Runoff checklist for successful continuous calibration modeling

Continuous calibration of storm and combined sewer systems models requires in-depth understanding of runoff basin hydrology. The model should reflect the real dynamics of runoff pathways between the surface features, as well as mimic the effects of back to back storms. In order to achieve this goal, a field visit at the site becomes essential. The field visit checklist should include acquiring knowledge of the runoff sources, identifying pathways followed by the runoff to enter into the system, potential inlet catchment constraints, subsurface gravel exposure due to aging and damaged pavement, and potential ponding areas. Field visit should occur during or immediately after a storm event to capture the most useful information of the complex runoff routing between the features of the basin. As necessary, runoff sources should be split in the model to incorporate field findings and to develop a model as close as possible to real conditions.

Efforts to improve model calibration based on field observations were made for the Dodge Park combined basin located in a very old largely unimproved section of downtown Columbus, OH.  Initial calibration using industry standard ranges for runoff parameters highly overestimated the basin response compared to observed flow monitoring data. Field visit during wet weather conditions lead to several observations. At many locations, runoff from alleys and streets without curbs drains into the lawn area adjacent to the pavement. Presence of brick roads generates a texture of impervious and pervious surface that attenuates the flow and allows more infiltration to the subsurface; parking lots made of gravel or with excessive pavement degradation behave as pervious areas. During storm events the area also experiences diffused ponding at several locations.

Detailed representation of the runoff sources such as buildings, streets, and lawn provided flexibility to achieve a better incorporation of field findings. Ponded area at the storm inlets was modeled as a storage unit with infiltration and evaporation and an orifice was used to realistically represent the constraint that the storm inlets present to flow entering the collection system. The high level of details achieved lead to significant improvements in the model calibration, proving the fact that combined system modeling requires close understanding of the subarea features of the basin to have good continuous calibration. Moreover, the detailed representation of runoff dynamics provides a model suitable for integrated planning, facilitating representation of projects like green infrastructures and inflow redirection.