1 Flood Hazard Analysis Session 1 Dr. Heiko Apel Risk Analysis Flood Hazard Assessment

2 Learning objectives Risk Analysis Flood Hazard Assessment  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 3 Why Care About Floods? Risk Analysis Flood Hazard Assessment

4 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 Risk Analysis Flood Hazard Assessment  Extraordinary rainfall  Excess of retention  Capacity of catchment  Accelerated & increased drainage  Excess of drainage capacity

5 Basic hydrology  Flood pathways and additional structural flood causes Risk Analysis Flood Hazard Assessment 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 exceedance flooding Flooding through the floodplains Dike or dam breach Impervious paved area Urban growth: increased paving Blockage or sewer collapse

6 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. Risk Analysis Flood Hazard Assessment TypeLead TimeDurationVelocity River Flash FloodsShort Fast Flooding due to dam/dike failure ShortShort-LongSlow-Medium Coastal Storm SurgesMedium-LongShort-MediumMedium Tsunamis (seismic sea waves) Short Fast Urban Drainage ProblemsMedium-Long Slow High GroundwaterLongMedium-LongSlow

7 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

8 Flood hydrographs  From rainfall runoff modeling or  Statistics on discharge time series Risk Analysis Flood Hazard Assessment  Normalize observed flood hydrographs for comparability  Cluster analysis  Characteristic flood hydrograph  Scale to desired flood magnitude

9 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 Risk Analysis Flood Hazard Assessment

10 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 Risk Analysis Flood Hazard Assessment

11 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 Risk Analysis Flood Hazard Assessment Cross section over channel and floodplain Interpolated cross sections

12 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 km 2 Source: Apel et al Risk Analysis Flood Hazard Assessment

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 Source: S. Vorogushyn 2008 Dynamic probabilistic dike breach modelling system IHAM 13 Risk Analysis Flood Hazard Assessment

14 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 Source: S. Vorogushyn 2008 Median of maximum inundation depth 90 th percentile map 10 th percentile map Source: S. Vorogushyn 2008 Risk Analysis Flood Hazard Assessment

15 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