Flood Risk Analysis

1
Flood Risk Analysis
Dr. Brett A. Bryan
2
Overview

• Aspects of flooding flood risk
• Monitoring
• Prediction
• Flood Risk Analysis
• Northern Adelaide Plains
• Northern Spencer Gulf

3
Flood Hazards

• Two main types of flooding
• Inland flooding
• Inundation
• Snow melt
• Watershed drainage system overload
• El Nino
• Natural processes
• Coastal flooding
• Storm surges
• Impacts of sea-level rise
• Secondary effects erosion, slip, mudslides etc.

4
Importance and Protection

• Flooding occurs primarily near water bodies
• People live primarily near water bodies
• Enormous potential impacts
• Property damage
• Loss of life
• Agricultural ecological effects
• Land degradation
• Great Mississippi Flood of 1993, 12 billion
  dollars, 50 lives
• Many activities aim to reduce the impact of
  floods
• Levees, dams, stream regulation
• Coastal walls etc.

5
Photos of Flood Impacts
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Flood Monitoring

• Real time information required
• Rapid decision-making process
• All sectors require sound, appropriately detailed
  information

• Is this sort of spatial information available?
• Needs to be coupled with
• Stream guage data
• Meteorological predictive data

7
Global Flood Monitoring

• Dartmouth Flood Observatory http//www.dartmouth.e
  du/artsci/geog/floods/

8
Australian Flood Monitoring

• Australian flood monitoring not so prolific
• Western Australia has an online system
• None for SA Why?

9
Flood Prediction Forecasting

• Flood forecasting through weather forecast
  information Bureau of Meteorology
• MIKE 11 Flood Watch
• a management system for real-time flood
  forecasting and warning
• Danish Hydrological Institute http//www.dhi.dk
• DSS combining an advanced time series database
  with the MIKE 11 FF hydrodynamic modelling and
  forecasting system and ArcView GIS

10
Mike 11 Screenshots
11
EarthSats FloodThreat

• FloodThreat - EarthSat Corporation
• http//www.earthsat.com/flood/floodthrt.html

12
Flood Risk Analysis

• Aims of Flood Risk Analysis
• Identify areas at risk from impacts of
  floods/SLRise
• Possibly, estimate damage from potential impacts
• Spatial technology (GIS,RS) integral to FRA
• 2 main techniques for FRA
• Inference extent of future flooding inferred
  from historical flood extents e.g. using RS, old
  maps
• Modelling prediction of extent of flooding from
  spatial data modelling e.g. topography

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Flood Risk Analysis - Inference
14
Ataturk dam
Birecik Dam on the Euphrates River in southeast
Turkey and northwest Syria Began filling at the
end of April, 2000 By May 18, 2000 the waters of
the Euphrates were rising by 30 centimetres a day
and had already submerged the villages of Belkis
and Apamee (from Dartmouth Flood Observatory)
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The Great Flood of 93 - Mississippi
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Northern Adelaide Plains

• Problem Gawler river prone to flooding
• Several agencies interested in flood risk
  analysis
• Local councils Mallala
• Insurance agencies
• State government - disaster management
• Last 2 major floods
• 1987 1 in 50 year flood
• 1993 1 in 100 year flood
• Extent of both mapped from aerial photography
• Other data also available contours, drainage.
  But

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Study Area

• Northern Adelaide Plains study area

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Northern Adelaide Plains

• 1 in 50 year flood 1987

19
Northern Adelaide Plains

• 1 in 100 year flood 1993

20
Northern Adelaide Plains

• Other available data

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NAP - Flood Risk Analysis

• Flood risk analysis for Mallala council
• What land parcels are at risk from flooding of
  the Gawler river?
• What land capital is at risk?
• How are flood risk areas related to current
  planning zones?
• Methods
• Overlay flood risk zones with cadastre and
  planning zones
• Assess area statistics and spatial distribution
  of risk zones

22
Cadastre and Land Value

• Relational database joins
• DCDB, valuation data

• Many to one
• One to many

23
Land Value
24
Flood Risk and Land Parcels
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Flood Risk and Planning Zones
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Flood Risk Analysis - Modelling

• Digital elevation models (DEMs) are a fundamental
  data source
• Flood risk generally depends on elevation of land
  surface
• The lower the topography the higher the risk of
  inundation

• Many studies have used DEMs to create flood risk
  maps based on DEMs
• E.g. USGS study in Washington state

27
Flood Risk Analysis - Modelling

• Problem assess coastal vulnerability to
  sea-level rise
• Study area Northern Spencer Gulf, SA
• Objective identify areas at risk of inundation
  and erosion from sea-level rise
• Methods
• Coastal geologic mapping
• Distributed coastal process modelling

28
The Study

• Broad scale analysis
• Northern Spencer Gulf
• Fine scale analysis
• False Bay

See Harvey, Belperio, Bourman Bryan (2000) A
GIS-BASED APPROACH TO REGIONAL COASTAL
VULNERABILITY ASSESSMENT USING HOLOCENE
GEOLOGICAL MAPPING OF THE NORTHERN SPENCER GULF,
SOUTH AUSTRALIA. In Asia-Pacific Journal of
Environment and Development.
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Geologic Mapping

• The premise
• Homogeneous coastal geological/ ecological units
  experience similar physical environmental
  conditions
• Similar coastal units have similar vulnerability
  to sea-level rise
• Coastal geologic units mapped from aerial
  photography
• Digitised into ArcInfo polygon database
• Attributed through intensive field survey
• 14 geological units identified in NSG

30
Geologic Mapping

• Distribution of 14 coastal geological/ecological
  zones in the NSG
• Elevation sequence of units based on 2 levelled
  transects

31
Geologic Mapping
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Coastal Vulnerability/Risk Zones

• 5 risk zones defined from coastal geological units

33
3D Visualisation of Risk Zones
34
False Bay

• Hi-res elevation data captured for False Bay
  using photogrammetry
• DEM built using TIN model
• Coast-normal elevation transects taken to assess
  elevation of risk zones

35
Elevation of Risk Zones

• Coastal vulnerability has a clear relationship
  with elevation

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Discussion

• Hi-res elevation data restrictive in extent
  cost
• Geological units provide good surrogate for
  physical coastal processes
• Levelling provides good indicator of elevation
  vulnerability
• Holocene geological mapping suitable technique
  for regional coastal vulnerability assessment
• Simple, accessible, relatively cheap
• Restricted to tide-dominated sedimentary coasts

37
Summary

• Analysis of floods using SIS includes monitoring,
  prediction and FRA
• Many examples of montoring, prediction and
  analysis of floods using many different data
  types and analytical frameworks - WebGIS, RS, GIS
• FRA is a useful tool for both inland and coastal
  assessment
• Northern Adelaide Plains study provided an
  example of the inference of flood risk from
  existing data
• NSG study provided an example of modelling of
  flood risk