WHAT LEVEE RISK? SIMPLIFIED LEVEE BREACH ANALYSIS FOR … · damages and has an estimated repair...

June 21, 2018

Slide 1 of 272018 ASFPM National Conference

What Levee Risk? Simplified Levee Breach Analysis for Non-Accredited FEMA Levees

WHAT LEVEE RISK?

SIMPLIFIED LEVEE BREACH ANALYSIS FOR NON-

ACCREDITED FEMA LEVEES

Brent Travis, Ph.D, P.E., D.WRE

Senior Hydraulic Engineer, WEST Consultants, Inc.

480-345-2155

[email protected]

Frank Brown, P.E., CFM, PACP

Civil Engineer

Civil Structures Branch in the Engineering Division

Flood Control District of Maricopa County

Alissa Miller, P.E., CFM

Staff Engineer, WEST Consultants, Inc.

Brian Wahlin, Ph.D, P.E., D.WRE

Vice-President, WEST Consultants, Inc.

mailto:[email protected]

June 21, 2018


What Levee Risk? Simplified Levee Breach Analysis for Non-Accredited FEMA LeveesTres Rios North Levee

Crest

Gila River

Leveed Areaor

Levee Protected Area

June 21, 2018



June 21, 2018



LAMP: Levee Analysis and Mapping Procedures

June 21, 2018



2013 FEMA LAMP Opportunities• Some non-accredited levees may qualify for improved floodplain

designation.

• A critical step in the LAMP submittal process is levee breach modeling.

• 2D flow modeling typically needed for dependable breach flow

predictions.

• LAMP requires a minimum of 2 breach simulations along a short levee.

• Longer levees will likely require multiple breach locations.

• Unfortunately, 2D hydraulic modeling can be both expensive and time

consuming, and hence many levee owners may not have the means to

submit LAMP applications.

• Solution: Develop a simplified method for rapid, conservative

floodwave predictions.

June 21, 2018



Levee and Canal Embankment Failures• Levee and canal breach failures occur frequently worldwide, causing

damage to infrastructure and risk to human life.

• Some specific statistics:

– The 2012 Rohri Canal failure in Pakistan flooded 25 villages and displaced

40,000 people.

– The 2015 Columbia Canal failure in South Carolina caused $132,000,000 in

damages and has an estimated repair cost of $100,000,000.

– The 53 levee breaches in New Orleans during Hurricane Katrina killed more

than 700 people, displaced hundreds of thousands, and caused in excess of

$100,000,000,000 in damages.

• The following slides show some other levee and canal failures that have

occurred over recent years.

June 21, 2018



North Jordan Canal, Utah, 2013

June 21, 2018



Taliperu Canal, India, 2014

June 21, 2018



Secchia River Levee, India, 2014

June 21, 2018



Bhagalpur Canal, India, September, 2017

June 21, 2018



How to model a breach floodwave• Unlike dams, flow from levee or canal breaches are

– 2D rather than 1D.

– Long lasting: Breached flow is continually replenished by channelized inflow.

– Challenging to monitor: Can be hundreds of miles long.

– Poorly understood: occur in remote locations.

• 2D modeling is almost always best for canal / levee breach analysis.

• However, 2D modeling is also

– Is expensive in terms of consulting fees;

– Requires detailed terrain data;

– Takes days or weeks to create;

– Requires hours or even days to execute;

– Requires knowing the most important failure location.

• What if there are multiple?

• Or you just don’t know?

June 21, 2018



Benefits of proposed approach

• This project provides a reasonable alternative to 2D flow analysis for

simple applications.

• It is helpful as a

– Quick analysis of multiple breach locations along long canal / levees.

– Methodology to identify the worst case breach failure location.

– Simple way to check 2D floodwave predictions.

June 21, 2018



Simplified model overview

June 21, 2018



Simplified Model Equation formulation

• To simplify, note that usually depth and velocity decrease as wave

propagates.

• Hence we can assume:

– Minimal velocity in integral: v(x) = v(r) (conservative)

– Maximum depth in integral (h = h0) (conservative)

• Result:

0

2 2

2

0 0 0 0

11

2 2

r

r

v r v xC h s r s r h r f dx

g h x g

2 2

2 00 0 0 0

0

12 2

r rv vC h s r s r h f

g h g

• Energy equation between breached area (w = 2r) and wave front:

June 21, 2018



Numerical Modeling by Zoppou and Roberts (1991)

June 21, 2018



Numerical modeling by Roger et al (2009)

June 21, 2018



Equation formulation

• For these same variables, the Saint-Venant equations are

2

0

0

1 11

1 1

C H S RQ H

C F R

H H H H Q Q

Q T R Q R R

• After establishing non-dimensional variables (e.g. H = h / h0, R = r /

h0, etc.) and rearranging:

June 21, 2018



• To simplify:

– Assume a flat plane (i.e. S0 <<1 so S0 ≈ 0);

– Set H = 1 in radical (conservative):

• The solution is obtained by Method of Characteristics to be:

Equation formulation

2

0 0

1 1

1 1 1 1

C H HQ H

C F R F R

0

3302

2/31 30 02

1 1

1

1

1 1 1

F RH

R

dR

dT F T

R F F T

June 21, 2018



HEC-RAS 2D Model

• To verify, a break of the TRNL North Levee were modeled in RAS 2D.

• Two break propagation speeds were assumed: 6 min. and 60 min.

• Predictions:

– Simplified method should always predict faster arrival times than RAS 2D;

– Simplified method should be closest to 6 min. break RAS 2D prediction.

• Model specifics:

– A 100’ wide final breach width was assumed;

– Levee height at breach = 12 feet;

– Manning’s roughness varied throughout the floodplain, from 0.035 to 0.07.

(Assumed n = 0.05 as an average value for simplified method.)

• Also shown:

– Original simplified wave model (quasi-unsteady; didn’t include roughness)

June 21, 2018


What Levee Risk? Simplified Levee Breach Analysis for Non-Accredited FEMA Levees60 Minute Breach

June 21, 2018



Floodwave distance versus time: 60 minute breach

Always higher

as expected

Sometimes flow angled

from levee is faster than

perpendicular floodwave.

June 21, 2018


What Levee Risk? Simplified Levee Breach Analysis for Non-Accredited FEMA Levees6 Minute Breach

June 21, 2018



Floodwave distance versus time: 6 minute breach

Always higher

as expected

June 21, 2018



Comparison with original method: 60 minute breach

June 21, 2018



Comparison with original method: 6 minute breach

June 21, 2018



Conclusions

• Updated simplified method an improvement over original, even though it

requires fewer inputs.

• Simplified method always predicts faster flood waves then predicted in

HEC-RAS 2D.

• Sometimes oblique waves travel faster to a location than waves

perpendicular to levee break.

June 21, 2018



Next Steps

• Rebuild equation with momentum equation rather than energy

(avoids unaccounted for energy losses and may improve fit).

• Finalize generalization of method to account for varying slope and

variable propagation breach times.

• Test against actual levee / canal failure data.

WHAT LEVEE RISK? SIMPLIFIED LEVEE BREACH ANALYSIS FOR … · damages and has an estimated repair...

Documents

Transcript of WHAT LEVEE RISK? SIMPLIFIED LEVEE BREACH ANALYSIS FOR … · damages and has an estimated repair...