Flood Risk Assessment Using GIS Tools, By Dr. Omar Elbadawy, CEDARE, Land and Water Days in Near East & North Africa, 15-18 December 2013, Amman, Jordan
10. Selected parameters
Area (A)
weighted mean of bifurcation ratio (WMRB)
stream frequency (F)
drainage density (D)
shape index (Ish)
slope index (Sl)
relief ratio (Rr)
ruggedness number (Rn)
texture ratio (Rt).
12. Watershed Factors
Watershed shape - runoff rates tend to be
lower for long narrow watersheds than for
compact water sheds having the same area
Long “time
of concentration”
Short “time
of concentration”
14. Risk value 4 *
( x xmin )
1
( xmax xmin )
Risk Calculation
( x xmin )
Risk value 4 *
1
( xmax xmin )
x = the parameter value
xmin = the minimum value of the parameter according to all studied basins
xmax = the maximum value of the parameter according to all studied basins
27. Conclusions
The proposed methodology proved to be suitable for the arid wadi system
especially when detailed data is not available
The selected geomorphological parameters for risk assessment are well
matched with the results from estimated runoff hydrograph when both peak
discharge and time to peak are considered
GIS has proved to be an easy and efficient tool for watersheds flood risk
assessment.
The risk classification presented provides a prioritization skim for flood
control and flood protection programmes.
The study presents an integrated approach for flood risk assessment for AlSokhna area, and should be reflected in development plans for the area
29. Geomorphologic
• Uses stream network topology and probability concepts
• Law of Stream Numbers
range: 3-5
• Law of Stream Lengths
range: 1.5-3.5
• Law of Stream Areas
range:3-6
N 1
RB
N
RBu
( N u N u 1 )
RBu 1
WMRb
N
L
RL
L 1
A
RA
A 1
29
31. Instantaneous Unit Hydrogaph
GIUH
IUH
0.55
0.44 L R B
tp=
RA
qp =
1.31
L
R
- 0.38
L
qp=
0.871
i
1
(
)
hours
.4
2.5
0.43
L
R
i=
L
1.5
i r A R L
ir is the intensity of effective rainfall in cm/h
AΩ is the area of the watershed in km2
αΩ the kinematic parameter for the stream of highest order
1/2
SΩ is the average slope of the highest order stream
nΩ is the average Manning roughness coefficient of Ω
bΩ is the average width of the highest order stream, in m
S
=
2/3
n b
32. Risk Assessment Parameters
Watershed characteristics
Area
Perimeter
Basin length
Valley length
Length of overland flow
Ruggedness number
Texture ratio
Stream morphology
Stream frequency
Drainage density
Stream order
Sum of stream number
Sum of stream length
RB
RL
RA
WMRb
Topographic and shape
Slope index
Circularity ratio
Elongation ratio
Relief
Internal relief
Relief ratio
Shape index
sinuosity