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Title: Accounting for wave attenuation over flooded marsh and agricultural land

Author: Brian Wardman and Brad Hall, Northwest Hydraulics Consultants (NHC)

Short Story (for e-newsletter): The San Francisco Bay region contains extensive tracts of marsh and agricultural lands which have the potential to dissipate, or reduce, offshore waves as they propagate landward. The primary model used to calculate overland wave propagation over these land types is the WHAFIS (Wave Height Analysis for Flood Insurance Studies) transect-based model. NHC completed field measurements and supporting analysis to develop appropriate WHAFIS input parameters to enhance the WHAFIS model’s ability to more accurately dissipate waves over vegetation specific to the Pacific Coast and San Francisco Bay.

Long Story (for CCAMP website): The San Francisco Bay region contains extensive tracts of marsh and agricultural lands which have the potential to dissipate, or reduce, offshore waves as the waves propagate over them. The primary model used to calculate overland wave propagation over these, and other, land types is the WHAFIS (Wave Height Analysis for Flood Insurance Studies) transect-based model. Although the WHAFIS model has been used in FEMA flood studies for over three decades, the WHAFIS model only includes default plant parameters for common Atlantic Coast vegetation types. In order to enhance the WHAFIS model’s ability to more accurately dissipate waves over vegetation specific to the Pacific Coast and San Francisco Bay, NHC completed field reconnaissance to identify the predominant vegetation species and measure their respective physical characteristics. NHC used the results of their field study to develop WHAFIS input parameters for the predominant vegetation types.

Natural marsh vegetation in the San Francisco Bay Area is generally classified as either salt or brackish. Salt marshes are characterized by the presence of pacific and smooth cordgrass (or their hybrid) in the low marsh zone, pickleweed in the middle marsh zone, and bulrush in the high marsh zone. Brackish marshes, which are more common where salinity levels are relatively low, are characterized by tule and cattails in the low marsh zone, and salt grass in the middle and high marsh zones. Diked agricultural lands in the Bay Area are typically used to cultivate oats. Numerous other plant species also exist in these regions; however, due to their more limited aerial extent, it is not expected that the other vegetation types would contribute significantly to wave dissipation.

The field reconnaissance measured and classified the following plant characteristics for each vegetation type noted above: stem diameter (at the base, mid-stem, and top of the plant), stems per square foot, plant height, and frontal area ratios (FAR). The FAR is an estimate of the ratio of the frontal area of the leaves to the frontal area of the stem. WHAFIS uses these parameters to calculate the area blocked by the vegetation as a wave passes over the marsh. Additional details on the WHAFIS calculations are found in the WHAFIS documentation.

Although NHC originally completed their field research for use within the north San Francisco Bay study area (north of the Richmond-San Rafael Bridge), their recommended WHAFIS parameters for Pacific coast vegetation types have been reviewed and adopted for use within the entire CCAMP San Francisco Bay Area Coastal Study area. The parameters are also under review for potential use within marsh dominated areas of the Open Pacific Coast study area. Salt marsh near San Rafael, CA. Example of set-up to measure blocked area of a square foot of pickleweed. Typical stem density of Pacific Cordgrass.