Concepts and Approaches to Flood Risk and its Management

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Concepts and Approaches to Flood Risk and its
Management

• Mark Morris, Managing Director (Ireland)

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Catchment data and review

• What data would you need to start a flood mapping
  exercise?
• What is readily available to you in your role?
• Who or where else could you obtain data from?

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Overview
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Mullingar South
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Process Overview
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Catchment review

• Site visit to help determine
• Slope and topography
• Distance to watercourse
• Size of river
• Ground conditions
• Likely flow routes / obstructions
• Cross section positioning

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Catchment review

• Flood history
• www.floodmaps.ie
• Locations (and extent where possible) of past
  floods
• Local Authority / OPW records and reports
• Benefiting lands maps
• Newspapers and journals
• Local knowledge

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Other Useful Datasets

• GSI alluvial soils maps
• Digital ground model OSi and/or local
  topographic survey
• Coastal floodmaps where available (contact point
  is OPW)
• Broadscale fluvial floodmaps where available
• Corine land cover maps from the EPA
• Annual and extreme rainfall data from Met
  Éireann
• Hydrometric station data EPA, OPW, Local
  Authority and ESB
• Previous models and studies
• Other info as may be available from OPW

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Topography can tell you a huge amount
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Floodplain data
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Overview
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Hydrological Assessment

• What information about the catchment is needed to
  calculate river flows?
• What calculation methods could be used?
• What sort of allowance should be made for climate
  change?

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Return Period and Probability

• Return period, T
• average interval between years containing one or
  more floods exceeding a flow Q
• Annual exceedance probability, AEP
• probability of flood greater than Q occurring in
  any year
• AEP 1/T
• AMAX
• annual maximum flow
• QBAR
• Average of the AMAX flow for the record length

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Hydrological Cycle
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Speed of response

• Depends on
• soils
• geology
• topography
• catchment size and shape
• wetness of soils
• very important for flood warning
• small rivers (urban) can respond in minutes
• large rivers can be watched for days

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The Impact of Urbanisation

• Natural Catchment

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The Impact of Urbanisation

• Natural Catchment

• Developed Catchment

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Catchment Characteristics
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Hydrological DTM
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Average Annual Rainfall
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Soil Map, supplemented by local data where
possible
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CORINE land cover map urban areas in red
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Waterbodies
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Improvements to Hydrology of Soil Type (HOST)
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Hydrological Method
Choice depends on catchment characteristics
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Hydrological Method
Choice depends on catchment size
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Flood Studies Update

• Replacing FSR
• Due out early 2010
• Best European practice
• Design flows will be more accurate
• Free website for carrying out calculations
• Hydrograph shapes fitted to match peak flows
• Only one method for estimation of peak flows in
  river catchments
• Potential for automation
• Downside potential for misuse by people who
  dont understand hydrology and want a quick
  answer

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Importance of local data

• Say the correct value of QMED is 10 m3/s
• 95 confidence limits for QMED
• from catchment descriptor model (original)
• 4.2 - 24.0 m3/s
• from 10 years of gauge data
• 7.9 - 12.8 m3/s
• from 20 years of gauge data
• 8.6 - 11.7 m3/s
• i.e. within 17 of correct answer

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Things to watch out for

• Flood flow estimate is usually the largest area
  of uncertainty
• Need to always check catchment boundaries and
  areas (even with digital data)
• Look for local data and look again
• Try more than one method

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• Having calculated flow, how do you derive a water
  level?

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Process Overview
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Steady Models
Backwater Model (steady)

• Flow can vary in space (but not time)
• Only require peak flow rates (no hydrograph)
• Inherently conservative
• Best uses
• Flood risk assessment
• Flood mapping
• Design work

Outputs
Inputs
Q
H
H
Q
H
Q
Q H
H
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Routing Models

• Used for transferring a flow hydrograph through a
  catchment.
• Calculates flow at each cross section.
• Generates a water level at the downstream end.
• Useful for flood forecasting

Routing Model (unsteady)
Inputs
Outputs
QT
QT
QT
QT
HT
QT
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Hydrodynamic Models

• River flow and level can change with time
• They are best for
• Looking at flood storage
• Tidal rivers with significant reverse flow
• Pumped systems
• Systems with time-based controls
• Rivers where there is very good survey, gauge
  data (to improve flow estimates) and calibration
  data

Hydrodynamic Model (unsteady)
Inputs
Outputs
QT HT
QT
QT HT
QT HT
QT HT
HT
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Model Dimensions

• 1D - Most widely used
• 2D - Increasingly used since advent of cheap DTMs
• 3D - Largely research/ sediment/water quality
  work

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1D Models

• Based on nodes or cross-sections

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Upstream Boundary
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Channel data
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Floodplain data
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Downstream Boundary

• Lough Ennel Westmeath CC level gauge

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2D Models
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Choosing a Model

• How do people go about choosing a car
• Looks?
• Performance?
• Economy?
• Safety?

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Model Uncertainty
1-2 m
Estimated water level
Numerical Uncertainty Choice of
model Topography Flow estimate
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Process Overview
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Flood Mapping
Water levels
GIS Interpolation routine
Flood Outline
DTM
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Flood Outline
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Visualisation
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Where to go for help

• There is plenty of advice available
• The OPW may be contacted for guidance on data and
  general advice
• Mark Morris mark.morris_at_jbaconsulting.ie
• JBA Consulting www.jbaconsulting.ie
• Flood risk management seminars
  www.engineersireland.ie