Watershed District (MCWD or District) within the cities of Minnetrista and Mound. The subwatershed includes Langdon, Black and Saunders Lakes. The eastern portion of the subwatershed is mostly developed, while the western portion of the subwatershed contains low-density development and openspace, including part of Gale Woods Regional Park and numerous wetlands. Langdon Lake outlets via storm sewer to Lost Lake, which then outlets by open channel to Cooks Bay of Lake Minnetonka.

Langdon Lake does not meet its water quality goal established in the 2007 MCWD Water Resources Management Plan (WRMP) of 55-70 μg/L. The 2008 in-lake phosphorus concentration, on average, was 166 μg/L. The MCWD began monitoring Saunders Lake in 2009; the lake is currently meeting state-wide standards. Based on the MCWD Hydraulic & Hydrologic Pollutant Loading Study it was determined that offsite loading could be reduced through retrofit of existing stormwater infrastructure with BMPs, enhancing the outlets of Saunders and Black Lakes, working with homeowners to install BMPs and wetland restoration.

Much of the corridor between Black and Saunders Lakes, composed of wetland and maple-basswood forest, has been identified by the DNR as a regionally significant area with outstanding ecological value. Most of this area has been incorporated into Gale Woods Regional Park by the Three Rivers Park District. The western half of the Langdon Lake subwatershed has been identified by the Department of Natural Resources as a Metropolitan Wildlife Corridor Focus Area and by the City of Minnetrista as a natural resources corridor.

Neighborhood Raingardens

Two neighborhood raingarden retrofit projects could afford additional water quantity treatment for developed areas that currently have no volume control. The goals of these projects would be to install enough raingarden storage to meet the current MCWD infiltration requirement of 1-inch. Sub-catchment runoff volume was used to determine the number and size of raingardens necessary to achieve this reduction.

PLOAD was used to model the impact of constructing these raingardens in two developed neighborhoods adjacent to Saunders Lake. For this analysis, the raingardens were lumped together and assumed to achieve the design value of 70% reduction (catching the first 1-inch of rainfall) over each neighborhood subcatchment. It was estimated that 25% of the runoff from the neighborhoods would not be able to be captured by raingardens due to slope and space limitations. The remaining 75% of runoff was routed to the raingardens and a 70% removal was applied to the load.