Hydrologic and Nutrient Budgets
Culver Lake is a 539-acre, 50-foot-deep, kettle-hole lake surrounded by a 3,991-acre watershed that is characterized by a number of features that affect the lake’s hydrology and pollutant load. Two large wetland complexes and one lake are encompassed within the watershed’s boundaries. While large sections of the watershed are characterized by steep terrain, there are other areas of less dramatic slopes. The soils tend to be shallow and rocky and the depth to bedrock is shallow. Finally, most of the development has occurred proximal to the lake. A number of gullies, sloughs, stormwater pipe outfalls, and tributaries convey stormwater runoff from developed and undeveloped areas into the lake. Each of these watershed variables affects the lake’s hydrologic and pollutant loading attributes and is a determinant factor in how best to manage the runoff and associated pollutant load entering the lake.
The goal of this project was to provide the data and technical guidance needed to make insightful and cost-effective stormwater management decisions. The up-dated hydrologic and pollutant loading data were used to guide the Association’s future stormwater management projects, providing them with a practical understanding of the intricacies of stormwater management, and the utility and limitations of different Best Management Practices (BMPs).
Princeton Hydro utilized MapShed as the primary modeling tool to update the hydrologic and pollutant loads for Culver Lake. This simple model makes use of GIS-based land use, precipitation, hydrologic, and terrain data to generate accurate total phosphorous (TP), total nitrogen (TN), and total suspended solids (TSS) loading data. The model is also able to compute septic-related nutrient loads and the sediment load caused by stream bed and bank erosion. The model showed that one subwatershed was the highest per-unit-area contributor of TP, TN, and TSS. This and the next-highest contributing watershed also had the greatest percentage of manageable pollutant loading. As such, targeting load reduction efforts in those subwatersheds will prove the most effective for the Association. The greatest stormwater load reductions will also be achieved through stormwater BMPs capable of managing the one-year event, along with the first flush of larger events. Our team of engineers then identified for the Association the optimal types of BMPs to utilize to properly manage the runoff and pollutant loads generated from the targeted subwatersheds.