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HomeMy WebLinkAboutRP 94-06; CARLSBAD BY THE SEA; COASTAL DESIGN CRITERIA; 1994-10-18e HETHERINGTON ENGINEERING, INC. GEOTECHNICAL CONSULTANTS O'Day Consultants 7220 Avenida Encinas Suite 204 Carlsbad, CA 92009 Attention: Mr. Jeffrey Mitchell October 18, 1994 Project No. 1235.1 Log No. 1477 Subject: COASTAL DESIGN CRITERIA FOR PROPOSED SEAWALL California Lutheran Complex References: Ocean Street North of Grand Avenue Carlsbad, California Attached Dear Mr. Mitchell: In accordance with your request, we have performed coastal engineering services relative to the ·proposed construction of a seawall along the seaward margin of the subject property. The purpose of our work was to provide pertinent coastal engineering parameters to be incorporated by the project engineering team into the design of the seawall. The work performed consisted of research and review of available pertinent oceanographic and geotechnical reports and plans relative to the project, analysis of coastal design criteria relative to the proposed seawall, and the preparation of this report providing the results of our analyses, and our conclusions and recommendations. SITE DESCRIPTION The subject site consists of an approximate 0.29 acre undeveloped ocean-front parcel of land located west of Ocean Street and north of Grand Avenue, Carlsbad, California. The parcel is part of the large Carlsbad-By-The-Sea residential retirement complex that encompasses the property east of the subject site. The subject site presently consists' of a terraced scenic view area that contains a concrete patio, stairs, benches, and is vegetated with a heavy growth of ice-plant. The lot is fronted by a sandy to cobbly beach, with the landward edge of the beach consisting of a heavy concentration of sub-rounded gravel to cobble clasts. Vertical concrete seawalls that form the seaward extent of hotel/apartment buildings exist both northwest and southeast of the subject property. ; f 5245 AVENIDA ENCINAS • SUITE G • CARLSBAD, CALIFORNIA 92008 • (619) 931-1917 COASTAL DESIGN CRITERIA FOR PROPOSED SEAWALL Project No. 1235.1 Log No. 1477 October 18, 1994 Page 2 PROPOSED DEVELOPMENT We understand that development of the ocean front lot will consist generally of a split-level one to three story condominium complex with a vertical concrete seawall built along the southwestern margin of the structure. The seawall will evidently also function as the foundational support for the southwest wall of the condominium structure, with a deck for the lowest level of structure cantilevered out over the top of the wall. No plans for the seawall were available for our review at the time of this report, however, we anticipate the seawall will consist of a smooth-faced, reinforced concrete vertical wall similar to existing walls in the site vicinity. GEOTECHNICAL CONDITIONS Geotechnical conditions for the site are discussed in "Geotechnical Investigation, Carlsbad-By-The-Sea, California Lutheran Homes ... ," prepared by Leighton and Associates (see Reference 5). In general, for the area of the proposed seawall, subsurface conditions reportedly consist of beach deposits overlying Terrace Deposits or Santiago Formation bedrock. Subsurface work by Leighton and Associates, Inc., did not penetrate to bedrock within the area of the proposed seawall. Based on review of available geotechnical reports as well as previous work by our firm within the site vicinity, we have estimated Santiago Formation bedrock to exist at an elevation of approximately +2 feet {MSL) beneath the area of the proposed seawall. This estimate is considered to be conservative and should be verified prior to construction so that design changes to the wall may be made if. necessary. The likely presence of groundwater within the area of the seawall along the Terrace Deposits/Santiago Formation interface is also a condition that should be considered prior to construction so that proper mitigative measures can be taken to facilitate planned excavations. COASTAL DESIGN CRITERIA The following coastal design parameters are based on data taken from the referenced oceanographic literature that is considered appropriate for the subject location. Calculations have been performed in accordance with.the guidelines presented in "Coastal Flood Plain Development, Orange County Coastline ... " (Reference 3) and "Shore Protection Manual. .. ," (Reference 8) . . , f HETHERINGTON ENGINEERING, INC. COASTAL DESIGN CRITERIA FOR PROPOSED SEAWALL Project No. 1235.1 Log No. 1477 October 18, 1994 Page 3 Still Water Elevation -San Diego Harbor (Hw) = +8.35 ft. MLLW or +5.78 ft. NGVD. Scour Elevation -1.0 foot below estimated bedrock contact (Hsc) = +l. 0 ft. NGVD Water Depth (ds) = +4.78 ft. NGVD Design Wave Period for elevation of breaking wave (T) = 22 sec. Design Wave Period for run up elevation= 8 sec. Elevation of Breaking Wave (Hb) = 10.56 ft. Run-up Elevation (HR)= 17.40 ft. (For smooth vertical wall) CONCLUSIONS AND RECOMMENDATIONS Parameters relating to the coastal environment that have been provided in this report should be incorporated into the final seawall design by the project engineering team. The results of our study indicate that the wall should extend to a crest elevation of at least +17 feet MSL, or up to the bottom of the cantilevered portion of the lowest level of the proposed structure if the building design depicted in the referenced "Parcel 1 and 2, Section A" is utilized. The portion of the lowest level of the structure that is cantilevered out seaward over the top of the seawall should be designed to resist uplift pressure produced by wave runup during severe storm events. Uplift pressures will be determined by the final design elevation of the cantilevered deck of the lowest level. The wall should be built "in-line" with the existing seawalls to the northwest and southeast in order to prevent accelerated erosion or scour within corner areas between adjacent walls. Foundational support, backfill, and drainage should be addressed by the Geotechnical Consultant. Plans for the seawall should be reviewed by our firm to confirm conformance with the recommendations included in this report and to provide modification as necessary. If you have any questions, please call. Very truly yours, N ~--,1,y._'77'1'----~4~ civil Engi Geotechnical (expire 3/ MDH/ss I N O. 19·, Date BRANDON A. BOKA Project Geologist HETHERINGTON ENGINEERING, INC. REFERENCES 1) "Assessment and Atlas of Shoreline Erosion Along the California Coast," Department of Navigation and Ocean Development, July, 1977. 2) "Coastal Design Criteria In Southern California," Walker, James R., Nathan, Robert A., Seymour, Richard J., and Strange, R. Read, in Coastal Engineering, 1984 proceedings. 3) "Coastal Flood Plain Development, Orange County Coastline," Moffatt and Nichol, Engineers, January, 1985. 4) "Coastal storm Damage, Winter 1983," US Army Corps of Engineers Task Force Report, April 1984. 5) "Geotechnical Investigation, Carlsbad-By-The-Sea, California Lutheran Homes, Carlsbad, California," by Leighton and Associates, June 30, 1994. 6) "Plot Plan and Parcel 1 and 2 -Section A, Parcel 2, Building 2," by O'Day .consultants, undated. 7) "San Luis Rey Quadrangle," USGS Topographic Survey, Revised 1975. 8) "Shore Protection Manual," US Army Corps of Engineers, 1984. 9) "Geotechnical Investigation, St. Tropez West Condominiums, 2901 Ocean Street, Carlsbad, California," Western Soil and Foundation Engineering, Inc., April 16, 1990 . . , ; HETHERINGTON ENGINEERING, INC.