Tips on Using Hec Ras in Dambreak

8/12/2019 Tips on Using Hec Ras in Dambreak

1/31

This paper is partially derived from short course lectures developed by the staffand engineers at the USACE Hydrologic Engineering Center (HEC)

Tips on Using the Dambreak

Option in HEC-RASDavid T. Williams, Ph.D., P.E., P.H., CFM, D.WRE

National Technical Director, Water Resources, PBS&J

[email protected]

Michael DePue, P.E., CFM, D.WRE

Associate Vice President, Water Resources, PBS&J

[email protected]

Illinois Association for Floodplain and Stormwater Management Conference

March 12-13, 2008, Tinley Park, Illinois


2/31

Teton Dam Failure


3/31


4/31


5/31


6/31


7/31

Teton Dam Failure


8/31

Lawn Lake Dambreak (10 minutes

to full breach), U/S of Estes Park


9/31

Morita Dam, Wailapa Stream, Hawaii


10/31

St. Francis Dam in Santa Clarita Valley, CA


11/31

Models and Methods Used

HEC-1 (COE)

HEC-RAS (COE)

DAMBRK (NWS)

FLDWAV (NWS)

Simplified DAMBRK (NWS)

TR-61 (also known as WSP2, NRCS)

TR-66 (Simplified Dam Breach Routing Procedure)

BREACH (HR Wallingford, England)


12/31

General Comments

In HEC-RAS, a dam breach analysis isperformed on an inline weir and a levee breachis performed on a lateral weir.

The breach editor can be accessed from eitherthe unsteady flow analysis editor or the breachbutton on the inline or lateral weir editor.

To compare a breached with a not breachedrun, the project should have two plans, but theycan (and in many cases should) have the samegeometry.


13/31

Dam & Levee Breach Input Menus


14/31

Dam Breach Data


15/31

Breach Progression

The Breach Progression editor defines the breach growthrate. By default, the breach growth is assumed to belinear from start to maximum size (Full Formation Time).

A non-linear breach growth curve can be developed by

entering a Time Fraction (from zero to 1.0) and a BreachFraction (from zero to 1.0) and is plotted in the graphicnext to the table.

Adjusting the curve in makes a more realistic breachgrowth; however, the curve can sometimes be used inorder to improve stability. If the program is havingstability problems during part of the breach growth, therate of growth during that time can be reduced byadjusting the curve.


16/31

Non-Linear Breach Growth


17/31

Estimating Dam Break Parameters

Determining the size and growth rate for breaches is a

very inexact science.

The use of several different procedures, to get a range

of values, is generally recommended.

The following are good references and give guidanceon breach analyses


18/31

Estimating Dam Break Parameters

Bureau of Reclamation, U.S. Department of the Interior

(Dam Safety Research Report)

Prediction of Embankment Dam Breach Parameters

http://www.usbr.gov/research/dam_safety/reports.html

developed best-fit and envelope curves for both breach

parameters and peak outflow from breached earthfill

dams as a function of a breach formation factor.

MacDonald and Langridge-Monopolis (Journal of Hydraulic

Engineering, Vol. 110, No. 5, May, 1984, p. 567-586)

Breaching Characteristics of Dam Failures
http://www.usbr.gov/research/dam_safety/reports.htmlhttp://www.usbr.gov/research/dam_safety/reports.html


19/31

Estimating Dam Break Parameters Froehlich (Water Resources Engineering, 1995 ASCE

Conference, San Antonio, Texas, August 14-18, 1995, p.887-891)

Embankment Dam Breach Parameters Revisited

developed a best-fit regression equation for peak flow

based on a breach formation factor.

Victor M. Ponce (his website), Case Studies of Dam-

breach Geometry and Hydraulics

shape factor representing dam-breach geometry is

formulated and a relationship between Froude

number based on peak discharge and the new shape

factor is identified.

http://ponce.sdsu.edu/dambreachgeometry.html


20/31

Breach Parameter Range

and Sensitivity

When performing a dam breach analysis, it is useful to

develop a range of values for the various breach

parameters.

For instance, the user might create a table that has a high,

medium, and low estimate for each breach parameter.

By combining the parameters in various combinations, a

range of dam break hydrographs can be created that

[hopefully] represents a maximum and minimum

hydrograph limit, as well as a most probable estimate.


21/31

Breach Sensitivity

High Medium Low

Max

Bottom

Width

200 150 100

Side

Slopes 1.5 1.25 1.0

Time to

Breach

15 min 45 min 1.5 hours


22/31

Effect of Breach Parameters

on Downstream Areas

How important the breach parameters are dependslargely on how far downstream from the dam thestudy area is located.

At the toe of the dam, the computed peak flow oftenvaries wildly within the assumed limits of the breachparameters.

However, the study area further downstream, thebreach parameters have less and less effect on theflow hydrograph.


23/31

Why Do the Hydrographs

Converge in Downstream Areas?

The total volume of water in each of the differenthydrographs is basically the same (being the stored waterbehind the dam).

As the hydrographs move downstream, a sharphydrograph (such as a dam break) will attenuate muchmore quickly than a flat hydrograph.

How close they get to each other depends on the distancethey are routed, steepness of the stream, roughness ofthe river and floodplain, and amount of floodplain storageavailable for attenuating the hydrograph.


24/31

Dambreak Floodwave Progression


25/31

Effect of Breach Parameters

on Downstream Areas

Hydrographs from different assumed breach parameterscan converge to produce the same peak flow (and for thatmatter, the same rise and fall) over a surprisingly smalldistance. In the above example, Fread found that the

hydrographs have substantially converged within fivemiles and are indistinguishable by mile 13.

Levee breaches in general and dam breaks in particular,have rapidly varying water surfaces and flows.

Because of this, a small time step is required to maintainstability and accuracy; often on the order of a few secondsfor a dam break problem.


26/31

Dam & Levee Breach

Stability Considerations

Cross sections that are spaced too far apart below a dambreak can cause stability and accuracy problems.

The Courant condition should be kept close to or less than

one (e.g., if the velocity is 20 ft/sec and a 10 sec. timestep is used, the cross section spacing should be nocloser than 200 feet).

Dam breaks are usually associated with large amount ofsediment, debris and turbulence.

The Mannings n should be increased (particularly atcross sections near the dam) to account for these effects.


27/31

Dam & Levee Breach

Stability Considerations

The reported breach time, especially for piping initiatedbreaches, often includes a significant period of timewhen the flow is, relatively speaking, a trickle. This timeshould not be included in the Critical Breach Timeunless the user makes appropriate adjustments to thenon-linear growth time.

If the dam break has stability problems, turning theMixed Flow option on may help (even if the flow is notgoing supercritical). This may reduce the accuracy ofthe water surfaces, particularly immediatelydownstream of the dam. This error is usually minorcompared with the error caused by the unknowns in the

breach parameters.


28/31

Computational Considerations

Theta Weighting Factor

A weighting applied to the finite differenceapproximations available in HEC-RAS ComputationOptions and Tolerance menu

Theoretically can vary from 0.5 to 1.0 Practical limit is from 0.6 to 1.0

1.0 = most stable; 0.6 = most accurate

The default in RAS is 1.0

Start at 1.0 and reduce towards 0.6 as long as themodel stays stable and produce reasonable results


29/31

Dam & Levee Breach Stability

Considerations - Summary

Small time steps

Appropriate cross section spacing downstream of dambreach (Courant


30/31

References (from Dr. Ponces website)

CASE STUDIES OF DAM-BREACH GEOMETRY AND HYDRAULICS

Victor M. Ponce

Professor, Department of Civil and Environmental Engineering,San Diego State University, San Diego, California 92182-1324

Ahmad Taher-Shamsi

Associate Professor, Amirkabir University of Technology, Tehran, Iran;Currently on sabbatical leave at the Department of Civil and EnvironmentalEngineering,San Diego State University, San Diego, California 92182-1324

Ampar V. Shetty

Research Associate, Department of Civil and Environmental Engineering,

San Diego State University, San Diego, California 92182-1324ABSTRACT

A review of dam-breach parameters and prediction methods developed fromthe analysis of historic embankment-dam failures is performed. A new


31/31

Example Dam Break Animation

Documents

Tips on Using Hec Ras in Dambreak