Use PCSWMM’s SRTC Tool to Calibrate the Time-Area Runoff

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Use PCSWMM’s SRTC Tool to Calibrate the Time-Area Runoff Hydrology in Mike Urban52nd International Conference On Water Management Modeling

Suibing Liu, PHD PEJacobs, Cincinnati, OH, USA

Senior Technologist ProfessionalSenior Technology Consultant for Hydrology and Hydraulic Modelling and Wastewater Collection System Modelling For National and International Projects

February 28, 2019Toronto, Canada

Agenda

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1. Background2. Hydrology3. Workflow4. Results5. System and Alternative Analysis6. Conclusions and Recommendations

1. Background

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Background• Client:

– Water and Sewer Department of the City of Tuscaloosa, Alabama• Project:

– Collection System Hydraulic Model Update for Long-term Capital Planning

• System:– > 550 miles of gravity sewer– >13,000 manholes

• Pump stations: 19• Flow meters: 47

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System Layout

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Google Earth - 4/27/2011

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Tuscaloosa–Birmingham Tornado aftermath• The City suffered major impacts resulting from the

tornado on April 27, 2011.– $2.4 billion of property damage, the costliest

single tornado in United States history at that time.

• Some areas suddenly became almost uninhabited for a period of time.

• The flows monitored during this time did not accurately reflect the pre-tornado land use because there were few people living in some areas.

• There was uncertainty in how redevelopment might increase the population density in some impacted areas.

• TWSD needed a hydraulic model to help manage flows for future development in these areas.

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Flow Monitors

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2. Hydrology

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Hydraulic Model• The modeling platform is DHI Mike Urban Mouse

engine.• Hydrology: Time-area runoff• The original model represented the wet weather

runoffs very well.• It was time consuming to adjust the Time-Area

curve parameters– No tool is available to do efficient calibration.

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Time Area Method

11 Source: Mouse Runoff Reference Manual, Mike by DHI.

RTK Unit Hydrograph• T1, T2, and T3: time to the peak of

respective unit hydrographs• K1, K2, and K3: recession coefficients

of the respective unit hydrographs• T1+K1*T1: last time unit of the first unit

hydrograph• T2+K2*T2: last time unit of the second

unit hydrograph• T3+K3*T3: last time unit of the third unit

hydrograph and the total unit hydrograph

• R1, R2, and R3: R-values of respective unit hydrographs; R = R1+R2+R3

12 Source: Computer Tools for Sanitary Sewer System Capacity Analysis and Planning, EPA/600/R-07/111, October 2007

3. Workflow

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Workflow• Calibrate the dry weather flows and patterns.• Calibrate the wet weather flow with the RTK method using the SRTC tool.• Simulate the RTK unit hydrographs with a one-inch rainfall.• Develop the Time-Area curve using the simulated flow.

– Time of concentration is the maximum of (T+T*K) from the three triangular RTK unit hydrographs.

– The dimensionless concentration time = (Simulation time from the first timestep with flows) / (Time of concentration)

– The Accumulated dimensionless area = (Accumulated volume at a time step)/(Total volume generated by the RTK unit hydrograph with a one-inch rainfall)

• Simulate the Time-Area curve and verify the results• Final refinements.

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RTK Unit Hydrograph

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RTK to Time-Area Curve Conversion

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0

20

40

60

80

100

0 20 40 60 80 100

% A

rea

t/Tc (%)

Time Area Curve

SWMM_RTK_1inch Results MU Time Area Curve Input

Flow Comparison

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0.00.10.20.30.40.50.60.70.80.91.00.0

0.5

1.0

1.5

2.0

2.5

3.0

5/14/12 5/16/12 5/18/12 5/20/12 5/22/12 5/24/12 5/26/12 5/28/12

Rai

nfal

l (in

ch)

Flow

(mgd

)SWMM-RTK Mike Urban Time-Area Rainfall

4. Results

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Calibration Criteria• For WWF, the following standards were used for

calibration guidelines– Match the general hydrograph shape. – Predicted time of peaks and troughs is within 1 hour of

the observed flow.– Predicted peak flow is within -15% to +25% of the

observed flow.– Predicted volume is within +20% to -10% of the

observed flow.19

Spreadsheet Converting RTKs

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Converted Time-area Curves

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5. System and Alternative Analysis

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System AnalysisExisting Conditions Future Conditions

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Alternative AnalysisUpsizing Storage and Upsizing

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6. Conclusions and Recommendations

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Conclusions and Recommendations• The RTK method can be converted to and modeled by

the Time-Area method.• Modeling platform is not only a part of the

deliverables, it is also a tool.• No modeling platform is perfect, but has something

unique.• Utilize the best tools, if available.• Results from different modeling platforms with

equivalent engines are similar but not necessarily identical.

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Thank you!

Acknowledgement:– Russ Barton, Director of Logistics & Asset Management, City of Tuscaloosa, AL.

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