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

2. 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

3. 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

4. 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 • Hydrology • Describes the processes in the catchment • Provides estimates of flood magnitudes by rainfall-runoff modeling Risk Analysis Flood Hazard Assessment

5. 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 Dike or dam breach Surcharge sewer causes basement flooding Direct overland flow and ponding in low pits (sinks) Flooding through the floodplains Sewer exceedance flooding Urban growth: increased paving 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

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

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 • Normalize observed flood hydrographs for comparability • Cluster analysis • Characteristic flood hydrograph • Scale to desired flood magnitude Risk Analysis Flood Hazard Assessment

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 Complexity Application scale simple large • 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 complex model setup data requirements computational demand small Risk Analysis Flood Hazard Assessment

11. Flood simulation Cross section over channel and floodplain • 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 Interpolated cross sections Source: HEC-RAS user manual Risk Analysis Flood Hazard Assessment

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 km2 Risk Analysis Flood Hazard Assessment Source: Apel et al. 2009

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

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

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