Sediment Export to Nearshore Waters - Hawaii

This raster data layer represents sediment plumes originating from stream mouths and coastal pour points. The Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) model for sediment retention was modified for Hawaii, parameterized, and run for each of the Main Hawaiian Islands to determine sediment export from subwatershed hydrologic units (Falinski, 2016). Results from this model were aggregated into larger drainage areas that flow to single coastal pour points. From these points sediment was dispersed offshore using the Kernel Density tool in ArcGIS with a 1.5-km search radius. The resulting raster depicts simplistic sediment plumes with units in tons of sediment per year per hectare. The InVEST model predicts the average annual amount of sediment (tons/yr) retained in and exported from each map pixel as a function of many landscape variables. Data inputs to InVEST included: 1) USGS 10-m Digital Elevation Model (DEM); 2) NOAA Coastal Change Analysis Program (C-CAP) land use/land cover data; 3) R factor (old USGS maps and interpolation); 4) K factor (USDA Natural Resources Conservation Service (NRCS) Soil Survey Geographic database (SSURGO)); 5) University of Hawaii at Manoa (UH) rainfall atlas; 6) ArcHydro-derived subwatersheds such that flow lines approximately match the State of Hawaii streams layer; and 7) derived products from the above and more. See Falinski (2016) for detailed methodology. Coastal pour points were created by intersecting streams and coastline features from the National Hydrography Dataset (NHD), resulting in points where streams flow to the shoreline. The NHD was used rather than flow lines generated from the DEM because there are many instances in Hawaii where streams flow into man-made ditch systems and never reach the coast or simply dry up and go underground before reaching the coast. To determine the amount of sediment load at the coastline, resulting coastal points were given a unique drainage identifier. Next, the stream segment features were buffered by 1 m and dissolved so that connecting stream networks became single features. These polygon stream features were then assigned the drainage ID from the coastal points using a spatial join and subsequently used to assign that drainage ID to the subwatershed polygons. Finally, subwatersheds were dissolved by drainage ID and sediment export from each subwatershed was summed up to yield the total sediment export for each larger drainage basin, which was then joined back to the corresponding coastal drainage points. Each step in the process required quality control to ensure that: no pour points are left out, subwatersheds are not erroneously connected to the wrong drainage or left out, each drainage has only 1 pour point, and drainages do not erroneously span a ridgeline that should divide basins.