iahr 2015 - predicting flooding events on gravel coasts, mccall, deltares, 30062015
TRANSCRIPT
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
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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
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