A County-Wide Shoreline Sediment-Input Study: Identifying Sources, Analyzing Priorities, Inviting Public Participation, and Selecting Feasible Sites

A county-wide study to identify shoreline sediment-input sources provided the foundation for reach- and drift-cell-scale analysis to prioritize sediment supply restoration and protection in Kitsap County. Thirty-six percent (~80 miles) of the county’s 223 miles of shoreline (excluding Bainbridge Island) is armored, a condition understood to inhibit sediment input necessary to maintain healthy beaches and high-functioning coastal habitat. Using field knowledge to support GIS-based mapping (integrating LiDAR, shoreline oblique photos, geologic mapping, and T-sheets) we developed five comprehensive sediment-input categories: shallow landslides, deep-seated landslides, combined landslide types, bluffs, and tributary streams. The presence or absence of armoring distinguished existing sediment sources (i.e., unarmored) from potential or historic sediment sources(i.e., armored). Tributaries were visually characterized as a potential sediment source if basin morphology suggested active mass-wasting processes with effective headwater-to-mouth sediment transport, and delta morphology suggested a wave-dominated environment. Applying calculated bluff heights and wave energy to each map unit provided relative rates and volumes of sediment input for each reach. The position of an input source in a drift cell provided a measure of the source’s benefit to the drift cell. Collectively, landform features, attributes, and modifiers contributed to the final reach and drift cell priority designations. Concurrent with the mapping and analysis, a public outreach survey sought shoreline landowners interested in considering armoring removal or other restoration actions. Four private properties, each located within a priority reach or drift cell, emerged as possible project sites. Each site has now undergone a geotechnical feasibility study leading to the completed design for bulkhead removal at one site, a pending contract for design of another bulkhead removal, and plans for staged restoration of the two sites with slightly higher risk levels that may preclude bulkhead removal. If all stakeholder objectives are met, some form of restoration work could occur at all four sites in the next year. Outreach continues to identify additional interested landowners for further restoration and protection