HomeMy Public PortalAbout2003 Sea Grant Landings ReportCAPE COD COOPERATIVE EXTENSION
Deeds & Probate Building
P.O. Box 367
Barnstable, MA 02630-0367
Phone 508-375-6690
Fax 508-362-4518
BREWSTER GC iSERVA.ILN COMMISSION
NOG
O
\./ Sea Grant
Woods Hole
W000s HOLE OCEANOGRAPHIC INSTITUTION
January 15, 2003
Seth Wilkinson, Agent
Brewster Conservation Commission
Town Hall
2198 Main Street
Brewster, MA 02631
RE: Analysis of Storm Damage Potential and ErciSion Control Alternatives for Selected
Brewster Town Landings
Dear Seth:
This report is a follow-up to site visits we conducted to several Brewster Town Landings.
Present were you, Town Administrator Charles Sumner, Conservation Commission Chair
Steve McKenna, Director of Natural Resources Bob Mant, and DPW Foreman Jim
Bearse. The purpose of the site visits were to view shoreline conditions, discuss erosion
and storm damage potential at selected landings, and identify possible erosion control and
storm damage reduction alternatives for each of the Town Landings visited.
A moderate coastal storm occurred shortly before our visits causing wave-induced
erosion to several of the landings. This necessitated town action in the form of placing fill
at Ellis Landing, the first town landing we visited.
Ellis Landing
Ellis Landing, located at the end of Ellis Landing Road on Cape Cod Bay, is of
considerable concern to the town primarily due to limitations to beach and tidal flat
access resulting from relatively frequent storm wave-induced erosion of the access (fill)
ramp. The long-term average annual erosion rate at Ellis Landing is approximately —0.66
ft /yr based on several plotted shorelines analyzed between the mid-1800s to 1994. The
landing faces north/northwest and is located directly on Cape Cod Bay (Photo 1 aerial
photo).
Due to the close proximity of the seaward end of the landing to the high water line it will
continue to receive relatively frequent storm wave-induced erosion and scarping. This
scarping and erosion inhibits beach and tidal flat access, and periodically uncovers a
leaching box located under the access (fill) ramp, subjecting it to possible damage. The
town is interested in suggestions that may reduce the frequency of storm and erosion
ated damage.
1930
1 05 Oyster Pond Road, \IS 42, Woods Hole, MA 025431525
Tel {5081289- • Ex< 15081 457-2172
2
I visited and photographed Ellis Landing on November 1 (see Photos 2 & 4). Note on
Photos 2 & 4 (top photos) the close proximity of the high water line (identified by the
landward limit of the wrack) to the seaward limit of the access ramp. There is a direct
correlation between frequent storm wave-induced erosion with the closeness to high
water.
On November 1, a small scarp was visible at the seaward end of the access (fill) ramp on
the west side of the landing (right side of the fill as shown on Photos 2 & 4) indicating
recent wave erosion from a relatively minor storm. Note that only the west portion of the
ramp shows erosion or scarp. This is primarily because that section of the ramp (fill)
protruded more seaward than the eastern section and was, thus, closer to the high water
line and thus wave action.
A minor to low-moderate coastal storm occurred between my November 1 visit and our
site visit on November 14 that caused considerable erosion, with a scarp approximately 2
—3 feet in elevation at the landing (see Figures 3 & 5). The scarp shown in Figures 3 & 5
cut across the entire landing but the town placed fill across the western section to provide
beach and tidal flat access, and to cover an existing storm drain leaching box.
Importantly, note that even shortly after the new fill was placed a small wave-induced
scarp (erosion of the fill) had already formed.
Erosion Control Alternatives
The primary goals of the Town at Ellis Landing are to provide continuous access to the
beach and tidal flats, and to prevent damage to the leaching box located under the access
(fill) ramp. This requires that wave-induced erosion be minimized and, thus, the
frequency of adding fill and associated costs also be reduced.
Alternative A — Continue to add fill:
Historically, the Town simply places/dumps fill at the ramp to create a smooth transition
to the beach for pedestrian and vehicle access to the beach and tidal flats. The fill
material appears to consist of a mixture of sand, pebble and finer material. While this
mixture is not entirely beach compatible, it adds to the stability of the fill for pedestrian
and vehicle access down the ramp. This fill is necessitated to compensate for material lost
to wave-induced erosion. According to the Town, this has been a frequent and expensive
undertaking. A positive contribution of this alternative is that when the fill is eroded from
the ramp this material is added to the 'littoral system' (dune, beach and nearshore areas).
Alternative B — harden or armor the ramp:
The ramp could be re-constructed with concrete or other 'hard' material. If properly
designed and embedded deep enough, particularly at the seaward end (i.e. deeper than
anticipated storm wave induced lowering of the beach) this would provide stability and
potential longevity to the ramp (absent a major storm). However, a hard structure more
than likely would increase wave reflection and refraction around the end of the ramp and
cause increased 'end scour' or flanking erosion of adjacent areas. For example, increased
wave-induced erosion would likely occur to the dune area immediate east of the ramp as
a result of wave refraction associated with strong north/northwest winds and waves.
3
Northeast wave reflection would also more than likely cause accelerated erosion of the
dunes on the east side. Armoring the fill with gently sloping rip-rap (revetment) could
reduce but not eliminate this end-scour or flanking erosion effect but would not
necessarily be conducive to easy pedestrian or vehicle access.
A hardened structure also may not necessarily be consistent with the performance
standards of the Wetlands Protection regulations for dunes or banks at 310 CMR 10.28 &
10.30.
Alternative C — Relocate the access ramp further landward:
Relocating the ramp/fill further landward would increase its distance from high water and
thus would decrease the number of times the ramp is eroded due to storm waves. As a
result, the necessity of adding fill should also be reduced, lowering associated costs. This
would also necessitate the relocation of the storm drainage system buried under the ramp.
This action would require an initial expense but on a long-term basis may be more cost-
effective than the current practice. With on-going erosion, the storm-water drainage
system located under the access ramp fill will eventually be damaged and need to be
addressed, and more than likely will need to be relocated in the future.
Discussion
With on-going chronic erosion at the site and a potential increase in future storm activity
there will more than likely be a concomitant increase in the frequency of adding fill to the
ramp and thus increased costs (Alternative A) in the future. Re-constructing and
hardening the ramp with concrete or other materials (Alternative B) would reduce erosion
of the fill but more then likely would cause increased erosion of adjacent areas
(particularly the dune on the east side) due to end scour or flanking erosion. Following a
major coastal storm reconstruction expenses would be high. Relocating the ramp further
landward (Alternative C) would reduce the frequency of wave interaction and thus
erosion of the fill and reduce the frequency of having to add fill. Following moderate
coastal storms, however, fill would continue to be required but more than likely at a
lower rate.
In summary, weighing the pros and cons of the 3 alternatives discussed, relocating the
ramp further landward (Alternative C) appears to optimally balance functional,
environmental and fiscal components.
Breakwater Landing
Breakwater Landing is located at the end of Breakwater Road on Cape Cod Bay (Figure
1). I visited Breakwater landings on November 1 and again on November 14 during our
site visit (see comparison Photos # 6 & 7).
The long-term average annual erosion rate at Breakwater Landing is approximately —0.39
ft/yr, although the erosion rate between 1951 & 1994 appears to have accelerated
showing an average annual erosion rate of —2.03 ft/yr.
4
The access ramp to the beach from the parking lot is very gradual and there does not
appear to be the problem with erosion/scalping or loss of material of the access/landing
due to minor• storms, as is the case with Ellis Landing. This could be due to the elevation
of the beach being higher relative to the parking lot. In addition and importantly, the
access ramp is further landward in relation to high water, with its seaward end gradually
transitioning into the beach and ending in line with the seaward toe of the adjacent dunes.
The main problem appears to be erosion of the dune adjacent (west) to the access ramp.
Riprap fronts this dune (see Figures 6 & 7). The main question the Town is considering
is whether the presence of the riprap fronting the dune is causing accelerated erosion of
the dune, and if so, should it be removed.
Photo 6 shows the dune and riprap on November 1. Note the dune toe is relatively close
to the riprap, approximately 5 feet. A minor coastal storm hit the area shortly before our
November 14 site visit and the dune toe was eroded back approximately 15 feet from its
November 1 location (see Photo 7). Note in comparing the storm-induced dune scarp
location on both sides of the access-way in Photo 7 that the dune scarp appears to be in a
similar location. This suggests that the riprap is not necessarily causing increased erosion
of the dune itself. However, I did note a small trough immediately landward of the riprap
(between the riprap and the dune) that Photo 7 does not show. This indicates that the
riprap is causing a slight hydraulic head build-up and causing some increased beach
erosion in between the riprap and dune. Photo 8 shows dune erosion on the west side of
the dune where there is a gap between the riprap and the adjacent revetment. This could
be caused by end scour or pedestrian use from the parking lot.
The main adverse impact of the presence of the riprap is that it is preventing aeolian
(wind-blown) sand transport from the beach to the dune and thus could severely limit the
natural reconstruction of the dune following storms. For this reason, removal of the riprap
would be beneficial.
I would also suggest closing the pathway or end scour area on the west side of the dune
(see Photo 8) and installing snow/sand fencing and/or planting dune grass to assist nature
in re-building the dune in this area by trapping wind-blown sand.
Alternative
In summary, removing the riprap fronting the dune will allow the dune to more quickly
and naturally rebuild following storms. Installing sand/snow fencing approximately
halfway up the eroded dune on the west side of the riprap leading to the parking lot will
assist nature in trapping wind-blown sand and rebuild that portion of the dune quicker.
Supplementing planting dune grass in this blowout with the sand/snow fence would be
optimum in dune rebuilding.
If the blowout on the west end of the dune is being caused by pedestrian access, a `no
access dune sign' could be beneficial in educating the public of the sensitive nature of
dunes and the beneficial role of dune grass in assisting natural dune rebuilding. WHOI
Sea Grant has dune signs and can provide these at no cost.
5
Saints Landing
Saints Landing is located at the end of Robbins Hill Road on Cape Cod Bay. The long-
term average annual erosion rate for this site based on data from the mid-1800s to 1994 is
approximately —0.6 ft/yr.
The primary problem at this location is storm water discharge through an existing pipe
with the drain beginning in the parking lot and the pipe exiting through the dune face.
The runoff/discharge then splashes onto the beach from the elevated pipe exiting through
the dime face causing erosion of the dune and beach (see Photos 9A, 9B, 9C). The
parking lot is graded such that rain runoff cascades rapidly toward the storm water drain
pipe (Photo 10) resulting in velocity runoff cascading through the discharge pipe and
onto the beach. The discharge pipe is elevated above beach grade causing considerable
erosion of the dune and beach (Figure 9a, b, & c). The outflow is causing not only
erosion of the dune but is cutting a gully across the beach to the ocean.
When major storms occur the rainfall discharged from the pipe produce maximum
scouring of the dune and beach simultaneously with the arrival of large highly erosive
storm waves. The gully in the beach caused by the discharge is lower than the adjoining
beach grade so the stone wave bore rolls further up the gully, increasing beach and dune
erosion.
The goal in this situation would be to:
1. Eliminate the parking lot runoff through the pipe by handling the runoff
differently; or,
2. Slow the runoff entering and thus exiting the storm water discharge pipe; and,
3. Reduce the elevation of the runoff exiting from the discharge pipe, which will
reduce the energy of the discharge runoff hitting the dune & beach.
If these are achieved, the beach and dune erosion caused by the discharge will be
mitigated, or the energy of cascading waters causing erosion of the beach and dune
should concomitantly be reduced thereby minimizing erosion of the dune and beach.
Alternatives
The construction of a vegetated or gravel-packed leaching trench could be considered in
the location of the existing drain (Photo 10). The parking lot could be re-graded to direct
runoff towards the leaching trench, although the existing parking lot grade presently
appears to direct the runoff towards the drain, so this may not be necessary. This is
evident in Photo 9A showing very little, if any, evidence of runoff cascading down the
pedestrian access way. This would allow runoff to slowly leach into the ground and avoid
the erosive discharge onto the beach and dune. A future problem may be if the vegetated
swale or gravel/sand filter beds become clogged with mud, silt or other clogging
elements.
Alternatively, a connection could be maintained to the existing discharge pipe at the
bottom of the vegetated swale or gravel-leaching trench that would be operative during
6
major rainfall events to convey the overflow. This should considerably slow or, if sized
appropriately, eliminate the volume and velocity of entering and exiting discharge runoff
into and out of the existing pipe.
Beach compatible material could be placed to fill the existing dune scarp and beach gully
that is presently being caused by the discharged runoff out of the pipe (Photo 9a,b,c). If
discharge continues out of the pipe, a splash apron consisting of riprap underlain with
filter cloth could further reduce the storm water runoff energy onto the beach, thereby
reducing the gulleying effect and erosion of the dune and beach. Some riprap already
exists on the beach at the discharge point.
The vegetated or gravel leaching trench appears to meet the primary goal of eliminating
the runoff from eroding the dune and beach, except perhaps in very large rain fall events.
A mechanism to handle the overflow from the vegetated swale or gravel trench in large
rainfall events could be constructed. If flow continues out of the pipe, a vertical pipe
attached underneath the existing pipe could be added that penetrates below beach grade
and thus allows the discharge to infiltrate the beach, rather than fall from the existing
elevation.
Robbins Hill Beach
Robbins Hill Beach Landing is located directly on Cape Cod Bay. The long-term average
annual erosion rate at this location is approximately 2 ft/yr based on data analyzed
between the mid-1800s to 1994.
The main problems here appear to be:
1. Aeolian (wind-blown) sand and overwash sands being deposited into the parking lot;
and,
2. It appears that pedestrians find it easier to walk over the dune at the northwest corner
of the parking lot to the beach rather than walk the very short distance to the
established beach access way causing dune grass destruction.
It was evidenced during our site visit that the town was scraping-up the aeolian and over-
washed sands from the parking lot and depositing that volume onto the backside and top
of the primary dune where the pedestrians were causing dune vegetation impacts by
walking over the dune at the northwest corner. This practice of placing the overwash sand
from the parking lot back on the foredune seems appropriate.
Alternatives:
The practice of scraping-up the aeolian and over-washed sand from the parking lot and
depositing that volume onto eroded areas of the foredune seems appropriate.
A potential way of reducing the volume of aeolian or over-washed sand from entering the
parking lot through the pedestrian access-way to the beach (Photo 11) may be to install
snow/sand fencing in the middle of the pedestrian access-way. Perhaps 2 rows of
staggered fences could be installed, parallel to the dune orientation. Staggering would
allow for continued pedestrian access through/around the sand/snow fencing to the beach,
7
while capturing wind-blown and over-washed sand in the location where a foredune
would otherwise exist absent pedestrian use. The town of Dennis uses this technique
(sand/snow fencing at each public access-way to the beach) during winter months and it
is very successful in capturing wind-blown sand: approximately 2-3 vertical feet of
aeolian sand has been captured over the past 3 months at Cold Storage Beach access-
ways (Photo 12).
A consideration of reducing pedestrian access over the dune at the northwest corner of he
parking lot may be to simply plant beach grass in bare areas caused by pedestrian use and
to install a `no pedestrian access' sign at that location. Sea Grant has such signs (`these
dunes aren't made for walking') free of charge. I will send or drop several signs for your
consideration.
Linnell Landing
There are two adjacent pedestrian access-ways to the beach from the Linnell Landing
parking lot (Photo 13 & 14). Perhaps one beach access-way is sufficient? Two access-
ways invites additional aeolian and storm overwash sand to be carried into the parking
lot.
Alternatives
A. Perhaps one access-way (on the west side) could be closed and the foredune
reconstructed with compatible, off-site sand, and planted with beach grass. This should
reduce the volume of aeolian and overwash sand into the parking lot. In addition, this
may reduce pedestrian impacts to adjacent, private dunes (Photo 15 & 16) by orienting
pedestrian away from the adjacent dunes. At present the west side access-way results in
pedestrians walking very close to the neighboring dune toe.
B. The foredune in both access-way locations could be reconstructed with compatible
sand. One restored and planted with beach grass (west side), the other, after sand is
deposited, could have a pedestrian dune crossover installed to prevent dune gullying by
pedestrian use. I have enclosed literature describing pedestrian crossovers. This should
reduce the volume of aeolian and overwash sands entering the parking lot from the beach.
Crosby Landing
Crosby Landing appears to be in fine shape. The dunes at Crosby Landing are extensive
(although low in elevation), and the well-constructed, elevated boardwalk keeps
pedestrians from impacting the dunes (Photo 17). The only issue at this beach appears to
be the groins (photos 18, 19 & 20).
The groins are interfering with natural alongshore sand transport. It is apparent that the
net sediment transport is from west to east evident by the fillet (or build-up of sand) on
the west side of the groins (see Photos 18 & 19). A couple of groins are causing an
offset of the high water line of approximately 45 feet, indicating slight downdrift erosion
and updrift accretion. An approximate 3-foot drop of beach grade at the high water line
Si cerely,
V-- 0 dui
Jfrn O'Connell, Coastal Processes Specialist
WHOI Sea Grant and
Cape Cod Cooperative Extension
8
existed on the downdrift side of the groins, particularly on the east end of the beach.
Because of the extensive (although low elevation) dunes, and the Town being the owner
of the beach and dune areas, the offsets are not of great concern.
If the groins were removed or modified, the beach would even out in its seaward extent,
i.e. reducing the beach width on the updrift side of the groin and adding to beach width
on the downdrift side. This evening out of beach width would establish more natural
conditions and allow sediment transport to continue downdrift unimpeded. It would also
facilitate seaward dune growth on the downdrift sides of the groins, but slight erosion of
the updrift dunes until natural equilibrium conditions were achieved.
On the easternmost groin, however, altering the groin may cause impacts to the existing
downdrift saltmarsh by covering the marsh with sand that migrate from the updrift fillet.
Notching or slightly unraveling the riprap comprising the groins could be good
compromise in closely establishing natural conditions. Perhaps if the riprap is unraveled
due to a major storm, the groins could be left in this condition and slowly re-establish
natural sand transport and dune equilibrium.
Summary
In summary, there are actions the town could initiate as described above that could
reduce storm and erosion-related impacts to the town landings on Cape Cod Bay.
Hopefully, expenditures could be concomitantly reduced as well. I would be happy to go
on site again with town officials to further discuss implementation of any of the
alternatives described above.
I hope this analysis of Brewster's town landing is helpful to the town. Feel free to call me
at (508) 289-2993.
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