As part of the Future San Francisco Bay Tidal Marshes project, we used our models of tidal marsh elevation to estimate the erosion protection tidal marsh habitats are providing to levees at sites throughout the San Francisco Estuary. We conducted a review of existing research on marsh wave attenuation to assign values to the different vegetation classes. From the literature surveyed (Cooper, 2005; Houser and Hill, 2010; Knutson 1982; Kobayashi, 1993; Lee, 2004; Moller and Spencer, 2002; Moller et al, 1996; Moller et al 1999; Wayne, 1976), we came up with estimated attenuation values of 10% per meter for upland, 6% per meter for hi marsh, 3%per meter for mid marsh, 1% per meter for low marsh, 0.1% per meter for mudflats, and 0.001% per meter for subtidal/open water. To represent the uncertainty in this estimate, we created values for both higher-than-expected attenuation and lower-than-expected attenuation by doubling and halving those values, respectively.
We then created a least-cost path for waves from San Francisco Bay and major streams/rivers to reach sites along the coast. We used wave attenuation (i.e. wave travel cost) grids as the cost surface and the direction to the nearest open water as the horizontal factor to restrict wave movement to within 45 degrees of its direction of propagation. These additive path costs were then turned into estimated wave attenuation values by first dividing the cost to reach each pixel by the distance that pixel was from open water to get an average attenuation and then raising this value to the distance travelled to get the true, multiplicative effect of wave attenuation. Subtracting the wave attenuation values from one produced the wave retention grids, which show the percentage of a wave’s initial energy (upon leaving open water) that remains upon reaching a given pixel.
