Loading...
HomeMy WebLinkAboutLincoln Beach - Coastal Processes and Flood Hazard Impact Analysis 2001.07.31I COASTAL Ggorocrc SgauTCgS, INC. July 31 ,2001 tMs. Judy Surber Port Townsend Building and Community Development Dept Waterman & Katz Building Port Townsend, WA Re: Coastal Processes and Flood Hazard lmpact Analysis for Lincoln Beach Protective Berm Proposal at John & Sylvia Thies, Springstead, Martin, and Lynn Families Lincoln Beach Rd. Area, Port Townsend, WA This brief report is intended for documentation of my assessment of coastal geologic conditions and changes in the flood hazards due to the proposal at the shoreline area at Lincoln Beach. The project area is defined as starting at the Springstead property on the SE end and including the Lawrence Street right of way (RO\A/), the Martin property, the Thies property, and the Lynn property at the NW end, for a total length of approximately 375 ft. Specifically, this report addresses the need for protection of the homes from coastal erosion and coastalflooding. The proposal, based on analysis performed in the winter of 2000-2001 but not fully documented priortothis report, isto construct a protective berm of gravel landward of the mean higherhigh water (MHHV\| line. Details of the proposal are included in the Shoreline Permit Application and the SEPA checklist. Goastal Processes The net shore-drift (long-term littoral drift) along the proposal area shore is to the NW (Keuler, 1988). However, this net shore-drift cell has been significantly altered by urban development in the City of Port Townsend. The natural net shore-drift system in the area brought littoral sediment from the eroding bluffs at Fort Townsend State Park and the Glenn Cove area past the shore of Port Townsend, Point Hudson, and on towards Point Wilson (Keuler, 1988; Figure 1). The earliest available maps of Port Townsend show a portion of the Lincoln Beach area as a spit fronting a large wetland. Point Hudson and the entire low shore that includes Lincoln Beach appears to have been an accretion landform under natural conditions, that is, prior to the development of the Port Townsend waterfront. The amount of net shore-drift sediment reaching the Lincoln Beach area has been substantially diminished by the extensive modifications to the shore in the city. The Port Townsend Boat Haven Marina is one example of a development that runs across the tidelands and almost completely blocks net shore-drift. The very large breakwater and channel at the marina act as a near total barrier to net shore-drift. The fill at lndian Point also acts to decrease whatever small amount of northward net shore-drift may have been in this part of the system. The extensive revetments along the downtown area of Port Townsend preclude the input of sediment to the coastal drift system and may still be trapping sediment in the voids between the ubiquitous rocks. The marina at Point Hudson also is also a major interruption of natural net shore-drift. There is likely some amount of seasonal shore drift from the north (originating from bluff retreat) along Lincoln Beach, but the net littoral drift is driven by southerly quadrant windstorms and that drive the net shore-drift system in most areas of the Puget Sound and Northwest Straits. Therefore, urban development up-drift (south of) Lincoln Beach area appears to have substantially impacted the Lincoln beach area such that the beach is no longer an actively accreting landform, and is likely erosional overthe long-term now. This explains why the upper beach has been incrementally receding in the late 1980's and more recently. The upper beach and berm/dune experienced significant retreat in the December 15, 2000 NW windstorm, further 7ot Wilson Ave., Bellingham, WA gBzzS-ZS4B G6o) 642-tB4S Fax: 67t.6694 coastalgeo@home.com Mr. Jack Thies; Lincoln Beach July 31, 2001 Page 2 of 3 Coesret GEoLoGIC Snnvtcns, twc, decreasing the volume of the berm/dune buffer that acts to protect the houses from coastal erosion and damage, and the low elevation area from coastal flooding. The protective berm proposal is intended to add more resistant material (gravel) to the berm and dune area and to address the apparent shortage of net shore-drift sediment in the system by augmenting the property beaches in the backshore area. This addition of gravel and placement of sand would enhance the beaches ability to absorb storm wave energy through the berm reshaping itself to have an active storm berm at an elevation slightly higher than the still water level. Dunegrass, once reestablished, would also help anchor sand and gravel and should reduce coastal erosion. Coastaland Upland Source Flooding Analysis An analysis of coastal and upland source flooding hazards was carried out at the request of the City of Port Townsend Public Works Department. The question that is addressed in this analysis is: Witt the proposed protective berm/dune negativety impact the surrounding low elevation area in terms of upland flooding? One source of floodwatertolhe lowelevation area could bethe lowelevation area around Lincoln Beach Rd. could occur as a result of substantial precipitation coupled with overland flow from a significant area west and south of the study area. The second source of floodwater entering the two-street "basin" could be coastal flooding. One of the conclusions of the analysis done prior to the completion of the proposed design is that the higher elevation finished berm/dune would significantly decrease the risk of coastal flooding due to wave overtopping. This is simply because a higher elevation berm/dune will act as a barrierto coastal water during times with an elevated still water level (due to a storm surge). The proposal would have a finished berm/dune elevation of +15.0 ft MLLW. The current elevation of the berm/dune in the project area is greater than +13.0 ft MLLW except at the SE end of the Lawrence St. ROW, where minimum elevations are +12.65 ft MLLW. Therefore, without the proposed work, the ROW would be the source of coastal floodwater entering the immediate area if and when the waters of Admiralty lnlet rise to a sufficient level to allow wave runnup to exceed +12.65 ft MLLW. The low elevation area landward of berm/dune was surveyed on July 27,2001 (see attached topographic map). Most of the effort was concentrated on the street ROWs and the low elevation ditch and wetland areas along the Lawrence St. ROW, Conditions in the "basin" (volumes under hypothetical flood conditions) including the lower elevation portions of Lincoln St. and Lawrence St, ROW areas were determined using survey data with AutoCAD 2000 Land Development Desktop. The volume in the basin up to elevation +12.65 ft MLLW (elevation of the berm/dune top under existing conditions) is 2,282 cy (61 ,614 cubic ft or 1 .42 acre-ft). The volume of the two- street area basin up to elevation +15.0 ft MLLW (elevation to the berm/dune top under proposed conditions) is 3,844 cy (103,788 cubic ft or 2.38 acre-ft). lt is important to note that these 2 computed volumes are significantly lower than the total basin volumes since the recent survey work did not include much of the fenced yard areas. The total volumes including the rest of the yard areas in the low elevation area may be on the order of twice the above stated volumes. The "basin" landward of the berm represents room for a very significant amount of floodwater. Witnesses to the 1982 flooding (including Jack Thies) have stated that there was no more than approximately 8-10 inches of standing water on the streets in the project area during flooding. This would be equivalent to approximately elevation +10 ft MLLW, which is 2.65 ft below the existing berm elevation. Mr. lack Thies; Lincoln Beach July 31, 2001 Page 3 of 3 CoesrII GgoLOcTC SERVICES, INC, Floor elevations were surveyed in the project area. The waterfront houses in the project area have floor elevations of between approximately +12.9 ft MLLW and +14.0 ft MLLW (the Lynn floor elevation was not measured). The Curnow house, which is located landward of Lincoln Beach Road, has a surveyed floor elevation of approximately +11.2f|MLLW. The elevation of Lincoln Beach Rd. nearthe Curnow house ip +9.0 to +10.0 ft MLLW, and lower Lawrence St. is at +8.2to +8.5 ft MLLW. Therefore, there would have to be 3 ft of standing water on the low elevation portions of the streets to reach the floor elevations of the lowest house (Curnow). Additionally, the Curnow floor elevation is 1.4 ft below the existing minimum berm/dune elevation, meaning that the proposed raising of the berm/dune would not change the occurrence of coastal flooding at the Curnow house. Street drainage upgrades have been completed at the portions of Lincoln St. that is west and up- slope of the project area. City staff and local residents agree that these upgrades have decreased the amount of runoff that reaches the Lincoln Beach Rd. area. Although the last few winters have not been particularly stormy, no flooding has occurred in the area since these upgrade have take place. ln conclusion, given the street drainage improvements, the fact that the Lawrence St. ROW berm/dune is already at minimum elevation +12.65 ft MLLW, and that the worst flooding on record was at a much lower elevation than the existing berm/dune, it is highly unlikely that that proposal will increase the hazard posed by flooding from upland sources in the area in any way. lt is also concluded that the proposal will decrease the hazard of coastal flooding and coastal erosion significantly, which should decrease the overall hazard in the houses in the project area, and should not effect the surrounding houses. Please call if there are any questions at (360) 647-1845. Thank you. References Keuler, R.F., 1988, Map showing coastal erosion, sediment supply, and longshore transport in the Port Townsend 30- by 60-minute quadrangle, Puget Sound region, Washington: U.S. Geological Survey Miscellaneous lnvestigations Map l-1198-E, scale 1:100,000. Coastal Geologic Services lnc. Jim Johannessen, MS Principal Geologist ATTACHMENTS: Net shore-drift on the Port Townsend area (from: Keuler, 1988) Topographic map of the Lincoln Beach berm project area. Mr.JilkThis; LirwlnWt luly 31, 2001 attachment COASTAL GEOLOGIC SER INC. MHHW +E.45'LynnHousePropertylineScale: 1" = 50'Vertical Datum: MLLW (NOS)Date: Surveyed: 4ll7 /AIDrawn: 612510lNOTE: All property lines areapproximate (Based on TAASSurvey Associates 1983 and MLSanstrom 1976 survevs)Adrnirclty lnletAProperty line(old Meonder line)SpringsteodHouseFloor Ehv. -+13,4 MLLWTBMlThiesHouseFloor Bcv. =+l,Lo MLLWFence/Rood edgeMortlnHouseFloor Elcv. =+12.9 MLLWPropertylines12'TBMS+12.26MLLW+12.73MLLWPropertylinesCurnowHouseFloor Elsv. =+11.2 MLLWConcreteWollWolkwoyTopogrophic SurveyClients: Jack Thies, LymyMartr4 Springstead, andCity of Port Townsend2LtncolnBeach Rd.Port Townsend, WA 98368TBM2Elev.=+9.46MLLW701 Wilson AveBellingham, W498225(w) (360) 647-1845(fax) (360) 671-6654coastalgeo@home.com\n Beoch-l