Preservation of Watershed Regime for Low-Impact Development through Detention

Low-impact development (LID) allows for greater development potential with less environmental impacts using on-site distributed storm-water controls that achieve a good balance among conservation, growth, ecosystem protection, and public safety. The qualitative statement of the ultimate goal for LID can serve as guidance for engineering designs, but it is inadequate for comparison and selection among the innovative alternatives. This paper presents an innovative method by which the long-term runoff statistics are employed as the basis to quantify the impact of the development on the watershed hydrologic regime. In this study, the standard LID detention volume is defined by the storm-water storage volume required to preserve the predevelopment mean and standard deviation for runoff volume population. Consequently, a detention basin is considered oversized if the after-detention runoff volume population has a lower mean flow while the undersized counterpart produces a mean runoff volume higher than that under the predevelopment condition. This simple but quantifiable method is very useful for detention alternative comparisons, and can serve as a guide to retrofit an existing detention basin, according to the proposed LID initiative.