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FLOOD FORECASTING
Siti Kamariah Md Sa’at
PPK Bioprocess..2010
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Satellite flood image: http://www.crisp.nus.edu.sg/coverages/floods2007/index_p2.html
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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
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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
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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
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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
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Main features of a river valley.
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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
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Upstream floods are generally local, with short lag times
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Flash Floods
Floods with exceptionally short lag time
Peak discharge reached only hours or minutes after storm has passed
Deadly
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Downstream Flooding
Usually from storms that last a long time and extend over large area
Total discharge increases downstream as tributaries collect floodwaters
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Downstream floods are regional in extent with longer lag times and higher peak discharges.
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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
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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
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Examples of Flood Hazards
Disruption of transportation/electrical services
Gas leaks Lack of clean water
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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
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Flood Forecasting
To estimate the magnitude of flood peak, the following alternative methods are available:
Empirical Formula Rational Method Frequency Analysis
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Empirical Formula Q = CAn
Where Q=Maximum flood discharge A=Catchment Area C=Constant that depend on
catchment & precipitation n=Index
Rarely used
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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
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Rational Method
For small size (<50 km2) catchments
This not cover what is MSMA (Manual Saliran Mesra Alam Malaysia)/Urban Stormwater Management Manual.
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Rational Method
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Runoff Coefficient
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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
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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
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Time of concentration, tc
Sometimes its written as tc=0.01947 K1
0.77
Where K1=√(L3/∆H)
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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
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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
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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)
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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
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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
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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
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Frequency Analysis
In next Chapter... 2 method
Extreme Gumbel Log Pearson Type III
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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
Duration (min)
5 10 20 30 40 60
Depth of rainfall (mm)
17 26 40 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
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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
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Cont....
Flood Routing..