The geomorphology of Puget Sound beaches

Puget Sound beaches are distinct from low-energy beaches previously studied in terms of their mixture of sand and gravel sediments, their profile morphology (which is steeper and more concave) and their tidal environment. These beaches can be grouped into four profile morphotypes based on foreshore sediment and morphological characteristics. However, these morphotypes do not appear to be correlated with estimates of storm wave height, period, fetch or tidal range suggesting that they are controlled primarily by antecedent morphologic conditions.; The low-energy beaches of Puget Sound are dominated by swash processes. At Cama Beach, Washington, wave energy is concentrated in a narrow band on the foreshore located just above mean sea level as a result of the long-term distribution of tides. The most important sediment transport events are associated with infrequent, strong storms which may have a return interval of several years or more. An appropriate response model for morphodynamic change on low-energy, macro-tidal environments must include the effects of tide level distribution in concentrating wave action and hence sediment transport, to a vertical corridor determined by the tide level distribution. Sediment transport outside of this zone may be limited.; The concentration of wave energy high on the beach foreshore results in a reduction, due to wave attenuation, of shear stress across the low-tide terraces of Puget Sound. A sediment entrainment parameter used as a proxy for seabed disturbance of eelgrass was compared with high-resolution maps of eelgrass density. There are strong vertical gradients in both wave energy and eelgrass density that may be correlated, however, a number of other limiting factors for eelgrass are also correlated with beach elevation (i.e., dessication and photo-damage) so it is difficult to identify the specific role wave energy plays in setting the upper limit of eelgrass on the beach face. The longshore pattern of eelgrass density was correlated with beach aspect, which itself is correlated with wave entrainment, but the direct correlation between longshore eelgrass density and longshore wave exposure was low. The conclusion is that wave energy is a second-order factor in determining eelgrass density at this location.