Coastal Zone Processes and Geomorphology of Skagit County, Washington

Geomorphic mapping of 130 km of marine shoreline in Skagit County reveals repeated morphologic and sedimentologic trends along many segments of the coast. The shoreline segments within which the trends are repeated are the littoral drift cells or shore drift sectors that act as nearly closed systems with respect to longshore sediment transport. The longshore trends include changes in mean grain size of beaches, sediment sorting, foreshore morphology, back- shore width and morphology, bluff morphology, and mean beach slope. The last parameter, slope, can be used as an index or surrogate measure of simultaneous changes in the other longshore trends.The longshore trends, besides being a convenient method to describe the coastal geomorphology, are found to be equally useful as tools to map directions of littoral sediment transport on a net, long-term basis, and, to help define the boundaries of drift sectors. Transport direction and littoral cell boundaries are included on the accompanying maps.Wave erosion of shore bluffs, as opposed to fluvial delivery, is the primary source of beach sediment. Mean minimum long term erosion rates are 5 cm/yr for unconsolidated bluffs, 0.7 cm/yr for jointed rocks fronted by wave cut platforms, and less than 0.1 cm/year for massive, resistant rock types. Shoreline segments with large, hazardous mass movements are relatively few, but within those segments large slope failures appear to provide a high percentage of the sediment contribution to beaches.