Soil Erosion and Sediment Yield Modeling with the Hydrologic Modeling System (HEC-HMS)

The effects of surface erosion and stream sediment loading in watersheds have become increasingly important in water quality best management practices (BMPs), watershed management, and natural resources conservation planning. Many water resources studies must now consider the erosion related effects of watershed activities. Surface erosion models describe the detachment, deposition and transport of soil particles by the erosive forces of raindrops and surface flow of water from their point of origin to the watershed outlet. The current version of the Hydrologic Modeling System (HEC-HMS) contains no capacity to simulate surface erosion processes; however, the need of erosion and sediment yield modeling exists throughout the Corps of Engineers, especially as the Corps moves towards watershed level investigations, including Total Maximum Daily Load (TMDL) studies. Therefore, the Hydrologic Engineering Center (HEC) has added existing, developed new and tested soil erosion and sediment yield methods to include in HEC-HMS. The goal is to develop tools within HEC-HMS that provide output necessary for making informed decision about managing soil erosion within the watershed. This paper discusses the addition of soil detachment, deposition and transport methodologies to the HEC-HMS program These new sediment modeling tools will increase the application of HEC-HMS for sediment modeling studies by directly computing sediment yield. An effort was made to ensure that sediment output from HEC-HMS could be easily used as boundary conditions in HEC-RAS for more detailed river mechanics modeling. Two surface erosion methods were included in the HEC-HMS subbasin element to model sediment erosion/wash-off for both pervious and impervious areas. In addition, an in-channel sediment routing method was included in the HEC-HMS reach element. These methods model the translation and attenuation of the sediment load along with deposition and erosion processes occurring in the channel.