FLOOD FORECASTING

1. ERT 246 Hydrology & Water Resources Eng. FLOOD FORECASTING

4. Satellite flood image: http://www.crisp.nus.edu.sg/coverages/floods2007/index_p2.html

5. Why flood happen? • Occurs when the level of a body of water exceeds its natural or artificial confines • Water then submerges land in surrounding areas

6. Causes of Flooding • Unbalance in hydrologic cycle • Very high precipitation gives floods • Very low precipitation gives droughts • Often combined effects: • Snow melt • Inadequate drainage • Water-saturated ground • Dam failures • High tides

7. River Flooding • Stage—height of a river • Bankfull stage (or flood stage)—when a river’s discharge increases to fill channel completely • Flood—water exceeds river’s banks

8. Floodplain • Area surrounding river influenced by flooding • Typically broad and flat, built of fine silt and mud from floodwaters • Usually very good agricultural land • Eg.: Mississippi River floodplain covers 80,000 square kilometers

9. Main features of a river valley

10. Upstream Flooding • Intense, infrequent storms of short duration • Cause flooding that is severe but local in extent • Called “upstream flooding” because effects of the storm runoff usually do not extend to the larger streams further downstream

11. Upstream floods are generally local, with short lag times

12. Flash Floods • Floods with exceptionally short lag time • Peak discharge reached only hours or minutes after storm has passed • Deadly

13. Downstream Flooding • Usually from storms that last a long time and extend over large area • Total discharge increases downstream as tributaries collect floodwaters

14. Downstream floods are regional in extent with longer lag times and higher peak discharges.

15. Examples of Flood Hazards • Primary Effects • Water damage to household items • Structural damage to buildings • Destruction of roads, rail lines, bridges, levees, boats, barges • Historical sites destroyed • Crop loss • Cemeteries flooded, graves disrupted • Loss of life

16. Examples of Flood Hazards • Secondary and Teritiary Impacts • Destruction of farmlands • Destructions of parklands and wildlife habitat • Health impacts • Disease related to pollution • Injuries (back, electric shock, etc.) • Fatigue • Stress, depression

17. Examples of Flood Hazards • Disruption of transportation/electrical services • Gas leaks • Lack of clean water

18. Secondary, Tertiary, continued • Impacts on crop prices; food shortages • Job loss and worker displacement • Economic impacts on industries • Construction (beneficial impact) • Insurance (negative impact) • Legal (beneficial impact) • Farming (negative impact) • Misuse of government relief funds • Changes in river channels • Collapse of whole community structures

19. Flood Forecasting • To estimate the magnitude of flood peak, the following alternative methods are available: • Empirical Formula • Rational Method • Frequency Analysis

20. Empirical Formula • Q = CAn • Where • Q=Maximum flood discharge • A=Catchment Area • C=Constant that depend on catchment & precipitation • n=Index • Rarely used

21. Rational Method • Q = C i A • Where • Q=peak discharge (m3/s) • C=coefficeint of runoff • i = mean intensity of precipitation (mm/hr) for duration equal to tc • A=drainage area,km2 • To compute Q, requires tc,i and C

22. Rational Method • For small size (<50 km2) catchments • This not cover what is MSMA (Manual Saliran Mesra Alam Malaysia)/Urban Stormwater Management Manual.

23. Rational Method

24. Runoff Coefficient

25. Runoff Coefficient • Coefficient that represents the fraction of runoff to rainfall • Depends on type of surface • When a drainage area has distinct parts with different coefficients… • Use weighted average

27. Time of concentration, tc • For small drainage basin, tc=tp • For other catchment, use Kirpich Equation (1940) tc=0.01947 L0.77S-0.385 • tc in min • L= maximum length of travel time in m • S= slope catchment = ∆H/L • ∆H = difference of elevation between the most remote point on the catchment and the outlet

28. Time of concentration, tc • Sometimes its written as • tc= 0.01947 K10.77 • Where K1=

29. Rainfall Intensity, i • Corresponding to a duration tc and the desired probability of exceedence P • Return period, T=1/P • Found from rainfall intensity—duration-frequency (IDF) curve

30. Rainfall Intensity, i • Average intensity for a selected frequency and duration • Based on “design” event (i.e. 50-year storm) • Overdesign is costly (what else?) • Underdesign may be inadequate • Duration

31. Rainfall Intensity, i • Based on values of tc and T • tc = time of concentration • T = recurrence interval or design frequency • As a minimum equal to the time of concentration, tc, (mm/hr)

32. Recurrence Interval (Design Event) • 2-year interval -- Design of intakes and spread of water on pavement for primary highways and city streets • 10-year interval -- Design of intakes and spread of water on pavement for freeways and interstate highways • 50 - year -- Design of subways (underpasses) and sag vertical curves where storm sewer pipe is the only outlet • 100 – year interval -- Major storm check on all projects

33. Time of Concentration (tc) • Time for water to flow from hydraulically most distant point on the watershed to the point of interest • Assumes peak runoff occurs when I lasts as long or longer than tc

34. Time of Concentration (tc) • Depends on: • Size and shape of drainage area • Type of surface • Slope of drainage area • Rainfall intensity • Whether flow is entirely overland or whether some is channelized

35. Example 1:Rational Method • An urban catchment has an area of 85 ha. The slope of the catchment is 0.006 and the maximum length of travel of water is 950m. The maximum depth of rainfall with a 25-year return period is as below • If a culvert for drainage at the outlet of this area is to be designed for a return period of 25years, estimate the required peak-flow rate, by assuming runoff coefficient is 0.3

36. Example 2:Rational Method • If the urban area of example 1, the land use of the area and the corresponding runoff coefficients are as given below, calculate the equivalent runoff coefficient.