City of Mexico Beach stormwater management plan

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

CITY OF MEXICO BEACH
STORMWATER
MANAGEMENT
PLAN

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CITY OF MEXICO BEACH

STORMWATER MANAGEMENT PLAN

C3- Prepared by
BCM Converse Inc.
and
Buchanan and Harper, Inc.
December, 1989

'Financial assistance for this publication was provided by the
Coastal Zone Management Act of 1972, as amended, administered by
the Office of Ocean and Coastal Resource Management, National
oceanic and Atmospheric Administration and the Florida Department
of Environmental Regulation.

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TABLE OF CONTENTS

SECTION TOPIC PAGE N 0.

INTRODUCTION ...................... 1
1.0 STORMNATER. MANAGEMENT PLANNING .... :::::::::::.*2
2.0 PLANNING CONSIDERATIONS ....................... 7
3.0 STORMWATER BASIN ............................. 29
4.0 STORMWATER FACILITIES ........................ 30
5.0 STOP14WATER ANALYSIS AND
MODEL DEVELOPMENT ........................... 32
6.0 ANALYSIS OF EXISTING SYSTE14 .................. 39
7.0 IMPROVEMENT ALTERNATIVES
FOR EXISTING SYSTEM ......................... 41
8.0 SURFACE WATER QUALITY ............. o..... o .... 64
9oo NONPOINT SOURCE IMPROVEMENTS ................. 72
BEST MANAGEMENT PRACTICES ............. o ...... 73
11.0 STORMNATER MANAGEMENT
10*0 REGULATION .................................. 78

LIST OF FIGURES

Figure No. Description Page No.

1 Existing Land Use 12
2 Geohydrologic Cross-Section is
3 Soil Types 21
4 Septic Tank Suitability 23
5 Development Potential 26
6 Trunkline Outfall System 31
7 Link/Node System 34
8 Drainage Sub-basins 35
9 Hydraulic Grade Line - Alternatives 44
10 Hydraulic Grade Line - Alternatives,
T=4.0 hrs. 45
11 Hydraulic Grade Line - Alternative 1 47
12 Hydraulic Grade Line - Alt.1,
T=4.0 hrs. 48
13 Hydraulic Grade Line - Alternative 2 50
14 Hydraulic Grade Line - Alt. 2,
T=4.0 hrs. 51
15 Hydraulic Grade Line - Alternative 3 53
16 Hydraulic Grade Line - Alt. 3,
T=4.0 hrs. 54
17 Hydraulic Grade Line - Alternative 4 56
18 Hydraulic Grade Line - Alt. 4,
T=4.0 hrs. 57
19 Hydraulic Grade Line - Alternative 5 59
20 Hydraulic Grade Line - Alt. 5,
T=4.0 hrs. 60
21 Hydraulic Grade Line - Alternative 6 62
22 Hydraulic Grade Line - Alt. 6,
T=40 hrs. 63
23 Hydraulic Grade Line - Existing vs.
Future Land Use 65

INTRODUCTION

This! Stormwater Management Plan was undertaken to assess the degree

of stormwater problems in Mexico Beach and to further provide

alternatives for improvements based on this assessment. The

assessment included delineation of topographic contours, drainage

basins, drainage sub-basins, existing conveyance systems and future

land use. These parameters were then evaluated using the

Stormwater Management Model (SWMM), Version 4 developed by the

U. S. Environmental Protection Agency.

This plan involves two basic components: 1) improvements associated

with correcting existing problems with the conveyance system; and,

2) regulation of development to reduce the potential for future

problems. Analyses included both water quantity and quality

considerations intended to address both flooding and pollution

aspects of stormwater management.

1.0 STORMWATER MANAGEMENT PLANNING*

The concept of preventing and reducing the source of stormwater

pollution best applies to developing urban areas, for these are

areas where development encroachment is yet minimal, or at least

controllable, and drainage essenti ally conforms to natural patterns

and levels. Such lands, in consequence, offer the greatest

flexibility of approach in preventing pollution. What is required,

therefore, is to manage development in such a way that a runoff

regime may be retained close to natural levels. It is in these new

areas where proper management can prevent long-term problems.

The goal of planning is to develop a macroscopic management concept

to prevent the problems resulting from short-sighted development

of individual areas. when considering stormwater management, the

planner is interested in controlling the volume and rate of runoff

as well as the pollutional characteristics. The goal is to

preserve the initial ecological balance so that expensive

downstream facilities can be minimized. Since the size of storm

sewer networks and treatment plants is quite sensitive to the flow,

quantity, and particularly the peak flowrate, a reduction in total

volume or a smoothing out of the peaks will result in low

construction costs.

*Adapted from: Urban Stormwater Management and Technology,

U.S. Environmental Protection Agency, Pub. No. EPA-600/8-77-

014.

Having set goals f or the watershed, the planning agency has two

basic choices for achieving the water quality standards. Either

the individual sites can be forced to comply with individual

practices and performance standards that fit into the master plan,

or the basin system can be designed and maintained as a public

utility. The decision on how to blend the options to meet specific

site conditions is the key to implementing a basin plan. Isolated

development tracts can be controlled by requiring developers to

follow specific source control practices, or a simple set of

performance standards can be applied and the choice of practices

can be left up to the developer. For example, the agency can

require that the runoff from the developed site must not exceed

predevelopment intensity. The developer will have to minimize

runoff producing areas and provide detention facilities at the

site.

When dealing with individual or small-scale construction in an

urbanizing area, a public utility must ensure that stormwater

control planning is implemented. The utility needs the power to

acquire land to preserve natural floodways and infiltration areas

before development overruns the best sites. Dealing with the

small-scale development is a difficult political problem when

stressing nonstructural controls. Plans must be developed, and

specific sites must be set aside for greenways, detention ponds,

and floodways before urbanization begins. This involves buying the

land or inverse condemnation before the tax base has been developed

to pay for it.

Planners also must consider the effects of their actions on areas

outside the individual watershed. For example, detaining storm

flow in a downstream watershed while it remains unregulated

upstream can cause higher flood levels than a completely

unregulated system.

The traditional urbanization process upsets the existing water

balance of a site by replacing natural infiltration areas with

roadways, parking lots, roofs and other impervious areas. The

increased quantity of runoff is carried away in concrete culverts

or compacted earth channels instead of in natural channels and

grassy floodways. The net impact is increased runoff, decreased

infiltration to the groundwater, and increased flowrates. The

increased flow velocities will mean increased channel erosion and

the transport of surface material to receiving waters. Although

most of the surface material is natural and harmless on the land,

it will become a water pollutant contributing to stream

degradation. If the natural drainage features can be preserved,

flowrate increases will be minimized and pollution loads contained.

The key to preserving a natural drainage system for an urbanizing

area is understanding the predevelopment water balance and

designing to minimize interference with the system. The soils and

hydrology of the site must be studied so that high-density, highly

impervious construction, such as shopping centers and industrial

complexes, is located in areas with naturally low infiltration

potential, and the best areas are preserved as open, undisturbed

space in parks and woodlands. Runoff from developed areas should

be directed to the recharge areas and detained to make the best use

of the full infiltration potential. Any necessary drainage

channels should be modeled on the natural swales of the undeveloped

site. The broad, grassy swales will slow down the runoff and

maximize inf iltration. The drainage plan can include variable

depth detention ponds that will rise during a runof f event and

return to a base level during dry weather.

Realizing that the goal of the design is maximizing infiltration-

recharge and minimizing runoff, the planner should be able to

incorporate the following techniques into the site plan:

Roof leaders should discharge to pervious areas or

seepage pits.

As much area as possible should be left in a natural

undisturbed state. Earthwork and construction traffic

compact the soil and decrease infiltration.

Steep slopes should be avoided. They will contribute to

erosion and lessen recharge.

large expanses of impervious area should be avoided.
Parking lots can be built in smaller units and drained

to pervious areas.

No development should be permitted in flood plains.

If natural drainage techniques are developed at a site, the

resulting system should provide a water balance closely

approximating the predevelopment conditions. The site will be less

densely populated than most planned areas; however, the planner

will have a community that should be more desirable to live in.

In any event, an effective stormwater management program can only

be achieved through a balance of regulation and physical

improvements. Regulation is intended to avoid problems which might

be caused by future development while physical improvements are

employed to correct past mistakes.

2.0 PLANNING CONSIDERATIONS

A variety of fundamental factors are instrumental to the

preparation of a stormwater management plan. These factors include

natural features such as topography, slope, soil conditions and

drainageways as well as other conditions such as land use,

intensity of urban development, drainage structures and future land

use*

2.1 Land Use

The City of Mexico Beach is a 'small, resort-oriented community

located adjacent to the Gulf of Mexico in the southeastern corner

of Bay County. The City's 1988 population is estimated at 1,193

persons with a summer seasonal population of approximately 3,000-

4,000 persons.

Land use is predominantly low density residential interspersed with

some service-oriented and retail commercial uses. Approximately

44% of the land area within the City is currently vacant.

Available land within the City could potentially accommodate a

population of 6,000-8,000 persons.

During the period 1980-1986 Mexico Beach experienced a tremendous

population increase growing from 632 to 1176 persons. This

translates into an 86% growth rate for the six-year period or an

annual growth rate of 14.4%. In this regard, Mexico Beach was the

fastest growing municipality in Bay County during the 1980-1986

period. During this time regulations for septic tank

installations have changed which will tend to reduce further

medium-density (townhouse) development.

In general, the development potential of Mexico Beach is

constrained by lack of a central sewage system and predominate soil

types which present moderate to severe limitations on septic tank

use. Unless funding for design and construction of a central sewer

system is obtained the City will continue to develop as a

predominately low-density residential community.

As shown on the existing land use map the City is primarily

residential in character predominated by low-density residential

land use interspersed with some medium and high density uses.

Scattered commercial uses are also located along the City's major

roadway US 98. Land uses within the City are described as follows.

Residential:

Residential land use in Mexico Beach is comprised of low-density

single-family detached and duplex residences, medium density mobile

home parks, and higher density townhouse and apartment-units.

During the early 1980's multi-family townhouse and apartment uses

were increasing; however, since that time changes in septic tank

regulations have decreased this trend.

Residential development in the City is following the classic

pattern experienced in most seasonal resort communities. As land

costs near the waterfront increase, more units per acre are

constructed to produce a higher sales return. As property values

decrease f or inland or non-waterfront locations, per unit densities

also decrease accordingly.

Current densities average 5 dwelling units per acre for low density

residential uses and 8-17 units per acre for medium to high density

uses. Figure 1 shows these uses in relation to waterfront and non-

waterfront locations.

Commercial:

Commercial activities in the City consist primarily of

resort/motel/restaurant and retail sales/service land uses. The

majority of these front upon the U.S. 98 corridor and are oriented

towards seasonal business activity. In general, fulltime residents

travel to other nearby cities for major shopping.

Industrial:

There are two industrial land uses in or near the Mexico Beach City

limits. Wallace Pump and Supply Company is a light industrial

f acility located on SR 386-A. The other industrial facility,

Harmon's Heavy Equipment Company, is located outside the City

limits on SR 386.

Recreation:

Water-oriented activities associated with the Gulf of Mexico and

the beach are the primary sources of recreation in Mexico Beach.

Swimming, sunbathing and fishing are the main activities for both

residents and visitors. Recreational facilities include a

municipal fi shing pier, three gulf-front parking lots for beach

assess, boat launches, two tennis courts, and the Mexico Beach

Canal Park. There are also two large R/V campgrounds which

provide private recreational,facilities.

Public/Institutional:

Public buildings and grounds and other public facilities have been

combined within this land use category. Public areas within the

City are the water treatment plant and City Hall. City Hall houses

the Fire Department, the Police Department, the public meeting hall

and the City's administrative offices. Institutional land uses are

churches and other non-profit organizations.

Conservation:

Conservation uses include the area south of U.S. 98 from 8th Street

eastward to the Gulf County line, excluding the Parker Tract, and

the beach area seaward of the c oastal construction control line.

These areas are under local or state jurisdiction for the purpose

of reducing storm-related damage, protecting natural resources and

providing beach access. The privately-held Parker Tract is on the

Save-Our-Coast list for possible state purchase.

Vacant:

Vacant lots and some large parcels are interspersed throughout the

City. In general, the majority of vacant land is located north of

U.S. 98, toward the northern boundary of the city limits. These

areas are generally low and swampy requiring fill for

development. There is also a possibility that jurisdictional

wetlands are located in these areas which would necessitate

state/federal permit for development.

Adjacent Land Uses:

The majority of the land surrounding Mexico Beach is undeveloped.

To the north are some individual ly-held vacant parcels, however,

most of the land is owned by the St. Joe Paper Company.

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FIGURE 1 G U L F 0 F M E x C 0
EXI.STING LANDUSE

RESIDENTIAL
Law Density
Medium Density

COMMERCIAL

INDUSTRIAL

RECREATION
Public
Private
PUBLIC/ INSTITUTIONAL
E X I C -0- -:13 EAACH--
CONSERVATION -- BCM
NOTE: There are no "Agricultural" or ILLJ _0
"Historic" land uses within VACANT E3.AY couNTY 0CM CONVERSE,INC.
the City.
F-1 - MARCH,1987 lentf3ts
SCALE: I"= 1200' Enq.1n*&r.$,fkqnners, Sc

To the west is a privately-held parcel called St. Michael's

Landing. At this time, it is uncertain whether this parcel will

be developed or be acquired by the State for a public park.

The eastern limit of Mexico Beach is also the jurisdictional

boundary between Bay and Gulf Counties. Land uses in Gulf County

adjacent to Mexico Beach are low to medium density residential

interspersed with limited commercial uses.

2.2 Urban Development

As mentioned previously, the City of Mexico Beach could hardly be

considered an intensely urbanized area. Developed areas, including

public facilities, account for approximately 55% of the total area

of the City. The approximate intensity of urban development by

land use has been estimated as follows.

of Total Area)

Land Use Intensity

Residential 20%

(Low Density, 1-5 DU/Acre) (14)

(Medium Density, 6-20 DU/Acre) (6)

commercial 1%

Industrial 0.5%

Public/Institutional 0.4%

Other 32%

(Roads, Canal, Beac h)

Source: BCM Converse Inc.

The preceding estimates indicate that residential areas and other

facilities, such as roads, comprise the majority of urban

development within the City. These types of development can

reasonably be expected to produce 40% - 60% impervious or paved

surface (lot coverage) which could potentially increase runoff and

pollutant loads.

The 40% to 60% lot coverage is considered relatively low-intensity

development. Unpaved portions (60% to 40%) of each lot provide
some available area for runoff ietention and soil percolation.

Under these circumstances, the intensity of development within the

City is not the major contributor of increased runoff as much as

other factors such as poor site conditions.

2.3 Physical Features

Physical features are naturally occurring site conditions which

influence development potential and drainage patterns. These

features include physiography and topography, geology, groundwater

hydrology, and soil conditions. Each of these is described as

follows.

2.3.1 Physiography and Topography

Mexico Beach is located in the coastal dunes portion of the East

Gulf Coast section of the Coastal Plains physiographic province

(Hunt, 1967). The beach dune deposits paralleling the coast are

the result of coastal currents and on-shore wind that has deposited

sand into 10 to 25 foot dunes paralleling the coast that have

relief on the order of 10 to 25 feet. Immediately inland are found

extensive wooded marshes interspersed with some drier areas.

Scattered small lakes and dry drainage'ditches assist in creating

drier land. As expected, the slopes are extremely low except on

the dune face.

Topographic contours range from approximately 20 foot elevations

principally along US 98 to sea level. Average elevations

throughout the City are in the 12-14 foot range. In general, the

topography is basically flat except for areas immediately adjacent

to the shoreline. Topographic contours are discussed further in

subsequent sections of this report.

2.3.2 Geology

The underlying geology of the Study Area is comprised of coastal

plain sediments of marine and estuarine origin. From oldest to

youngest, these formations can be generalized into three groupings:

limestone and dolomite, marls and clayey sands, and marine and

alluvial sands and terraces. The surface deposits are sand and

muck with dunes paralleling the coast and swampy terrain found one-

quarter to three-quarter mile inland.

The eight formations of the Eocene Series and Oligocene Series are

limestone and dolomite beds, distinguished primarily on the basis

of fossil evidence. These formations, together with the lower

stage of the Miocene Series, constitute the Florid an Aquif er.

These are discussed more fully in the section "Groundwater and

Hydrology".

The middle and upper stages of the Miocene series include

formations composed of marl and silty and clayey sands, forming an

impervious cap to the lower limestone aquifer. The Citronelle

Formation of Plio-Pleistocene Series, the Pleistocene Series and

recent and Pleistocene deposits include alluvial sands, gravels and

clays, marine terraces, and recent marine sediments. The dunes are

built up by the interaction of waves and on-shore winds. They

stand up to 10 to 20 feet above the inland areas which consist of

muck-filled swamp and slightly higher ground which remains fairly

dry due to the high permeability of the sandy soils prevalent in

such areas.

2.3.3 Groundwater Hydrology

Due to the stratigraphic and water bearing characteristics of the

geologic formations underlying the City, three distinct aquifers

are formed. Very porous, permeable surface sands, ranging from 65

to 140 feet thick along the coast, and 10 to 30 feet further

inland, form the unconfined, water table aquifer.

Underlying this is an impermeable layer of sandy clay and clayey

shell material which forms an aquaclude, blocking the vertical

movement of water. An artesian or conf ined aquif er is f ormed below

this aquaclude, known as the Floridan Aquifer. This aquifer is

composed of limestone formations that are as much as 1,200 feet

thick (Musgrove et al., 1965).

Between the base of the water table aquif er and the top of the

Floridan Aquifer is a bed of shell-hash interlayered with sand and

limestone lenses. A cap of sandy clay material acts as a semi-

confining layer which maintains water in this shell-hash under

pressure, forming a secondary artesian aquifer. Figure 2

FIGURE 2
Geohydrologic Sections of Econf'ina Creek Basin Area

140

6
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WAIWF15E Aouic'uo I . . . . . . . . . . I . . . . .
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_U_
L 0R IOAN@ZAQ IfER,, E M INA110M
3m I 1 5
SWI , L L L L
v . . . MCI., . ,A.11
. . . . . . . . . . . . . , " . . . . . .
6A.A L04 Sim Ii

so
AMR .7Z.,
LIKE
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AOUICLUOE AOUICLUDE
()E

T,
-440
r

440-
$_XtZ

0 f Z 3 4 5 0 odes

SOURCE: Bay County 201 Facilities Plan
1000-

FLORIOAN AQUIFER
1 0 R
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OA h I.
1, M.

illustrates a cross-sectional view of the arrangement of these

three aquifers.

Groundwater of the shallow unconfined aquifer originates from

rainfall percolating directly down from the land surface. As this

water moves through the aquifer and is blocked by the impermeable

sand and clay aquaclude at the base, it seeps out into surface

streams and maintains base flow during periods of no rainfall.

Since the boundary between the water table aquifer and the

secondary artesian aquifer is only semi-confining, some water may

seep downward to provide a source of groundwater in this lower

aquifer.

Water in the Floridan Aquifer underlying the study area enters the

aquifer to the north of the study area through sink holes and areas

where the 1imestone formations are at the ground surface (Musgrove

et al., 1965).

2.3.4 Soil Conditions

Soils found in Mexico Beach can be generally grouped into three

basic categories: 1) coarse sandy soils found along dunes and

beaches; 2) fill and disturbed soils associated with urban

developme nt; and, 3) nearly level, poorly drained soils normally

associated with low-lying areas. Soil conditions relative to

drainage and stormwater management for soils in Mexico Beach are

described as follows and shown on Figure 3.

Soil Type Conditions
Limitations on Depth to

Texture Drainage Slmg Water Table Permeability

Leon (13) Sarxi- Cutbacks 0-2% 0-11 Poorly drained
fine sand cave

Mandarin (27) Sand- Cutbacks 0-2% 1.51-3.51 Poorly drained
fine sand cave

Rutlege (29) Sand- Ponding, 0-2% +21-11 Very poorly
loamy cutbacks drained
sand cave

Pott&urg (30) Sand- Cutbacks 0-2% 0-11 Poorly drained
fine sand cave

Arents (40) Sand Deep to 0-5% 761 Variable
water

Beaches (44) Sand Deep to Variable - Variable
watar

Fripp (48) Sand- Deep to 2-30% 761 Moderately
fine sand water well drained

Palmico (51) Muck- Flooding NA 0-11 Very poorly
loamy cutbacks drained
sand cave

Source: Soil Survey of Bay County Florida, Soil Conservation Service, 1984.

A%
C, <
'30

06,

27 29 29 51 51
51 30
51 %-%

30 .3"

4-0

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6 U L F 0 F Al E X C 0

FIGURE 3
SOIL TYPES

MEXICO BEACH
BCM
SAY COUN-ry 13CM CONVERSE, INC.
SCALE'. I"= 1200'-' Engineers, Pionners , Scientists

Based on the preceding conditions and distribution of soils shown

on Figure 3, approximately 50% of areas within the City present
potential drainage problems. All soils listed above are described

as presenting slight potential for erosion.

It should be noted that the majority of poorly-drained, low-lying

soils are generally found in the predominately vacant areas of the

City (see Figure 1). As these areas are developed and/or filled

drainage patterns and volumes of runoff could be altered.

2.4 Future Land Use

Two predominate factors will influence the type, distribution, and

extent of future land use in Mexico Beach. These are : 1) number

of residential units and size of commercial buildings allowed by

State septic tank regulations (Chapter 1OD-6, FAC); and, 2) the

extent of subdivided, platted lots currently in existence.

2.4.1 Septic Tanks

The City does not own, operate or otherwise participate in a

central sewage system. All development within the City uses septic

tanks for sewage treatment. Figure 4 shows soil types within the

City and their suitability for septic tank use. As shown, a

considerable portion of the City contains soils which present

severe limitations on use of septic tanks. In these areas

installation of septic tanks requires fill or other site

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29

............
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51

... ...........
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t. . . .. ......... ...
..........

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6 U L F 0 F M E X C 0

FIGURE 4
SEPTIC TANK SUITABILITY

SEVERE LIMITATIONS ON USE

MEXICO BEPLCH
BCM
E3 Aly u @j T y acm CONVERSE,INC.
SCALE'l"x 1200'@' Englneer3, Plonner-,, Sclentists

modification in order to meet percolation requirements. Lack of

a central sewer system and soil conditions f or septic tank use

present a major problem for future growth. Also, continued use of

septic tanks could result in groundwater contamination, pollution

of the canal system or other health hazards. Recognizing this

problem, the City has continually emphasized the construction of

a sewage system as a priority item.

The Department of Health and Rehabilitative Services (HRS) is

authorized to regulate individual sewage disposal facilities

pursuant to Section 381,272 F.S. and the attendant Florida

Administrative Code rules and standards. Part VIII (Onsite Sewage

Disposal) of the Water Quality Assurance Act of 1983 created more

stringent regulations for septic tank installation. In

subdivisions with 0.25 acre lots and a public water system, septic

tanks can be used if the average daily sewage flow does not exceed

2,500 gallons per acre per day. This part also established a

septic tank setback of 75 feet from surface waters. For lots

platted prior to 1972, it established a 50 foot minimum surface

water setback and an exemption from size requirements provided that

sewage flows do not exceed 2,500 gallons per acre day for lots with

public water systems.

Based on current septic tank regulations it is very probable that

residential densities will not exceed 6 units per acre for existing

subdivisions and 4 units per acre in unplatted areas.

2.4.2 Platted Lots

An important future land use consideration is the extent and size

of platted areas. A considerable portion of the City has been

platted into residential or commercial lots. Although lot

dimensions differ from area to area the most common lot size is

approximately 7500 square feet, with the majority of lots being

platted prior to 1972. In most cases, deed restrictions recorded

for platted areas limit use of lots to residential or commercial

lodging purposes.

Figure 5 shows platted areas within the City. Also shown and

numbered are larger, unplatted vacant areas capable of supporting

future development, except for vacant area 1 which is platted.

With the excep tion of vacant area 1, all other vacant areas exhibit

soil characteristics which pose limitations on building site

development and use of septic tanks. These conditions can, and

have, been overcome through use of fill and associated site

modifications.

2.5 Comprehensive Plan

The City is currently in the process of revising its comprehensive

plan pursuant to Chapter 163, F.S. and Chapter 9J-5, FAC. The plan

was submitted to the Department of Community Affairs on December

1, 1989 and, as such, has not been officially adopted.

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0010
400

G U L F 0 F E X C 0

FIGURE 5
DEVELOPMENT POTENTIAL

VACANT AREAS

PLATTED AREAS
MEXICO E31EACH
BCM
BAY COUNTY
BCM CONVERSE, INC.
SCALE: I"= 1200' Engineers, Planners, Scientists

The draft comprehensive plan does provide proposed land use

districts, dwelling unit densities, and land use intensities which

provide useful guidelines in determining future development

potential.

2.6 Development Potential

General estimates of development potential were prepared*for the

vacant areas shown on Figure 5. Factors considered include amount

of vacant acreage, densities proposed on the Comprehensive Plan-

Future Land Use Map, and the amount of impervious surface (roof,

driveways, road, etc.) generated by potential development. These

estimates are as follows.

Est. Acres of

Vacant Est. Gross Development Potential* Impervious Area**

Area Acreage 4 du/acre 6 duZacre 4 du/acre 6du/acre

1 83 264 396 33 35.7

2 21 64 96 8.5 9

3 10 30 45 4 4.3

4 62 184 276 25 26.6

5 9 28 42 3.6 3.9

6 46 140 210 18.4 19.8

7 7 20 30 2.8 3

8 17 52 78 6.8 7.3

*Number of residential dwelling units

**Roofs, driveways, roads, etc.

Source: BCM Converse Inc.

only vacant area 1 has been platted including dedicated streets and

ROW. subdivision of the remaining vacant areas would be subject

to the City's subdivision regulations, DER stormwater regulations,

and septic tank regulations limiting density to four lots per acre.

In general, it is anticipated that the City will continue to

develop as a low-density, residential community. Residential

development will be limited to single-family, duplex, and mobile

home dwellings with lot coverage limited to 40%. Small-scale

commercial activity and development will continue to occur along

us 98. These commercial activities will be allowed 90% lot

coverage and will be the principle generators of parking lots and

other major areas of impervious surface.

3.0 STORMWATER BASIN

The contributory area for stormwater is best described as a single

large basin (approximately 17 square miles) with many sub-basins.

The basin outf alls to a canal system which circumnavigates the

City. The canal system begins at the Gulf of Mexico and ends at

the Gulf of Mexico.

The basin is comprised of the city limits of the City of Mexico

Beach and adjacent silviculture areas located to the north. The

silviculture areas discharge to the canal system through both

sheet flow and two creek systems. The westernmost creek system is

known as Salt Creek with a contributory area of approximately 1.6

square miles. The easternmost creek system is known as Cypress

Creek with a contributory area of approximately 14 square miles.

The remainder of the silviculture area, approximately 1.4 square

miles, sheet flows to the canal system.

The silviculture areas are comprised of forested timberland,

forested wetland, and hardwood swamps.

The City of Mexico Beach is comprised of undeveloped timberland and

wetland along with residential areas. Developed impervious areas

are predominantly isolated to the westernmost part of the city

along U.S. 98.

3.1 STREET TYPES AND CONDITIONS

The streets within the study area are comprised of both paved and

unpaved surfaces with roadside ditches. The roadside ditches act

as the main conveyance system in the more developed portion of the

City for transmittal of stormwater to the major outfall systems.

4.0 STOP14WATER FACILITIES

4.1 TRUNKLINE OUTFALL SYSTEM

The primary trunkline outfall system consists of a manmade

excavated canal system, approximately 3 miles in length, which is

comprised of one main canal with offshoots. The main canal

begins in the westernmost portion of the city at an outfall to

the Gulf of Mexico, thence north to the city limits, thence

east-northeast roughly following the city limits boundary, thence

turning south to discharge into the Gulf of Mexico approximately

9800 feet east of the westernmost outfall. Beginning at the

westernmost outfall, the canal system is well maintained and of
suf f icient size f or maritime traf f ic f or approximately one f ourth

its length. From there the system is predominantly a large ditch

system connecting isolated larger sections. Refer to Figure 6

for a layout of the system.

Discharging into the main trunkline system are several minor

trunkline systems located in the eastern portion of the City.

0d,

%
% %
% %

nE
j [7
".XAM& CITY

6 U L F 0 F E 0

FIGURE 6
TRUNKLIN@ OUTFALL SYSTEM

MEXICO r=3E.IXC;H
BCM
E3 4ckY C 0 U N -r,)r BCM CONVERSE,INC.
SCALE: 1"z 1200'@L: Englneer3, Plonner3, SclcntlSt3

3 1

The largest of these minor trunkline systems is the system which

conveys Cypress Creek. The remainder of the minor trunkline

systems consist of collector ditches for roadside ditches within

the more developed residential portions of the City.

4.2 FLOODING PROBLEMS

Based on conversations with the City of Mexico Beach personnel,

the greatest flooding problems are isolated in two locations.

The first location is the ditch system contained between Maryland

Blvd. and S.R. 386A. The second location is an area north of the

canal system and just west of where S.R. 386A crosses the canal

system. Refer to Figure 6 for locations.

5.0 STORMWATER ANALYSIS AND MODEL DEVELOPMENT

5.1 MODEL DEVELOPMENT

The model selected for use in analyzing the drainage system is

the Stormwater Management Model (SWMM), Version 4, developed by

the U.S. Environmental Protection Agency and updated by the

University of Florida. Version 4 of the model is the latest

release currently available. This model has been widely applied

to urban stormwater management studies throughout the country

with good success and is thought to be the most comprehensive
model currently available for analysis of urban systems.

Link/Node System

In order to begin the modeling process the existing drainage

system had to be equated to a link/node system as a method of

describing flow connection points and flow transmittal points.

Refer to Figure 7 for the layout of the link/node system and the

labeling system. Determination of the locations of nodes is

based on points of interest in the system, transition in

conveyance structure, intersection of conveyances, etc.

Development of Sub-basins'

once the link/node system has been developed the basin can be

divided into sub-basins tributary to the nodes. The sub-basin

areas and characteristics were developed from the aerial maps and

U.S.G.S. Quad sheets. Refer to Figure 8 for the delineation of

the sub-basins and the labeling system.

Model Parameters

Key input parameters required by the model include total area,

impervious area, generalized slope of the sub-basin, hydraulic

length of the sub-basin, width of the sub-basin, runoff

characteristics, and hydraulic parameters for the conveyance

system. For a detailed description of the various parameters refer

to the SWMM User's Manual.

2100 - REACH'NUMBER
510 - NODE NUMBER

C, <

1065 -1095

2050
.,-'0549 550 -
1060

1090- 2040
1058 %
% -2080
% 2010
1055 1070 %.
2000 595
1040 547 1067 1080."., 2005 2020 596;
546 46 r 585
1050 57 580 590
39 555 P60 6 2030
% 207 600-
1030 545 3 2090 0 AM
4
1020 3. UU
To ftMAMA -$TV
9 - 611
10 3010. 620
:526 To
G U L F 0 F C 0 3000 'ct
15
00
510 Reach From To Description Reach Fr oin To Description
Number Node Node Number Node Node

-----------------------------------------
----------------------------------------------------------
1000 520 510 Natural channel 109'0 570 569 600 CIP Culvert
1010 529 520 Natural Channel 1095 574 570 Natural Channel
1015 525 520 481 RCP Culvert 2000 576 574 Natural Channel
1020 530 U.S. 98 Bridge 2005 579 576 Natural Channel FIGURE 7
1030 539 530 Natural Channel 2010 580 579 C,R, 386A Bridge
LlNk/N0DE SYSTEM
1040 540 539 36th Street Bridge 2020 585 580 Natural Channel
1050 545 540 Natural Channel 2030 590 585 Natural Channel
1055 546 540 Natural Channel 2040 596 585 Natural Channe I.
1058 547 546 Natural Channel 2050 595 590 Natural Channel
1060 549 547 Natural Channel 2070 590 600 Natural Channel
1065 550 549 24N ACP Culvert 2080 605 600 Natural Channel E I C C) E3 C,` H
1067 555 550 Natural Channel 2090 600 610 Natural Channel
BCM
1070 560 555 Natural Channel 3000 610 611 U.S. 98 Bridge
1080 569 560 Natural Channel 3010 611 620 Natural Channel E3 jc@y r- 0 U N T'y
BCM CONVERSE, INC.
SCALE: I"= 1200' Engfneer3, Plonner3, Scientlsf3

A
---60 -70

-9
5 105-
8
40.
30

0-./
0 r MEXICO
90
2 0 Subcatch- Channel/Pipe Subcatchment Subcatchvent Total Area of
Bent or Inlet Area Inside Area Outside Subcatchment
No. Number for City L W Is City Limits (Acres)
Drainage (Acres) (Acres)

----------------------------------------------------------------------
10 510 0 0 0
20 520 10 0 10
25 525 0 1,050 1,050
30 530 70 12 19 FIGURE 8
40 540 3 35 65 1
45 545 37 0 37 SUB-BASINS
50 550 34 35 69
60 560 57 299 356
70 570 20 293 313
80 580 63 0 63
85 585 68 0 68
90 590 3 0 3
95 595 92 0 92 M EX I CM E3 E lk H
96 596 133 8,423 8,556 -BCM
100 600 14 0 14 E3 AY C M U 1)r
1
105 605 226 0 226 8CM CONVERSE, INC;-
110 610 0 0 0 SCALE: I" x 2000- EngInvers, Pionner3, Sclentlst3

Area Characteristics

The total area characteristics were determined by planimetering

the sub-basins and by referring to the Florida Department of

Transportation basin delineations developed as part of the design

of U.S. 98 and S.R. 386A. Directly connected impervious area

characteristics were determined based on the land use. For

existing land use the directly connected impervious area'was

estimated based on an evaluation of the aerial photographs. For

future land use the directly connected impervious area was

estimated based on an assumed percentage for each type of future

land use. Percentages for each type of future land use were

estimated based on development patterns described in Section 2 of

this document.

Hydraulic Length/Slope/Width

The hydraulic length is defined as the distance of overland flow

from the furthermost upstream point in the sub-basin to the point

of collection. Determination of the hydraulic lengths were made

based on the aerial photographs. The slope of the sub-basin was

determined by dividing the difference in elevation along the

hydraulic length by the hydraulic length. The width is a

parameter which approximates the shape of the basin. The width

is best estimated by dividing the sub-basin area by its hydraulic

length.

Hydrologic/Hydrogeologic Characteristics

Hydrologic/hydrogeologic parameters needed for the model

primarily consist of precipitation, infiltration, and depression

storage data. Precipitation data used in the model was taken

from the synthetic mass rainfall curves prepared by the Swannee

River Water Management District based on statistical rainfall

data. Based on analyses performed by the Florida Department of

Transportation, these rainfall curves are valid for events in

northwest Florida. Infiltration information was obtained from

the Bay County Soil Survey. Depression storage data was taken

from the aerial photographs.

Hydraulic Characteristics

Hydraulic characteristics for the conveyance system include the

type of conveyance, physical dimension, slope, and friction or

roughness parameters. The type of conveyance, physical

dimensions and slope were obtained from field data and the aerial

photographs. Roughness parameters were obtained from

standardized engineering references.

The remaining characteristics were determined from the

topographic maps.

5.2 MODEL ACCURACY

verification and calibration are generally a necessary integral

part of any stormwater management system model development.

Calibration is the adjustment of model parameters using one set

of known data and verification is the testing of the adjusted

model by using an independent set of known data. The necessity of

calibration and verification occurs due to the inaccuracy of

parameter interpretation. Additionally, analysis of water

quality data is based upon limited data and empirical evaluations.

Quantity predictions can be approximated without calibration and

verification, however, quality predictions may be off by orders of

magnitude. Quality predictions are not credible without

site-specific data for calibration and verification and may be used

only to judge relative ef f ects of pollution abatement alternatives.

Verification and calibration will not be possible for this model

due to budgetary constraints. As a result the data obtained from

the analysis should be considered an approximation of actual

conditions. Without verification and calibration, analysis of

water quality via the model would serve no purpose. Therefore,

the water quality portion of the model has not been "turned on"

and the SWMM model will be used to evaluate quantity only.

Quantity predictions, while not being totally accurate, should be

of sufficient accuracy to evaluate the existing and future

conditions and recommend improvements to the system. Water

quality may be evaluated by a rational analysis of the basin

without knowing site-specific data and, as a result, best

management practices can be recommended which will reduce

degradation of the waters.

The natural extension of this project will be to continue it with

a second phase whereby, the necessary site-specific data is

obtained. The water quality portion of the model is "turned on"

and verification and calibration is performed.

6.0 ANALYSIS OF EXISTING CONVEYANCE SYSTMI

This section will consist of a description of the methodology

used in analyzing the existing conveyance system. The analysis

primarily consisted of selection of an appropriate design storm,

simulation of existing and future conditions, and a summary of

resultant predictions.

6.1 bESIGN STORM EVENT

The selection of the design storm event is a two part process,

selection of an appropriate event return frequency and an

appropriate event duration.

Selection of an event return frequency or recurrence interval for

a municipality is dependent upon public safety, convenience and

economics. Since the system is comprised primarily of open

channels the economics of capital improvements is less

significant than in a system comprised of rigid structures.

Therefore, a larger event return frequency of 25 years was

chosen.

The event duration is a more basin specific parameter than the

event return frequency. The resultant analysis data from a

specific duration storm event will vary with the duration of the

storm event in a bell-shaped curve. The duration storms

contained at the top of the bell-shaped curve will produce the

largest stages and flows within the system. Determination of

this critical duration is a trial and error procedure. An

initial duration was selected and the analysis generated.

Additional durations were then analyzed for the existing system

until a critical duration was determined.

6.2 SUMMARY OF RESULTANT PREDICTIONS

The existing conveyance system was modeled for existing land use

conditions. The results of the analysis indicated that the

existing system exhibited problems from 36th Street east to the

Gulf outfall. Flooding was encountered in this area and in some

locations exceeded 2.5 feet. The majority of the flooding is the

result of an obstruction and an undersized culvert. Improvement

alternatives need to be considered in order to reduce the

flooding and increase the overall efficiency of the conveyance

system.

7.0 IMPROVEMENT ALTERNATIVES FOR EXISTING SYSTEM

This section contains descriptions, costs and recommendations for

various improvements to the existing conveyance system. These

improvement alternatives are based upon the concept that the

conveyance system shou ld function in an efficient manner and all

flooding which, in the opinion of the modeler, would result in

the endangerment of life or property should be eliminated within

the basin. Also, when economically feasible all flooding of

uplands shall be eliminated. No effort will be made to reduce

flooding in lowland areas.

7.1 GENERAL

Based upon a visual observation of the conveyance system it is

apparent that a maintenance program should be implemented. The

existing canal system contains locations where erosion has

occurred resulting in an obstruction or partial blockage of flow.

Also, locations exist where vegetation or debris has collected

resulting in a blockage or reduction in conveyance capacity.

A maintenance program should be implemented with priorities

established for the following items:

1. cleaning of structures - one of the first items which should

be addressed is that all culverts should be examined and cleaned

to remove all debris, blockage, silt, etc.

2. Removal of Obstructions - The entire system should be

inspected for obstructions or partial blockages to flow resulting

from erosion, debris, etc. The obstructions should be removed

and if required the area"stabilized with vegetation or other

structural means. Vegetative means of stabilization should be

utilized whenever possible due to their inherent water quality

benefits. In the event the section shape prohibits the use of

vegetation, reshaping of the section should be considered in

order that vegetation may be utilized.

3. Easements - Rights of ways and easements should be reviewed

for all parts of the system to insure that adequate areas exist

for the conveyance and f or maintenance. Easements or rights of way

should be obtained for all areas with inadequate or

non-existent easements.

4. Cleaning of the system - The entire system should be cleaned

to remove debris and excessive vegetation and, if necessary, to

re-contour and stabilize areas subject to erosion. A regular

schedule of maintenance should be established and strictly

adhered to. Initially, cleaning of the system will be labor

intensive. However, once the system has been cleaned, future

cleaning should be straightforward. The benefits of a regular

cleaning schedule will be realized in a reduction in specific

problems which must be addressed. Also, routine maintenance will

be preventative and cost effective compared to remedial

maintenance for the correction of specific problems. The

frequency of cleaning operations are somewhat dependent upon the

system and will be determined based upon need. However,

generally speaking most system require cleaning one to two times

per year.

7.2 IMPROVEMENT ALTERNATIVES

specific improvement alternatives to the existing system were

evaluated by making changes to the computer model and executing

an analysis of the effects of the changes on the performance

characteristics of the conveyance system. Locations for

potential improvements were based upon inefficiencies noted in

the computer analysis of the existing system. Inefficiencies in

the conveyance system can be seen by analyzing the change in

elevation or slope of the hydraulic grade line of the conveyance

system. Abrupt changes in elevation or steep slopes in the

hydraulic grade line indicate areas which are functioning

inefficiently. The effect of each alternative or combinations of

alternatives upon the hydraulic grade line may be seen by

referring to Figures 9 and 10. The desired effect for any

MAN MIN ANN SAM MAN AM ANN SAM MAN VIA ANN SO Am Now MAN MAN

FIGURE 9
M
ja
IMUM HYDRAULIC GRADE LINE
FOR VARIOUS IMPROVEMENTALTERNATIVES

ALT. NO. 6
AM ALT NO. 5
= ALT NO. 4
M ALT. NO. 3
919 ALT NO. 2
= ALT NO. 1
12.0-/ AM EMS77NO COMMON
Ea FLOOD EL

9.0-/
:3
7.5

6.0-/

4.5-
=3

3.0-/ FLOOD Ei-
EXIS77NO COIVD17ION
ALT NO. 1
All ALT. NO. 2
1.5- ALT NO. 3
AL.T NO. 4
ALT NO. 5
0
z 7 7- -@-@-7 -7-7- -7- ALT NO. 6
.0 1 1
510 529 539 546 549 555 569 574 579 585 600 611
NODE
J-

m m m = m = =

FIGURE 10
HYDRAULIC GRADE LINE CcP TIME T= 4.0 HOUR
FOR VARIOUS IMPROVEMENTALTERNATIVES

F--l M-MWAME NO. e
= M-7Ef WAME NO. 5
E@@3 A-7EIWAVVE NO. 4
01D AL7ERVA71VE A10. 3
GM AL7ERVA71VE NO. 2
Q AMMAN
MOSM0
12.0- ff!@ APROX

10-15-/

luLl
9.0
.3
7.5
6.0-//

In IF
:
4.5
3

3.0 - APROX FWQD ELEV
MfJSnMG C0)VDMN
ALTERNA71VE NO. 1
AL7ERNA77VE NO. 2
3c 1.5 AVERMANVE NO. 3
ALTERNA77VE NO. 4
77 ALTERNMVE Ala 5
AL VE NO. e
0.0 F-- t
510 529 539 546 549 555 569 574 579 585 600 .611
NODE

alternative would be a reduction of the hydraulic grade line.

The design storm event used in modeling the improvement

alternatives was the 25 year, 4 hour duration storm event with a

simulation time of 8 hours. The 4 hour duration event was

determined as the storm event which resulted in the maximum water

surface elevations for the existing system. The determination

was made by a trial and error analysis of storm events with

different times of duration. No attempt was made to verify that

the same duration storm event will result in maximum water

surface elevations once changes are made to the existing system.

It is our opinion that the critical duration event for each

alternative will not vary sufficiently from the 4 hour event to

justify the additional time and expense necessary to determine

the critical event for each improvement alternative.

Alternative No. 1

Alternative No. 1 consists of the removal of an obstruction in

the canal system located adjacent to the City limits bo undary,

east of the 37th Street Bridge. The obstruction, denoted as

Reach 1058 on Figure x, appears to have been caused by erosion

and has resulted in a partial closing of the canal. Based on an

analysis of the conveyance system, it appears that the removal of

the obstruction results in an upstream decrease in the hydraulic

grade line of 2.32 feet for maximum elevations and 4.19 feet at

the time of 4 hours after the storm event began which is also the

duration of the rainfall. We estimate that the cost of removal

FIGURE I I
MAXIMUM HYDRMLIC GRADE UNE
AUERNATIVE NO. I Vu'S'. EXIS77NG COND17ION

ALTERNA77VE NO. I
611- -X- EXIS77NO COND177ON
-a APROX FLOOD El".
600-

585-
579-

574-
ui - . ....
0 569-
0
z 555-

549-

546-

539-

529-

510

0 1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRAUUC GRADE UNE (FI)

FIGURE 12
HYDRAULJC GRADE UNE 0 TIME T= 4.0 HOUR
AUERNATIVE NO. I VS. EXIS77NG COND177ON

-4- ALTERM477VE NO. 1
611 -X- EX187ING COND1770M
-0-- APROX. FLOOD El".
600

585

579

574
Lu
0 569
0
z 555

549

546

539

529

510
0 1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRALJUC GFU%DE UNE (FT)

of the obstruction would be approximately $2,100.00. This would

result in a very cost ef f ective improvement to the conveyance

system. Therefore, this obstruction should be removed and the

section reshaped to match existing adjacent canal cross-sections.

The side slopes should be stabilized with vegetation.

Alternative No. 2

Alternative No. 2 consists of changing the 31st Street canal

crossing from a 24 inch RCP to a 60 inch RCP in addition to the

work of Alternative No. 1. The culvert crossing is denoted as

Reach 1065 on Figure x. Based on an analysis of the conveyance

system with the above noted modifications, it appears that the

resultant hydraulic grade line will exhibit an upstream decrease

for maximum elevations of 4.02 feet and at a time of 4 hours a

decrease of 5.89 feet. We estimate that in addition to the cost

of Alternative No. 1, the cost for replacing the crossing with a

60 inch RCP would be approximately $21,500.00.

Based on the effect of this alternative on the hydraulic grade

line, we recommend that the City of Mexico Beach, as a minimum,

consider replacing the 24 inch RCP. This culvert is severely

undersized and was installed after completion of the canal

system. The size was most likely selected based on economics.

The result of this undersized culvert is that virtually all areas

located to the east must discharge to the Gulf of Mexico at the

FIGURE 13
MAKIMUM HYDRAUUC GRADE UNE
ALTERNATIVE NO. 2 VS. EXIS77NG CONDITION

ALTERNA71VE NO. 2
611 EXISTING CONDITION
-E- APROX. FLOOD ELEV.
600-

585-

579-

574-
Lu -
0 569-
0
555-

549-

546-

539-

529-

510---
ol 1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRAUUC GRADE UNE (F7)

FIGURE 14
HYDRAULIC GRADE LlNE (0 77ME T= 4.0 HOUR
ALTERNATIVE NO. 2 VS. EXIS77NG COND177ON

ALTERM477VE NO. 2
611 -X- E)aS77NQ 0ONDITION
-0- APROX FLOOD ELEV.
600-

585-

579-

574-
uu -
Q 569-
0
555-

549-

546-

539-

529-
510-
T-
o 1 2 3 4 5 is 7 8 9 10 11 12 13
HYDRAMCP GFU%DE UNE (M)

easternmost outfall. The best possible solution, which is

considered as Alternate No. 3, would be to eliminate the canal

crossing at this location and re-establish a uniform canal

section.

Alternative No. 3

Alternative No. 3 consists of removing the 31st Street canal

crossing, a 24 inch RCP, and reshaping the section to match

existing adjacent canal cross-sections, in addition to the work

of Alternative No. 1. Additionally, the side slopes of the

reshaped section should be stabilized with vegetation. The

crossing is denoted as Reach 1065 on Figure x. Based on an

analysis of the conveyance system with the above noted

modifications, it appears that the resultant hydraulic grade line

will exhibit an upstream decrease for maximum elevations of 5.61

feet and at a time of 4 hours a decrease of 6.51 feet. We

estimate that in addition to the cost of Alternative No. 1, the

cost for removing the existing culvert and reshaping the section

along with stabilization would be approximately $5,500.00.

We recommend that this alternative be selected over Alternative

No. 2. The cost savings is substantial and this alternative is

much more efficient resulting in a decrease in the hydraulic

grade line of approximately 1.5 feet more than Alternative No. 2.

It appears that alternate routes are available and removal of

this crossing would not result in an adverse impact to traffic.

FIGURE 15
M
A;(
IMUM HYDRAUIJC GRADE UNE
ALTERNATIVE NO. 3 VS. EXIS77NG COND17ION

E3- -4- ALTEPAA77W NO. a
611 -X-- EXISnNG OONDITION
APROX FLOOD ELEM
600-
585-

579-

574-

Q 569-
555-

549-
X-4

546- . .....

539-

529-
5101
0 1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRAUUC GRADE UNE (FI)
X-X-

FIGURE 16
HYDRAULIC GRADE UNE @ 77ME T= 4.0 HOUR
AUERNATIVE NO. 3 VS. EXIS77NG CONDITION

--i- ALTERM77VE NO. 3
611 -* EXIS77NG COND1770N
--0- APROX. FLOOD El".
600-

585-

579-

574-
ul -
0 569-
0
z 555-

549-
546-

539-

529-

510-
0 1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRALJUC GRADE UNE (FI)

Alternative No. 4

Alternative No. 4 consists of Alternative No. 3 and cleaning all

portions of the conveyance system as described above. Based on

an analysis of the conveyance system with the above noted

modifications, it appears that the resultant hydraulic grade line

will exhibit an upstream decrease for maximum elevations of 5.61

feet and at a time of 4 hours a decrease of 6.83 feet. Cleaning

of the conveyance system does not exhibit any significant

decrease in the hydraulic grade line for maximum elevations.

However, it does result in a decrease in the hydraulic grade line

throughout the overall duration of the storm event. Additionally,

the value of this task goes beyond any decreases it causes in the

hydraulic grade line as discussed above. Therefore, we believe

that the execution of this alternative is necessary. We estimate

that in addition'to the cost of Alternative No. 3, the cost for

cleaning the system would be approximately $161, 000. 00. Estimation

of the costs for cleaning the system are difficult at best and

should be considered as a rough estimate for planning purposes

only. This estimate also assumes that the work would be contracted

out. The costs would be substantially reduced if the work was

performed by the City.

FIGURE 17
M
IMUM HYDRAULIC GRADE UNE
AL7ERNATIVE NO. 4 VS. EXIS77NG COND177ON

ALTERNA71VE NO. 4
611 EXISVINO COMMON
APROX. FLOOD S".
600-

585-
579-

574-
us - . .............. . ......
0 569-
0
z 555-

_x
549-

546-

539-

529

510- x
I -T- I
1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRAUUC GRADE UNE (FT)

FIGURE 18
HYDRAUUC GRADE UNE @ TIME T= 4.0 HOUR
ALTERNATIVE NO. 4 VS. EXISTING COND177ON

--i- ALTERM77VE AlO. 4
611 -X- OUS71NO COMMON
-0- APROX. FLOOD EUV.
600

585

579

574
UJ
ul Q 569
-4 O
555

549

546

539

529

510

0 1 2 3 4 5 6 7 8 9 10 I 1 12 13
HYDRAUUC GRADE UNE (M)

Alternative No. 5

Alternative No. 5 consists of Alternative No. 3 along with:the

regrading of approximately 3400 linear feet of canal from 31st

Street to 19th Street. These sections of canal are denoted as

Reach 1067 and Reach 1070 on Figure x. These two sections, or

portions of the sections, of canal are at a significantly higher

elevation than the remainder of the canal. This results in an

obstruction or hindrance to stormwater flow, thereby, decreasing

the efficiency of over one-half of the system. Based on an

analysis of the conveyance system with the two sections regraded,

it appears that the resultant hydraulic grade line will exhibit a

decrease in the maximum elevation of 6.51 feet and at a time of 4

hours a decrease of 7.51 feet from the elevations of the existing

system. It is probable that the entire lengths of the sections

would not need to be regraded in order to increase the depth to

an elevation consistent with the remainder of the canal system.

We estimate that the necessary additional excavation is

approximately 2 feet. An accurate estimate of the cost for

regrading the two sections would not be possible without a more

accurate survey of the two sections such that the actual areas

which need to be deepened could be defined. However for planning

purposes we estimate the costs for regrading the two sections

would be approximately $60,000.00. It should be noted that the

anticipated work could most likely be done as part of the

cleaning of the system and a majority of the cost associated with

that task.

FIGURE 19
M
Mr
IMUM JiMPALILIC GRADE UNE
ALTERNATIVE NO. 5 VS. EXISTING CONDITION

-4- AL7EFMWVE Nas
-X- EWSnNQ CONDMON
-a- APROX FLOOD H"

15BS

1579-

11514-

0
1519-
0

1549

1546-

1539

1529

1510
-T- I
o 1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRAUM GRADE UNE (FI)

FIGURE 20
HYDRAULIC GRADE LINE 0 TIME T= 4.0 HOUR
ALTERNATIVE NO. 5 WSPA. EXISTiNG COND177ON

--i- ALTERMWE NO. 5
611 -X- EXIS77NO COND17ION
-0- APROX. FLOOD ELEV.
600-

585-

579-

574-
ui -
0 569-
0
555-

549-

546-

539- <>

529

510-

0 1 2 3 4 5 16 7 8 9 10 11 12 13
HYDRAMCP GRADE UNE (FI)

Alternative No. 6

Alternative No. 6 consists of Alternative No. 5 along with

cleaning of the conveyance system. This alternative represents a

combination of the recommended changes to the system and

demonstrates the overall anticipated improvements in the

performance of the system. Based on an analysis it appears that

the resultant hydraulic grade line will exhibit a decrease in the

maximum elevation of 6.53 feet and at a time of 4 hours a

decrease of 7.78 feet from the elevations of the existing system.

Costs associated with the tasks of this alternative may be

derived by addition of the costs for the previous alternatives.

7.3 STORMWATER IMPACTS

Stormwater impacts from future development within the City are

anticipated to have little overall effect to the existing system.

This is primarily a result of the large amounts of off-site

property which are tributary to the system and account for a

majority of the stormwater. The largest potential impact to the

stormwater management system from development is not from the

associated runoff but from the potential physical damage to the

system itself. Examples of items which could have adverse

impacts on the system include additional crossings which

constrict flow, erosion from points of discharge, etc.

FIGURE 21
MMIMUM HYDR@WUC GRADE LINE
ALTERNATIVE NO. 6 Viclp'. EXISTING CONDITION

E3-- --A- ALTERMANVE NO. 6
611 -X-- EXISM0 00NDIWOM
APROX. FLOOD El".
600-

585- a_,__

579-

574-
Lu -
0 569-
0
555-

549-

546- x-

539- El
El
529-
510-
0 1 2 3 4 5 16 7 6 9 10 11 .12 13
HYDRAUUC GRADE UNE (FT)

FIGURE 22
HYDRAUUC GRADE UNE @ 77ME T= 4.0 HOUR
ALTERNATIVE NO. 6 VS. EXIS77NG COND177ON

-d- ALTERM477VE NO. 6
611 -X- EXISMO COMMON
---0-- APROX. FLOOD ELE V.
600-

585-

579-

574

Q 569-
0
555-

549

546-

539-

529-
510--
0 1 2 3 4 5 16 7 8 9 10 11 12 13
HYDRAUUC GRME UNE (FT)

Since it was anticipated that analysis results from future

development would not differ significantly from existing

conditions, improvement alternatives were not modeled using

future land use parameters, except as necessary to verify this

assumption. To verify the assumption an analysis was made of the

existing system using future land use parameters. Results of the

analysis and differences between the existing land use analysis

may be seen by examining the hydraulic grade line elevations for

each analysis show on Ficmre 23.

8.0 SURFACE WATER QUALITY

8.1 GENERAL

As previously discussed, a surface water quality analysis was not

performed using the stormwater management model. Water quality was

evaluated using a rational analysis. The analysis attempted

to identify existing and potential nonpoint sources of pollution

and recommend practices to reduce degradation of surface waters.

Generally the quality of surface water should approximate the

quality of precipitation. Exceptions occur when the quality is

degraded or enhanced due to local features, either natural or

unnatural. An example of a naturally occurring degradation or

enhancement would occur when another naturally occurring source

of water of greater or lesser quality mixed with the surface

FIGURE 23
EXIS71NG LMD USE VS. FUTURE LAND USE

620 EXISM3 L. U,1E C
611 FUTURE L.U.IF-0.
610 -a FUTURE L U.1 . C.
EXIS71AIG L. U.1P. C.
600
590-
585
580
579
576-
574--
tu 570-
cl 569
560
555-
550
549-
547
546
540
539-
530
529
520
510

0 1 2 3 4 5 6 7 8 9 10 11 12 13
HYDRAULIC GRADE LINE (M)
L U. - L4ND USE
E. C. - EXIS WINO COND17ION
P.C. - PROPOSED COND177ON

water. An example of an unnatural degradation would be where

rainfall runoff causes agricultural pesticides to wash into the

surface water. Precipitation may also be a cause of degradation

by depositing air borne pollutants into the surface waters.
0

In most circumstances the two primary categories of degradation

to surface waters occur as a result of point and non-point source

discharges. Point source discharges occur when pollutants are

directly discharged to surface waters. An example of a point

source discharge is when a wastewater treatment plant discharges

reclaimed water or effluent to surface waters. Any resultant

degradation comes from an identifiable, singular point, with

known impacts, i.e. a point source. Nonpoint source pollution of

surface waters is best described as a category of pollution which

includes various sources of pollution generally not attributable

to singular, identifiable point sources and are most often

transmitted by surface runoff. The pesticide degradation

previously described is a nonpoint source discharge. Stormwater

management systems are predominantly concerned only with nonpoint

source pollutants.

The pollutants from nonpoint source discharges are classified as

sediments, nutrients, potential toxins, pathogens, heavy metals,

and organic compounds.

8.2 SEDIMENTS

Sediment is the single largest pollutant by weight of the:

nation's surface waters, exceeding the entire sewage load by 500

to 700 times. It has been estimated that in excess of $500

million is spent annually to remove sediment from surface waters.

The adverse effects of sediment include increased turbidity in

the receiving water, filling of conveyances and surface waters,

damage to aquatic organism food sources and spawning areas, and

modification of the oxygen content of the surface water.

Additionally, sediment may carry other pollutants such as heavy

metals, pesticides, and organic compounds which attach themselves

to the surface of the eroded particle.

Erosion or sediment transport is a naturally occurring phenomenon

and a certain amount should be expected to enter the surface

waters. However, due to the activities of man the amounts of

sediment transport is in most areas many times greater than

naturally occurring levels. Examples of events which have caused

this increase include construction activities leaving exposed

soil areas, unplanted agricultural areas, vegetative changes for

soil type, modifications to natural drainage features,

uncontrolled erosion and inadequate maintenance of conveyance

systems, etc.

Based upon our analysis, sediment transport is the largest single

contributor to degradation of the stormwater management system

and surface waters of the City of Mexico Beach. It has resulted

in a decrease in efficiency of the conveyance system by filling

in portions of the canal system, creating obstructions, and

increasing the turbidity of the surface waters. We also believe

that sediment transport from stormwater runoff is contributing to

the problems associated with the primary inlet to the Gulf of

Mexico. The exact contribution to the inlet problems cannot be

determined at this time and the majority of the sediment transport

affecting the inlet is most likely a result of lateral drift from

the Gulf of Mexico. However, a reduction of sediment transport

from stormwater runoff should result in a decrease in efforts to

maintain the inlet.

8.3 NUTRIENTS

The primary nutrient constituents are nitrogen and phosphorus.

These nutrients are generally derived from agricultural chemicals,

detergents, and animal waste. The entry of nutrients into surface

water can increase algal activity, result in noxious odors, and

reduce the oxygen content of the water.

Based upon our analysis we would not expect high concentrations

of unnaturally occurring nutrients. The majority of the nutrients

which will be transported to the surface waters will be from animal

wastes and fertilizers along with other household agricultural

products.

8.4 POTENTIAL TOXINS

Potential toxins found in stormwater are generally from pesticides

and chemical compounds of industrial and commercial origin.

Pesticides vary greatly in strength, formulation and toxicity.

Some of the organochloride pesticides are extremely resistant to

decomposition and can remain in the environment for more than 20
years creating a health hazard for all organisms. While most of

these types of pesticides are no longer in use they still represent

a potential problem due to their persistence.

Potential toxins from industrial and commercial sources include

illegal dumping and discharge of waste products such as acids,

some paints and discharges from industrial processes either by

airborne pollutants or point source discharges. Examples of

these type of toxins are PCBs and dioxin.

Based upon our analysis we would not expect unusually high

concentrations of toxins in the system. However, it should be

noted that some toxins in minute quantities are most likely

present. We believe that the most likely source for any toxins

would be air borne pollutants from the paper mills and chemical

plants in the area. It is doubtful that toxins exist in the

system in sufficient quantities to pose a threat to the health

and safety of the public.

8.5 PATHOGENS

Pathogenic hazards associated with surface water quality are a

result of bacterial contamination. The primary source of

bacterial contamination is from human and animal wastes.

Resulting diseases include anthrax, encephalitis, tetanus,

histoplasmosis, leptospirosis, bronchitis, and salmonellosis.

While pathogens present a significant hazard to public health and

safety, rarely do they represent a real hazard except to public

water supply, and livestock. Also, human and animal wastes are

one of the simplest substances to decompose. An analysis of

pathogenic hazards is not possible. The potential for hazards

may exist due to extensive use of septic systems in the City.

8.6 HEAVY METALS

Heavy metals found in stormwater runoff include-cadmium, copper,

chromium, lead, nickel, mercury, and zinc. The primary source

for heavy metals in stormwater runoff is runoff from streets. In

surface waters marine traffic can also contribute to the deposition
of heavy metals - Heavy metals are a @yproduct of vehicular traffic

and the internal combustion engine. The predominant heavy metals

found are lead and zinc. The results of high concentrations of

heavy metals are devastating to organisms with concentrations

increasing through transmittal to higher species through the food

chain. However, the ef f ects of low concentrations over long

periods of time is less well documented. Heavy metals tend to

settle out of surface waters and collect in the sediment deposited

on the bottom. From time to time these metals can be resuspended

in the water column through disturbance. In areas with high

traffic, both vehicular and marine, the concentrations of heavy

metals in the sediments could exceed safe levels.

Based upon our analysis we do not anticipate that the

concentrations of heavy metals in stormwater runoff is excessive.

We do believe that a potential exists for high concentrations of

heavy metals in the sediments of portions of the canal system

subject to heavy marine traffic. It is our opinion that these

heavy metals are the result of low concentrations over a long

period of time and that they do not represent a significant

hazard at the present time. However, we would advise that before

any dredging is done in the canal system where marinas, docking

or fuel facilities exist that sediment samples be analyzed for

heavy metals. In the event that heavy metals are present in

significant concentrations the spoil material will need to be

disposed of in an appropriate manner.

8.7 ORGANIC COMPOUNDS

organic compounds in stormwater runoff and surfa ce waters include

oxygen consuming material which can deplete the oxygen content of

the water. If the oxygen content is sufficiently reduced marine

organisms will perish. Quantitative measures of the potential

impact on a receiving water is expressed by biochemical oxygen

demand (BOD) and by chemical oxygen demand (COD) for

non-biodegradeable compounds. The BOD and COD measure the amount

of oxygen consumed in the decomposition process.

Based upon our analysis we do not anticipate that BOD and COD

will exceed normally anticipated levels.

9.0 NONPOINT SOURCE IMPROVE14ENTS

It does not appear that any excessive nonpoint source pollutants

are entering the system which need special attention except for

sediment transport. The nonpoint source pollutants which are

entering the system, including to a large degree sediment

transport, can be reduced through the use of best management

practices discussed herein.

In addition to best management practices for sediment transport,

additional improvements need to be implemented. The actions

recommended include the removal of existing deposits of sediment

in the conveyance system which serve as a source for sediment

transport and stabilization of the conveyance system. These

improvements have been previously discussed as part of the system

maintenance which is recommended.

10.0 BEST MANAGEMENT PRACTICES

Best management practices should be incorporated into the

stormwater management system to reduce the levels of pollutants

entering the system from existing sources. Policies should also

be implemented to require best management practices to prevent or

decrease impacts from future sources. Best management practices

or BMPs are a term allocated to certain practices which have been

found efficient in the removal of pollutants on a per unit cost

basis. BMPs can be divided into two general improvement methods,

structural practices and non-structural practices, for reducing

or preventing pollution.

10.1 NON-STRUCTURAL BMPS

Non-structural BMPs are practices which minimize or prevent

pollution before transport to the receiving water or primary

conveyance system. These practices address sources of pollution

at the furthermost upstream point.

1. Street Cleaning

Street cleaning through the use of mechanical or vacuum sweepers

can remove litter, dust and dirt from street surfaces. Varied

rates of success have been achieved with different types of

sweepers with the best results being obtained using the vacuum

type sweepers. While sweeping can be effective, it does not

remove any pollutants except those located on the street

surfaces. The greates t benefit from this practice would occur on

heavily traveled urban streets. It is our opinion that street

weeping at this time would not be cost effective in reducing

pollutants from entering the stormwater management system.
However, street sweeping 'may be an effective alternative in the

future as development progresses. Particularly if streets can be

identified which contribute heavy loads of pollutants.

2. Erosion/Sedimentation Control

Erosion/Sedimentation control measures should be implemented to

reduce or prevent sediment from exposed ground surfaces,

construction activities, and poorly constructed and maintained

drainage conveyances from di*scharging to the surface waters..

ordinances should be enacted and enforced to prevent sediment

from discharging from exposed ground surfaces and construction

activities. As previously discussed, existing conveyances

systems should be examined for locations of erosion and

maintenance procedures taken to stabilize those areas. Future

construction of conveyance facilities should be required to

stabilize those facilities with either structural or vegetative

means with vegetative means being encouraged.

10.2 STRUCTURAL BMPS

Structural BMPs are practices which minimize or reduce pollution

through physical modifications. These practices address specific

problem areas.

1. Detention/Retention Basins

Detention/Retention basins are large pond like structures which

are constructed to treat and/or attenuate stormwater runoff.

Treatment of the stormwater is achieved through settling,

biological assimilation and possibly filtration. Stormwater

attenuation may be achieved by designing a detention facility

large enough to store a portion of the runoff and discharge it

downstream at a reduced rate. Detention/Retention basins are the

primary treatment facility in use for stormwater management.

Based upon our analysis the use of detention/retention basins

will be most applicable to future development.. The City

presently has a well defined conveyance system which if modified

with the recommended improvements will not need additional

attenuation facilities to reduce flooding of the system.

Detention/Retention facilities could be used in the future if

locations are identified where flooding or sediment is being

introduced to the system at a known point.

2. Sedimentation Basins

Sedimentation basins are similar to Detention/Retention Bisins in

that they are large structures through which stormwater runoff is

directed. The p rimary difference is they are designed to

specifically target sediment for removal. This is accomplished

by reducing the velocity of the incoming runoff such that any

suspended particles will settle out prior to discharge.

Sedimentation basins may be of a long linear shape or more

uniformly rectangular or round.

The primary benefit of sedimentation basins for the City would be

in their installation at points along the existing conveyance

system to trap sediment and reduce maintenance efforts.

Potential locations for such facilities would be at points of

connection for minor ditch systems to the major trunkline system.

Construction would entail the over-excavation and widening a

section of the ditch system for a sufficient length to reduce

conveyance velocities to less than the settling velocities of any

suspended particles. In many instances this technique could be

used without the need for additional property.

3. Swales

Swales are ditch sections which contain a top width-to-depth

ratio of the cross-section equal to or greater than 6:1, or side

slopes equal to or greater than 3 horizontal to 1 vertical, and is

stabilized with vegetation. Swales have been found to be very

ef f ective in the removal of pollutants in areas with an appropriate

underlying soil.

Based upon our analysis, swales would most likely be the

treatment mechanism which could most benefit the City. Numerous

streets exist with shallow roadside ditches. The City should

consider implementing an improvement project which would reshape

these roadside ditches to meet the swale requirements and

stabilize them with sod. The benefits of such a project would be

in reduced maintenance of both these ditches and downstream

ditches. In many cases where shallow roadside swale sections are

utilized the homeowner will maintain and mow the section thereby

relieving the City of the majority of the maintenance requirements.

Benefits downstream will be realized in a reduction in sediments

and maintenance along with a reduction in other pollutants.

11.0 STORMWATER MANAGEMENT REGULATION*

Chapter 163.3177 F.S., requires that all local governments

address stormwater management in "a general sanitary sewer, solid

waste, drainage, potable water, and groundwater aquifer recharge

element... 11 in their comprehensive plans. Rule 9J-5 F.A.C. further

requires the development and implementation of a Level Of Service

(LOS) standard for all such infrastructure. It defines the LOS

standard as being "an indicator of the extent or degree of service

provided by, or proposed to be provided by a facility based on and

related to the operational characteristics of the facility". Rule

9J-5.003(23), (24) & (25) provide standards for the engineering-

based operational characteristics of a facility's capacity to

handle stormwater quantity (e.g. volume and rate of runoff from a

site) and quality (e.g. reduction in the amount of pollutants

carried of f -site in the runof f) . Such drainage facilities are

among the public facilities required to meet the concurrency

requirement (see. 9J-5.011(2)(c)2, F.A.C.). Thus, proposed

developments that will, if approved, result in a lowering in the

LOS below the level specified in the plan may not be permitted.

Adapted from: Land Development Regulations, Technical Assistance

Manual, Center for Governmental Responsibility, University of

Florida, 1989.

In addition to addressing the stormwater runoff issue in the

required comprehensive plans, local governments must also deal with

the issue in their land development regulations. Chapter 163.3202

F.S. provides that:

(2) Local land development regulations shall contain specific

and detailed provisions necessary or desirable to implement the

adopted comprehensive plan and shall as a minimum:

(d) Regulate areas subject to seasonal and periodic

flooding and provide for drainage and stormwater management;"

The state has now established a comprehensive regulatory

program designed t,o address the need for effective management of

stormwater runoff. A complex mosaic of regulations and regulatory

entities has been developed to address the issue. Local

governments addressing the issue in their required comprehensive

plans and land development regulations must take into account the

regulatory framework within which local requirements must operate.

The regulatory system already established for managing

stormwater runoff at the state and regional levels of government

were created to deal with the stormwater quality and quantity

issues.

11.0 S.tormwater Quality

An initial step toward addressing the water quality

problems caused by nonpoint sources was taken in 1982 with the

promulgation of Chapter 17-25, F.A.C. known as the Stormwater Rule,

by the Florida Department of Environmental Regulation (DER) which

is given responsibility to regulate stormwater discharges as a

source of pollution (see, 403.087,.088 F.S.) The purpose of the

rule is to prevent pollution of Florida waters by stormwater

discharged from new, expanded or modified development. The rule

establishes a minimum state-wide standard for all non-exempt

construction to conform to. A performance standard is used which

is designed to ensure that required stormwater systems remove at

least eighty percent (80%) of the annual pollution load of the

developed site. The rule permits delegation of stormwater quality

assurance permitting to the water management districts or local

governments. Except for the Northwest Florida Water Management

District, all of the water management districts have accepted this

delegation. The permitting requirements for meeting stormwater

quality standards compliment previously existing stormwater

quantity standards administered by the water management districts

implementing Part IV of Chapter 373 F.S. discussed next.

11.2 Stormwater Quantity

Part IV of Chapter 373 F.S. requires regulation of the

construction, alteration, maintenance, operation, and abandonment

of "dams," "appurtenant works," "Impoundments," "reservoirs," and

"works" affecting waters in the state. Thus, virtually all types

of real property improvements that are intended to or have the

effect of controlling, diverting, or impounding surface waters,

fall within the purview of this law. The surface water management

called for under Part IV and the implementing rules, are intended

to protect against flooding and provide protection of existing

upstream and downstream flows.

Rule 17-40 F.A.C. provides guidelines for implementing

Part IV of CHapter 373. The rule establishes surface water

management requirements and criteria for establishing minimum flows

and levels pursuant to Section 373.042 F.S. Each of the water

management districts are required to develop water management plans

consistent with the rule and Section 373.036 F.S. A review by DER

of existing water management district rules is required to

determine their consistency with 17-40 F.A.C. The DER has

conducted that review and determined all existing water management

district programs to be consistent. Pursuant to the rule, all

districts with permitting programs have been delegated the

responsibility to implement the program.

Under the program, all non-exempt projects which entail the

construction and operation of facilities which manage or store

surface waters, or other facilities which drain, divert, impound,

discharge into, or otherwise impact waters of the state must be

permitted by the water management or the DER (17-40.070) district

with jurisdiction or the DER (see 17-40.070 F.A.C.) . Statutory

exemptions are provided for; certain soil-related involving

agriculture, floriculture, silviculture, and horticulture; certain

agricultural closed systems; and the right of any natural person

to capture, discharge, and use water for purposes permitted by law

(Section 373.409 F.S.).

11.3 Local Regulation

All Florida local governments are required to address

stormwater management in their comprehensive plans and land

development regulations. The requirements established in the

drainage and capital improvement elements of the comprehensive plan

will largely determine what will be required in the development

regulations. For many localities, it may be possible, and in some

cases necessary, to rely upon the substantive requirements of the

DER and water management district to address stormwater quantity

and quality.

Rule 9J-24 F.A.C., which establish the procedures and

criteria for review of local government land development

regulations, requires the inclusion of "specific programs,

activities, standards, actions or prohibitions which regulate or

govern the ... provision of adequate drainage facilities to control

the individual and cumulative impacts of flooding and non-point

source pollution in drainage basins existing wholly or in part

within the jurisdiction.11 Thus, the land development regulations

adopted pursuant to Section 163.3202 F.S. and Rule 9J-24.033 F.A.C.

must address both the water quantity and quality aspects of

stormwater management.

Since existing laws require inclusion of stormwater

provisions in local land development regulations, and all such

regulations must be incorporated into a single code, the stormwater

regulations included herein are intended to be a section of the

overall land development regulations rather than a separate

ordinance. The City will prepare and submit its land development

regulations to DCA before December 1, 1990.

The following stormwater regulations are intended to

address both the quantative and qualitive aspects of stormwater

management. These regulations are further intended to be realistic

to the limited financial and administrative resources available to

the City for regulating stormwater.

It should be noted that the format of these model

regulations may change as dictated by the construction of the land

development regulations. Specific requirements and design criteria

will remain the same unless changes occur in governing laws during

the coming year.

11.4 MODEL STORKWATER. MANAGEMENT PROVISIONS

XX.01 Public Purpose

The discharge of untreated and uncontrolled stormwater can

reasonably be expected to be a source of pollution to waters of the

state, and a direct cause of flooding causing risk of harm to life

and property. It is the intent of the city to minimize adverse

impacts of pollution and flooding through regulation of stormwater

discharges caused by land development.

xx.02 Exemptions

The following development activities are exempt from the

requirements of subsection xx.03 , except that measures to control

erosion and sedimentation must be undertaken for all development

when considered necessary by the City.

1. Individual single-family, duplex, tri-plex and quadraplex

residential dwelling units constructed on a single lot or parcel

of land, provided the individual single-family, duplex, tri-plex

or quadraplex structure is not part of a larger common plan of

development or sale.

2. Accessory buildings or structures constructed in conjunction
with single-family, duplex, tri-plex or quadraplex re:sidential

dwelling units.

3. Any development within a residential subdivision provided the

following conditions have been met:

a) A stormwater management facility which has been

constructed in conformance with the standards prescribed

in this section is available to receive stormwater

discharges;

b. The development will be constructed in accordance with

the stormwater management provisions approved as a larger

common plan of development or sale provided such

provisions are valid as part of a final development order

or agreement and comply with the provisions of this,

Section;

C. Applicable exemptions as specified in Subsection 17-

25.030, Florida Administrative Code (DER Stormwater

Permit).

d. Applicable exemptions as specified in Subsection 14-

86.003, Florida Administrative Code (FDOT Drainage

Connection Permit).

xx.03 Stormwater Management Recruirements

All non-exempt development activities must be designed,

constructed, and maintained in accordance with the provisions of

this subsection.

x.031 Design Standards

1. Water Quality

At a minimum, all-non-exempt stormwater management

facilities shall be designed, constructed and

maintained in conformance with the provisions of

Chapter 17-25, Florida Administrative Code

(Regulation of Stormwater Discharge).

2. Water Quantity

At a minimum, all non-exempt stormwater management

facilities shall be designed and constructed so as

to attenuate stormwater runoff caused by the 25-

year, critical duration storm event, or in

conformance with the provisions of Chapter 14-86,

Florida Administrative Code (Drainage Connection).

The maximum hydraulic grade line elevations

contained in the Stormwater Management Plan shall

be used as a basis for design.

3. To the maximum extent practicable, natural systems

shall be used to accommodate stormwater.

4. The proposed stormwater management system shall be

designed to accommodate the stormwater that

originates within the development and stormwater

that flows onto or across the development from

adjacent lands. The stormwater system shall not be

required to attenuate or treat off-site flows, but

must convey off-site flows downstream. In no case

shall off-site flows be impeded.

5. The proposed stormwater management system shall be

designed to function properly for a minimum twenty

(20) year life.

6. No surface water may be channelled or directed into

a sanitary sewer.

7. The proposed stormwater management system shall be

compatible with the drainage systems or drainage

ways on surrounding properties or streets, taking

into account the possibility that substandard

systems may be improved in the future.

8. The banks of det ention and retention areas should

be sloped to accommodate, and should be planted

with, appropriate vegetation. Vegetated slopes

shall not exceed 3:1 for normally dry facilities and

4:1 for wet facilities for a depth of 2 feet below

normal water level.

9. Dredging, clearing of vegetation, deepening,

widening, straightening, stabilizing or otherwise

altering natural surface waters shall be minimized.

10. Natural surface waters shall not be used as sediment

traps during or after development.

11. For aesthetic reasons and to increase shoreline

habitat, the shorelines of retention areas shall

be sinuous rather than straight.

12. Water reuse and conservation shall, to the maximum

extent practicable, be achieved by incorporating the

stormwater management system into irrigati,on systems

serving the development.

13. Vegetated buffers of sufficient width to prevent

erosion shall be retained or created along the

shores, banks or edges of all natural or man-made

surface waters.

-14. In phased developments the stormwater management

system for each integrated stage of completion shall

be capable of functioning independently as required

by this Code.

15. All detention and retention basins, except natural

water bodies used for this purpose, shall be

accessible for maintenance from streets or public

right-of-way. For facilities where the City of

Mexico Beach is to accept maintenance, and area of

no less than 10 shall be provided from the top of

bank for access by equipment.

16. Necessary erosion and sediment control measures

shall be implemented to prevent sediment transport

off-site.

xx.032 Certification

I. Design and construction of stormwater management

facilities shall be certified by a professional

engineer licensed in the State of Florida, and;

2. Where applicable valid permits must be obtained

pursuant to Chapter 17-25, FAC and Chapter 14-86,

FAC prior to commencement of construction.

119

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