8 juli 2015

Predicting flooding events on gravel

coasts

Robert McCall, Gerd Masselink, Timothy Poate, Dano

Roelvink

Contents

• Background

• Model development and validation

• Comparison to current empirical model

• Conclusions

Storm waves attack Chesil Beach

2

3

Introduction

• Gravel beaches occur in many high-lattitude areas around the

world (Northern Europe, Russia, North America, Australia & NZ,

Argentina and Chile)

• In the UK approximately 1/3 of the coastline is protected by a

gravel beach or barrier

• Considered sustainable forms of

coastal defence due to ablilty to

absorb large amounts of wave

energy

Chesil Beach

4

Introduction

• Gravel beach failure and flooding does occur …

Chesil Beach, 1979 Waves overtopping Chesil Beach, during Hercules storm

January 2014

5

Objective

• Compared to sandy beaches, relatively few (process-based) model

exist, particularly for simulation of storm impact

• Not many tools available to coastal managers to predict coastal

flooding

• Develop new tools for coastal flooding and assess current tools

o Develop a new process-based storm impact model for gravel

coasts (XBeach-G) based on the XBeach model

o Validate using field and laboratory data and one set of model

parameters (no site-specific calibration)

o Compare XBeach-G to the current standard empirical model

and look for improvements for the empirical model

6

Model description

• New process-based model developed during the project based on

XBeach (Roelvink et al., 2009)

• XBeach-G: process-based depth-averaged model including:

o Waves and currents

o Groundwater (infiltration)

o Sediment transport

o Bed update

• Model results compared to data measured in laboratory experiment

(BARDEX) and collected at four natural gravel beaches

• Validation simulations show good accuracy in simulating (McCall et

al, 2014):

o Groundwater levels and variance

o Wave transformation

o Wave run-up

Model validation

8 juli 2015

Plymouth Rapid Coastal Response Unit BARDEX experiment Storm swell at Loe Bar

7

Model validation: overwash events

• Model shows good prediction of cross-shore profile change during

overtopping / overwash events (McCall et al., 2015)

Measured pre-storm

Measured post-storm

Modelled post-storm

Comparison with empirical model

• Empirical Barrier Inertia Model (BIM; Bradbury, 2000) is widely

used in UK

• Threshold for overwash (flooding) defined by ratio of wave forcing

and cross-sectional area of the barrier

9

Comparison with empirical model

• Empirical Barrier Inertia Model (BIM; Bradbury, 2000) is widely

used in UK

• Threshold for overwash (flooding) defined by ratio of wave forcing

and cross-sectional area of the barrier

3

s

c

m

sw

H

ARBI

L

HS

Hs = significant wave height

Lm = mean wave length

Rc = crest height above SWL

A = cross-sectional area of

the barrier above SWL

wS

BI“safe”

potential for overwash

10

Comparison with empirical model

• BIM was derived from data of one barrier and is only strictly valid

for a range of boundary conditions

• Simple hindcast of 25 documented storm events, including 3

“severe” and 3 “moderate” flooding events

• Impacts based on qualitative descriptions in scientific literature and

popular press

8 juli 2015

Chesil Beach 1979 Slapton Sands 2001

Hurst Spit 1979

11

Comparison with empirical model

• Each event is plotted in the location above or below the line

according to the empirical BIM model

• BIM model predicts 2 out of all flooding events (including one at

the barrier the model was derived for). All others in “safe” area

8 juli 2015

Documented as severe flooding Documented as moderate flooding

12

Comparison with empirical model

• The colour of each event corresponds to the overtopping discharge

predicted by XBeach-G

• XBeach-G predicts overtopping/overwash for all storms:

8 juli 2015

Documented as severe flooding Documented as moderate flooding

Overtopping/overwash discharge: >100 l/s/m >20 l/s/m <20 l/s/m <2 l/s/m

Prediction: 20-100 l/s/m Prediction: 2-20 l/s/m

13

Conclusions

• Empirical model did not correctly predict overwash in majority of

hindcast flooding events

• Empirical model particularly missing description of beach slope

(affecting wave run-up), foreshore profile (affecting wave height)

and grain size / hydraulic conductivity (affecting barrier resilience)

• More care should be taken when applying the empirical model in

flood safety assessment

• Process-based modelling can be used to improve estimates of

flooding events

14

Thank you