1. Siti Kamariah Md Sa’at PPK Bioprocess..2010
ERT 246 Hydrology & Water Resources Eng.
FLOOD FORECASTING
Siti Kamariah Md Sa’at
PPK Bioprocess..2010

4. Satellite flood image: http://www. crisp. nus. edu

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 Rarely used 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 To compute Q, requires tc,i and C
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
C = C1A1 + C2A2 + ….. + CnAn
ΣAi

27. Time of concentration, tc
For small drainage basin, tc=tp
For other catchment, use Kirpich Equation (1940)
tc= 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= K10.77
Where K1=√(L3/∆H)

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. Frequency Analysis In next Chapter... 2 method Extreme Gumbel
Log Pearson Type III

36. Example 1: Rational Method
An urban catchment has an area of 85 ha. The slope of the catchment is and the maximum length of travel of water is 950m. The maximum depth of rainfall with a 25-year return period is as below
Duration (min) 5 10 20 30 40 60
Depth of rainfall (mm) 17 26 50 57 62
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

37. 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.
Land Use
Area (ha)
Runoff coefficient
Roads 8
0.70
Lawn 17
0.10
Residential Area 50
0.30
Industrial Area 10
0.80

38. Cont....
Flood Routing..