Using PCSWMM's SRTC tool to calibrate the time-area runoff hydrology in MIKE URBAN

Jacobs was tasked to update and calibrate the collection system hydrology and hydraulic model for the Water and Sewer Department of the City of Tuscaloosa, Alabama. The modeling platform was predetermined as Mike Urban using Time-Area runoff method for its hydrology. The City had used the Time-Area runoff method in its original models and it represented the wet weather runoffs very well. However, it was time consuming to adjust the Time-Area curve data points and other associated hydrological parameters, because no tool is available to help calibrating the Time-Area parameters efficiently.

After comparing the Time-Area runoff method with the Unit Hydrograph RTK method, it was determined that it is possible to convert a RTK unit hydrograph to an equivalent Time-Area curve. Therefore, the metered wet weather flows could be first calibrated using the RTK method. Many tools are available to perform the RTK calibration, such as EPA SSOAP toolbox, InfoSWMM RTK Calibrator, and the SRTC tool in PCSWMM. The PCSWMM’s SRTC was chosen for this task.

The basic steps are listed below:

1. Disaggregate the flow metering data, calibrate the dry weather flows and patterns.
2. Calibrate the wet weather flow portion of the flow data using the RTK method utilizing the PCSWMM’s SRTC tool.
3. Simulate the calibrated RTK unit hydrographs with a one-inch rainfall in PCSWMM.
4. Calculate the Time-Area curve using the simulated flow data from step #3.
   1. Time of concentration is the maximum of (T+T*K) from the three triangular RTK unit hydrographs.
   2. The dimensionless concentration time = (Simulation time from the first timestep with flows) / (Time of concentration)
   3. The Accumulated dimensionless area = (Accumulated volume at a time step)/(Total volume generated by the RTK unit hydrograph with a one-inch rainfall)
5. Simulate the Time-Area curve in Mike Urban and verify the results
6. Make final refinement of the calibration in Mike Urban.

The following figure represents the comparison of the WWF calibrated directly from RTK method, and the simulation results of the Time-Area curve directly converted from the RTK unit hydrograph. It shows that the results of the two methods match very well.

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