Qualitative approach to define age of sewersheds using physically based models

Effective new development planning efforts are predicated on understanding the history of the site as well as the location and magnitude of any future development.  An essential piece of future development planning is the construction of a physically based hydrologic and hydraulic model.  Understanding the historic rainfall dependent inflows and infiltration (RDII) entering the collection system helps to ensure any future development will not compromise the existing collection system. 

Divided into 148 submodels, the approach to model the City of Columbus’ collection system included a three stage high-resolution physically based hydrologic surface and aquifer methodology to simulate RDII.  High-resolution implies that each house nearby infrastructure was incorporated.  The most important component of this modeling approach was the inclusion of the age of infrastructure and age of homes.  Age of homes played a significant role in model parameterization.  House ages were divided into three groups (< 1936, 1936 to 1964, and >1964); each employing a unique set of parameters for the modeled area.  Furthermore, three stages of RDII were defined to represent rapid inflows and delayed longer-term responses.  For each of the three stages, four predefined model parameters were used for calibration including: aquifer conductivity slope, subcatchment ground water flow coefficient (A1), aquifer deep losses, and monthly evapotranspiration intensity constants. 

Once the models were calibrated and validated to a continuous series of wet weather events, the objective of this analysis was to determine future development modeling parameters.  An analysis was conducted to look at RDII from several old areas where development originated before ~1935 and compare peak flow and total volume per acre to more recently developed areas after ~1990. As expected, the analysis revealed that older sewersheds contributed roughly 10x more volume per acre with peak flows nearly 30 times greater than the newly developed sewersheds. Considering building practices and overall storm water management strategies have been consistent since the mid 1990’s in the Columbus area, future conditions models were parameterized based on the more recently developed sewersheds as a conservative approach.