Wave Attenuation across a Tidal Marsh in the San Francisco Bay

Quantifying wave attenuation is critical to understanding how marshes function. Reduction in wave energy can create conditions for sediment trapping and deposition, countering the effects of erosion and increasing the surface elevation. A field campaign was performed to study wave attenuation and sediment transport over the edge of the marsh at China Camp State Park in San Francisco Bay, CA. Here the tide flows over mudflats and through three consecutive vegetation zones: cordgrass (Spartina foliosa), cordgrass with pickleweed (Salicornia pacifica), and pickleweed. High-frequency pressure and optical turbidity sensors were deployed at eight stations along a 150 m cross-shore transect, starting on the mudflat 50 m outside the vegetation and ending in the pickleweed. Measurements were collected during perigean spring tides in winter 2014 and spring 2016. The tidal range was approximately 2 m with a maximum water depth of 0.6 m at the start of the pickleweed. Wave statistics were calculated from high-frequency pressure data, and wave attenuation was modeled as an exponential decay over distance. Using the winter and spring deployments, we compared the variation driven by differing vegetation characteristics and wave climates. Pickleweed, which forms a thick entanglement of vegetation, attenuated waves to a greater degree than the cordgrass, which is taller but more rod-like. For similarly sized waves, exponential decay coefficients were an order of magnitude greater when the vegetation was emergent than at high water when shallowly submerged. Suspended sediment measurements suggest sediment is resuspended in the cordgrass and deposited in the pickleweed. Winter storms brought larger waves; the maximum RMS wave height was 0.3 m at the marsh edge in winter versus 0.1 m in spring. The pickleweed density and height were largely unchanged between the seasons; however, the cordgrass was sparser and shorter in winter. Even with the sparse cordgrass, the waves were completely attenuated by 100 m into the marsh for both seasons. These results highlight the effectiveness of vegetative drag and bathymetric change in attenuating wave action under a variety of conditions. This dataset will help us understand the interaction between suspended sediment, vegetation, and flow, all crucial components to marsh sustainability.