NUI Galway – UL Alliance First Annual ENGINEERING AND - ARAN ...

Modelling Extreme Flood Events due to Tides, Rivers and Storm Surges
Joanne Comer, Michael Hartnett
Department of Civil Engineering/ECI, National University of Ireland, Galway.
j.comer1@nuigalway.ie
Abstract
Flooding in towns and cities is commonplace in
Ireland, due to both coastal and river flooding. It is
anticipated that in the future this problem will increase
due to the effects of climate change. Flooding of coastal
hinterlands can be caused by a number of influences:
high river flow, high tides, storm surges or a
combination of any or all of the above. The dependence
between such factors as river flow, tides, and storm
surges is a pertinent point of research. It must be
known how these factors interact together before
flooding can be accurately predicted. To model the
effects of such flooding a complete modelling solution is
sought, this shall include a 3D ocean model to predict
storm surges, a 2D estuary model which uses input
from the surge model to model water levels in the
harbour, and a floodplain model used to assess water
levels within the area concerned, this project will focus
on Cork harbour.
1. Introduction
Coastal flooding is a function of different physical
phenomena therefore this research aims to investigate
the interactions between tides, river flows and storm
surges using extreme value joint probability theory.
The main aim of the research is to develop a modelling
system that will accurately predict coastal flooding due
to the joint probability of these three processes.
Model results will be used in conjunction with
statistical theory in two ways. Firstly, based on the
independent occurrences of flooding from tide, storm
surge and river flow joint probabilities will be derived.
Secondly, past extreme events will be modelled using a
detailed coastal model in which all three processes are
included then combined probability models will be
developed for extreme coastal flooding events. The
results from both approaches will be critically analysed
and compared. Finally, recommendations will be made
regarding the most appropriate model to be used. It is
also likely that different models may suit different
coastal regions; this will be investigated and again
recommendations made.
This research consists of two separate work
packages:
1. Investigate the dependence between river flows,
tides and storm surges, and thus their joint probabilities
of occurrence.
2. Develop a combined 3D, 2D and 1D model in
order to accurately predict water levels in Cork City
from a combined combination of storm surges, tides
and river flows.
The results of the two work packages will be
combined to produce flood risk and extent maps.
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2. Joint Probability Analysis
Joint probability analysis is an important area of
research within flood risk studies however it is a subject
that has not produced many studies when used on tides,
surges and river flows, especially in Ireland. A small
number of studies have been carried out in the UK with
the development of a best practice approach based on
methods used [1]. Research of this nature is very sitespecific,
there is no general model, and therefore
significant work must be carried out in order to obtain
results for a particular area.
The joint probabilities method is used to determine
the dependence of two or more variables on each other.
It determines the likelihood that if one variable is above
a certain threshold, then at the same time the other
variable will also be over its threshold. The thresholds
for each variable are set out initially and represent
extreme events.
3. Modelling
This research proposes to develop a complete
modelling system for coastal flood prediction. This will
be achieved using a 3D ECOMSED model of the North
Atlantic Ocean to produce accurate predictions of storm
surges within the relevant coastal area. ECOMSED is a
three-dimensional hydrodynamic, wave and sediment
model [2]. The coastal area of Cork Harbour will be
modelled in 2D using a nested DIVAST model. The
numerical model DIVAST (Depth Integrated Velocities
and Solute Transport) is a two-dimensional time variant
model, developed for estuarine and coastal modelling
[3]. These models will be linked with LISFLOOD-FP,
this is a two-dimensional hydrodynamic model
designed to simulate floodplain inundation over
complex topography [4], high resolution topography of
Cork city will be represented in the model using 2m
resolution LiDAR data.
4. References
[1] Hawkes and Svensson (2005). ''Joint Probability:
Dependence Mapping and Best Practice''. Defra /
Environment Agency R&D Technical Report FD2308/TR1.
[2] Blumberg and Mellor (1987). ''A Description of a Three-
Dimensional Coastal Ocean Circulation Model. In: Three-
Dimensional Coastal Ocean Models.'' American Geophysical
Union.
[3] Falconer, Struve, Wu (1998). ''DIVAST Manual''. Cardiff
Univeristy, Cardiff School of Engineering.
[4] Bates, Horritt, Wilson and Hunter (2005). ''LISFLOOD-FP
User Manual and technical note''. University of Bristol.