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HomeMy WebLinkAbout2961-437 c:. GIvtA- ~ '. Aspect;?~~~l~~e~ ~c\ c.t td" 2fz,c; I 0'1 f ~ ~% 2- q G, 1 @ February 24, 2009 Jefferson County Planning Commission 621 Sheridan Street Port Townsend, Washington 98368 Re: Feeder Bluffs and Beach Staircases Jefferson County Shoreline Master Program Dear Planning Commission: Aspect Consulting, LLC (Aspect) is a West Sound earth sciences and geotechnical engineering consulting firm. Two of Aspect's technical spedalties are environmental protection and remediation, and geotechnical evaluation and design in steep slope critical areas and other environmentally sensitive settings. In the past 14 years, we have completed geologic and geotechnical evaluations on approximately 200 shoreline projects in the Hood Canal, Puget Sound, and the Strait of Juan de Fuca areas. In addition, we have completed geologic/geotechnical evaluations and design programs for about 30 staircases on steep coastal bluffs. We understand that you are evaluating changes to the Shoreline Master Program which has generated some questions on the impact of beach staircases on natural processes of coastal feeder bluffs. Given our experience working on shoreline and steep slope projects in Jefferson County and nearby coastal areas, we would like to share our thoughts and experience on this matter. A bluff staircase investigation requires assessing the topography and slope angles, soil types and strengths, surface water and groundwater characteristics, vegetation types, shoreline processes, type of characteristic landslide, and history of landslide activity. Based on this, the geologist/engineer can determine potential impacts of the project on the slope, and the potential impacts of the slope on the project. Where the site characteristics are suitable for the proposed project, the engineer then provides design recommendations including setback of foundations from the top and bottom of the bluff, depth of the foundations, drainage improvements, etc. Beach staircases are not designed to stabilize the bluff soils. Because stairs do not stabilize the slope, the natural sediment transport processes operate independently of the stairs. If the bluff soils slide, the soil debris flows around the support posts for the staircase, or if the slide is deep or large enough, the staircase generally breaks loose and slides with the soil. In either case, the soil is not prevented from moving to the beach. In our experience, properly constructed beach stairs seldom have an impact on the basic stability of the bluff. Stairs can be constructed without significant impact on sediment transport processes. Well-constructed bluff stairs include the following elements: 1) landings set back suffidently far from the top and bottom of the bluff that they allow soil to move around the supports; 2) landings set above ground to allow sediment to move beneath them; 3) foundation 179 Madrone Lane North Bainbridge Island, WA 98110 Tel: (206) 780-9370 Fax: (206) 780-9438 www.aspectconsulting.com a limited liability company Jefferson County Planning Commission February 24,2009 posts embedded deep enough that thin and shallow slides will not reduce bearing support; 4) foundation posts strong enough and set far enough apart to allow sediment to flow around and between them; and 5) steps to the beach that can be raised up (drawbridge style) when not being used. To minimize hazards to people and the environment, if a landslide occurs, all soil and wood debris should be removed from the supports to prevent damage to the stairs, and soil allowed to move onto lower portions of the slope. The stairs should be inspected frequently and any damage repaired promptly. If a landslide damages the stairs to the point that part of it lands on the beach, staircase debris should be promptly removed. We would be happy to answer any questions or concerns that you have. You can reach us at 206-838-5834 (Dave) or 206-780-7720 (John). Sincerely, Aspect consulting, LLC ~iW-;Jn1/ cTc/~ John L. Peterson, PE Senior Associate Geotechnical Engineer jpeterson@aspectconsulting.com David H. McCormack, LEG Associate Engineering Geologist dmccormack@aspectconsulting.com cc: Brockman Builders S:\Jefferson County SMP\Jefferson Co Beach Stairs_rev1.doc Page 2