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Flood Hazard Analysis
Session 1
Dr. Heiko Apel
Risk Analysis
Flood Hazard Assessment
1
Learning objectives
 Learn

Terminology, definitions and key concepts of flood hazard analysis

Flood hazard mapping procedure
 Understand

The hydrological cycle and the main causes of floods

The different types and characteristics of floods

The basics of flood modeling

The impacts of dike failures on flood hazard

The basics of climate change impacts on floods
Risk Analysis
Flood Hazard Assessment
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Why Care About Floods?
 Second most frequent natural disaster
 Floods are occurring more frequently resulting in increasingly large
losses
 The total damage caused by minor and medium floods can be as high
as the total damage caused by major floods
Risk Analysis
Flood Hazard Assessment
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Basic hydrology
 Generation of floods – Extremes in the hydrological cycle
Extraordinary rainfall
Excess of retention
Capacity of catchment
Accelerated &
increased drainage
 Excess of drainage
capacity

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 Hydrology
 Describes the processes in the catchment
 Provides estimates of flood magnitudes by rainfall-runoff
modeling
Risk Analysis
Flood Hazard Assessment
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Basic hydrology
 Flood pathways and additional structural flood causes
Overland runoff and
muddy flooding due
to intensive rainfall
Groundwater
flooding due to
raised water table
Surcharge sewer
causes basement
flooding
Direct overland
flow and ponding
in low pits (sinks)
Sewer
exceedance
flooding
Urban growth:
increased paving
Dike or dam
breach
Flooding
through the
floodplains
Impervious paved area
Blockage or sewer collapse
Source: The Planning System and Flood Risk Management,
Ministry of Environment, Heritage and Local Government, Ireland
Risk Analysis
Flood Hazard Assessment
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Flood Types, Causes, and Characteristics
Type
River
Lead Time
Duration
Velocity
Flash Floods
Short
Short
Fast
Flooding due to
dam/dike failure
Short
Short-Long
Slow-Medium
Storm Surges
Coastal
Medium-Long Short-Medium
Tsunamis (seismic sea
waves)
Medium
Short
Short
Fast
Drainage Problems
Medium-Long
Medium-Long
Slow
High Groundwater
Long
Medium-Long
Slow
Urban
Short refers to less than one day; Medium refers to between one day and one week; Long refers to more than one week.
Slow refers to less than 1 m/s; Medium refers to between 1 m/s and 2 m/s; fast refers to greater than 2 m/s.
Risk Analysis
Flood Hazard Assessment
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Flood magnitude
Estimates of flood magnitude can be determined using one of two
methods:
 Rainfall-runoff modeling
 Frequency analysis
 In principle: estimation of the probability of occurrence of a flood event
of a given magnitude (maximum discharge)
 Standard method: Extreme Value statistics
 Fitting a distribution function to a time series of discharges, extrapolate
from observations to extreme events (Caution: large uncertainties!)
 Reach scale risk assessments: heterogeneity of flood probability
 Different probabilities of occurrence for different reaches in the same event
(regional flood frequency analysis)
 Influence of dike breaches on downstream flood magnitude and probability
(probabilistic & dynamic dike failure modeling)
 Large scale risk assessments
 Correlation of floods in different basins
Risk Analysis
Flood Hazard Assessment
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Flood hydrographs
 From rainfall runoff modeling
or
 Statistics on discharge time series



Normalize observed flood
hydrographs for comparability
Cluster analysis
Characteristic flood hydrograph
Scale to desired flood magnitude
Flood peak discharge
1
Normalized discharge Q

Base flow
Risk Analysis
Flood Hazard Assessment
Flood volume
time
8
Mapping of inundation areas
 Spatial presentation of inundation areas for a defined
flood event showing maximum of:

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Inundation extend (A)
Inundation depths (h)
Flow velocities (v)
Intensity index (h*v)
Inundation timing
Inundation duration
 These values are derived from hydraulic modeling
 Use GIS to visualize inundations and risk
assessments
Risk Analysis
Flood Hazard Assessment
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Flood simulation
 Computational hydraulics approaches:
 1D hydrostatic
 1D hydrodynamic simplified (kinematic,
diffusion wave)
 1D full hydrodynamic
 1D/2D simplified hydrodynamic
 1D/2D full hydrodynamic
 2D full hydrodynamic
 3D full hydrodynamic
Complexity
Application scale
simple
large
complex
small
model setup
data requirements
computational demand
Risk Analysis
Flood Hazard Assessment
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Flood simulation
 1D full hydrodynamic
Cross section over
channel
floodplain
Mulde_Test1
Plan: Plan and
02 08/08/2008
.035
140
 Pros
.11
.
0
3
5
.11
.033
.035
 Many software packages available, including free
software, e.g. HEC-RAS
 Computationally efficient without consideration of
hydraulic structures
Elevation (m)
135
130
125
120
115
 Cons
0
500
1000
1500
2000
2500
Station (m)
Interpolated cross sections
 No representation of 2D floodplain flow
 Derivation of cross sections time consuming
 Interpolation to inundation areas
 Application
 River reaches with confined floodplains and
parallel to the river
 Large scale
Source: HEC-RAS user manual
Risk Analysis
Flood Hazard Assessment
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30
Flood simulation
 2D full hydrodynamic
 Pros
 Detailed process description
 Precise calculation of h and v in areas with complex flow patterns
 Realistic representation of floodplain processes, well suited for urban
environments
 Mostly commercial software
 Cons
 Computationally demanding
 Setup of computational mesh
 Mostly commercial software
 Application
 Small scale, up to 500 km2
Risk Analysis
Flood Hazard Assessment
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Source: Apel et al. 2009
Failure of dikes
 Failure of dikes or dams cause severe inundations
 Old dike systems need special attention
 Dike failure is difficult to incorporate in Flood Risk
Assessments
 Static approach (the usual way)
 Definition of breach scenarios (location, timing, breach width)
 Sufficient for small scales (e.g. a town) but not for larger scales (e.g. river
Dynamic probabilistic dike breach
reaches)
 Dynamic approach (research)

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Consideration of different failure modes
Probabilistic failure determination
No predefined failure locations
Data and computation intensive
modelling system IHAM
1D-HN Model
RIV1H
Risk Analysis
Flood Hazard Assessment
(www.epdriv1.com,
USACE, 1995 )
Dike breach model
Raster-based
inundation model
(modified from Apel,
Merz, 1996)
Source: S. Vorogushyn 2008
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Failure of dikes (cont.)
 Output of probabilistic dike breach and flood hazard
assessment:
 Dike failure probabilities (global and per failure mode)
 Spatially differentiated inundation probabilities
 Spatially differentiated inundation depths, velocities, duration,
and intensity with uncertainty estimates
th
10 percentile map
Median of maximum inundation depth
90th percentile map
Source: S. Vorogushyn 2008
Source: S. Vorogushyn 2008
Risk Analysis
Flood Hazard Assessment
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Climate change and floods
 Long term flood mitigation and management plans should take into
account climate change and floods
 Temperature increase leads to intensification of hydrological cycle
 Global increase in temperature of estimated 2.8 – 5.2 °C leads to a
global increase in evaporation and precipitation: 7 – 15%
 Increasing probability of extreme events
 Regional differences
 Large spatial and seasonal variation, high uncertainty
 Differences have been observed in discharge time series (non-stationary approaches
needed!)
 Global climate change scenario simulations, downscaling procedures
and hydrological models can estimate regional variation
 But uncertainty for flood projections, especially magnitude, very large
Risk Analysis
Flood Hazard Assessment
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