Mill Pond Study

Hydrologic and Hydraulic Analysis to Identify and Solve Sediment Accumulation and Water Quality Issues

Weston & Sampson conducted a multi-faceted study of Mill Pond in Durham, New Hampshire, to determine the potential causes of and solutions to ongoing sediment accumulation and associated decreases to water quality. We developed a detailed hydraulic model that included approximately 1.8 miles of the Oyster River, from the base of the Oyster River Dam, through Mill Pond, and into the tidal zone beyond the Rte. 108 bridge crossing. The hydraulic model was used to evaluate long-term trends in sediment transport, identifying areas of the Oyster River and Mill Pond that tend to accumulate or produce sediment. Ultimately, the analysis determined that neither modifications to gate operations nor modifying the spillway to lower the normal pool would decrease sediment deposition in Mill Pond.

Weston & Sampson also conducted a hydrologic and hydraulic analysis of Mill Pond Dam, the pond itself, and the upgradient watershed to identify conceptual-level alternatives for modifications to the existing gate structure and spillway that could increase the dam’s hydraulic capacity and bring the dam into compliance with NHDES standards. This effort determined that overflows across Rte. 108 from the Lamprey River watershed add considerably to runoff generate in the Oyster River watershed. As a result, it was determined that only large scale changes to the dam’s spillway and normal pool level would allow it to safely pass its regulatory design flood.

Our team also conducted an analysis of nutrient loading in the Mill Pond watershed that may be contributing to decreased water quality in the pond. We developed a menu of lowest cost alternative nitrogen control strategies for non-point source reduction, resulting in a list of potential nutrient control measures through BMP optimization. We also developed an Implementation Plan to address Mill Pond Water Quality, identifying future actions and next steps as well as a potential implementation schedule down to the subwatershed level. The plan also included planning level cost estimates for both capital cost and operations and maintenance costs to support the possibility for future state revolving fund (SRF) or other grant or loan financing.

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