Distribution and abundance of rippled scour depressions along the California coast

Rippled scour depressions (RSDs) are prominent sediment features found on continental shelves worldwide. RSDs are generally characterized as elongate nearshore deposits of coarser-grained sediment with long-wavelength bedforms depressed 0.4–1.0 m below the surrounding finer-grained sediment plateau, thereby adding complexity and patchiness to relatively homogeneous unconsolidated sedimentary substrates on the inner continental shelf. Most research corroborates the hypothesis that RSDs are formed and maintained by currents and wave interaction with the seafloor sediment. While many localized studies have described RSDs, we use bathymetric and acoustic backscatter data from the state-wide California Seafloor Mapping Program (CSMP) to describe the spatial distribution of RSDs at the regional scale. The goals were to: 1) quantify the abundance and patterns of distribution of RSDs along the entire 1200 km California coast, and 2) test the generality of previously described or predicted relationships between RSD occurrence and geographic, oceanographic and geomorphic parameters, including depth, wave energy, latitude, shelf width, and proximity to bedrock reefs and headlands. Our general approach was to develop and apply a Topographic Position Index-based (TPI) landscape analysis tool to identify the distinct edges of RSDs in bathymetry data to differentiate the features from other sedimentary and rocky substrates. Spatial analysis was then used to quantify the distribution and abundance of RSDs and determine the percentage of bedrock reef, sedimentary and RSD substrates on the continental shelf within state waters. RSD substrate accounted for 3.6% of the California continental shelf, compared to 8.4% for bedrock reef substrate. The percent coverage of RSD substrate varied with depth, with 88% occurring in the 20–80 m depth range, and increased with proximity to bedrock reef substrate. RSD cover also varied significantly with shelf width, but not with proximity to headlands. Given the recent findings on the ecological significance of RSD, the results are relevant to marine spatial planning and ecosystem based management in terms of evaluating how well the 68 individual marine protected areas (MPAs) within California's newly designated state-wide MPA network collectively represent regional percentages of bedrock, sedimentary, and RSD substrate.