1 flood hazard analysis session 1 dr. heiko apel risk analysis flood hazard assessment

1

Flood Hazard Analysis

Session 1

Dr. Heiko Apel

Risk AnalysisFlood Hazard Assessment

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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

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

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Why Care About Floods?


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Basic hydrology

Generation of floods – Extremes in the hydrological cycle

Hydrology Describes the processes in the catchment Provides estimates of flood magnitudes by rainfall-

runoff modeling


Extraordinary rainfall

Excess of retention Capacity of

catchment Accelerated &

increased drainage Excess of drainage

capacity

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Basic hydrology

Flood pathways and additional structural flood causes


Source: The Planning System and Flood Risk Management, Ministry of Environment, Heritage and Local Government, Ireland

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 exceedan

ce flooding

Flooding through the floodplains

Dike or dam breach

Impervious paved area

Urban growth: increased paving

Blockage or sewer collapse

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Flood Types, Causes, and Characteristics

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.


Type Lead Time Duration Velocity

River

Flash Floods Short Short Fast

Flooding due to dam/dike failure

Short Short-Long Slow-Medium

Coastal

Storm Surges Medium-Long Short-Medium Medium

Tsunamis (seismic sea waves)

Short Short Fast

Urban

Drainage Problems Medium-Long Medium-Long Slow

High Groundwater Long Medium-Long Slow

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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 basinsRisk Analysis

Flood Hazard Assessment

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Flood hydrographs

From rainfall runoff modelingor

Statistics on discharge time series


Normalize observed flood hydrographs for comparability

Cluster analysis Characteristic flood

hydrograph Scale to desired flood

magnitude timeN

orm

aliz

ed d

isch

arge

Q

Flood peak discharge

Flood volume

Base flow

1

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Mapping of inundation areas

Spatial presentation of inundation areas for a defined flood event showing maximum of: 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


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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

simple

complex

model setup data requirements

computational demand

Application scale

large

small


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Flood simulation

1D full hydrodynamic Pros

Many software packages available, including free software, e.g. HEC-RAS

Computationally efficient without consideration of hydraulic structures

Cons 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


0 500 1000 1500 2000 2500 3000115

120

125

130

135

140

Mulde_Test1 Plan: Plan 02 08/08/2008

Station (m)

Ele

vatio

n (m

)

Legend

EG Max WS

WS Max WS

Ground

Bank Sta

.035 .11 .035

.11 .033 .035

Cross section over channel and floodplain

Interpolated cross sections

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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

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 reaches)

Dynamic approach (research) Consideration of different failure modes Probabilistic failure determination No predefined failure locations Data and computation intensive

1D-HN Model RIV1H

(www.epdriv1.com,USACE, 1995 )

Dike breach model

Raster-based inundation model

(modified from Apel,Merz, 1996)

Source: S. Vorogushyn 2008

Dynamic probabilistic dike breach modelling system IHAM

13Risk AnalysisFlood Hazard Assessment

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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


Median of maximum inundation depthMedian of maximum inundation depth

9090thth percentile map percentile map



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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

1 flood hazard analysis session 1 dr. heiko apel risk analysis flood hazard assessment

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