Simplified Detention Requirements with Watershed-Wide Benefits

The City of Overland Park, Kansas, has required stormwater detention for new developments in watersheds with known flooding problems since the early 1980s. Although technically sound, the City's current detention requirements are rather complicated and time-consuming to apply and administer. Simplified detention requirements that would streamline the design process and reduce the administrative workload were desired. We explored a simplified method that used standardized release rates in 100-year storms. Simplified, representative watersheds ranging in size from 200 acres to 40 mi² were tested for detention control using standardized hydrologic methods and detention basin designs. The first set of tests focused on the maximum 100-year release rate that would prevent an increase in nuisance flooding directly downstream of a 10-acre development. We found that a 100-year release rate of 3.0 cfs per acre would provide reasonable assurance of no increase in the 10-year discharge from the development site. The objective of the second set of tests was to find a release rate that would also prevent increases in Q10 and Q100 at a problem site with a drainage area of 200 acres as the watershed is developed. Because the impact of a development on flooding at a downstream problem site depends on the location of the development within the larger watershed, multiple development patterns were tested. We found that a 100-year release rate of 3.0 cfs per acre for new developments would prevent increases in Q100 and Q10 at the 200-acre point in nearly all cases. Additional tests focused on the downstream impacts of in-fill development and redevelopment projects. We found that higher release rates are acceptable for redevelopment projects. The effectiveness of development-scale detention for larger-scale flooding problems was investigated on hypothetical watersheds with drainage areas up to 40 mi². We found that the downstream benefits of on-site detention diminish with increasing watershed size. Development-scale detention does not prevent an increase in flooding at a problem site with a drainage area over a few square miles. However, it does mitigate the increase in discharge.