Wave-Induced Erosion on a Tidal Marsh Shoreline in Corte Madera Bay

One of the primary factors controlling the formation and configuration of a shoreline is the interaction of waves with the shore. A monitoring program was set up at the Corte Madera Ecological Reserve in order to monitor shoreline erosion of the marshfront over a period of four years, 1989-1992. Information regarding wave climate was recorded using a wave gage set up at a stationary position within the Corte Madera Bay, supplemented by wind data collected from a weather station set atop the Larkspur Ferry Terminal. The average erosion rate along the bayfront edge of the Corte Madera Ecological Reserve for the 4-year period is 1.1 m-year (3.6 ft/year). The source of wave energies within Corte Madera Bay was subdivided into five different sources: erosive and non-erosive wind waves, storm waves, ferry boat wakes, commercial vessel waves, and recreational boat waves. Erosive wind and storm waves constitute approximately 70% of the total erosive energy; erosive wind waves are considered to be those coming in from NNE to S directions. Ferry boat waves and commercial ship waves constitute approximately 18% and 12% of the total erosive energy, respectively. Contribution from recreational boat waves in insignificant. The study period coincides with a 6-year continuous drought in California and hence wind and storm energies may be significantly under-represented.