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ADDRESS COWI Marine North America
35 Corporate Drive
Suite 1200
Trumbull, CT 06611
TEL 203-268-5007
FAX 203-268-8821
WWW cowi-na.com
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As requested, COWI North America, Inc. (COWI – formerly Ocean and Coastal
Consultants, Inc.) has reviewed information related to the June 9, 2014 FEMA Flood
Insurance Study (FIS) with respect to the design of the geotube stabilization project
along Sconset Bluff.
COWI's involvement with the Sconset geotube project design has been ongoing since
2010. Over that time, new information has become available as coastal and climate
change science progresses. The 2014 FIS represents new information that was not
available at the time of original design or most recent design revisions. The geotube
design is based on the best available information at the time the design was
completed.
In light of this new information, COWI reviewed the new Still Water Elevations (SWEL)
provided in the FIS for the area near the Sconset geotube project (Transect 13). The
1% Chance Occurrence (100 year) SWEL for Transect 13 is 5.8 feet (NAVD88 datum).
In consultation with the project surveyor, Blackwell & Associates, the FEMA flood
elevation was converted to the project datum of MLW using a conversion of 1.52 feet,
resulting in a 100 year SWEL of 7.32 feet (MLW). This is less than the 10.2 feet (MLW)
100 year SWEL that was utilized for the original geotube design.
A check was run to see how this change in SWEL would impact the geotube design,
particularly with regard to the crest or top elevation of the geotubes. The crest
elevation for the geotubes were selected based on three primary components: the
SWEL, wave setup and wave runup. Allowances for freeboard, sea level rise and
constructability are also considered.
The 100 year wave setup is calculated based on deep water wave conditions and the
offshore slope, neither of which is impacted by the change in SWEL. As such, the
previously calculated wave setup value of 3.9 feet is still valid.
In order to determine an appropriate wave runup value for design, COWI calculated the
100-year wave runup utilizing several methods that accounted for both offshore and
nearshore conditions. In the original design, both the nearshore and offshore runup
analysis yielded similar results. The revised SWEL does not change the original
MEMO
TITLE Sconset Coastal Analysis Vertical Datum Review
DATE 9 September 2015
TO SBPF
COPY Epsilon Associates
FROM Joseph F. Marrone, PE
PROJECT NO 210019.1
P:\2010\OCC_210019.1\3_Project_Files\Reports\210019.1_20150909_Datum Memo_1.0.docx
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offshore analysis.. As such, the previously calculated runup value of 10.1 feet is still
considered valid.
Based on the above, the minimum top of geotube elevation to prevent overtopping
would be: 7.3 feet (SWEL) + 3.9 feet (wave setup) + 10.1 feet (wave runup) = 21.3 feet
(MLW datum). In addition, COWI recommends a 1 foot freeboard allowance and 1 foot
allowance for sea level rise and increased storm frequency due to climate change, for
a minimum top of geotube elevation of 23.3 feet (MLW). This is above the currently
installed (3 tube configuration) top of geotube elevation which ranges from 20.1 feet to
21.3 feet. As such, our previous recommendation to add the 4th geotube remains the
same.
In the current era of rising sea levels and intensifying storms, having a reasonable
margin of safety for coastal protection is well advised. The proposed design provides
that necessary margin.
Please contact me with any questions.