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Introduction to hydrological extremes Johannes Cullmann, Federal Institute of Hydrology

Introduction to hydrological extremes Johannes Cullmann, Federal Institute of Hydrology. 1 Introduction. Hydrological extremes are natural phenomena

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Introduction to hydrological extremes Johannes Cullmann, Federal Institute of Hydrology

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  1. Introduction to hydrological extremes Johannes Cullmann, Federal Institute of Hydrology

  2. 1 Introduction • Hydrological extremes are natural phenomena • Floods – Dyck (1995): “A flood is the swelling of flow above base flow, restricted in time and characterised by rising above a specific threshold”. • Droughts -Tallaksen and van Lanen (2004) define drought as “a sustained and regionally extensive occurrence of below average natural water availability”. • Hydrological extremes impact on society -> disaster

  3. 1 Introduction

  4. 1 Introduction

  5. 1 Introduction

  6. Source: CRED

  7. Source: CRED

  8. Source: FAO

  9. Source: FAO

  10. 1 Introduction - Characteristics • Drought • Slow process • Large areas affected • Difficult to predict • Drought prevention and management is difficult • Possible economic impact slow but potentially large • Perilous due to consequences (food supply) • Flood • Highly dynamic • Spatially limited • Difficult to predict • Flood mitigation and management is relatively easy • Heavy local economic damages • Perilous due to speed of events and consequences (disease)

  11. 2 Drought types – the scheme Figure: Types of drought Source: Adopted from National Drought Mitigation Centre, and G. Rossi, B. Bonaccorso, A. Cancelliere, (2003)

  12. 2 Describing droughts - Indices Indices • needed for assessment • needed for statistics • What about management? • What about mitigation? Figure: Possible ways of deriving drought characteristics (Hisdal et al., 2004)

  13. 2 What can we do? • Agricultural and Water Resources droughts can be alleviated by management • Infrastructure (water backbone systems, storage units) • Functional management (water distribution, on farm management) • http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0010/163990/on-farm-water-storages.pdf

  14. 2 Drought types - mitigation Figure: Types of drought Source: Adopted from National Drought Mitigation Centre, and G. Rossi, B. Bonaccorso, A. Cancelliere, (2003)

  15. 2 Drought forecasting • Forecasting drought is difficult. Statistical models usually fail to predict extreme events. • Drought forecasting system can be based on global circulation patterns (el Nino) and weather outlooks/long-range forecasts. • Some usefull links: • http://www.drought.unl.edu/ • http://www.ifad.org/lrkm/range/drought.htm • http://climate.mssl.ucl.ac.uk/ • http://www.netweather.tv/index.cgi?action=longrange;sess=

  16. 3 Flood types Coastal flood : • Flooding occurs in coastal zones as a consequence of high tide, backwater effects or tsunami. Riverine flood : • Floods occur in river basins. Inundations : • Flooding of large areas as a consequence of extreme precipitation and/or snowmelt. These floods are slow and easier to predict. Flash floods : • Extremely dynamic flood type. Occurs in small headwater catchments as a consequence of intensive rainfall. Dam failure and ice jams may also cause a flash flood.

  17. 3 Vulnerability and risk Flood Risk - The statistical probability of an event combined with the scale of the potential consequences Vulnerability = Exposure + Susceptibility - Resilience

  18. 3 Describing floods and impacts

  19. 3 Flood mitigation – based on vulnerability Structural measures • Retention (dams and area), dykes, flood channels. • Design based on flood frequency analysis (statistics) or models. Both approaches have shortfalls, especially in areas with scarce data. • http://acwi.gov/hydrology/Frequency/B17bFAQ.html

  20. 3 Flood frequency analysis

  21. 3 Flood mitigation – based on vulnerability • Non-structural measures • Enhancement of infiltration capacity, flood preparedness • Vulnerability analysis, early warning systems (models) • Preparedness, insurance

  22. 3 Flood management – based on vulnerability Societal aspects Technical aspects

  23. Online - Measurement of precipitation Precipitation forecast Q(t,x) P(t,[x,y]) DWD: local precipitation model Rainfall- Runoff Modell Hydrodynamic flood routing modell Precipitation radar Forecast: Flood Discharge Q [m3/sec] Water level h [m] at time t [h] and gauging site x [river km] 3 Flood management – modelling

  24. 3 Planning – the consequences of hazard

  25. Resume • Extremes will always occur • Statistical indices, models and approaches are conditional to stationarity • There is no absolute protection – extremes have to be managed • Warning systems are possible and preconditions for effective management • Society is affected – society should be prepared

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