniform flow.
               Buffers with concentrated flow were 40 to 60%, 70 to 95%, and-
               61 to 70% less effective with respect to sediment, P, and N
               removal than uniform flow plots.
          6.   Most on-farm vegetative buffers observed during the Virginia
               Tech study were judged to be ineffective for sediment and
               nutrient removal.   The majority,of flow entering the grass
               portion of the buffers was judged to be concentrated because
               runoff tended to accumulate in natural drainageways long'
               before reaching the vegetative buffers. This was more of a
               problem in hilly areas and less of a problem in flatter areas
               such as the Coastal Plain.

          The Virginia Tech researchers concluded that the effectiveness of
          the experimental vegetative buffers should not be used as a direct
          indicator of real world vegetative buffer effectiveness because of
          the concentrated flow problems previously discussed. Concentrated
          flow effects under real agricultural conditions were estimated to
          be orders of magnitude greater than those encountered during the"
          experimental field studies (Dillaha et al., 1989).

          Vegetative Buffer Models and Design Procedures

          Kentucky Filter Strip Model: Barfield et al. (1979) developed a
          steady state model, the Kentucky filter strip model, for
          determining the sediment filtration -capacity of grass media as' a
          function of flow, sediment, load, particle size, flow duration,
          slope, and media density. Outflow concentrations were primarily a
          function of slope and media spacing for a given flow condition.

                                           9








          The Kentucky filter strip model was extended for unsteady flow and
          non-homogeneous sediment by Hayes et al. (1979a) .      A graphical
          solution of the Kentucky model was described by Hayes et al.
          (1982). However, the complexity of the procedure makes solution of
          the equations difficult unless the sediment is well graded.
          Methods for determining the values of the hydraulic parameters
          required by the Kentucky model for real grasses were presented.
          Using three  different types of grasses, model predictions were
          reported to be in close agreement with laboratory data (Hayes et
          al., 1978).

          A simplified procedure derived from the Kentucky filter strip model
          was developed for the SCS to estimate the long-term effectiveness
          of grass filter strips (Hayes and Dillaha, 1992; Dillaha and Hayes,
          1992). The procedure estimates the trapping efficiency of grass
          buffers with respect to sediment but does not consider other types
          of contaminants or buffer vegetation. The model is fairly simple
          to use but requires the use of the WEPP model (Lane and Nearing,
          1989), which is not yet available to the public, to estimate
          sediment and surface runoff loadings to the buffer.

          CREAMS Model: Agricultural Research Service researchers (Flanagan
          et al., 1986; Williams and Nicks, 1988) have attempted to evaluate
          the effectiveness of vegetative buffers for erosion control using
          the CREAMS model (Knisel, 1980). Williams and Nicks (1988) applied
          CREAMS to a 1.6 ha watershed in Oklahoma.             Filter strip
          effectiveness was found to be dependent on strip length, Manning's
          n, slope, and slope shape. The authors concluded that CREAMS can
          be-a useful tool for evaluating vegetative buffer effectiveness in
          reducing sediment yield.      This model, like others mentioned
          previously, cannot consider the long-term effectiveness of
          vegetative buffers because it has no way of accounting for sediment
          accumulations within the vegetative buffer. Consequently, CREAMS
          would be expected to overestimate long-term sediment trapping.
          CREAMS also is severely limited by its sediment transport model
          that tends to overestimate sediment transport.      The model also
          cannot account for concentrated flow effects, and Manning's n is
          the only factor used to simulate the effects of vegetative buffer
          vegetation.   CREAMS does have nutrient transport submodels, but
          their use with vegetative buffers has not been reported.           In
          summary, CREAMS was not developed to describe vegetative buffers
          and use of the model for vegetative buffer design is highly
          questionable since it does not simulate the principal physical
          processes affecting transport in vegetative buffers.

          GRAPH Model:   Lee et al. (1989) developed an event-based model,
          GRAPH (GRAss PHosphorus), to simulate P transport in vegetative
          buffers by incorporating chemical transport submodels into the
          grass filter strip model in SEDIMOT II (Wilson et al., .1984; Warner
          et al., 1984), a stormwater.and sediment transport model developed
          for strip mine reclamation. The grass filter model in SEDIMOT II
          was derived from the Kentucky filter strip model, GRASSF, developed

                                           10








          by Hayes (1979).       GRAPH considers the effects of advection
          processes, infiltration, biological uptake, P desorption from the
          land surface to runoff, adsorption of dissolved P to suspended
          solids in runoff, and the effects of changes in sediment size
          distribution on P transport. Required data for the model includes:
          rainfall intensity and duration, an inflow hydrograph, a sediment
          graph, sediment size distribution, vegetative buffer dimensions and
          hydraulic characteristics, inflow graphs for dissolved P, P
          desorption and adsorption reaction coefficients for soil and plant
          matter, and the P content of each soil particle size class. GRAPH
          simulates time varying infiltration, surface runoff, sediment
          yield, particle size distribution, and dissolved and sediment-bound
          P discharge along with P and sediment trapping efficiencies in
          vegetative buffers. GRAPH was verified with data from vegetative
          buffer field plots. Model predictions and observed P transport 'in
          grass buffers compared favorably.

          Phillips Model: Phillips   (1989a,b) presented a theoretical method*
          for evaluating the relative effectiveness of buffer zones in
          removing sediment, sediment adsorbed chemicals, and dissolved
          chemicals from surface and subsurface flow. The method does not
          make absolute predictions of buffer effectiveness but rather
          estimates the effectiveness of a given buffer relative to a
          reference   buffer. Philips's      method predicts the          relative
          effectiveness of buffers for water quality improvement using two
          models, the hydraulic model and the detention model.        Neither' of
          the Phillips models have been validated or tested with field data
          because they attempt to characterize the long-term effectiveness of
          buffers and no long-term data on buffers has been collected with
          which to verify these or any other long-term models.

          Phillips Hydraulic Model: The Phillips hydraulic model was
          developed to describe the transport of sediment and sediment-bound
          chemicals through vegetative buffer zones. The model assumes that
          sediment transport through the filter is a function of the energy
          of overland flow and is based on the Bagnold stream power equation.
          The detention model equation as originally proposed by Phillips is:
                                            0.4( Sb)-I.3(.1
                                 Bb =( KII)(_@b)       b)0.6
                                 BZ K, Lz      SZ     n.
          where:  K =   saturated hydraulic conductivity of the buffer soils
                  L =   buffer length
                  s =   sin 6, where 6 is the slope angle relative to the
                        horizontal
                  n =   Manning roughness coefficient
                  b =   subscript denoting the buffer    of interest,-and
                  r =   subscript denoting the reference buffer

          The hydraulic model is derived as follows.        For a given mass of
          water, Bagnold's stream power equation (Bagnold, 1977) can be used
          to estimate the sediment transport capacity. That is, the time rate









          of energy expendature per unit weight of flowing water is:
                                        PgAL VS -VS                         (2]
                                             pgALr
          where:   p  =  density of water
                   g  =  gravitational constant
                   A  =  cross-sectional area of flow
                   Lr =  length of the flow reach
                   V  =  mean flow velocity
                   s  =  slope of the hydraulic grade line (-sin 0)

          For steady state flow conditions, the flow rate per unit width can
          be expressed as:
                                            q= VA = Vy                            (3]

          where y is the steady-state f low depth.            Equation [ 3 ] can be
          rearranged to:
                                              V= q                                [4]
                                                 Y

          The average flow velocity can be expressed with Manning's equation
          as:
                                          V= -1 R2/3S1/2                          [)
                                              n

          where R is the hydraulic radius of the flow.           For shallow sheet
          flow conditions, R = y. Multiplying both sides of Equation (5] by
          the area per unit width gives the flow rate per unit width:
                                        q= VA= 1 y2/3 S 1/2A                     [6]
                                              n 

          For steady, shallow sheet flow      conditions, A = y and Equation      [6]
          can be rearranged to:
                                               nq 3/5                            [7]
                                              S1/2
          Equations [2], [4] and [7] can      then be combined to:

                                        Pu= 0.4S1. 3n -0. 6                  [8]
          Phillips incorporated buffer length, L, into Equation (8] by
          assuming q = Li, where i is the steady-state excess rainfall rate.
          Equation (8] can then be expressed as:

                                      PU=LO 0.4I0.4s1.3 n-0.6                   [9]
          Then, denoting the saturated hydraulic          conductivity, K, as an
          indicator of the infiltration capacity of      the soil, a general index
          of buffer effectiveness relative to a reference buffer is obtained:



                                               12





                                                                                                 A





                                 Bb = Kb(Lb) 0.4( ib)o.4(  -1.3(0.6                (10]
                                                       
                                 Br   Kr  Lr ir         Sr    nr.
           Assuming that rainfall intensity is        the same on both the buffer of
           interest and the reference buffer, ib= ir, Equation (10 reduces to
           Phillips, hydraulic model (Equation [1]).

           Phillips Detention Model: The Phillips' detention model estimates
           the relative effectiveness of buffers in removing dissolved
           substances from surface and subsurface flow through vegetative
           buffer. Contaminate removal in the buffer is defined as a function
           of the total contact time of both surface and subsurface flow in
           the buffer. The model is derived from Darcy's law and the Manning
           equation.
                    

           where M is the soil moisture storage capacity and the other terms
           are defined as before.         The soil moisture storage capacity is
           defined as the available soil moisture content (soil moisture
           content at field capacity minus soil moisture content at wilting
           point) times the lessor of the seasonable high water table depth or
           the depth to a confining soil layer.

           Phillips derived the detention model as follows.                 The  total
           contact time due to surface runoff through the buffer can be
           expressed as:

                                                T= L                                [12]
                                                    V

           where T, is the detention time due to surface runoff in the filter.
         Combining Equations [2], [8], and [12] gives:

                                         Ts=ns-0.3q O.4 L                          (13]
           where q. is the surface discharge.

           For subsurface flow, Phillips used Darcys law to estimate the
           velocity of subsurface flow:
                                                V=Ks                                 [14]

           where K = saturated hydraulic conductivity. Detention time due to
           subsurface flow,, Tq., was then estimated as:
                                               Tg=KsL                                [15]
           Considering both surface and subsurface throughflow, Phillips
           defined an index of detention, T*, for a given flow of:



                                                 13







                         T*=T,*T,=[nO-6Ls-0.3 (qlq,,) -1-1]*[.KsL(qq1q)J    [16]
         where q. is the subsurface discharge component of flow.        Phillips
         assumed that q, q.,, and % were a function of K. Phillips further
         assumed that "the portion of discharge which travels overland or in
         subsurface flow is a fucntion of the infiltration capacity, which
         is assumed to be a fucntion of K. For the overland flow component,
         detention time varies as the -0. 4 power of q.. For a given
         stormwater mass, since K is an index or surrogate of q,, T=f (K-0*4).
         Since the portion of the discharge traveling on the surface is an
         inverse function of K, the sign is reversed and T=f(K")." The
         relative abilities of buffers to hold infiltrated water was given
         by MIM, where M is the soil moisture storage capacity obtained by
         multiplying the available soil moisture capacity (field capacity
         minus wilting point) by the depth to the water table or a confining
         soil layer. Equation (16] can then be expresses as:
                                   b     b)2(L        -0.7( M
                             T* .6            b)0.4( Sb)   b)
                                                                            [17]
                             T*.r nr    Lr   K,    S,     M.-
         which is functionally identical to Equation (11].

         other Design Approaches: Procedures for the design of vegetative
         buffers with respect to organics removal have been presented by
         Norman et al. (1978) and Young et al. (1982). However, these
         procedures were based primarily on infiltration or limited organics
         removal data.    Regression type design equations for P reduction
         were presented    by Young et al.     (1982), but details of their
         development were not presented and they have not been verified.

         State and Federal Vegetative Buffer Programs

         Chesapeake Bay Preservation Act: Regulations have been proposed in
         Virginia as part of the Chesapeake Bay Program to require
         vegetative buffers along all water bodies in designated Resource
         Protection Areas (Chesapeake Bay Preservation Area Designation and
         Management Regulations, Chesapeake Bay Local Assistance Board).
         The Resource Protection Areas are defined as "sensitive lands at or
         near the shoreline that have intrinsic water quality value ... and
         are sensitive to impacts which may cause significant degradation to
         the quality of state waters or loss of aquatic habitat."           This
         definition includes all tidal and nontidal wetlands, tidal
         shorelines, and all tributary streams within Virginia's Chesapeake
         Bay drainage basin.

         Along all tidal waters, a 100 ft (30.5 m) vegetative buffer zone is
         required, and a 50 ft (15.2 m) buffer is required along nontidal
         waters.   The buffer length is measured from the mean high water
         level of  nonvegetated wetlands and from the wetland for vegetated
         wetlands. If agricultural lands are adjacent to waters, then "the
         buffer zone area shall maintain as a minimum best management


                                           14








          practice, a 25 ft (7.6 m) wide vegetative buffer measured landward
          from the mean high water level of tidal waters or tributary
          streams, or from the landward edge of any wetlands." Buffers will
          not be required for agricultural drainage ditches if the adjacent
          land has best management practices in place in accordance with a
          conservation plan approved by the local Soil and Water Conservation
          District.   The regulation specifies that the vegetative buffer
          shall be composed of either trees with a dense ground cover, grass,
          or an approved legume cover which can be managed to prevent
          concentrated flows from breaching the vegetative buffer.         The
          vegetative buffer must be maintained until the landowner has
          implemented an approved BMP program which provides water quality
          protection at least the equivalent of that provided by the
          vegetative buffer. The regulation specifies that for the purposes
          of the Act, 100 ft buffers remove 75 and 40% of the incoming
          sediment and nutrients, respectively.

          The proposed regulations are a step in the right direction, but it
          is highly unlikely that they will result in 75% sediment and 40%
          nutrient reductions.   In many areas,    little if any runoff will
          flow across the buffers and in other areas most runoff will cross
          the vegetative buffer as concentrated flow. This will be a major
          problem with the proposed regulations because vegetative buffers
          will only be required along perennial streams depicted on USGS
          topographic quadrangle maps.    Consequently, most surface runoff
          will collect in ephemeral drainageways before reaching. the
          vegetative buffers and cross as concentrated flow. The state of
          Maryland has similar buffer zone requirements along shorelines and
          major tributaries but does not require vegetative buffers
          explicitly for removing pollutants from surface runoff.

          Conservation Reserve Program: The use of constructed vegetative
          buffers in the United States has increased significantly in the
          past few years, because vegetative buffers were an approved USDA
          cost-share practice under the Conservation Reserve Program (CRP) of
          the Food Security Act of 1995.        The CRP was established to
          encourage farmers to take highly erodible land out of crop
          production and convert the land to permanent (10 year) cover. This
          program was designed to reduce soil erosion, improve water quality
          and wildlife habitat, as well as eliminate production of excess
          commodities. Farmers participating in the CRP receive an annual
          rental payment for land enrolled in the program.      As originally
          implemented, only land classified as highly erodible was eligible
          for participation in the CRP.    In 1988, the CRP was modified to
          include vegetative buffers because of their potential environmental
          benefits.   The requirement that the land be highly erodible was
          eliminated for vegetative buffers. Requirements for vegetative
          buffers under the CRP include:

          1.  The land must be adjacent and parallel to a stream, river,
              lake, estuary, or wetland greater than 2 ha (5 acres) in area.
          2. The land must have been planted in an agricultural commodity

                                           15









              in at least two years from 1981 through 1985.
         3.   The land must still be suitable for crop production.
         4.   The land with a vegetative buffer must be capable of reducing
              sediment delivery to adjacent water bodies.
         5.   The land must be planted to permanent grasses, trees, or
              shrubs.
         6.   The vegetative buffer must be a minimum of 20 m (66 ft) in
              length and no more than 30 m (99  ft) in length.
         7.   The vegetative buffer may not be grazed or harvested during
              the 10 years of the contract.

         National Vegetative Filter Strip Conservation Practice Standard:
         The U.S. Soil Conservation Service is currently in the process of
         updating the national conservation practice standard for vegetative
         buffers to overcome some.of the limitations of vegetative buffers.
         The proposed standards define vegetative buffers as vegetated areas
         which are    designed   to remove sediment, nutrients, pathogens,
         organic materials, pesticides, and other contaminants from surface
         runoff by filtration, deposition, infiltration, adsorption,
         adsorption, decomposition, and volatilization. The key word here
         is "designed".     This implies that vegetative buffers are not
         suitable for every site and that their length and position will be
         a function of local site conditions and hydrology.

         An important part of the proposed standard is the statement: "The
         practice (vegetative buffer) applies    ... in locations above the
         occurrence of concentrated flow and above conservation practices
         such as terraces or diversions which concentrate flow." The new
         standard relaxes previous requirements that vegetative buffers be
         located immediately adjacent to streams and instead says that they
         should be located where they will be the most effective for
         pollutant removal. This may be at the lower boundary of a field,
         or it may be within a.field.

         The proposed standards suggest that the design of vegetative
         buffers and the suitability of a particular site for vegetative
         buffers must consider (Dillaha, 1989):

         1.   Adequacy of soil drainage and depth to water table to ensure
              satisfactory   vegetative   growth   and   prevent    prolonged
              saturation of the soil.
         2.   Provisions for preventing hillside seeps and other continuous
              discharge of water through the vegetative buffers.
         3.   Reduced effectiveness of vegetative buffers under snow or
              frozen ground conditions.
         4.   Vegetative buffer length required to provide the desired
              pollutant reduction over the design life of the vegetative
              buffer. In other words, pollutant accumulation and subsequent
              release from vegetative buffers must be a design constraint.
         5.   The effects of slope onvegetative buffer effectiveness.
         6.   Provisions for mowing, to maintain the effectiveness of
              vegetative buffers composed of grass and similar vegetation.

                                          16








         7.  Effects of grazing on.vegetative buffer.performance.
         8.  Effects of application of herbicides to vegetative buffers or
             adjacent fields for weed control. , If herbicides are applied
             to fields, sprayers should be turned off before crossing
             vegetative buffers.or using them for turn rows.
         9.  Vegetative buffers should be installed on the contour as much
             as possible to filter runoff before it concentrates in natural
             drainageways.
         10. Care should be taken during tillage operations to avoid
             tilling into vegetative buffers and causing localized flow
             problems.
         11. Large fields with significant natural drainageways or grassed
             waterways are acceptable for vegetative buffers only if the
             vegetative buffers are installed on both sides of internal
             field drainageways. This will allow pollutants to be trapped
             before they can enter the drainageways.
         12. Some sites may require limited grading to correct flow
             problems within the vegetative buffers caused by gullies or
             high areas within or immediately downslope of the vegetative
             buffers.
         13. Shrub and wildlife strips should not be permitted because they
             are relatively ineffective for water quality improvement when
             compared to grass and legume vegetative buffers.
         14. At sites with significant flow along or parallel to vegetative
             buffers, shallow berms or diversions may be needed at 15 to 30
             m intervals to intercept runoff and force it to flow through
             the vegetative buffer before it can concentrate further.
         15. Vegetative buffers should not be installed in areas higher
             than the fields they are intended to protect.

         Sediment and Nutrient Loadings Due to Eroding Banks

         A major difficulty with the Chesapeake Bay Preservation Act as
         currently implimented is that the Act makes it difficlt to modify
         buffer zones even if the modifications would reduce sediment and
         nutrient loadings to the Bay and other water bodies. Consider the
         case of eroding banks. Ibison et al. (1990) examined the loss of
         sediments and nutrients from eroding tidal shorelines along the
         Chesapeake Bay and its tributaries. Eroding banks were reported to
         be responsible for 5.2 and 23.6% of the controllable N and P,
         respectively, entering Virginia's portion of the Chesapeake Bay.
         In a follow up study, Ibison et al. (1992) confirmed the results of
         the previous study and reported that the sheer mass of materials
         lost through shoreline erosion results in-nutrient loading rates
         (on an areal basis) to , the Chesapeake Bay several orders of
         magnitude higher than upland loading rates. For example, N and P
         losses from shoreline erosion were estimated to be approximately
         25,000 kg-N/ha-yr and 15,000 kg-P/ha-yr versus losses of 2 to 80
         kg-N/ha-yr and 0.3 to 19 kg-P/ha-yr for cultivated farm land.
         Consequently, stabilizing one hectare of eroding bank may reduce
         nutrient loadings to the Bay as much as stopping all nutrient loss


                                         17










          from 300 to 800 ha of cultivated farmland.

          Many of the actively eroding banks along the Chesapeake Bay and its
          tidal tributaries are characterized by high steep banks. The banks
          erode at an average rate of 0.2 m/yr (Byrne and Anderson, 1977)
          with reported rates as high as 3.3 m/yr (Ibison et al., 1992). To
          stabilize these banks, disturbance of the existing riparian zone is
          often required for the construction of shoreline structures or to
          grade the bank back to a stable slope of 2 to 1 (run to rise) or
          50%.   This necessitates removal of all vegetation during grading
          and then replanting after grading is complete.          It may also be
          necessary to remove large trees on and in the vicinity of the bank
          to prevent mass slumping and loss of soil during large storms that
          cause trees to fall down the banks, bringing tons of soil with
          them.   The Shoreline Programs Bureau recommends that all large
          trees be removed from steep slopes and that large trees also be
          removed from the zone within two bank heights distance from the top
          of steep banks.     Removal of trees under these circumstances is
          estimated to reduce average annual soil loss from banks by
          approximately 10% (Hill, 1992).

          Literature Review Sununary

          As   discussed    previously,    vegetative    buffers    as    presently
          implemented are unlikely to be very effective in removing sediment
          and nutrients from surface runoff because they are usually
          installed with little consideration of site conditions which affect
          their performance.       Equations which have been developed for
          vegetative buffer design assume that runoff is uniformly
          distributed across the width of vegetative buffer as shallow sheet
          flow.   This will rarely be the case in real world situations as
          flows in all but the most uniformly sloping fields tend to
          concentrate in internal field drainageways before reaching field
          boundaries    where   vegetative    buffers    are   usually     located.
          Consequently, significant portions of field runoff will cross the
          vegetative buffers as concentrated flow, locally inundating the
          vegetative buffers, and greatly reducing vegetative buffer
          effectiveness for sediment and nutrient removal.           In addition,
          almost all vegetative buffer research reported has been of a
          short-term    nature    which   did   not    consider    the    long-term
          effectiveness of vegetative buffers for pollutant       reduction. The
          design equations and models developed from these studies do not
          consider the effects of sediment and nutrient accumulation in
          vegetative buffers. Consequently, they will probably over predict
          vegetative buffer effectiveness over the long run.

          Of all the models reviewed, only the Phillips method was developed
          for vegetation other than grasses. The Phillips method, because of
          its theoretical basis and simplicity, is also easily modified to
          account for important factors such as concentrated flow and
          vegetative uptake which were not considered by any of the models


                                             18








        discussed.  Consequently, the Phillips method appears to be the
        most reasonable model for estimating relative buffer strip
        effectiveness and satisfying the objectives of this project.











































                                       19








                             MODEL DEVELOPMENT

         The method developed by Phillips (1989a,b)       was selected for
         evaluating buffer zone effectiveness. The method was modified to
         correct several errors and weaknesses in the model and to better
         reflect Virginia conditions. In addition, a procedure is presented
         to account for the benefits of shoreline protection structures in
         riparian buffer zones.


         Phillips Hydraulic Model

         Several limitations were encountered with Phillips's hydraulic
         model. First, the buffer length term, L, is used to estimate the
         flow rate per unit width to the buffer zone.     But buffer length
         does not give an estimation of unit loading to a buffer. What is
         needed to estimate loading is the length of the upslope area
         contributing runoff to the buffer plus the length of the buffer.
         Consequently, an additional term, L*, must be defined that
         represents "the length of the upslope area contributing runoff to
         the buffer." The L in the Phillip hydraulic model can therefore be
         redefined as L + L*.

         A second limitation involves the saturated hydraulic conductivity
         term, K, which is used to estimate infiltration in the buffer zone.
         Phillips's approach does not consider the effects of buffer zone
         length on infiltration, ie. infiltration losses from surface runoff
         would be as significant with a one meter length buffer as they
         would be for a 100 m buffer. To incorporate the effects of buffer
         length on total infiltration, the hydraulic conductivity term can
         be multiplied by the buffer length, L.

         A third limitation of Phillips's approach is the failure to
         consider the effects of concentrated flow.       Concentrated flow
         effects can be incorporated into the model by incorporating the
         ratio of the fraction of flow entering the buffer and the reference
         buffer as shallow sheet flow.

         The new hydraulic model with the above modifications can now be
         expressed as:
                          Bb =( KbLb)@
                                    L,*+L    Sr     n
                                        .r


         where: X     saturated hydraulic conductivity of buffer soils
              L     buffer zone length
              L*    length of area upslope of the buffer contributing runoff
                    to the buffer
              S     sin 0, where 0 is the slope angle relative to the

                                          20










                        horizontal
                 n =    manning roughness coefficient
                 C =    fraction of surface runoff crossing the buffer as sheet
                        flow
                 b =    subscript denoting buffer of interest, and
                 r=     subscript denoting reference buffer

            Phillips     Detention Model

           In checking the derivation of the Phillips detention model, an
            error was found in the derivation.               The model is rederived as
            follows.     Given Equation (12] and Darcys law (Equation [14]),
            Equation (15] is actually:
                                                  Tg=L                                   (19                                                      Ks
								    Ks
            Equation [16] therefore becomes:
                                  T*=[n 0.6 Ls -0.3 (q,/qq)-0.4]* L (qg/q)                [20]
                                                                 Ks

           As with the hydraulic model, if K is substituted for (6qq6q/6q q) in the
            subsurface flow portion of Equation [201, the equation reduces to:

                                        T*=nO.6L2 S-1.3 (qs,/q q) -0.4                      [21]
            This equation is identical to Phillips' except the exponent of the
          slope term changes from -0.7 to -1.3.                The saturated hydraulic
            conductivity can also be substituted for qs/q, but in this case,
            the sign of the exponent of K must be reversed because surface
            runoff is inversely proportional to K since higher values of K
            indicate higher potential for infiltration and therefore reduced
            runoff. Equation (21] can therefore be simplified to:

                                            T*=n O.6L2s-1.3KO.4                             [22]

            The resulting hydraulic model with the addition of                      the    soil
            moisture storage capacity term discussed previously is:
                           Bb=nb0.6Lb2Kb 0.4 Sb -1.3 Mb
                           Br nr Lr   Kr       Sr       Mr                                        [23]
                                   

            As with the hydraulic model, a concentrated              flow term should      also
            be added to account for short circuiting due to concentrated                   flow
            effects.     An additional term is also added to account for the
            relative ability of different types of vegetation and vegetative
            growth stages to assimilate nutrients. Since little information is
            available on nutrient uptake for the ecosystems of interest, the
            net productivity of relevant ecosystems is used as an indicator of
            nutrient uptake since they are proportional.                    This factor is


                                                    21








                                                                                                     A


           incorporated into the model as a ratio similar to the way
           concentrated flow effects are handled. The resulting equation is:
                             Bb .( n.)0'6( Lb)2                  Cb)( Vb)             [24]
                             Br nr       Lr   K.-           M.- Cr V@
           where V is the vegetative uptake or net pr, Am @tivity of the
           vegetative buffers being compared.           It is essential to consider
           both the type of vegetation and the maturity of the vegetative
           system when estimating net productivity. For example, a riparian
           zone in a rapidly growing early secession stage would be expected
           to be able to assimilate and hold more nutrients than a more stable
           mature riparian buffer approaching or at climax.

           The original Phillips detention model did not consider the effects
           of the length of the buffer on the soil moisture capacity.
           Intuitively, one would expect the soil moisture capacity to be
           proportional to the area of the buffer since a buffer twice as long
           as another buffer, all other conditions equal, would be expected to
           have twice the soil moisture storage capacity. Consequently, M
           should be multiplied by L to account for the area of the buffer                *
           The vegetative uptake factor, V, is also multiplied by L for
           identical reasons. The resulting detention model is:
                                     0 6    4    1 4( b)-I,I(    C")( V
                             B           Lb) ( b)                     b)              [25]
                             B. n.       Lr K,       s,     Mr C, V,

           Readers should keep in mind that the         the original Phillips models
           as well as the proposed model are theoretical and have not been
           verified due to a lack of adequate field data for validation.

           Shoreline Erosion Model

           Shoreline erosion: The proposed shoreline erosion model is simple
           but should provide a fairly good estimate of sediment and nutrient
           losses per meter of shoreline for both protected. and unprotected
           shoreline.      The model does not estimate sediment and nutrient
           losses from partially protected shorelines.                  For stabilized
           shorelines, shoreline erosion and nutrient losses are assumed to be
           zero.


           If a shoreline is not protected and actively eroding, the sediment
           loss is estimated in a manner similar to that recommended by Ibison
           et al. (1992):
                                            YB z HB EB pB                             [26]
           where: Y. = bank sediment loss per unit width (kg/m-yr)
                 E..= bank or shoreline erosion rate (m/yr)
                 H, = bank height (m)
                 PB = bulk density of bank soil (kg/m')


                                                 22











              If the nutrient content of the bank is known or can be estimated
              then:
                                                         Y,I     YB NB                                      [27]
                                                                 1000
                                                         YBP     YB PB                                      [28]
                                                                 1000
              where:      YB,,       nitrogen lost in eroding bank sediment (kg/m-yr)
                     YB,     phosphorus lost in eroding bank sediment (kg/m-yr)
                     X.      concentration of nitrogen in bank material (mg/g)
                     PB      concentration of phosphorus in bank material (mg/g)

              Upland      Erosion:         To determine the relative contributions. of
              sediment and nutrients from shoreline erosion and disturbed upland
              requires an estimate of shoreline and upland contributions. Upland
              losses need to be estimated per unit width like the shoreline
              losses for comparison purposes.                          Since the actual trapping
              efficiency of the buffer is unknown, the conservative assumption
              is made that the buffer trapping efficiency is 100% for the portion
              of the runoff entering the buffer as shallow sheet flow.                                        All
              sediment and nutrients in concentrated flow are assumed to pass
              through the buffer unattenuated. Sediment, nitrogen and phosphorus
              movement through the buffer can therefore be represented as:

                                                       Yj*   YU (1-C)                                       (29)
                                                      Y@N    YUN ( i - C                                    [30]
                                                      G      YUP (1-C                                       (31]
              where: Y,,*            mass of sediment from upland area passing through
                                     buffer (kg/m-yr)
                     Yu      sediment        loading to the buffer                     from the upland
                             contributing area (kg/m-yr)
                     C           fraction of surface runoff from the field entering the
                                 buffer as sheet flow
                     Yum*        mass of nitrogen from upland area moving through
                                 buffer (kg/m-yr)
                     Y,,.        loading of nitrogen to buffer from upland area (kg/m-
                                 yr)
                     Y,,*        mass of phosphorus from upland area moving through
                                 buffer (kg/m-yr)
                     Ym          loading of phosphorus to buffer from upland area
                                 (kg/m-yr)

              The sediment loading to the buffer from the upland contributing
              area, Yu, can be estimated from published values for various
              landused- or it can be calculated directly using the Universal Soil
              Loss Equation (Wischmier and Smith, 1976), the Revised Universal
              Soil Loss Equation (Renard et al., 1992), WEPP (Lane and Nearing,

                                                               23








           1989), or other similar erosion predictor.                The estimate will
           probably be given in tons/acre-yr and will need to be converted to
           kg/m-yr to be consistent with the shoreline erosion units. If the
           soil loss units are in tons/acre-yr, then the estimated sediment
           loading to the buffer can be converted to kg/m-yr with the
           following equation:
                                           Y,= 2242   A,, E                             [32]
                                                        WB
           where: A,,         upland area contributing sediment to the buffer,
                              acres
                 Eu = sediment loss from upland area contributing sediment to
                        the buffer as predicted by USLE or other method,
                        tons/acre-yr
                 W. = buffer width (long dimension of buffer, dimension
                        perpendicular to field slope direction), m

           Similarly, if the nutrient loadings to the buffer are in lbs/acre-
           yr, they can be converted to kg/m-yr with the following equations:
                                            YU'V = C1 AU NU                             (33]
                                                       WB

                                            YUP = C1  AU PU                             [34]
                                                       WB                                               Z

           where: NU          nitrogen loading to buffer from upland area, M/L'
                 P, = phosphorus loading to buffer from upland area, M/L1
                 C1 =   units conversion factor
                        = 0.454 for nutrient loadings (N, or Fu) in lb/acre and
                           upland area (Au) in acres
                        = 1.0 for nutrient loadings (Mu or Pu) in kg/ha and
                           upland area (Au) in hectares

           Impact of Shoreline Stabilization: If modifications made to the
           buffer due to shoreline stabilization reduce the effectiveness of
           the buffer as predicted by Equations [18] and [25], then the
           combined      effects     of    buffer     effectiveness      and      shoreline
           stabilization must be evaluated to assess the effectiveness of the
           system as a whole. The combined effects of buffer modification and
           shoreline stabilization can be estimated as follows. To compare
           the effectiveness of the buffer and shoreline stabilization, it is
           necessary to come up with an estimate of the absolute (not
           relative) effectiveness of the proposed buffer.                  The modif ied
           Phillips models do not do this directly, but they can be used in an
           indirect way to do so.

           First assume that the reference buffer is 100% effective in
           removing sediment from shallow sheet flow and totally ineffective

                                                  24









         in removing contaminants from concentrated flow.     The amount of
         sediment and nutrients moving through the reference buffer can then
         be estimated using Equations [29], [30] and [31].      The relative
         effectiveness of the reference and proposed buffer is calculated
         using the modified hydraulic model, Equation [18].               The
         effectiveness of the buffer/shoreline stabilization system, E,, can
         be estimated as:
                                  Es@       YV*                           [35]
                                      (Bb / B') ( YB + YU.-

         where BbIBI is the relative buffer effectiveness predicted with the
         hydraulic model. If E,<l, the buffer/shoreline stabilization system
         has a net positive effect.   If Es>l, the negative consequences of
         buffer alteration outweigh the benefits of shoreline stabilization.



































                                          25








                      Buffer Evaluation Procedure

         The  steps to follow in applying the model are as follows:

         1.   Decide upon an acceptable effectiveness of the buffer relative
              to the reference buffer in advance.     For example, if it is
              decided that the buffer must be at least as effective as the
              reference buffer then the ratio of B,/B, for both the hydraulic
              and detention models must be greater than or equal to 1.0. If
              the given buffer only needs to be 90% as effective as the
              reference buffer then the ratio of BbIB, would need to be
              greater than or equal to 0.9. The important idea is to decide
              on the required ratio before analysis.

         2.   Collect model data required to define the characteristics of
              the reference buffer and the buffer of interest.             The
              reference buffer can be either a reference buffer whose
              characteristics are defined by a local regulatory authority or
              it may be the characteristics of the buffer you are
              considering modifying before modifications. Parameters that
              must be defined for both buffers include:

              K  =  saturated hydraulic conductivity of buffer soils
              L  =  buffer zone length
              L* =  length of area upslope of the buffer contributing runoff
                    to the buffer
              s  =  sin 0, where 6 is the slope angle relative to the
                    horizontal
              n  =  manning roughness coefficient
              C  =  fraction of surface runoff crossing the buffer as sheet
                    flow (modify for proposed hydraulic modifications in
                    proposed buffer)
              N  =  soil moisture storage capacity
              V  =  vegetative uptake or net productivity factor

              Procedures, sources of information and tables for estimating
              these values are given in the following section.

         3.   Estimate the effectiveness of the proposed buffer for removing
              sediment and sediment-bound chemicals using Equation [18], the
              hydraulic model. If the proposed buffer is not as effective
              as desired, consider increasing the buffer length and changing
              the proposed vegetation (higher density vegetation may be used
              to increase Manning's n). It may also be possible to reshape
              the upland area or install hydraulic structures to reduce
              concentrated flow and increase the proportion of sheet flow
              entering the buffer. It is unlikely that properties such as
              slope and hydraulic conductivity can be changed.

         4. Estimate the effectiveness of the proposed buffer relative to
              the reference buffer for removing dissolved chemicals using

                                          26









               Equation [25], the detention model, and the data values
               collected in step 1.      If the proposed buffer is not as
               effective as desired, consider increasing the buffer length
               and changing the proposed vegetation (higher density
               vegetation may be used to increase Manning's n and the
               vegetative uptake factor, V).     It may-also be possible to
               reshape the upland area or install hydraulic structures to
               reduce concentrated flow and increase the proportion of sheet
               flow entering the buffer. It is unlikely that properties such
               as slope and hydraulic conductivity can be changed.

          5.   If shoreline erosion is not a factor, the evaluation and/or
               design is complete. If shoreline erosion is to be considered,
               continue with steps 6 through 9.

          6.   Calculate sediment, nitrogen and phosphorus losses due to
               shoreline erosion using Equations (26], [27] and (28],
               respectively.

          7.   Estimate sediment,  nitrogen, and phosphorus losses from the
               field contributing surface runoff to the buffer using the USLE
               or other soil loss estimation technique, estimates of soil
               nutrient concentrations, or general estimates of sediment and
               nutrient losses from the literature for various landuses.

          8.   Calcu late sediment, nitrogen and phosphorus transport through
               the buffer using Equations [29], [30], and [31].

          9.   Calculate the overall effectiveness of the buff er/shore line
               protection system using Equation (35].       If E,<l then the
               combined buffer/shoreline stabilization system is more
               beneficial than the original buffer alone.   If E,>1, shoreline
               stabilization results in increased sediment and nutrient
               losses.























                                           27









                DATA FOR BUFFER EVALUATION PROCEDURE

          Data Requirements for the Hydraulic and Detention Models

          The following values must be estimated to use the modified
          hydraulic and detention models:

              Saturated hydraulic conductivity of buffer soils, K
              Buffer length, L
              Length of area upslope of the buffer contributing runoff to the
              buffer, L*
              Sin 0. where 0 = slope angle relative to the horizontal, s
              Manning roughness coefficient, n
              Fraction of surface runoff crossing buffer as sheet flow, C
              Soil moisture storage capacity, M
              Vegetative uptake or net productivity factor, V

          These values can be estimated as follows.

          Saturated Hydraulic Conductivity, K: can best be estimated from
              permeability values given in modern county soil survey reports.
              These values are generally presented in the "Physical and
              Chemical Properties of the Soils" table. Permeability values
              are usually given as a range in inches\hour.          For planning
              purposes, use the midpoint of the range unless better
              information on the permeability value is available.          Typical
              permeability ranges given in soil surveys are given in Table I.
              Hydraulic conductivity can also be estimate if the general soil
              texture is known as shown in Table Il. Soil survey estimates
              are preferred, however.

                        Table I. Estimation        of    hydraulic
                                    conductivity values from Soil
                                    Survey permeability estimates.


                                 Permeability Suggested
                                    Range,       Hydraulic
                                     in/hr     Conductivity

                                 ------------ ------------
                                    0.06-0.2        0.13
                                    0.2-0.6         0.40
                                    0.6-2.0         1.30
                                    0.6-6.0         3.30
                                    6.0-10.0        8.00










                                             28








               Table II. Hydraulic conductivity as a function of soil
                          texture.


                                        Hydraulic Conductivity, m/day
                    Soil Texture              Range       Suggested

                    Clay soils, surface      0.01-0.2       0.10
                    Loam soils, surface       0.1-1         0.50
                    Fine sand                   1-5         2.00
                    Medium sand                5-20         10.00
                    Coarse sand               20-100        40.00
                    Clay, sand,  gravel mix 0.001-0.1       0.05



          Buffer Length, L: is the distance measured along the land surface
              between the upland edge of the buffer and the down slope edge
              of the buffer. Buffer length is best measured in the field but
              it may also be estimated from areal photos. Any units may be
              used as long as they are consistent with the units used in
              estimating the upslope slope-length contributing runoff to the
              buffer, L*.

          Upslope Slope-length Contributing Runoff to the Buffer, L*: is the
              average length that surface runoff tranverses in the field or
              area before reaching the buffer. It is best estimated from on
              site field estimates, but it can also be estimated from areal
              photographs. The average upslope   Contributing slope-length can
              also be estimated as:
                                         L* = AU                             [361
                                               WB


              where A. is the area of the field contributing runoff to the
              buffer, andWB is the width of the buffer (long dimension of the
              buffer).

          Sin 0, s: is a measure of the land     slope across the buffer where
              0 is the slope angle relative to the horizontal. The value of
              s is best measured from in buffer surveys but it can also be
              estimated from topographic maps.     Values for Sin 0, s can be
              estimated from estimates of buffer slope in percentage or
              degrees using the data given in Table III.









                                            29









          Table III. Estimates of Sin(O) for various buffer slopes in
                        percent and degrees.


              Buffer   Buffer       S,      Buffer   Buffer      S,
              Slope,    Slope, Sin(O)       Slope,   Slope,    Sin(O)
             percent Degrees              percent- Degrees

                0.5       0.3     0.0050      40       21.8    0.3714
                 1        0.6     0.0100      45       24.2    0.4104
                 2        1.1     0.0200      50       26.6    0.4472
                 3        1.7     0.0300      60       31.0    0.5145
                 4        2.3     0.0400      70       35.0    0.5735
                 5        2.9     0.0499      80       38.6    0.6247
                 6        3.4     0.0599      90       42.0    0.6690
                 7        4.0     0.0698      100      45.0    0.7071
                 8        4.6     0.0797      120      50.2    0.7682
                 9        5.1     0.0896      140      54.4    0.8137
                 10       5.7     0.0995      160      58.0    0.8480
                 12       6.8     0.1191      180      60.9    0.8742
                 14       8.0     0.1386      200      63.4    0.8944
                 16       9.1     0.1580      300      71.5    0.9487
                 18       10.2    0.1772      600      80.5    0.9864
                 20       11.3    0.1961     1200      85.2    0.9965
                 25       14.0    0.2425     2400      87.6    0.9991
                 30       16.7    0.2873     4800      88.8    0.9998
                 35       19.3    0.3304     9600      89.4    0.9999





          Manning's Roughness Coefficient, n: is a measure of the roughness
               of the lands surface in the buffer. It is used to estimate how
               much surface runoff is retarded (reduction in velocity of
               overland flow) as it passes through the buffer.            Manning's
               roughness coefficient is difficult if not impossible to measure
               directly, so it is usually estimated from tabular values that
               give the roughness coefficient as a function of land use and
               condition.    The roughness coefficients used in the buffer
               evaluation procedure should be for shallow flow conditions, not
               for the more readily available channel flow conditions.
               Manning's roughness coefficient for shallow sheet flow
               conditions can be estimated from Table IV.,










                                              30









                  Table IV. Estimates       of    Mannings      roughness
                             coefficient as a function of landuse and
                              condition    for   shallow    sheet    flow
                             conditions (Engman, 1986).


                          Landuse and Condition              Suggested
                                                                Value

                          --------------------------        ----------
                          Forest (light underbrush)             0.30
                          Forest (dense undergrowth)            0.40
                          Bare sand                             0.01
                          Bare clay loam (eroded)               0.02
                          Fallow - no  residue                  0.05
                          Chisel plow  <0.25t/ac residue        0.07
                          Chisel plow  0.25-1t/ac residue       0.18
                          Chisel plow  1-3 t/ac residue         0.30
                          Chisel plow  >3 t/ac residue          0.40
                          Disk/harrow  <0.25 t/ac residue       0.08
                          Disk/harrow  0.25-1 t/ac residue      0.16
                          Disk/harrow  1-3 t/ac residue         0.25
                          Disk/harrow  >3 t/ac residue          0.30
                          No-till <0.25 t/ac residue            0.04
                          No-till 0.25-1 t/ac residue           0.07
                          No-till 1-3 t/ac residue              0.30
                          Moldboard plow (fall)                 0.06
                          Coulter                               0.10
                          Range (natural)                       0.13
                          Range (clipped)                       0.10
                          Grass (Bluegrass sod)                 0.45
                          Short grass (prairie)                 0.15
                          Dense grass                           0.24
                          Bermuda grass                         0.41




          Fraction of surface runoff crossing buffer as sheet flow, C: is an
              estimate of the amount of surface runoff from the field
              contributing runoff to the buffer that enters and presumably
              crosses the buffer as shallow sheet flow.        it is a dif f icult
              and somewhat arbitrary value to estimate. The          f raction of
              surface runoff crossing the buffer as sheet flow       can best be
              estimated using the following procedure:

              1.  Survey the area upslope of the buffer that           contributes
                  surface runoff to the buffer and estimate the       area of the
                  upslope contributing area, A,.
              2.  Delineate the major drainageways within the upslope
                  contributing area.
              3.  Delineate and measure the area, Aj, contributing surface
                  runoff to each drainageway, i.
              4.  The fraction of surface runoff crossing the buffer as

                                                31










                  shallow sheet flow can then be estimated as:
                                          C =AUE  A,                         (37]          A
                                                AU
        Soil moisture storage capacity, M: is def ined as the available soil
            moisture storage capacity of the buffer soils times the lessor
            of the depth to the seasonal high water table or the depth to an
            impeding soil layer (i.e. impervious or semi impervious soil
            layer.   Available soil moisture is defined as the difference
            between the soil water content at field capacity and the wilting
            point.   Estimates of available soil moisture are given in the
            "Physical and Chemical Properties of the Soils" table of modern
            soil survey reports or in soil property databases (i.e. the
            Soils-5 database) as a function of soil type. Soil moisture
            storage capacity can therefore be estimated as:

                                         M = 8A D                            [38]

            where OA is the  available soil moisture of the buffer soils and
            D is the lessor   of the depth to the seasonably   high water table
            or the depth to   an impeding soil layer.

        Vegetative uptake or net productivity factor, V:         is used as an
            index of the buffer vegetation's ability to assimilate
            nutrients. Since little information was available on the
            nutrient assimilative capacity of vegetative ecosystems in
            Virginia, it was decided to use net primary productivity as an
            indicator of net nutrient uptake.        This is reasonable since
            nutrient uptake and net primary productivity are highly
            correlated.      Unfortunately, data on net productivity for
            Virginia ecosystems is almost as rare as data on net nutrient
            uptake, particularly as a function of sucessional stage.           In
            instances where the buffer under study and the reference buffer
            are composed of the same type and age of vegetation, the
            vegetative uptake factor will have no effect on buffer
            performance. The values in Table V are gross estimates of net
            primary production.      The user is encouraged to find better
            sources.of information on net primary production or net nutrient
            uptake for the region   being investigated if possible.












                                            32








        Table V. Net primary productivity (adapted from: Leith, 1975)

            Vegetation                 Net Primary Productivity,
                                                 g/m'-yr

            Mixed forest, dense under story        1000
            Mixed forest, sparce under story       500
            woodland, good cover                   600
            Woodland, sparce cover                 400
            Grass, good stand                      500
            Grass, poor stand                      300
            Woodland, shrubs,   grass              800
            Cultivated land                        650
            Swamps and marsh                       2000



        Data Requirements for Shoreline Stabilization Model

        The following additional values must be estimated to evaluate the
        benefits of shoreline stabilization:

            Bank or shoreline erosion rate (m/yr), E,,
            Bank height (m), H,
            Bulk density of bank soil (kg/ml), p,,
            Concentration of nitrogen in bank material (mg/g), N,
            Concentration of phosphorus in bank material (mg/g), P.
            Sediment loading to the buffer from the upland contributing area
            (kg/m-yr), Y.
            Nitrogen loading to buffer from upland area, M,,
            Phosphorus loading to buffer from upland area, P,,


        These values can be estimated as follows.

        Bank or shoreline erosion rates (m/yr), E,,: are very difficult to
            measure. Measurement techniques include interpretation of areal
            photos over a period of years or actual measurements of
            shorelines relative to fixed measurement points. In any case,
            years of measurements are required to obtain average annual
            erosion rates. Consequently, estimates of shoreline erosion for
            planning purposes are best obtained from published reports on
            shoreline erosion in Virginia (Byrne and Anderson, 1977; Ibison
            et al., 1990; 1992; Hardaway et al., 1992).

        Bank height (m), H,: measurements must be made in the field or from
            detailed topographic surveys of the study site if they exist.
            Bank height should be measured vertically from the base to the
            top of the bank.



                                          33








        Bulk density of bank soil (kg   /M3) ,p.: can be determined in the
            laboratory from undisturbed field samples or from one or more
            field techniques.   It can also be estimated from soil survey
            report descriptions of soil horizons.    Soil bulk densities of
            coastal soils generally range from 1.3 to 1.8 g/cm3 (multiply
            g/CM3 by 1000 to get units of kg    /M3 ) . If a soil bank has
            distinct horizons with varying bulk densities, the bulk
            densities of each horizon should be area weighted to calculate
            an average bulk density for the whole profile. if bulk density
            information is not available, assume an average value of 1.5
            g/CM3 ( 1500 kg /M3 ) .

        Concentration of nitrogen, N,,, and phosphorus, P,, in the bank
            material (mg/g): must be estimated from laboratory analyses as
            suggested by Ibison et al. (1992).      To reduce the costs of
            laboratory analyses, a single composite soil sample can be
            created by combining soil samples from each soil horizon in
            proportion to the thickness of each horizon.       If laboratory
            analysis of soil samples is not possible, then estimates of soil
            nutrient levels can be approximated with concentrations reported
            by Ibison et al. (1990, 1992) for similar landuses and
            locations.

        Sediment loading to the buffer from the upland contributing area
            (kg/m-yr), Y,: are best determined by direct calculation using
            the Universal Soil Loss Equation (Wischmier and Smith, 1976),
            the Revised Universal Soil Loss Equation (Renard et al., 1991),
            WEPP (Lane and Nearing, 1989), or other similar erosion
            predictor. The estimate will probably be given in tons/acre-yr
            (except WEPP predictions, which are given in kg/m-yr) and will
            need to be converted to kg/m-yr to be consistent with the
            shoreline erosion units.      If the soil loss units are in
            tons/acre-yr, then the estimated sediment loading to the buffer
            in kg/m-yr can be determined with Equation [32].            Rough
            estimates of sediment loading as a function of landuse can also
            be obtained from the literature.

        Nitrogen, Nu, and phosphoru5f Pul loading to the buffer: from upland
            areas can be estimated in several ways. First, estimates can be
            by collecting and analyzing soil samples from the area
            contributing runoff to the buffer.    The average soil nutrient
            concentration can then be multiplied by the estimated soil loss
            from the area with appropriate unit conversions to estimate
            nutrient loadings to the buffer. Nutrient loadings can also be
            estimated using models such as CREAMS (Knisel, 1980). Lastly,
            nutrient loadings to the buffer can be estimated from published
            unit area nutrient losses for various landuses.     For example,
            Beaulac and Reckhow (1982, cited by Ibison et al., 1992),
            reported nutrient losses from cropland of 1.9 to 71 lbs-N/ac-yr
            (2.1 to 80 kg-N/ha-yr) and 0.23 to 17 lbs-P/ac-yr (0.26 to 19
            kg-P/ha-yr). Areal nutrient loadings can be converted to a per
            unit width basis using Equations (33] and (34].

                                         34








                         PRACTICAL APPLICATION

        Example Problem I

        Problem Statement:    For a site with the following conditions,
        determine how long a buffer is required to be at least as effective
        as the given reference buffer.

        Data:


        Reference buffer characteristics

           Buffer soil: Suffolk sandy loam (K = 1.3 m/day)
           Buffer length, L = 100 ft (30.5 m)
           Field slope-length, L*    400 ft (122 m)
           Buffer slope = 10% (s    0.0995 from Table III)
           Buffer vegetation is forest with heavy undergrowth (n = 0.4 from
           Table IV)
           Fraction of runoff crossing buffer as sheet flow (C = 0.5)
           Soil moisture storage capacity (available soil moisture = 0.15
           from soil survey and depth to water table = 5 m, M = 0.15*5
           0.75)
           Vegetative uptake for forest (V = 1000 from Table V)

        Study buffer

           All conditions are the same except the landowner wants to thin
           the trees, without improving ground cover, to improve his view
           of the water.

           For forest with light under brush, n = 0.3 and V = 500.       All
           other factors are the same except L and L*.

        Solution:

        Since the new buffer must be as effective as the reference buffer,
        BbIB, must be greater tan or equal to 1.    Substituting the known
        values into the hydraulic model, Equation [18]:
               1. 0=( 1 . 3Lb )( 00)-1.4( 0 , 0995)-1,3( 0 .3)0.6 (0-5 0.0084 Lb
                    1.3*100 500      0.0995    0.4    0.5)=
        Solving the above equation gives L, = 119 ft (36 m). Therefore, the
        landowner would be required to have a 118 ft buffer rather than a
        100 ft buffer if the landowner wants to thin the forest vegetation
        according to the hydraulic model.

        In a similar manner, the detention model, Equation [25], gives:

        Solving the detention equation gives Lb = 124 ft (38 m). This value

                                         35





             1 =(0.3)0.6( Lb)4 1. 3 0.4 0.0995)-1-3 0.75)(0.5 500 )=4.29*10-9L4
                0.4       100  1.3     0.0995     0.75  0.5  1000
                   
        is very similar to that recommended     by the hydraulic model.













































                                            36








        Example Problem II

        Problem Statement: Now suppose the landowner in Example Problem I
        decides that she wants to protect her property from shoreline
        erosion. Given the following information, what would the impact of
        shoreline stabilization be on sediment and nutrient losses from her
        combined buffer/shoreline protection system?

        Data:

        Assume that the landowner's property has a steep bank immediately
        next to the shore. Characteristics of the bank are as follows:

            Bank or shoreline erosion rate (m/yr) , E, = 0. 5 m/yr (1. 64 ft/yr)
            Bank height (m) , Hs = 5 m (16.4 ft)
            Bulk density of bank soil (kg/m') , PB = 1500 kg   /M3
            Concentration of nitrogen in bank material (mg/g), N,=0.365 mg/g
            Concentration of phosphorus in bank material (mg/g) , PB=O. 25 mg/g
            Sediment loading to the buffer from the upland contributing area
            (kg/m-yr), Y,, = 338 kg/m-yr for a 305 m wide buffer and a 9.18
            acre field contributing runoff to the buffer with a soil loss
            rate of 5 tons/ac-yr
            Nitrogen loading to buffer from upland area, N,,;:: 10 kg/ha-yr
            Phosphorus loading to buffer from upland area, P,= 5 kg/ha-yr

        Solution:

        To stabilize the shoreline requires that the bank be graded back to
        a grade of 2:1 and that all large tree be removed from a distance of
        2 bank heights from the top of the new graded slope.         This means
        that of the original 38 m (124 ft) buffer (from Example Problem I),
        10 m will be graded and planted to grass, small trees and shrubs;
        the next 10 m will have all the large trees removed, and the
        remaining 18 meters will be the thinned trees described in Example
        Problem I.

        For the 2:1 graded region, the slope will now be 30' or s=0.5. For
        the rest of the slope, s=0.0995. Area weighting the slope gives a
        mean slope of s=(10*0.5+28*0.0995)/38=0.2049

        The weighted Manning's roughness coefficient for the buffer will be
        (n=0.24 for the stabilized grass slope, n=0.3 for the forested
        area): n=(10*0.24+28*0.3)/38=0.284. The vegetation factor will be
        v=500 since both light woods and grass have the same net primary
        productivity value.


        Buffer effectiveness according to the hydraulic model is:
                  Bb=(1-3*124)(500)-0.4 0.2048)-l-3( 0.284 01 0 . 5)= 0.395
                  Br  1.3*100   500   (0.0995       0.4     0.5

                                           37










         Buffer effectiveness according to the detention model is:
              Bb   (0.284 )0.6( 124 )4( 1. 3 )0.4( 0 . 2049 )-1.3( 0 . 75 )(0.5)( 500)= 0.376
              Br    0.4         (100)   (1.3)    (0.0995)      0. 7 5 0     0. 5  1000
         Both the hydraulic and detention models predict greatly reduced
         buffer effectiveness due to the grading done to stabilize the
         shoreline bank. Both models predict that the buffer will only be
         about 40% as effective as the reference buffer.

         However, if the benefits of shoreline stabilization are considered,
         the overall system may still be beneficial.

         Sediment and nutrient losses due to shoreline erosion:

         With the information given above, sediment loss due to shoreline
         erosion is:
                         YB = HB EB PB = 5*0.5*1500 = 3750 kg/m-yr

         In a similar manner, nutrient losses due to shoreline erosion are:
                        YBN - YB NB  3750*0.65 = 2.437 kg-N/m-yr
                              1000       1000
                        YBP - YB PB   3750*0.25 =  0.937 kg-P/m-_yr
                              1000         1000


         Sediment and nutrient transport through the buffer:

         Sediment passing through the buffer can be estimated as:
            Yu*=Yu(l-C) = 2242 Au Eu  (1-C) = 2242 8.63*5 (1-0.5) = 159 kg/m-yr
                                 WB               304.8
         Similarly, nutrients passing through the buffer are:
            Yun=Yun(1-,_C)=C1 Au    Nuq(1-c) = 13.49*10  (1 -0. 5 ) =0. 057 kg-N/m-yr
                                WB              305


             Yup=Yup (1-_C)= C1, AU Pu  (1-C) =1 3. 49 *5 (1-0. 5 ) =0. 029 kp-P /m-yr
                                 WB            305


         The overall effectiveness of the buffer/ shoreline stabilization
         system for sediment reduction can     then be represented by Equation
         [35]:
         Since Es,<1, shoreline stabilization  results in greater sediment loss
         reduction than the buffer alo  ne. The shoreline stabilization system
         results in a 90% decrease in sediment losses.         Equation [35] can

                                            38






                      ES       Y@               159       =0. 102
                         (BI, / B,) ( Yr, + YU* 0.395*(3750+159)

        also be applied to nutrient losses. The resulting values of ES for
        nitrogen and phosphorus are 0.058 and 0.076, respectively.       This
        means that the shoreline stabilization/buf f er system reduced overall
        nitrogen and phosphorus losses by 94 and 92%, respectively.











































                                          39









                        SUMMARY AND CONCLUSIONS

        A procedure is presented for evaluating the impacts of proposed
        vegetative buffer modifications on buffer effectiveness.          The
        procedure is based on the hydraulic and detention models developed
        by Phillips for evaluating buffer effectiveness.           Phillips's
        original models were modified to correct several limitations
        encountered.     The modified models consider the effects of
        concentrated flow and vegetative uptake on buffer performance.

        The proposed model is relative simple in concept and application and
        is suitable for use by planners.   All of the data required by the
        model can be collected on site or can be estimated from the
        literature. Labratory analysis of soil and bank samples, however,
        will greatly improve model reliability with respect to nutrient
        losses. In areas with shoreline erosion, the procedure also allows
        the benefits of shoreline control to be considered.











































                                         40








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