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Dam Breach: HMS vs. RAS

EWRI HEC User Conference – April 12, 2010

Wayne Tschirhart, PE, CFM

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Background

• San Antonio River Authority

– Texas enabling legislation passed in 1937.

– Charged with protection and management of

the San Antonio River and its tributaries.

– Owns/maintains 40 SCS intermediate/small

flood control dams.

• 27 in Bexar County

• 13 in Karnes County

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Project Motivation

• Texas Commission on Environmental Quality established new dam safety rules in January, 2009.

• All high and significant hazard dams must have an emergency action plan by January 1, 2011.

• Emergency action plans must contain dam breach inundation maps.

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Dam Breach Models

• Public domain:– DAMBRK (NWS)– FLDWAV (NWS)– SMPDBK (NWS)– SITES (NRCS)– SIMBA (USDA-ARS)– HEC-1 (ACOE)– HEC-HMS (USACE)– HEC-RAS (USACE)

• Proprietary:– MIKE 11 (DHL)– SOBEK (Delft Hydraulics)– ISIS (Wallingford)– HR BREACH (Wallingford)

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Dam Breach Models

• Public domain:– DAMBRK (NWS)– FLDWAV (NWS)– SMPDBK (NWS)– SITES (NRCS)– SIMBA (USDA-ARS)– HEC-1 (ACOE)– HEC-HMS (USACE)– HEC-RAS (USACE)

• Proprietary:– MIKE 11 (DHL)– SOBEK (Delft Hydraulics)– ISIS (Wallingford)– HR BREACH (Wallingford)

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The Question: Does it matter?

HMSRAS

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Test Subject: Calaveras 10

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Test Subject: Calaveras 10

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RAS Process

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HMS Process

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Breach Hydrographs

7,180 ac-ft74,000 cfsRAS

VQp

0.2 %7.2 %% Diff

7,150 ac-ft85,500 cfsHMS

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Breach Profiles

% DiffDiff

(fps)

% DiffDiff

(ft)

2.2

2.0

5.8

0.3

Velocity

0.3

0.3

0.8

0.1

2.0

1.8

2.9

1.4

1.0

1.1

1.2

0.8

Depth

Max

Median

Mean

Min

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Conclusion

• Whether one uses HMS or RAS to produce the

breach hydrograph is a matter of preference.

– The breach parameters are basically the same.

– The differences between the breach hydrographs

are not significant enough to declare one better

than the other.

– HMS reservoir parameters are easier to enter and

more intuitive.

• RAS is still needed to perform routing.

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References

• Texas Commission of Environmental Quality, 2007. Hydrologic and Hydraulic Guidelines for Dams in Texas. Austin, TX: Texas Commission of Environmental Quality Dam Safety Program

• U.S. Army Corp of Engineers. 2009. Hydrologic Modeling System HEC-HMS User Manual. Davis, CA: USACE Hydrologic Engineering Center.

• U.S. Army Corp of Engineers. 2006. River Analysis System HEC-RAS User Manual. Davis, CA: USACE Hydrologic Engineering Center.

• Wahl, T. 1998. Prediction of Embankment Dam Breach Parameters – A Literature Review and Needs Assessment. Dam Safety Report No. DSO-09-004. Denver, CO: Bureau of Reclamation Water Resources Research Laboratory.

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Discussion/Questions