Development of a Watershed Restoration Plan and Sediment TMDL in Vermont

Watershed assessments in Vermont have identified sedimentation as one of the largest categories of pollution in the state, with stream bank erosion as a major source of sediment. Increased development and stormwater are often linked to the increase in bank erosion. The Allen Brook watershed is located in Williston, the fastest growing town in Vermont. In 1999, Allen Brook was listed on the state impairment list for failing to meet water quality standards, requiring the state to develop a watershed restoration plan and sediment Total Maximum Daily Load (TMDL). A multi-component procedure was developed to identify the sediment sources and quantify loads. First, rapid quantitative and qualitative field assessments were conducted for 31 stream sites within the watershed. Next, total suspended solids (TSS) for two sites were measured during baseflow and storm events over a six-month period. The Soil and Water Assessment Tool (SWAT) model was used to quantify flows and sediment within stream reaches as well as determine sediment contributions from subwatersheds based on landuse and soil characteristics. Finally, pollution prevention and stormwater retrofit surveys were conducted. The sampling and modeling data were used to quantify sediment loads and set numerical reduction targets. A restoration plan to meet these targets was developed including quantifiable control actions to be implemented at sites identified during pollution prevention and retrofit opportunity surveys.