Soil Erosion and Sediment Analysis for the Mameyes River Watershed Using HEC-HMS and GIS

Abstract

<b><sc>Abstract </sc></b> The Mameyes River watershed comprises 10.4% of the El Yunque National Forest. The steep slopes in the watershed and the humid climate make it a candidate for significant production of soil erosion. This river is important for its biological and historic value; however, watershed erosion is impacting water quality and the ecosystem. This research presents a computerized method to estimate the average soil losses and sediment yield in any watershed of Puerto Rico. Different formulations have been developed over the years to estimate annual soil loss. In this research, the U.S. Army Corps of Engineers (USACE) – Hydrologic Engineering Center - Hydrologic Modeling System (HEC-HMS) (USACE, 2022) V.4.10 was used to estimate soil loss and sediment yield using the Universal Soil Loss Equation (USLE) and the Modified Universal Soil Loss Equation (MUSLE) for rainfall events of 24 hours and different return periods. Precipitation data was obtained from the NOAA ATLAS-14 Point Precipitation Frequency Estimates. The rainfall-runoff erosivity factor (R) was taken directly from the United States Environmental Protection Agency (EPA) Rainfall Erosivity Factor Calculator. The values of soil erodibility (K) for the soils were obtained from the United States Department of Agriculture (USDA) Web Soil Survey as well as the gradation data, which is an additional parameter for MUSLE. The factors of slope length (L) and slope steepness (S) were determined using a Digital Elevation Model (DEM) obtained from the National Oceanic and Atmospheric Administration (NOAA) Data Access Viewer, and processed using the open-source Geographic Information System (GIS) software, QGIS. All the parameters needed for MUSLE were spatially distributed and processed in QGIS. The parameters of cover-management (C) and support practice (P) factors were approximated using satellite imagery obtained from Google and land use and land cover maps were also obtained from Google and their Earth Engine. Data for all of the parameters were supplied as input to the HEC-HMS model. HEC-HMS simulations can be used to obtain runoff estimates, and when the soil erosion component is added, it calculates the sediment loadings and concentrations of the watershed and its sub-basins. Results of the model simulations are presented through tables and graphs generated automatically and can be exported to other software for better visualization. The output obtained from the hydrologic model shows the plots for the hydrographs, sediment loading, and soil erosion for the watershed given a certain storm event and lapse of time. As needed, many simulations can be executed for different events to compare the results and see the differences in erosion. From these results, we can analyze how severe erosion is and propose mitigation plans for certain conditions. Results are presented for design storms in Puerto Rico for exceedance probabilities of 2, 10, 25, 50, and 100 years and a duration of 24 hours.