APPENDIX B2-Dredge Prism Design-26Sept2022.pdf
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- Attached to
- ABSTRACT OF OFFERORS IFB- MAINTENANCE DREDGING AT STATION CAPE DISAPPOINTMENT, ILWACO, WA
- Solicitation number
- 70Z08823R00000354
About this file
This document includes a final dredge prism design and slope stability summary for maintenance dredging at the United States Coast Guard Station Cape Disappointment in Ilwaco, Washington. The design firm Wood submitted the dredge prism design and slope stability analysis to the Coast Guard to support proposed dredging. The dredge prism design set includes existing conditions and proposed dredge plans, cross sections, and notes. The slope stability summary memo evaluates the proposed dredging plan and recommends dredging no steeper than a 3:1 slope beginning no higher than 0 feet mean lower low water and extending no deeper than -12 feet mean lower low water, to ensure stability of the shoreline riprap. Care should also be taken not to disturb or undercut the existing riprap during dredging operations.
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Text version
September 26, 2022
Ms. Gilda Barboza
United States Coast Guard
Civil Engineering Unit Oakland
1301 Clay Street, Suite 700N
Oakland, CA 94612-5203
Re: Dredge Prism Design
US Coast Guard Station Cape Disappointment
Task Order 70Z08822FESDV0023
Dear Ms. Barboza, Wood is please to submit our Final Dredge Prism Design to support proposed dredging at USCG Station Cape Disappointment near Ilwaco, Pacific County, Washington. In the attached you will find the following:
▶ Existing Conditions Plan (Sheet C-1)
▶ Proposed Dredge Prism Design (Sheet C-2)
▶ Dredge Prism Design Cross-Sections (Sheet C-3)
▶ Slope Stability Summary Memo
Please let us know if you have any questions/comments.
Sincerely, Kimbrie Gobbi Project Manager kimbrie.gobbi@woodplc.com
Attachments
1. Dredge Prism Design Set
2. Slope Stability Summary Memo
15862 SW 72nd Ave., Suite 150 Portland, Oregon, 97224
(503) 639-3400 wsp.com
ATTACHMENT
DREDGE
PRISM DESIGN
SET
TOP OF SLOPE
CONTROL POINTS
NOTES
VERTICAL DATUM
BENCHMARK
HORIZONTAL DATUM
A
B
B'
E
F
F'
E'
D'
D
TOP OF SLOPE
DO NOT REMOVE OR
DISTURB ANY RIPRAP
OR SHORELINE
PROTECTION
EXISTING MAJOR CONTOUR
EXISTING MINOR CONTOUR
PROPOSED MAJOR CONTOUR
PROPOSED MINOR CONTOUR
PROPOSED DREDGE AREA FOR PERMITTING
PROPOSED DREDGE TOE
PROPOSED AREAS BELOW DREDGE TOE TO BE DREDGED
TO -10' MLLW
PROPOSED AREAS ABOVE DREDGE TOE TO BE DREDGED
BASED OFF 3:1 SLOPE FROM 0' MLLW WATERWARD TO
DREDGE TOE.
RIPRAP SHORELINE PROTECTION (APPROXIMATE)
LEGEND
NOTES
A
B
A'
B'
C D
E F F'E'
D'C'
DREDGE PRISM (-10 MLLW) DREDGE OVERDEPTH (-12 MLLW)
DREDGE PRISM (-10 MLLW) DREDGE OVERDEPTH (-12 MLLW)
DREDGE PRISM (-10 MLLW) DREDGE OVERDEPTH (-12 MLLW) DREDGE PRISM (-10 MLLW) DREDGE OVERDEPTH (-12 MLLW)
DREDGE PRISM (-10 MLLW) DREDGE OVERDEPTH (-12 MLLW)
DREDGE PRISM (-10 MLLW)
DREDGE OVERDEPTH (-12 MLLW)
EXISTING GRADE
EXISTING GRADE
EXISTING GRADE EXISTING GRADE
EXISTING GRADE
EXISTING GRADE
APPROXIMATE RIPRAP EXTENT, DEPTH
UNKNOWN. DO NOT REMOVE OR DISTURB
ANY RIPRAP OR SHORELINE PROTECTION
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
EDGE OF PERMITTED
DREDGE AREA
APPROXIMATE RIPRAP EXTENT, DEPTH
UNKNOWN. DO NOT REMOVE OR DISTURB
ANY RIPRAP OR SHORELINE PROTECTION
APPROXIMATE RIPRAP EXTENT, DEPTH
UNKNOWN. DO NOT REMOVE OR DISTURB
ANY RIPRAP OR SHORELINE PROTECTION
ATTACHMENT
SLOPE
STABILITY
SUMMARY
MEMO
September 26, 2022
Re: Slope Stability Review
US Coast Guard Station Cape Disappointment
Task Order 70Z08822FESDV0023
Background This memorandum presents our review of slope stability related to the planned dredging for the 2022- 2023 season at United States Coast Guard (USCG) Station Cape Disappointment, Pacific County, Washington.
We have reviewed the following items:
▶ Recent (August 2022) photographs taken of the site conditions along the shoreline adjacent to the planned areas to be dredged.
▶ The recent (September 21, 2022) hydrographic survey, proposed dredging plan, and cross sections (Drawings C-1 through C-3); and
▶ The results of our Slope Stability Evaluation prepared prior to the February 2022 dredging, dated October 5, 2020.
Based on our review, the shoreline protection along the areas to be dredged appears consistent with previous photographs (such as Photo 1 in the October 5, 2020 memo). The recent topographic survey of this area shows the rock-covered slope to be steeper than the previous survey; however, based on the photographs, it appears the difference is due to more refined survey measurements rather than any change in the rock shoreline protection geometry.
The October 5, 2020 stability analysis evaluated two cross sections, one at the northwest slope west of the fixed dock (near cross section D-D’ on the project dredging plan), and one midway between the Haul Out Pier and the Fixed Dock (near cross section E-E’ on the project dredging plan). The planned dredging was found to be stable. In the vicinity of the shoreline edge protection, it was recommended dredging no steeper than 3H:1V (Horizontal: Vertical) beginning no higher than a top elevation of 0 feet mean lower low water (MLLW) and extending no deeper than elevation -12 feet MLLW. Care should be taken to ensure that the dredging activities do not disturb or undercut the rip rap armoring at any portion along the shoreline.
These recommendations for dredging remain the same for the current dredging plan.
It is noted that increased dredging volumes are required due to shoaling and a migrating sand bar on the eastern portion of the dredging prism. As a result, a 3H:1V dredge cut slope will not “daylight” at the eastern edge of the permitted dredge area. This is illustrated at the eastern end of cross section A-A’ on Drawing C-3. Based on our review, dredging at a steeper inclination will not adversely affect the stability in
15862 SW 72nd Ave., Suite 150 Portland, Oregon, 97224
(503) 639-3400 wsp.com this area. In our opinion, if a vertical excavation is made at the edge of the dredge prism as shown, grades would soon adjust to a natural angle of repose of about 2H:1V.
Sincerely, James Dransfield, P.E.
Principal Engineer-Geotechnical james.dransfield@woodplc.com
Dan Schall, P.E.
Senior Civil Engineer daniel.schall@woodplc.com
| ATTACHMENT 1. DREDGE PRISM DESIGN SET |
| ATTACHMENT 2. SLOPE STABILITY SUMMARY MEMO |
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