Continuous Hydrology with Subbasin Specificity and LID: The Flow Duration Design Model

Traditional hydrologic analysis involves simulating a single design storm event for a specific site and using a "peak flow standard" to match the absolute hydrograph peak of pre- and post-development scenarios. Experience with this type of regulatory scheme reveals several problems. Use of a "flow duration standard" and regional-scale detention ponds eliminates many of the known shortcomings while providing better regional planning and protection of aquatic resources. Typically, applications of flow duration standards have involved use of regionally calibrated, HSPF-based hydrologic modeling. Until recently the weakness of this type of application has been the regional calibration. Unfortunately, jurisdictional boundaries typically don't follow watershed boundaries and variations occur due to soil, slope, elevation and others. In addition, the quality of aquatic resources varies spatially as impacts to these resources occur at various time and size scales. The limited financial resources of stormwater management agencies suggest that a more strategic application of funds is required to ensure the most appropriate use of capital. The Flow Duration Design Model allows tailoring model simulations to specific subbasin characteristics. It also incorporates a concept of "target conditions" rather than predevelopment or old-growth-forest conditions. These characteristics allow the model to be tailored to specific community and resource needs depending on constituent goals. In addition, the model incorporates the ability to simulate various low impact development (LID) techniques, such as porous pavement or amended soils. Collectively, these features allow the model to be integrated into and support specific objectives of watershed action plans, even when these action plans differ due to physical watershed differences or jurisdictional goals.