Formulation of a New Breach Model for Embankments

IMPACT Project Workshop, Wallingford 2002

Breach Formation Theme

Introduction

Do we need a

new model?

The PhD Project

• Started on 1st of October 1998

• Ended on the 31st of January 2002

• Objectives:– Improve the understanding of the process

embankment breaching.– Develop a model based on that understanding

The HR Breach Model

• Lateral and Longitudinal Breach Growth

• Flow Equations

• Sediment Transport Equations

Lateral Growth (existing models).

• Uniform Erosion

• Constant Shape.

• Top Width Adjustment• Constant bottom width

• Most stable section

• Assuming a shape

• Slope Instability (some)

Breach Growth (Real Cases 1)

Breach Growth (Real Cases 2)

Breach Growth (Real Cases 3)

Breach Growth (Physical Modeling at HR)

Growth Mechanisms

• Continuous Erosion

– Computed using a sediment transport equation.

• Mass Failure

– Shear failure

– Bending failure

Shear Failure (1)

Shear Failure (2)

s

b

WW

HHHdcFOS

tan*'tan)('* 321

Bending Failure (1)

Bending Failure (2)

2012)( /6/)( dMdHHactualt

Longitudinal Breach Growth(New Model)

• The sediment transport continuity equation.

qx

Qs

t

Ap

)1(

Sediment Transport Equations

• Difficult to decide which one?

• Selected Equations:– Yang 1979 (Non Cohesive)– Visser 1998 (Non Cohesive)– Chen and Anderson 1986 (Non Cohesive)

Flow Equations

• Weir Equation• Flow over the crest and on the downstream face

• Bernoulli Equation• Hydraulic calculations on the crest and the

downstream face

Material Uncertainty

Factor of Stability Probability Distribution Functions

0

0.25

0.5

0.75

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Factor of stability

Pro

bab

ity o

f fai

lure

Poor

Good

V. Good

DeterministicApproachEqual Probability

low

er li

mit

Up

pet

lim

it

Composite Embankments (1)

Composite Embankments (2)

• Core Failure Modes:– Sliding– Overturning– Bending

Piping (1)

Piping (2)

• Processes– Flow through the pipe– Pipe widening and deepening– Failure of the material above the pipe– Failure of the top of the dam– Similar processes to overtopping

Numerical Model Results

Case Study (1)

Case Study (1) Comparison

Case Study (2) Central

Case Study (2) Side

And .. Finally

• An acceptable approach has been achieved for a new breach model

• More research is needed