Taking NYC's LTCP Modeling to the Next Level: A Unique Approach to Recalibrating 13 InfoWorks Sewer System Models

The NYC Department of Environmental Protection (DEP) is developing a CSO Long-Term Control Plan (LTCP) that includes a combination of grey and green infrastructure practices. The InfoWorks hydrologic/hydraulic models of the collection system were recently updated and recalibrated using a host of unique methods to better suit them for use in green infrastructure analyses. This included application of detailed impervious land cover data, a two-phased flow monitoring program, validation of CSO storage facility performance, and a year-long continuous simulation to validate wastewater treatment plant (WWTP) flows.As the DEP placed increased attention on the use of green infrastructure to manage street runoff by either slowing it down as it enters the combined sewer network or preventing it from entering the network entirely, it became clear that a more detailed evaluation of impervious cover would be essential. DEP invested in the development of high quality satellite measurements of impervious surfaces at a 7.9 ft by 7.9 ft (2.4 meter) pixel level to provide more detailed and physically-based impervious data.A unique two-phased calibration approach was utilized that focused on the hydrology module (runoff) of the model. Flow monitoring data was collected in upstream areas of the collection systems, remote from (and hence largely unaffected by) tidal influences and in-system flow regulation for use in understanding the runoff characteristics of the impervious surfaces. Data collected at recently-constructed CSO storage facilities in the system were also used to refine the performance of model predictions of stored and discharged flow. Finally, a validation step was completed by simulating a continuous calendar year (2011) to evaluate the conveyance of flow to the WWTPs using rainfall and operational data.