HomeMy WebLinkAboutCOASTAL PROCESSES REPORTPOINT WHITNEY BOAT ACCESS RENOVATION
COASTAL PROCESSES ASSESSMENT AND DESIGN CONSIDERATIONS
FINAL REPORT
Prepared for:
KPFF
Tacoma, Washington
On behalf of:
Washington Department of Fish and Wildlife
Olympia, Washington
Prepared by:
Northwest Hydraulic Consultants Inc.
Seattle, Washington
14 August 2018
NHC Ref No. 2003950
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Prepared by or under the supervision of:
K.M. Leytham, PE
Principal
Digitally signed by Keith M.
Keith M.
Leytham
018.08.1508:32:48
Leytham 0700'
DISCLAIMER
This report has been prepared by Northwest Hydraulic Consultants Inc. for the benefit of OFF
Consulting Engineers and Washington Department of Fish and Wildlife for specific application to the
Point Whitney Boat Access Renovation Project. The information and data contained herein represent
Northwest Hydraulic Consultants Inc. best professional judgment in light of the knowledge and
information available to Northwest Hydraulic Consultants Inc. at the time of preparation, and was
prepared in accordance with generally accepted engineering and geoscience practices.
Except as required by law, this report and the information and data contained herein are to be treated
as confidential and may be used and relied upon only by KPFF Consulting Engineers and Washington
Department of Fish and Wildlife, its officers and employees. Northwest Hydraulic Consultants Inc. denies
any liability whatsoever to other parties who may obtain access to this report for any injury, loss or
damage suffered by such parties arising from their use of, or reliance upon, this report or any of its
contents.
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TABLE OF CONTENTS
1 INTRODUCTION.....................................................................................................................................1
1.1 Background......................................................................................................................................1
1.2 Approach.......................................................................................... ............................... .......2
1.3 Site Location....................................................................................................................................2
2 SITE VISIT AND GEOMORPHIC ASSESSMENT.........................................................................................
3
2.1 Site Surveys—.. .................................................................................................................................
4
2.2 Geomorphic Interpretation.............................................................................................................
4
2.3 Boat Ramp Sedimentation...............................................................................................................
7
3 SEDIMENT TRANSPORT ANALYSIS.........................................................................................................
8
3.1 Wind Analysis..................................................................................................................................8
3.2 Wave Analysis................................................................................................................................12
3.3 Longshore Sediment Transport Analysis.......................................................................................
17
4 DESIGN CONSIDERATIONS- .............................................................. .................................................
21
4.1 Ramp Improvements.....................................................................................................................22
4.1.1 Elevated Ramp..........................................................................................................................
22
4.1.2 Windward Berm......................................................................................................................
22
4.1.3 Combined Elevated Ramp and Berm........................................................................................
23
4.2 Sediment Management Plan.. ............................................................. .........................................
24
5 CONCLUSION.......................................................................................................................................24
6 REFERENCES........................................................................................................................................ 25
LIST OF TABLES
Table 1: Summary of observations at sediment pits on the ramp.......................................................7
Table 2: Longshore sediment transport rates for 2017. ..................................................................... 18
Table 3: Longshore sediment transport rates — grain size sensitivity test ......................................... 20
Table 4: Longshore sediment transport rates —foreshore slope sensitivity test ............................... 20
Table 5: Longshore sediment transport rates — wave attenuation sensitivity test ............................ 20
LIST OF FIGURES
Figure 1: Point Whitney boat ramp (source: WDFW)............................................................................ 2
Figure 2: Location map showing Point Whitney and shoreline to the south (Google Earth)................3
Figure 3: Site map showing RTK measurement positions (green crosses) and contours generated
from topographic survey. Elevations reference NAVD 88......................................................4
Figure 4: Various photos of the beach and bluffs to the south of Point Whitney ................................. 6
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Figure 5:
Views of beach in the vicinity of the boat ramp. Looking upslope at the ramp (left) and
west along the beach (right)...................................................................................................
6
Figure 6:
Pit #1 excavated near top of ramp..........................................................................................
8
Figure 7:
Pit #2 excavated part -way down the ramp.............................................................................
$
Figure8:
Location of WPOW1 station....................................................................................................
9
Figure 9:
Wind distribution plot (wind rose) — WPOW1, 1984 to 2018...............................................10
Figure 10:
Monthly wind distribution plot (wind rose) — WPOW1.......................................................11
Figure 11:
Observed WPOW1 wind speed (top panel) and hindcast wave height (bottom panel) - 2017
.................................................. ... .... ..................................................... .... I ... I ........................
12
Figure12:
Dabob Bay SWAN model grid extent...................................................................................
13
Figure 13:
Wave height distribution map — winds from North-North-West..........................................14
Figure 14:
Wave height distribution map — winds from North..............................................................
14
Figure 15:
Wave height distribution map —winds from North-North-East...........................................15
Figure 16:
Wave height distribution map —winds from South-South-East.........................................„.15
Figure 17:
Wave height distribution plot — winds from South................................................................
16
Figure 18:
Wave height distribution plot — winds from South -South-West ..........................................
16
Figure 19:
Input environmental parameters and computed longshore transport rate — van Rijn ........
18
Figure 20:
Dimensions of a hypothetical deposition area east of the boat ramp ...................................
19
Figure 21:
Annual net potential longshore transport ............................................................................
21
Figure 22:
Plan and profile schematic of proposed elevated ramp and rock berm. Elevations reference
NAVD88................................................................................................................................
23
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1 INTRODUCTION
Northwest Hydraulic Consultants Inc. (NHC) was engaged by KPFF Consulting Engineers (KPFF) to
undertake an assessment of coastal processes that are resulting in sedimentation of the boat ramp at
Point Whitney, near Brinnon, Washington. The boat ramp is operated by Washington Department of Fish
and Wildlife (WDFW) who are responsible for maintenance of the facility, including periodic sediment
removal. This report presents the findings of the assessment and design considerations for renovation of
the boat ramp.
1.1 Background
The Point Whitney boat ramp near Brinnon, WA, is presently experiencing sedimentation problems that
are burying portions of the ramp below sand and gravels (Figure 1). WDFW is seeking to improve the
boat ramp functionality and, if possible, reduce the requirement for maintenance clearing of the ramp as
part of redevelopment of the site. At present the concrete boat ramp does not extend far enough into
the inter -tidal portion of the beach and users often experience challenges with pulling boat trailers back
up the beach. Also, during times when the ramp is covered in gravel, users will back trailers laterally
onto the lower portions of the beach to avoid the ruts left by previous users who were stuck, thus
extending the area of disturbance to the beach.
Developing an understanding of the shoreline coastal processes and sediment transport at the site is an
important step to developing a functional renovation plan. WDFW does not want to relocate the boat
ramp as this would necessitate additional permitting effort that does not fit within the proposed project
schedule. Design considerations have focused on solutions that can be constructed within the existing
boat ramp footprint.
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Figure 1: Point Whitney boat ramp (source: WDFW).
1.2 Approach
NHC undertook a coastal processes study combining a wind and wave assessment of the project area
with a geomorphological assessment of the shorelines to prepare estimates of the annual average
longshore sediment transport. Our approach used existing information, for instance historical airphotos
and records of wind and tides, to develop an understanding of the dominant processes affecting the site.
Site specific information, including topographic survey, measurement of sediment grainsize, and
observations of coastal processes, was collected during a site visit on June 12, 2018. An empirical
approach to quantifying sediment transport rates was adopted using several alternative sediment
transport formulae, which in the absence of detailed data for model validation and calibration, were
considered the most appropriate tools available. Conceptual design options are presented that best
meet WDFW's objectives.
1.3 Site Location
The Point Whitney boat ramp is located on a short section of northward -facing gravel beach along a
coastline that is dominantly oriented north -south (Figure 2). Figure 8 shows the overall location of the
site within the context of Puget Sound and Hood Canal. The shoreline to the south of Point Whitney is
dominated by a gravel beach that terminates at approximately the high tide line in a steep bluff
extending to approximately 50 ft to 80 ft above sea level. A small creek discharges to the beach
approximately 0.5 mi south of Point Whitney. Erosion of the bluff and sediment introduced by this creek
appear to be the two main local sources of sediment.
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The northward facing beach at Point Whitney is interrupted to the west by the outlet of a tidal lagoon.
Tidal flow through this outlet appears to maintain the channel, as it interacts with sediments moving
westward along the beach.
Figure 2: Location map showing Point Whitney and shoreline to the south (Google Earth).
2 SITE VISIT AND GEOMORPHIC ASSESSMENT
A site visit was conducted on June 12, 2018 for the purposes of collecting topographic and bathymetric
information, evaluating sediment sources and burial volumes, making observations of coastal processes,
and making qualitative measurements of sediment grain size. The site visit was attended by Mr. Derek
Ray, M.Sc., P.Geo. from NHC's North Vancouver, BC office, and Mr. Kevin Geoghegan, EIT from NHC's
Seattle, Washington office. During the onsite visit, NHC staff met with WDFW staff for a verbal
orientation of the site and informal history of the ongoing operational issues related to sediment
accumulation on the beach.
The site visit took place in the morning during a dropping tide. Water levels were approximately two
thirds of the way down the beach when NHC staff arrived on site, and by noon the extreme lower end of
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the beach was exposed or visible in shallow water. Winds were light and the sea state was at near calm
conditions.
2.1 Site Surveys
A topographic and bathymetric survey was undertaken using GNSS real-time kinematic (RTK) survey
equipment. Several points were surveyed along transects down the beach which provide detailed
topography of the ramp area. The lowest elevation points were collected while wading in water up to
waist deep. The surveyed points reference the Washington State Plane North horizontal datum and the
North American Vertical Datum of 1988 (NAVD 88). The NAVD 88 datum is 2.85 ft higher than MLLW in
Dabob Bay at NOAA station 9445246 (Mofjeld et al., 2002).
Figure 3: Site map showing RTK measurement positions (green crosses) and contours generated
from topographic survey. Elevations reference NAVD 88.
2.2 Geomorphic Interpretation
The beach extending approximately 0.25 mi to the south of Point Whitney was inspected on foot to gain
an understanding of the dominant processes and main sediment sources. Figure 4 shows various views
of the beach, which terminates near the high tide line in steep bluffs.
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The material comprising the lower portion of the bluffs is relatively resistant interbedded sands and
gravels that tilt towards the north on an angle of approximately 30°. Based on available geologic
mapping, these are interpreted to be pre -Fraser glacial deposits (and possibly older) in original position,
or that underwent post -deposition tilting. The bluffs are nearly vertical and dominantly vegetated with
occasional exposed portions (as seen in Figure 4), indicating more recent erosion.
The sediment comprising the beach to the south of Point Whitney is relatively coarse, ranging in size
from gravel to small boulders. The portion of the beach that is obviously experiencing active sediment
transport is limited to a narrow band approximately 10 ft to 13 ft wide at the base of the bluffs, while
the middle and lower portions of the beach appear to undergo active sediment transport very rarely. It is
not possible to comment on the onshore -offshore movement of material with the information available
at present. The composition of the northward -facing beach adjacent to the boat ramp is much finer
(Figure 5), indicating two dominant processes: a) that the finer fraction of material is generally
transported along the shoreline with little retention between wind events, and b) that the beach to the
south of the boat ramp which provides material supplying the beach at the boat ramp is supply limited —
e.g. the wave climate is capable of transporting more material than is presently available.
The beach in the vicinity of the boat ramp is transport limited, as opposed to supply limited. The
dominant wave direction from the south (which aligns with the longest fetch) results in waves refracting
around the point and depositing sediment that ranges in size from medium gravel to coarse sand, with
fine gravel being the dominant clast size. The lower wave energy environment of this section of the
beach has resulted in a sediment -rich environment; however, site observations indicate that the wave
environment frequently moves the material across the beach, as evidenced by the fact that the beach is
evenly graded with no large accumulation zone on the western corner. It is likely that this finer beach
material is mobilised relatively frequently within the existing wave climate.
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.-7z� W�-
Figure 4: Various photos of the beach and bluffs to the south of Point Whitney.
Figure 5: Views of beach in the vicinity of the boat ramp. Looking upslope at the ramp (left) and
west along the beach (right).
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2.3 Boat Ramp Sedimentation
Sediment deposition on the boat ramp was investigated during the site visit by digging five small pits by
hand. The five pits were distributed evenly from near the top of the ramp to near the water line
(numbered one through five from upslope to downslope). Observations at each pit are summarized in
Table 1. Typically the sediment within the top layer at each pit is relatively loose, indicating the depth of
the active transport zone, and coarser, indicating winnowing of fines. Sediment in the bottom portion of
each pit is more compact, ranging from very hard at the top of the ramp to moderately compact at the
bottom of the ramp. Excavation of the more compact layers in pit number 1, 2, and 3 required the use of
a pick and the material was removed in chunks, while the lower parts of pit 4 and 5 were not nearly as
compacted. This is interpreted to be partly related to less traffic over the lower parts of the ramp and
the presence of water piping through the lower pits that would tend to soften those layers. The average
depth of burial on the ramp is about 8 inches, though it should be noted that the bottom of pit 5 did not
terminate on the ramp surface, either because the ramp does not extend that far down the beach or
because it was not possible to excavate deep enough in the loose, wet sediment to reach the ramp.
Figure 6 and Figure 7 show conditions at pit 1 and 2 respectively.
Table 1: Summary of observations at sediment pits on the ramp
1 3.0 7.2 4 Lower part of pit very hard sediment
2 1.5 8.4 2 Lower part of pit very hard sediment
3 1.5 7.2 4 Dominated by very fine gravel
4 2.0 8.4 10 Lower part of pit much looser than upper ramp
5 3.0 10.8 15 Didn't reach concrete ramp; water piping into pit
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Figure 6: Pit #1 excavated near top of ramp.
Figure 7: Pit #2 excavated part -way down the ramp.
3 SEDIMENT TRANSPORT ANALYSIS
Longshore sediment transport is the process responsible for the movement of sediment along the
coastline resulting in deposition in the vicinity of the boat ramp. Knowledge of the wind and wave
climate is required to confirm the longshore sediment transport processes and estimate the sediment
transport rate. These processes are discussed in the following sections.
3.1 Wind Analysis
Wind -generated waves are responsible for most of the waves experienced at Point Whitney; however,
no meteorological station is situated in the vicinity. The closest meteorological station with long-term
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wind data near the project site is the National Data Buoy Center Station West Point (WPOW1) station
(1984 to 2018), which is located in Puget Sound (Figure 8). While West Point is located about 20 mi east
of Point Whitney, both sites are located in the same geographical area (southern Puget Sound) and the
orientation of the nearby water bodies to the wind forcing is similar (North -South). For the purpose of
this study, we have made the simplifying assumption that the wind climate experienced at Point
Whitney is similar to that experienced at West Point.
Figure 8: Location of WPOW1 station.
The local wind climate can be assessed by the use of a wind rose, a graphic presentation of winds for
specified areas, utilizing arrows at the cardinal and inter -cardinal compass points to show the direction
from which the winds blow and the magnitude and frequency for a given period of time. The wind rose
derived from the observed data at WPOW1 is shown in Figure 9. The results show that the prevailing
winds at WPOW1 are primarily from the north and south directions, corresponding to the orientation of
the Sound. The site experiences stronger southerly winds than northerly winds.
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o%
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Figure 9: Wind distribution plot (wind rose) — WPOW1, 1984 to 2018.
Monthly wind roses for WPOW1 are shown in Figure 10. The largest storms are from the south and
typically occur between late fall and early spring (October to March).
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3.2 Wave Analysis
A long-term hourly incident wave climate was hindcasted using the empirical JONSWAP method. Key
input parameters include hourly wind data from WPOW1 station and fetch length. A wind stick plot
illustrating the wind climate for 2017 and time -series plot of hindcast wave height are shown in Figure
11.
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This simplified approach computes deepwaterl waves only and assumes that the wave direction is the
same as the offshore wind direction. To examine the effects of shoaling, refraction and diffraction on
wave propagation direction in the vicinity of Point Whitney, a nearshore wave model, Simulating Waves
Nearshore (SWAN), of Dabob Bay was developed. SWAN incorporates physical processes such as wave
propagation, wave generation by wind, white -capping, shoaling, wave breaking, bottom friction,
sub -sea obstacles, wave setup and wave -wave interactions in its computations. The SWAN model was
implemented on a bathymetric grid resolved on a 164 ft by 164 ft (50 m by 50 m) orthogonal
grid spacing. The model grid was generated from NOAA National Centers for Environmental
Information. The grid extends 6.2 mi in the east -west direction and 17.4 mi in the north -south direction
(Figure 12).
Model simulations were conducted for northerly storm events (NNW, N, NNE) and southerly storm
events (SSW, S, SSE) with a constant wind speed of 34 mph (or 29 knots). The northerly storm results are
show in Figure 13, Figure 14 and Figure 15. The southerly storm results are show in Figure 16, Figure 17
and Figure 18.
1 Refers to waves that are unaffected by interactions with the bed.
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The results indicate that when the offshore winds are from the NNW and N directions, waves reach the
shore approximately head on and result in cross -shore transport (i.e., the sediment transport is
perpendicular to the shoreline). When the offshore winds are from the NE and southerly directions
(SSW,S,SSE), waves refract and diffract around Point Whitney and travel along the shore from east to
west. When waves approach the shore at an angle, a longshore current will be generated. On sandy -
gravel shorelines, the waves and current have the capacity to transport considerable amounts of
sediment along the shore, assuming there is a supply of material. This transport of sediments along the
shore from east to west during the southerly storm event is the dominant factor in the ongoing sand
deposition on the boat ramp.
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3.3 Longshore Sediment Transport Analysis
To provide input to a conceptual assessment of the coastal processes, a high level analysis of the
sediment transport was undertaken using empirical formulae with input from the wave climate analysis.
Potential net longshore transport (in yd3 per year) of sediment at the foreshore along Point Whitney
boat ramp was computed based on the offshore wave climate as input. Based on the findings from the
SWAN model simulations, the following assumptions were applied to the hindcast offshore wave data to
account for nearshore wave transformation:
• Waves from NNW and N directions reach the shoreline along Point Whitney boat ramp from 0°
true north.
• Waves from NE direction reach the shoreline along Point Whitney boat ramp from 45' true
north.
Waves from SSW, S and SSE directions reach the shoreline along Point Whiney boat ramp from
90° true north.
• Wave height in the nearshore zone is typically half of the offshore wave height.
Other assumptions made for the analysis include:
• Dso of 2 mm - very coarse sand.
• Beach slope of 8%.
• The sediment system is not supply limited.
The assumed Dso of 2 mm is the lower limit of the sediment samples collected from the boat ramp pits
(Error! Reference source not found.). Thus, using a 2 mm Dso in the sediment transport analysis would
produce a conservatively high estimate of the transport rate. The relationship between Dso and sediment
transport rate is captured in the sensitivity analysis below.
Three transport formulas were applied, including CERC (Shore Protection Manual, US Army Corps of
Engineers, 1984), modified Kamphuis (Mil-Homens, 2013), and van Rijn (2014). The CERC equation does
not account for particle size and beach slope and is only valid for sandy conditions. The modified
Kamphuis equation is only applicable to sandy beaches, and the van Rijn equation is applicable to sandy
gravel and shingle beaches (0.1 to 100 mm).
Figure 19 shows the time -series of input parameters and computed longshore transport rate for the
period between July and December 2017 using the van Rijn equation. Positive transport rate indicates
sediment transport from east to west. Negative transport rate indicates sediment transport from west
to east.
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7 7;
N �N
I'
to .-1 tnN
C � 3 r1 � •' t j � � I
CL
O O
_s N
N Jul Aug SOP Oct Nov Dec
u1 u?
Offshore Hs
Nearshore Hs
o q
= T T E
1
a I o
4
0 0
o Jul Aug Sep Oct Nov Dec
CO
� OO
Q n
3�
N
0 0
Jul Asa sep oa WV Dec
CC
w
So . .. L
�E
C L
QI q
~ o I — c
Ju Aug Sep Oct NOV Dec
Figure 19: Input environmental parameters and computed longshore transport rate —van Rijn
Computed sediment transport rates for 2017 based on the three equations are summarized in Table 2.
The large range of predicted longshore transport rate is due in part to the uncertainty in the coefficients
in each formula, as well as to the specific beach environment to which each equation was intended to be
applied.
Table 2: Longshore sediment transport rates for 2017
transportEquation Longshore
CERC (1984) 3,157
Modified Kamphuis (2013) 105
Van Rijn (2014) 183
011sh
Near
a wave angle
ofe wave ana
� 4
_
L
11
I
T — 1
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As there are no field data to assess the results of the predicted longshore transport rates, it is not
possible to determine which estimate most closely reproduces conditions at Point Whitney. For context,
if a hypothetical sediment deposition area with dimensions 100 ft x 40 ft was assumed east of the boat
ramp at the break in the north -south to an east -west aligned beach (Figure 20), the average change in
bed elevation in this area using CERC, modified Kamphuis, and van Rijn equations are estimated to be
22.0 ft, 0.7 ft, and 1.3 ft respectively. On this basis, the predicted sediment transport rate using the van
Rijn equation appears to most reasonably replicate the conditions.
Figure 20: Dimensions of a hypothetical deposition area east of the boat ramp.
Sensitivity tests were conducted using the van Rijn equation to determine the sensitivity that variation in
sediment size (Table 3), foreshore slope (Table 4) and percentage of wave attenuation between
deepwater and shallow water (Table 5) might have on the calculated longshore transport rate. The
results show that:
■ For the sediment size range examined, the transport rate increases by about 1.5 times when the
diameter is reduced by half.
• Longshore transport rate is not overly sensitive to foreshore slope.
• Longshore transport rate is sensitive to the assumed percentage of wave attenuation between
deepwater and shallow water. The estimation could be improved by conducting a long-term
two-dimensional nearshore wave model study of Dabob Bay to obtain the nearshore wave
characteristics at the Point Whitney shoreline.
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Table 3: Longshore sediment transport rates — grain size sensitivity test
size (mm)
Longshore
transportSediment
0.25 —fine sand
636
0.50 — medium sand
420
1.00 — coarse sand
277
2.00—very coarse sand
183
4.00 — very find gravel
120
Table 4: Longshore sediment transport rates — foreshore slope sensitivity test
6%
162
8%
183
10%
200
12%
215
Table 5: Longshore sediment transport rates — wave attenuation sensitivity test
50%
60%
70%
80%
90%
100%
183
301
458
693
955
1287
The annual net potential longshore transport volume between 1998 and 2017 at Point Whitney was
computed and is shown in Figure 21. The average net annual potential longshore transport volume is
about 170 m3. This translates to an average bed elevation increase of 1.3 ft in the assumed sediment
deposition area east of the boat ramp.
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7_17-L
200
180
160
140
M
E
am 120
E
3
9 100
t
O
a
80
ea
H
60
40
ce
0 1 ._-- , !._ . __ . _ , - - 1
1995 2000 2005 2010 2015 2020
Year
Figure 21: Annual net potential longshore transport.
Note that assessing longshore sediment transport rates with a high degree of certainty requires
extensive field data, a detailed analysis of sediment sources, and a fully calibrated numerical model. This
level of detail and precision is costly, with study costs exceeding the costs of construction and
maintenance of the boat ramp. And regardless of cost and effort, a degree of uncertainty would remain
within the study results.
4 DESIGN CONSIDERATIONS
Three options for mitigating the issue of sediment migrating along the beach and covering the ramp are
presented below. Each option builds on the current sediment management principle in which
accumulated sediment is excavated and deposited on the leeward side of the ramp, allowing the
longshore transport processes to continue westward. Given the dominant geomorphic processes, an
ongoing sediment management plan will be required to avoid long-term aggradation of the beach on the
windward side of the boat ramp and the eventual return to the existing conditions. The following
preliminary designs improve the existing ramp conditions by inducing sediment accumulation on the
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windward side of the ramp. The primary goal of the improvement is to capture the sediment away from
the ramp and reduce the frequency of required maintenance. Additional sediment management plan
considerations are also presented.
4.1 Ramp Improvements
The following preliminary design options consider elevating the replacement ramp above the existing
beach profile, constructing a berm on the windward side of the ramp, and implementing a combination
of both an elevated ramp and a berm. The major difference among the three options is the amount of
storage each will provide for sediment accumulation.
4.1.1 Elevated Ramp
Elevating the proposed ramp above the existing beach profile would maintain the current footprint but
would only provide nominal sediment storage. The ramp renovation footprint is an important
consideration for permitting purposes. Extending the repair efforts outside of the footprint may require
permits which could adversely impact the proposed project schedule. Elevating the proposed ramp
would maintain the existing footprint while still providing some sediment storage, but would result in a
slightly narrower driving surface.
Based on a preliminary analysis using the contours derived from the topographic survey, elevating the
ramp by 1 ft would provide an estimated 5 yd3 of sediment storage to the east (windward) side of the
ramp. According to the sediment transport rates estimated above, the added storage would not be
sufficient to contain a year's worth of sediment, but would provide some level of storage that is not
present for the existing ramp.
4.1.2 Windward Berm
Constructing a berm on the windward side of the ramp provides significantly more sediment storage,
but the new feature would extend outside of the existing footprint and may require a new permit. A 3-ft
tall rock berm, shown in Figure 22 along with the elevated boat ramp, would provide an estimated 170
yd3 of sediment storage assuming it is extended to the approximate lowest observed tide (-4 ft MLLW). It
would capture sediment east of the boat ramp and could provide approximately a year's worth of
storage, depending on actual sediment transport rates. This would reduce maintenance requirements,
possibly to a near annual basis, and would provide managers with advanced warning of impacts to the
ramp if the sediment trapping area is monitored. Even if the sediment transport rates are
underestimated, and the sediment trap fills more frequently, the berm would intercept sediment and
prevent the degree of accretion presently occurring between maintenance events. An added benefit of a
berm is the partial protection from waves.
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Sediment
Elevated boat ramp storage
3 ft rock berm
Direction of
sediment transport
Figure 22: Plan and profile schematic of proposed elevated ramp and rock berm. Elevations
reference NAVD 88.
Although the berm presents a simple and cost effective solution for mitigating the ramp sedimentation
issue, extending the repair work outside of the existing footprint could invoke permitting requirements.
The additional time and expense incurred from the permitting efforts could make this option less
desirable.
4.1.3 Combined Elevated Ramp and Berm
A possible solution to implementing a berm while avoiding the potential permitting impacts on the
proposed schedule is to proceed with the elevated ramp option while the berm option is being
permitted. This option would allow the schedule to continue unencumbered by the permitting process,
offer some sediment storage in the interim, and provide all the benefits of the berm upon its
completion. Once the berm is constructed, the elevated ramp would continue to offer additional storage
providing a second line of defense against sedimentation.
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4.2 Sediment Management Plan
Regardless of the selected repair option, routine maintenance will still be required to excavate
accumulated sediment. Given the estimated sediment transport processes established above, there are
no simple options to prevent the ramp from being buried (in the long term) with no intervention. The
current maintenance strategy should be maintained moving forward since there is no net removal of
sediment from the natural transport process. Displacing the sediment to the western side of the ramp
ensures that the sediment is available for transport to the shore west of the ramp, avoiding degradation
of the beach.
The key difference in the proposed maintenance management plan from the current plan is the reduced
frequency of maintenance requirements to prevent ramp burial. Sediment that would previously accrete
on the ramp would instead be stored in sediment traps. If the traps are excavated before they reach
capacity, the major sediment transport processes would not affect the ramp, which is distinct from the
existing conditions where the ramp is subjected to sedimentation immediately following excavation.
Managing the sediment to avoid burial of the ramp would have the added benefit of avoiding extensive
disturbance to the lower beach to each side of the ramp as users seek areas that are clear of ruts formed
by previous vehicles becoming stuck in the soft gravel.
Excavation of the trap area below the present grade would provide additional storage but it is important
to note that care should be taken avoid over -excavation during sediment clearing that might destabilize
the ramp structure. Removing too much sediment could lead to undercutting the proposed protective
structures. Sediment management operations should focus on the accreted sediment and avoid
disturbing the beach profile below the structures.
5 CONCLUSION
Point Whitney is subject to longshore sediment transport processes that currently present
sedimentation issues for the existing boat ramp located on the northern -facing shore. As part of
WDFW's efforts to improve ramp functionality and reduce the requirement for maintenance, NHC
developed preliminary design options for the proposed repair work to mitigate the sedimentation issue.
The design process included wind and wave analyses, which were used to estimate sediment transport
rates. Under the assumptions described above, the analysis using the van Rijn equation produced a likely
range from about 120 yd3/yr to around 400 yd3/yr. Although we are not able to narrow down the
estimated range to a precise rate due to limitations in data availability and the level of analysis,
understanding the sediment transport process occurring in the westerly direction is the fundamental
consideration in the repair design.
In response to the inevitability of sedimentation at the ramp, the preliminary designs presented above
share a common key feature of temporary storage to reduce the required frequency of sediment
management. Creating an area for the sediment to collect away from the ramp is an effective way to
both address the sedimentation issue and reduce the requirement for maintenance. Elevating the ramp
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by 1 ft would provide a nominal amount of storage, while installing a 3-ft tall rock berm on the windward
side of the ramp would create additional storage and shelter the ramp for longer durations between
maintenance activities. The third preliminary design option addresses the possible schedule delay
incurred from permitting a berm outside of the existing footprint. By proceeding with the elevated ramp
while simultaneously pursuing berm permitting, the project can continue as planned and benefit from
additional protection after berm construction.
6 REFERENCES
Mil-Homens, J., R. Ranasinghe, J.S.M. van Thiel de Vries, and M.J.F. Stive (2013). Re-evaluation and
improvement of three commonly used bulk longshore sediment transport formulas. Coastal Engineering,
75:29-39, May 2013. ISSN 0378-3839. doi:10.1016/j.coastaleng.2013.01.004.
Mofjeld, H.O., Venturato, A.J., Titov, V.V., Gonzalez, F.I., Newman, J.C. (2002). Tidal Datum Distributions
in Puget Sound, Washington, Based on a Tidal Model. NOAA Technical Memorandum OAR PMEL-122.
Seattle, WA. November 2002.
US Army Corps of Engineers (1984). Shore Protection Manual. Coastal Engineering Research Center,
Waterways Experiment Station, Vicksburg, MS.
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