Modeling Best Management Practices in Sensitive Areas

XP Solutions swmm program was used to simulate the routing of storm flows into best management practices before discharging to Lake Tahoe during several runoff events.

Located on the border of California and Nevada, Lake Tahoe is the largest alpine lake in North America. It is known for its scenic beauty and outstanding lake clarity. However, the clarity of the lake has been declining since the mid-1960s because of the deposition of fine particles and nutrients from urban runoff and the growth of aquatic organisms that feed off the nutrients in the runoff (U.C. Davis, 2010). A program to restore the clarity of Lake Tahoe has been initiated and involves the development of numerous projects to control the quality of runoff entering the lake.

One such project is the Kings Beach Watershed Improvement Project (WIP). This includes a planning -level process that analyzes runoff patterns in the Kings Beach community at the north end of Lake Tahoe and proposes methods to treat runoff generated in the community before it reaches Lake Tahoe. Methods to collect, control, and treat Kings Beach runoff include the installation of detention basins, infiltration galleries, rock bowls, porous pavement, sediment traps, sediment vaults, and advanced treatment filter vaults connected by a network of pipes and rock and grass - lined channels that will eventually outfall into Lake Tahoe.

A comprehensive software package (xpswmm) for modeling stormwater, sanitary and river systems was selected as a tool to model the WIP, represent the complicated nature of the runoff patterns and to track suspended solids generated in rainfall events.

XP Solutions xpswmm program was used to simulate a detailed system of pipes and channels with surface and subsurface flow. Within the model, treatment nodes were used to represent BMPs to simulate the removal of TSS from the system. The model proved to be a useful tool when examining improvements throughout the Kings Beach area, and for the fine detail of design of individual phases of the project.

Most importantly, the model allowed the entire drainage system to be modeled as an interconnected unit which would not have been possible with more traditional hydrologic techniques such as the rational method applied basin by basin.

To date, one phase has been constructed and another is in final plans. The model is continually updated to reflect new information on field conditions, client needs, and water quality considerations and has worked well as the initial planning level model and eventual design model. - Courtesy of Paul Wisheropp, P.E.; Andrea Manha, P.E.; and Cardno Entrix, Sacramento.