Changes due to jetties at Tillamook Bay, Oregon

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Center

CHNES DUE TO JETTIES AT
TILLAMOK BAY, OREGON

Pi'~ hms h A. Terich

.10 ~ ~~~~~~~~~~~~~Re printed by the Amer. Soc. Civ. Eng.,
Proc..15th. Coastal Eng. Conf., OCh. 104:
L... ~~~~~~~~~~~~~1791-181 1.

~ U.S DEPARTMENTOF COMMERCE NOAA
COASTAL SRICES CENTERi
~~ 2234 SOUTH HOBSON AVENUE

GC M
57.2
.076 -
no. 77-016 9i OREGON STATE
SEA GRANT
COLILEGE PROGRAM
Reprint no. ORESU-R-77-016

Reprinted by the American Society of
Civil Engineers from the Proceedings of
the 15th Coastal Engineering Conference,
Honolulu, Hawaii, July 11-17, 1976

CHAPTER 104

CHANGES DUE TO JETTIES AT TILLAMOOK BAY, OREGON

Paul D. Komar' and Thomas A. Terich2

ABSTRACT

Bayocean Spit, separating Tillamook Bay from the Pacific Ocean on
the north Oregon coast, underwent severe erosion following construction
of a north jetty at the bay entrance in 1914-17. This erosion ultimately
led to the complete breaching of the spit in 1952. Simultaneous to
the spit erosion south of the entrance, the shoreline north of the north
jetty advanced seaward by some 600 m (2000 ft).. This pattern of erosion
and deposition following jetty construction has generally been interpreted
as the jetty blocking a large north to south net littoral drift in the
area, estimated by a previous study at 620,000 m3/yr (800,000 yd3/yr).
Our reexamination of the shoreline changes and patterns of erosion and
deposition following jetty construction disagrees with this interpretation,
and instead we conclude that all of the changes resulted from local
rearrangements of the beach due to the disrupted equilibrium following
jetty construction, but at the same time maintaining an overall condition
of zero net littoral drift. This interpretation is supported by other
evidence that indicates a near-zero net drift on this portion of the
Oregon coast. Thus severe coastal erosion can result from jetty con-
struction even in areas of zero net littoral drift.
A new south jetty has been recently completed (1974). The result
has been further realignments of the shoreline with accretion and shore7
line advance immediately south of the south jetty. This provides
further confirmation that a zero net littoral drift exists in the area.
This study also demonstrates the effects of building only a single
jetty rather than a pair of jetties. Following construction of the north
jetty,- the outer bar or ebb-tide delta at the Tillamook Bay inlet grew
appreciably in size. Sand deposited there came from erosion of Bayocean

'School of Oceanography, Oregon State University, Corvallis, Oregon 97331.
2Department of Geography, Western Washington State College, Bellingham,
Washington 98225.

1791

1792 COASTAL ENGINEERING-1976

Spit further to the south. The shoal growth pushed the main channel
at the entrance against the north jetty where it has remained since
jetty completion. In the process, the channel became much deeper and
narrower than the channel geometry prior to jetty construction.

INTRODUCTION

Bayocean Spit on the northern Oregon coast, about 80 km (50 miles)
south of the Columbia River, separates Tillamook Bay from the Pacific
Ocean (Figure 1). This spit has had a long history of development and

:Tillamook S Potrllond

P 2 | t f OREGON

S : : A Z 5' f f : S : :i

Tiomokmoo

'Cap

Figure 1: Tillamook Bay :and Bayocean Spi t.

erosion. The development and ultimate financial failure of Bayocean Park,
a resort community built on the 'spit early in *this century, has been
discussed by Terich 1and Komar (1 974). ::Th 'final demise of the communit
resulted from erosion to the sand spit following construction of a north
jetty at the Tillamook Bay entrance in 1914-17. Erosion over some

JETTY-CAUSED CHANGES 1793

thirty-five years progressively narrowed the spit, and in 1952 it was
breached at its narrowest point. This breach has been subsequently
repaired by the construction of a dike, and recently a new south jetty
has been constructed. The purpose of the present paper is to analyze
the shoreline changes that resulted from the jetty construction and led
to the erosion of Bayocean Spit. :This example of shoreline erosion
resulting from jetty construction is unusual in that, as will be shown,
this area of the Oregon coast has a zero or near-zero net littoral drift
along its beaches. Thus the shoreline changes and erosion are not the
more familiar case of jetties blocking a net littoral drift, causing.
erosion in the downdrift direction. This study also demonstrates the
results of constructing only a single jetty under such conditions,
rather than a pair of jetties.

LITTORAL DRIFT ON THE OREGON COAST

With the exception of the section of coast near the mouth of the
Columbia River, the Oregon coast is a series of long pocket beaches
separated by pronounced rocky headlands.: All evidence indicates that
these areas are experiencing a seasonal reversal in the sand drift
along the beaches, but with a zero or near-zero net littoral drift over
a several years time span. This is best demonstrated by the effects of
jetty construction on patterns of shoreline erosion and accretion.
The study of Komar', et al. (1976) investigated these patterns for all
jetty systems on the Oregon coast with the exception of the Columbia
River jetties. Our study found that following jetty construction,
sand would in general accumulate adjacent to the jetties, both to the
north and south, filling in the pockets formed between the jetties and
the pre-jetty shorelines which curved inward at the entrances. Our
study showed that the amount of deposition adjacent to the jetties
depended on the sizes of the pockets formed by the jetty construction.
Deposition commonlyioccurred both north and south of the jetty systems,
and the relative amounts of shoreline accretion on opposite sides of
the jetties could in no way be taken as infering a net littoral drift
along the coast. Sand for this shoreline advance next to the jetties
came from erosion of the coast at greater distances from the jetties.

1794 COASTAL.ENGINEERING-1976

The shoreline alterations following jetty construction continued until
the shoreline became essentially straight and parallel to the prevailing,
wave crests, at which point zero net sand transport was again achieved
and a new-equilibrium reached. As will be shown in this paper, this
pattern of deposition adjacent to the jetties and erosion at greater
distances along the coast also explains the shoreline alterations at
the Tillamook Bay entrance that led to the destruction of Bayocean Spit.
The pattern of changes there, however, was complicated by the fact that
only a north jetty was constructed initially, not a pair of jetties.
Deposition around jetties therefore indicates that, except near
the Columbia River, the Oregon coast tbeaches :0have, as close ,,as we can
determine, a long term zero net littoral drift. This is also supported
by observations that there is a seasonal reversal in the general transport
directions due to the patterns of storm systems. During the summer
,months waves prevail from the northwest, causing a southerly sandl
transport 'along Oregon beaches. During the winter months the transport
is to the north due to waves arriving mainly from the southwest. The
wave data is inadequate, howeveri, to actually carry out any calculations
of littoral drift rates in an attempt to demonstrate a zero net drift.
Similar to the jetties, the rocky headlands show no indications
of blockage of a net littoral drift, there being no accumulations of
beach sand either to the north or ,south sides. What little study that
has been done of heavy minerals in the beach sands also indicates that
there is no bypassing of littoral sands around these headlands, which
is reasonable considering their sizes and that they extend to considerable
water depths. It is because of these barriers that the Oregon beaches
can be described as pocket beaches of varying size. The only net
littoral drift required within a pocket beach is the small amount
necessary to redistribute the beach sand away from its sources to the
complete stretch of beach. Previous studies (for example,/ Komar and
Rea, 1976) have shown that sea cliff erosion is the primary source of
beach sands in most areas, the river sands being trapped within the
estuaries. Therefore even a net drift required for sand redistribution
within the Oregon pocket beaches will be very small.
Because there is essentially ta zero net drift of littoral: sands

JETTY-CAUSED CHANGES' 1795

on the Oregon coast beaches, the erosion of Bayocean Spit following
jetty construction clearly is not another example such as the Santa
Barbara, California, breakwater, or the Port of Madras, India, where
the coastal erosion resulted from blockage of a large net littoral
drift by jetty construction..

SHORELINE CHANGES FOLLOWING CONSTRUCTION OF A NORTH JETTY

The principal sources of information on the shoreline changes
utilized in this study are: ;(1) surveys undertaken by the Corps of
Engineers, Portland District, before and after jetty construction;
(2) aerial photographs from'a variety of sources; (3) field studies of
old shorelines and other features that are still visible; and (4) the
writers' surveys in the case of the more recent construction of the
south jetty at Tillamook Bay., In addition to the shoreline changes, once
erosion became appreciable on Bayocean Spit, the Corps of Engineers,
Portland Dsitrict, monitored the rates of retreat of the dune bluffs
and coastal property on the spit; this data has also been utilized.
Since most of the shoreline changes and spit erosion occurred more than
twenty-five years ago, we have had to rely primarily on historic data
rather than on our own measurements. There is of course a certain
amount of uncertainty in doing this.
A north jetty at the entrance to Tillamook Bay was begun in 1914
and completed in 1917. For economic reasons, an accompanying south
jetty was not constructed at that time. The most apparent immediate
response to the north jetty construction was a shoreline advance to
the north of the jetty, documented in Figure 2. The buildout of the
shorelinethere nearly kept pace with the jetty extension. As in the
cases of jetty construction elsewhere on the Oregon .coast, and already
discussed in this paper, this deposition adjacent to the north jetty
occurred-primarily so that the pocket formed between the jetty and the
pre-jetty shoreline, which curved inward toward the bay entrance,
would be filled. The shoreline built outward only until it became parallel
to the prevailing wave crests, at which point long term changes ceased.
The overall position of the shoreline has changed very little in the

1796 COASTAL ENGINEERING-1976

PACIFIC OC- EAN

NORTH N
JETTY
1971

BAYOCEAN

TILLAMOOK BAY JETTY
High lids shorelines northiof
;0REEN : 0: ; � 0jetty, 1914-1971.

lim

Figure 2: The shoreline advance north of the north jetty at the
Tillamook Bay entrance after its completion in 1917. The 1914
shoreline gives a typical pre-jetty location.:

past forty-some years, the only alterations being due to partial jettyi:
degradation and reconstruction in the 1930':s.
The sand accumulation north of the north jetty amounted to some
6,000,000 m3: (8 x 106 yd3). The second most apparent effect of the
north jetty construction was the resulting erosion Of Bayocean Split to
the immediate south. Based on this pattern of deposition to the north
and erosion to the south, previous studies qenerally concluded that there
must be an appreciable southerly net :ittoral drift. Based on accumulation
rates north of the north jetty, this littoral drift was Xestimated at
600,000 m3/yr (800,000 yd3/yr) by the Corps of Engineers (1970), but
placed at a :lower estimate of 140,000 m3/yr (180,000 yd3/yr) by 0 'Brien
(1930). We have already summarized the evidence that argues against such
a net littoral drift on the Oregon coast. :In the case of the Bayocean
Spit and Tillamook Bay area, the pronounced headland Cape Meares exists to
the immediate south (Figure 1). If such a large net drift did exist,
this cape should also block the transport causing a buildout of the
beach there to the north; there is no n6ticeable buildout, the beach
in fact being principally gravel and cobbles; not sand as on the beach;

JETTY-CAUSED CHANGES: 1797

to the, north fronting the spit, and the cliffs near Cape Meares are
undergoing extensive erosion. As already indicated, the deposition
north of the north jetty following its construction can be better
interpreted as changes resulting from local rearrangements of the
beach due to the'disrupted equilibrium caused by the jetty's presence
but at the same time maintaining an overall condition of zero net drift.
It is difficult to determine when noticeable dune and property
erosion began on the spit itself following construction of the north
jetty in 1914-17. O'Brien (1930) indicated that there was considerable
erosion at that time just to the north of Cape Meares (Figure 1),; off
the spit itself. He also mentioned, however, that the spit itself had
suffered little change, "although the channel has moved northward against
the jetty, probably due to the decreased sand pressure from the north."
This channel migration following jetty construction and the growth of
the shoal outside the bay entrance will be documented later. The sand
deposition at the bay entrance came from erosion of the beach along
the length of Bayocean Spit. From O'Brien's comment it would appear
that there may have been some beach erosion at that time but the
erosion had not yet reached the dunes nor threatened any property in
Bayocean Park.
O'Brien (1930) also mentioned that the north jetty had weathered
down appreciably. For this reason, the north jetty was reconstructed and
lengthened in 1932-33. After this reconstruction erosion on the spit
became very' apparent. Even while reconstruction was in progress erosion
began to undermine the sidewalk fronting the natatorium of Bayocean.Park,
built close to the beach, and by 1936 the structure's roof had collapsed
(Figure 3:). Erosion of the spit was progressive, although some winters
were more severe than others and caused rapid dune and property retreat.
For example, during January 1939 a storm broke a small gap along the
narrow southern end of the spit, washing sand and gravel into the bay.
The natatorium was finally completely destroyed by this storm (Figure 3).
Maximum recession of the top of the dune bluff was 7.5 m (25 ft) with an
average recession of 2 m (7 ft). In addition to the loss of the natator-
ium, four houses were undermined and lost, nine were immediately threatened,

1798 COASTAL ENGINEERING-1976

Figure 3:: Progressive
erosion of the natator-
ium on Bayocean Spit.

and six had to be moved back for safety.
Washovers of the spit occurred during subsequent winter storms
until on 13 November1952 storm waves together with high tides entirely
breached the spit's narrowest southern section, initially removing a
1200 m (4000 ft) long segment of spit. This breach progressively widened,
becoming nearly a mile wide at high tide. The breach: developed into the
natural opening for the bay, the northern entrance with the jetty begin-
ing to shoal and close (Figure 4; 1955 survey). Waves rolled through
the breach producing swells within the bay, causing some erosion to farm-
lands on the bay's edge. For this reason it was decided to close the
breach, so in 1956 work was begun on a rubblemound dike, set back within
the bay. The construction of this dike and the difficulties in closure
are discussed by Brown, et al. (1958). It was anticipated that the

JETTY-CAUSED CHANGES 1799

jetty -

1904\ 1932/i in955 52 1n971

breach

0 I 2
I I I
kilometers

Figure 4: Progressive erosion to Bayocean Spit leading to its
breaching in 1952, necessitating the placement of a dike
closing the breach.

pocket in the shoreline seaward of the dike would fill a nd re-establish
a sand).beach fronting the dike, which it did as can be seen in the 1971
survey of Figure 4.

SHOAL DEVELOPMENT AND CHANNEL CHANGES AT THE BAY ENTRANCE

Although erosion occurred along most of the length of Bayocean
Spit following construction of the north jetty, there was some deposition
to the immediate south of the jetty (as well as to the north, as already
seen). This deposition to the jetty's south at the bay entrance was in

the form of a substantial growth to the outer bar or ebb-tide delta.
As will, be discussed later, it is estimated that some 3.3 x 106 m3
(4.3 x 106 yd3) of sand was added to this shoal following jetty construction.

1800 COASTAL ENGINEERING-1976

20 / ~~ TILLAMOOK BAY
ENTRANCE
October 1902N

6 epfoolofl fe

14.~~~~ ~6.)l 20

TILLAM~~~~OO 1000

V~~~~ETAC

(3~~~~~~~~~~~~~~~ locc o

22 September 1941 Nep

Figure 5: Tillamook Bay entrance before (upper) and after (lower):north
jetty construction in 1914-17.

JETTY-CAUSED CHANGES 1801

The changes in the shoal and bay entrance are illustrated by Figure 5,
the upper survey showing the entrance prior to jetty construction and
the lower being typical of the entrance after construction. It is seen
that there was an increase in the overall size of the outer bar shoal
and a reduction in water depths. Under moderate to high wave conditions,
this shoal became entirely covered by breaking waves and surf, making
it a hazard to boats using the entrance.
Figure 5 also shows that the channel leading from the bay entrance
was pushed northward against the jetty by the growth of the shoal. The
channels at the entrances to the Umpqua and Coquille Rivers on the Oregon
coast similarly migrated until they became adjacent to the single jetties
that were initially built there (Komar, et al., 1976; Kieslich and Mason,
1976). Such a response to a single jetty rather than a pair of jetties
has also been shown to occur on other coasts (Kieslich and Mason, 1976).
In addition to the growth of the outer bar and migration of the
main channel following construction of the north jetty at Tillamook Bay,
there were changes in the geometry of the channel outside the entrance.
This is shown in Figure 6 which compares channel cross-sections before
(June 1914) and after (June 1920) jetty construction. It is seen that
after jetty development the channel became much deeper and narrower than
existed prior to construction. These changes are presumably the response
of the channel to the pressure from the south by the shoal growth. However
changed in depths and widths, the channels before and after jetty construc-
tion did not differ significantly in cross-sectional areas. In addition,
the changes in geometry did not extend inward along the channel into the
entrance itself. Figure 7 shows a number of channel cross-sections
immediately north of the spit at the entrance's narrowest point, some
before and some after jetty completion. There are no noticeable effects
there due to the addition of the north jetty. Despite the changes in
channel geometry just outside the entrance and the growth of the shoal,
the entrance itself remained relatively unchanged and bore the same
relationship to the tidal prism within the bay according to O'Brien's
(1931, 1939) relationship.,
Although this study undertook no field investigations of the
currents and waves in the area of the entrance, the changes in'the channel
position and geometry, and the growth of the outer bar shoal can be

1802 COASTAL ENGINEERING-1976

TILLAMOOK ENTRANCE :rMOO
Channel cross-sections :0
before (June 1914)
after (June 1920)
jetty construction

N <

------: :: 100-5 2000
: /9: 2 0 0 f v : t:0: : D ; ; :600
June 1914 20E

PACI/F/C OCEAN

Figure 6: Deepening and :narrowing of the channel following
construction of a north jetty.

understood in terms of the findings of previous studies of inl'ets. Processes
in the vicinity of inlets are complex in that they are the combined results
of tidal-currents, wave action, and possible effects of salinity and
thermal stratification resulting' in a net inward flow at the bottom of
the channel. The main ebb flow currents generally act to carry sand from
the bay and nearshore areas to the offshore where it is deposited in the
form of a delta, sometimes cailed the'"ebb-tide delta. i' If located, on a
coast with a zero net littoral drift, this delta would be symmetrical and,
arcuate in shape. The existence of a net littoral drift would produce an"
asymmetry. Although the main ebb flow is directed offshore, currents move
inward toward the entrance in the nearshore, from both sides of the inlet.
Two eddies or gyres may develop on flanks of the ;main ebb channel
emanating from the inlet; one gyre would be a clockwise flow, the other

JETTY-CAUSED CHANGES 1803

A ; Horizontal Distance in Meters B
600 oo00 400 300 200 100 0

:� -- Sept 1902 1 . : | \: ~- F-15 /,
0)
Pre-Jetty '0 I ' '" '
" dJuly ,8I
-Sept 902
os t - Pot-Jety C ' S-' E -T15
May 1918 : V
- -- July 1925
...... June 1932

B
Section TILLAMOOK BAY ENTRANCE
,~ ff/CHANNEL CROSS-SECTION

,: :".'~ i

Figure 7: Cross-sections of the Tillamook Bay entrance before and
after constructionm of the north jetty.

counter-clockwise, but both would produce currents directed toward the inlet
on their shoreward sides. Lynch-Blosse and Kumar (1976) subscribe to such
gyres being important at inlets, and discuss modifications where longshore
currents are superimposed due to a general oblique wave approach to the
coast. Dean and Walton (1975) point out that the ebb current can be
viewed as a jet directed seaward, the high velocities in the central current
of the jet transferring momentum outward and entraining adjacent waters,
giving rise to the gyres described above. As a result, during ebb flow
from the inlet there will be inward moving currents close to the shore,
transporting sand toward the inlet and aiding the development of the
flanking outer bars or shoals. During tidal flood flows the water converges
toward the inlet from all sides, especially in flood channels to the flank
of the deeper ebb channel. Thus the flood currents also aid in the devel-
opment of flanking shoals.
Wave action generally acts to counter growth to the outer bar
of the inlet. It does this by transporting sand back onshore to the beaches.

1804 COASTAL ENGINEERING-1976

For this reason, on coasts of high wave conditions (like Oregon) the outer
bars of inlets tend to be smaller than on coasts of small waves (Dean and
Walton, 1975; Walton and Adams, this volume). However, wave refraction
over the shoal can also aid in the development of the outer bar, and some
studies have suggested that this process may be more important than the
current gyres already discussed (Hayes, et al., 1971; Hubbard, this volume).
Refraction of the waves around the outer bar causes a longshore current
directed toward the inlet from both sides, thus working in concert with
any ebb-tide gyres and flood-tide currents acting in the area. With an
overall oblique wave approach to the inlet area, the longshore current may
be inward toward the inlet on only the updrift side.
As seen in Figure 5 (upper), typical flanking outer bars existed
at the Tillamook Bay entrance prior to jetty construction. There was a
seaward offset of the northern shoreline which is sometimes taken to
indicate a net littoral drift (Hayes, et al., 1971; Lynch-Blosse and
Kumar, 1976), but there is no consistency as to whether the shoreline ;
offset is updrift or downdrift of the inlet. The presence of an offset
at the Tillamook Bay inlet in an area of zero net littoral drift casts
doubts on offset direction as an indicator of net drift direction and
on the theories of origin of the offset which- rely on the presence of
a net littoral drift.
The construction of the single north jetty at the Tillamook Bay
inlet provided additional protection from the wave activity. This
protection would allow for further growth of the south flanking shoal,
the north flanking shoal becoming covered by the shoreline advance to
the north of the north jetty. The growth of the south flanking shoal
was presumably due to the continuation of an eddy gyre in this region
during ebb tide, and perhaps due to wave refraction effects, both
described above, but With a decrease in the wave activity that had acted
to reduce the size of the shoal. This is partly verified by model
studies conducted at the Waterways Experiment Station on the Masonboro
Inlet, North Carolina, and reported in Dean and Walton (1975, page 137).
Like the Tillamook Bay entrance, the Masonboro Inlet has only a single
jetty. The model studies indicated that shoal growth resulted from
(a) the jetty's sheltering of the shoal area, thereby creating a sand

JETTY-CAUSED CHANGES 1805

trap, and (b) a large eddy or gyre carrying sand toward the inlet on
both ebb and flood currents.
It is seen that all factors affected by the installation of a north
jetty at Tillamook Bay would act to increase the growth of the south
flanking shoal. The shoal growth pushed the channel northward against
the jetty, and the pressure from the developing shoal also presumably
accounts for the narrowing and deepening of the channel.

INTERPRETATION OF SHORELINE CHANGES AND THE BUDGET OF SEDIMENTS

As already indicated, a closer inspection of the changes following
construction of the north jetty does not agree with the interpretation of
a large net littoral drift identified by earlier studies (Corps of
Engineers, 1970). The overall pattern of erosion and deposition is
illustrated schematically in Figure 8, and it is seen that the pattern
is symmetrical north and south of the jetty, complicated by the fact that
only a single j'etty was installed. Although erosion occurred along most

Monhatten Bech

Rockoway eoch
Figure 8: Patterns of erosion and
deposition following construction
L I of a north jetty but prior to the
south jetty development.

jetly

eo 0 mio
: : 121 erosion
;7i:: E- by deposition

Coape
Meares

1806 COASTAL ENGINEERING-1976

of the length of Bayocean Spit, there was a net deposition just to the south
of the jetty in the form of growth to the flanking outer bar..; Following
jetty construction, sand moved northward from most of the length of the
spit and accumulated in this shoal growth. As will be shown shortly,
the loss of sand from the spit erosion can be approximately balanced by;:
the amount of sand deposited on the shoal plus that carried into the bay
during the 1952-56 breach.
To the north of the jetty sand accumulated and the shoreline
advanced (Figure 2). Deposition there probably derived its sand from;' E
beach erosion further to the north, symmetrical with the northward
movement of sand on the spit to the inlet. This overall pattern of
deposition adjacent to jetties and erosion at greater distances was
shown to occur at other Oregon coast jetty systems (Komar, et al., 1976),
and was reviewed earlier in this paper. Erosion to Bayocean Spit was
much greater than erosion to the north of the jetty at Manhatten Beach
and Rockaway Beach (Figure 8) because of the longer stretch of beach
that exists to the north. Only a small amount of sand had to be
eroded per unit length of shoreline to supply sand to the accretion area
immediately north of the jetty. In contrast, Bayocean Spit is only a
small segment between the jetty and Cape Meares on the south (Figure 8)
so that a larger amount of sand had to be supplied by each unit length
of spit shoreline erosion to support the shoal growth next to the inlet.
We have attempted to put sediment volumes on the patterns of
erosion and deposition illustrated in Figure 8 by the development of
a budget of sediments. This budget is for changes following the
completion of the north jetty in 1917 but before the construction of
the south jetty in 1969. Our estimates are given in Figure 9.: Depo-
sition to the north of the jetty amounted to some 6 x:106 m3, which
agrees with previous estimates of the fill (Corps of Engineers, 1970).
As already indicated, this sand deposition came from beach erosion further
to the north; that erosion value is given in Figure 9 in parentheses
because we have no actual measure of the erosion other than the supposi-
tion that it will balance the deposition north of the jetty. Of course
any transfers of sand around the jetty during or after construction would
alter this exact balance; unfortunately we have no way to evaluate such a

JETTY-CAUSED CHANGES 1807

Figure 9: Budget of sediments for
ero~sion 4 iareas of erosion and deposition
following construction of the
north jetty at Tillamook Bay.

deposition

deposition p i ', ""

0I
B .a lom .

spit
57x1 erosion \ deposition
breach ) - 1.5 106X m
1952-56b ,

CAPE
MEARES

transfer around the north jetty, but believe it to be small, especially
in comparison with the volume of sand deposited to the north of the
jetty. This belief is in part supported by the continued existence of
a shoreline offset north and south of the jetties, even after construction
of a south jetty. This suggests that the jetties are an effective
barrier to longshore movements of sand on the beaches; otherwise sand
would presumably transfer from the north to the south'beach until they
have equal offshore extents.
We estimate that the flanking outer shoal to the south of the
jetty increased in volume by 3.3 x 106 m3 following jetty construction.
This estimate~ is based on comparisons of bathymetric surveys such as
those of Figure 5, before and after jetty completion. Our estimate
shows reasonable agreement with the results of Walton and Adams (this
volume). They place the total volume of the outer bar of Tillamook Bay
at 15 x 106 m3, so the measured growth volume is small in comparison.
Of special interest is that Walton and Adams show that outer bars on

1808 COASTAL ENGINEERING-1976

coasts with small wave energy are some 23% larger than bars on coasts
with large wave energies. If we reason that the growth of the Tillamook
inlet outer bar is due to the additional protection offered by the jetty
from wave attack, and employ their 23% resulting growth factor, one
obtains a volume increase of 3.5 x 106 ;m3, almost exactly the same as
our measured increase (3.3 x 106 m3).
Erosion of Bayocean Spit and the area to its immediate south up
to Cape Meares amounted to some 5.7 x 106 m3 (Figure 9). This estimate
is based on measured beach and dune bluff retreats and heights of the
dune bluffs and sea cliffs.
The other large transfer of sand in the area was into the bay
when the spit breached in 1952 and remained open until 1956. This volume
of deposition and loss from the seaward side of the spit is estimated at
1.5 x 106 m3, and is based on the volume of spit removed by the breaching
process (which gives a minimum transfer) and shoaling values within that
portion of the bay. This estimated volume is considered to be much poorer
than our estimates of spit erosion or deposition near the jetty.
There is also the possibility of some deposition on the inner
shoal (flood-tide delta), within the bay at the jettied entrance. The
surveys of the entrance area, before and after jetty construction, suggest
that deposition within the bay in this area was small, but we could not
make satisfactory measurements from the available data.
Altogether there is a reasonable accounting for the transfer of
sand from erosion areas to deposition areas in the region of Bayocean Spit
and Tillamook Bay following construction of the north jetty.;; The shoal
growth and sand entering the bay through the breach together account for
4.8 x 106 m3 of deposition. This is close to the 5.7 x 106 m3 of
estimated erosion from the spit. Considering the inaccuracies of these
estimates, the results demonstrate that the patterns of erosion and
deposition can be accounted for by local rearrangements of nearshore
sands following construction of the north jetty, without blockage of
an appreciable net littoral drift by the jetty. -

JETTY-CAUSED CHANGES 1809

CONSTRUCTION OF THE SOUTH JETTY

The hazardous channel conditions which developed due to the growth
of the outer bar following construction of the north jetty prompted renewed
considerations for construction of a south jetty. Construction of a south
jetty was begun in April 1969 and completed in 1974 with a total length
of 2000 m (6,:500 ft).
Even as the south jetty was being constructed, sand accumulated
between it and the curved pre-jetty shoreline, causing a shoreline advance
to the south. This accumulation continued until the shoreline became
straight and parallel to the prevailing wave crests, the same as des-
cribed earlier for other jetty systems on the Oregon coast as well as
for the earlier construction of the north jetty. This provides further
proof for a zero or near-zero net littoral drift in the area.

SUMMARY OF CONCLUSIONS

(1) With the exception of the section of coast near the Columbia
River, the Oregon coast is a series of pocket beaches separated by pro-
nounced rocky headlands. There is a zero or near-zero net littoral drift
within these pockets. Shoreline changes due to jetty construction are
therefore not due to blockage of a net drift.
(2) Following completion of a single north jetty in 1917 at the
Tillamook Bay entrance, the shoreline built outward to the immediate
north. This accretion occurred to fill the embayment created between the
jetty and the pre-jetty shoreline which curved inward toward the bay
entrance. Deposition continued until the shoreline became straight and
parallel to the prevailing wave crests, at which time a new equilibrium
was achieved with a zero net littoral drift.
(3) Bayocean Spit to the south of the Tillamook Bay entrance
suffered severe erosion following construction of the north jetty, cul-
minating in its breaching in 1952. Initially, sand eroded from the spit
moved northward and was deposited at the entrance in the form of growth
to the outer bar (ebb-tide delta). Later when the spit breached, consi'd-
erable quantities of littoral sediments were also carried into the bay.
(4) Growth of the outer inlet bar or shoal following construction

1810 i COASTAL ENGINEERING-1976

of a north jetty can be understood in terms of wave and current processes
acting at inlets. DThe single jetty offered increased protection from the
waves and perhaps augmented flanking currents flowing inward toward the inlet.
(5) Considerable coastal erosion can result from jetty construction
even in coastal areas that are not experiencing a sizeable net littoral
drift; blockage of a net drift by jetty construction is not a necessary
prerequisite for erosion.

ACKNOWLEDGEMENTS

This work is a result of research Sponsored (in part) by the Oregon ;
State University Sea Grant Program, supported by NOAA Office of Sea Grant,;
Department of Commerce, under Grant #04-6-158-44004.;

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JETTY-CAUSED CHANGES 1811

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