You are here:
MODELING FRAMEWORK FOR EVALUATING SEDIMENTATION IN STREAM NETWORKS: FOR USE IN SEDIMENT TMDL ANALYSIS
Citation:
Hayter, E J. MODELING FRAMEWORK FOR EVALUATING SEDIMENTATION IN STREAM NETWORKS: FOR USE IN SEDIMENT TMDL ANALYSIS. Presented at National TMDL Science and Policy Conference, Phoenix, AZ, November 13-16, 2002.
Impact/Purpose:
This research project sets out to design and conduct an assessment of the long-term ecological consequences of alternative watershed management choices. As the first project to be done at this scale using predictive ecological endpoints, we will seek to identify the appropriate components of such an analysis. We will use experience gained in the conduct of this analysis to identify key research and data needs for future analyses. We will extend this analysis beyond previous and ongoing studies in two ways: by incorporating biological endpoints, primarily properties of fish communities, and by introducing the concept of sustainability of ecological state under future scenarios contrasted with the present state of those same ecological resources. Requirements that are identified during the course of this study will permit the recommendation of specific capabilities that should be incorporated in a general modeling system currently under development to support environmental assessments. Finally, the analysis is intended to be of value for establishing environmental management choices that will be beneficial and those that would be detrimental to the sustainability of ecological resources. Specific objectives are listed below:
1. Develop watershed-based modeling systems to forecast the effectiveness of alternative management plans in meeting sediment-related, nutrient-related, pathogen-related, and toxics-related criteria and standards, and biologically-based criteria and standards; and
2. Develop and maintain a comprehensive technical support capability that directly links environmental TMDL exposure research activities and products for the EPA Office of Water, EPA Regional Offices, and the States to be used for implementation of policy, regulatory development, remediation, and enforcement needs.
Description:
A modeling framework that can be used to evaluate sedimentation in stream networks is described. This methodology can be used to determine sediment Total Maximum Daily Loads (TMDLs) in sediment impaired waters, and provide the necessary hydrodynamic and sediment-related data to a fish population model. Specifically, it can be used to calculate the maximum allowable load of sediment that can be carried to the streams from the surrounding watershed without exceeding the water quality target for the modeled streams. Watershed sediment loads are represented as nonpoint source loadings, and are calculated in units of tons of sediment per acre per year. The framework also accounts for instream sediment processes such as bank erosion and aggregation and degradation of the sediment beds in the modeled stream network. An application of the modeling framework to a reach of the Housatonic River in Massachusetts is described.
The framework consists of coupled models that simulate both watershed and instream physical processes. The watershed portion of the framework consists of the U.S. EPA's Total Maximum Daily Load (TMDL) Universal Soil Loss Equation (USLE) model, that is a Windows-based software application for estimating diffuse (i.e., nonpoint) sediment source loads within a watershed. Specifically, this TMDL USLE program is useful for estimating the expected relative magnitude of land surface sediment loadings from different land use types within a watershed. The strength of the methodology is in estimating sediment loadings generated by erosion on agricultural lands. However, the USLE has also been applied to rangeland, forest lands, landfills, construction sites, mining sites, reclaimed lands, military training lands, parks, and other land uses where mineral soil material is exposed to the erosive forces of raindrop impact and overland flow.
The instream portion of the framework consists of a model that simulates the hydrodynamics and sediment transport in stream networks. EFDC1D, a one-dimensional (1D) version of the three-dimensional (3D) version EFDC, can simulate bi-directional unsteady flows and has the ability to accommodate unsteady inflows and outflows associated with upstream inflows, lateral inflows and withdrawals, groundwater-surface water interaction, evaporation and direct rainfall (Hamrick 2001). EFDC1D also includes representation of hydraulic structures such as dams and culverts. For sediment transport, the model includes settling, deposition and resuspension of multiple size classes of cohesive and noncohesive sediments. The sediment bed is represented by multiple layers of mixed sediment classes. A bed consolidation model is implemented to predict time variations of bed depth, void ratio, bulk density and shear strength. The sediment bed representation is dynamically coupled to the cross-sectional area representation to account for area changes due to deposition and resuspension.