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GG3021: Flood Risk Estimation

2. Flood Risk Estimation.

JasminFlorian
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GG3021: Flood Risk Estimation

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    1. 1 GG3021: Flood Risk & Estimation Methods for estimating flood probabilities READING: 1. Jones (1997) Global Hydrology, Chapters 4. Longman. Harlow 2. Ward & Smith (1998) Floods: Physical Processes and Human Impacts. Chapter 6. Wiley. Chichester. L 3. Acreman (ed) The Hydrology of the UK: A study of change. Chapters 1 & 5. Routledge, London. 4. Howe, Slaymaker & Harding (1967) Some aspects of the flood hydrology of the upper catchments of the Severn and Wye. Transactions Institute British Geographers, Publication 41. 5. Malamud, B, Turcotte,D & Barton, C (1996) The 1993 Mississippi Flood: a one hundred or a one thousand year event? Environmental & Engineering Geoscience, II, 4, 479-486.

    2. 2 Flood Risk Estimation ‘To manage flood risk successfully, knowledge is needed of both the magnitude of a given flood and an estimate of the likelihood of this flood occurring’ (Foresight, 2001) This requires the use of PROBABILITY. Methodology: 1. Obtain probability distribution of existing flow magnitudes. 2. Use this to assign probabilities of a given flood size. Issues: Length of flow record, and representation of extreme flows in that record.

    3. 3 Flood records are derived from river gauging stations

    4. 4 Gauging Station Networks in Europe Red Circles: >30 Years record. Note UK. Blue Circles: <10 years record

    5. 5 England: Southern Region Gauging Station Network Source: http://www.ceh.ac.uk/data/nrfa/index.html

    6. 6 Example of Flood Magnitude-Frequency Distribution Most flood magnitude distributions are NOT gaussian (top) but are positive skewed (Lower) In the UK most river flow records are short, 50 years or less. So the problem is that it is unlikely the LARGE (RARE) floods are included in Flow records! UNCERTAINTY……

    7. 7 Flood Risk – Recurrence Intervals

    8. 8 Problems with Annual Maxima Flood series Several floods in a given year may exceed the annual maximum for another year. The above problem is overcome by using a PARTIAL DURATION SERIES of flood events. Need adequate separation of flood peaks, eg 30 days minimum (Malamud et al 1996), or flow must fall >50% below a peak before another peak is chosen. A set number (eg 6 or more) flows may be chosen per year. These are ordered (Largest = rank 1) then the largest N (corresponding to N years of record) are taken for the partial duration series. OR…. Set Threshold = Q cumecs and select ALL flows in a year greater than the threshold. Rank as for Annual Max Series and take N flows for N years data.

    9. 9 Flood Probability 1. Flood Probability in any ONE year = 1 / (Return period) Eg: Flood Return period = 10 years Thus Probability = 1/10 = 0.1 or 10% probability in any year. Foresight (2001) notes that flood return periods often lead to complacency in general populations because the PROBABILITY is not stressed, rather the recurrence interval. 2. Mean Annual Flood (MAF) for n year flow record = Sum(Annual Max Discharges) / n. 3. Probability of a flood with Recurrence Interval of T years, occurring, or being exceeded, in the next n years is: 1- (1-1/T)n Eg Probability of 1 in 100 year flood in the next 5 years is: 1-(1- 1/100)5 = 1-0.995 = 0.049 (eg almost 1/20 chance)

    10. 10 Typical recurrence interval-discharge plots

    11. 11 Problems with recurrence interval curves different distributions to describe a flood record give different estimates of flood magnitude for a given return period. (Malamud et al (1996)) July 10 ’93 event 12300 m3 /s: changes the recurrence interval estimate for each method Different distributions give different Recurrence interval estimates

    12. 12 Flow-Duration Curves Exceedance Probability for a flow (P) = 100/T % (where T is Recurrence Interval) Flow Duration curve plots Flow Discharge vs P Shape of curves are indicative of catchment response to rainfall. Steep curve: Variable flow little storage and flow reflects rainfall pattern small mountainous catchment Shallow Curve: Damping of flow variation. STORAGE eg in porous materials eg Chalk, Limestone wide well-developed floodplains Flow less dominated by rainfall pattern unless catchment saturated

    13. 13 Flow Duration Curves: R.Tamar, Cornwall

    14. 14 Flow Duration Curves for UK Rivers: Yr 2000. The Blue lines are for 2000. Black Lines are 1921-1990 average duration curve. Note: 2000 data all plots at HIGHER discharges for given flow frequency

    15. 15 Reconstruction of past floods: Palaeostages

    16. 16 Adding new events to the flood frequency curve

    17. 17 Probabilistic Flood Estimation: Recurrence Intervals - Assumptions Independence of events– Probably OK for Annual Maximum Series? (May be problem for Peaks over Threshold series). Floods are random quickflow events to which an underlying statistical distribution can be fitted. STATIONARY SERIES– no trend or periodicity (or trend is explicable and can be used as a model for the future) – NON-STATIONARITY – Climate Change, Land use change Each flood is from the same underlying population. May be difficult to tell for a flow record. Floods could be due to a COMBINATION of factors eg 25yr rainfall & 40 year groundwater level => 1/1000 year event due to the COMBINED probabilities (Klemes). Chichester Floods.

    18. 18 INDEPENDENCE OF EVENTS (Jones 1998)

    19. 19 SINGLE POPULATION: Cold Climate Rivers: Mixed flood populations due to multiple flood generating processes (Church, 1988)

    20. 20 NON-STATIONARITY

    21. 21 Problems with recurrence intervals: NON STATIONARITY. Mississippi River- Changes in recurrence intervals due to climate variability (Knox).

    22. 22 Flood Growth Curves: Methodology Take a series of recurrence intervals from the recurrence interval plot: eg 2,5,10,20,40,50,100 year floods Use the recurrence interval plot to find the Discharge (or stage) for each recurrence interval. Divide each discharge by the Mean Annual Flood eg: Q5/QMAF, etc. Plot against the recurrence intervals: This gives the GROWTH CURVE.

    23. 23 GB Regions for flood growth analysis Regions are assumed ‘homogeneous’ in terms of their flood hydrology. Figures are runoff coefficients (%rainfall as runoff in river).

    24. 24 Regionalising the Flood Record Growth Curves for the UK regions based on local flood records. Y-axis = Qri/Qmaf. Regions 5-7 are relatively urbanised regions. Problem of ‘homogeneity’ ..does a catchment produce runoff in a similar way to its neighbour? Also- problems of land use change…so how long are these curves viable?

    25. 25 Regional Flood Growth Curves: Global Variation

    26. 26 Regional Growth Curves Scottish Regional Growth Curves. Implications of climate change on flood hydrology and the regional growth curve. (Source: Scottish EPA)

    27. 27 Envelope Curves for ‘MAXIMUM’ FLOOD DISCHARGES Requires many flow records for different sized catchments. Main problem: Often based on flow records may EXCLUDE extreme events as most records are relatively short. Alternative data: Flood mark data may be included to ‘extrapolate’ the data. Datable sedimentary evidence eg slack water deposits

    28. 28 UNGAUGED RIVERS: Plot Mean Annual Flood versus drainage area (an easily measured catchment variable) for gauged rivers in an area.

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