Transcript Flood and Runoff Estimation Methods

Flood and Runoff estimation
methods
Indirect
Methods
(Equations)
Direct Methods
(measurements)
Talbot’s
CN (SCS)
Rational
Indirect Methods
Manning’s Formula
Unit Hydrograph
Creager’s Equation
Current meter
Direct
Methods
Crest stage gauge
Staff gauge
SCS Method
• SCS Method
• In this method the runoff volume (Q) resulting from a
given rainfall storm (P) is calculated using the
following formula.
( P  0.2S ) 2
Q
P  0.8S
25400
S
 254
CN
Talbot Method
• Talbot Method
• Design flood can be related to morphological
properties of the catchment. Design flood discharge
(Q),
in m3 /sec, can be calculated using the
following formula
Q  aCA
n
Talbot Method
• Talbot Method
Runoff coefficients according to catchments characteristics
Catchments characteristics
C
C1 = Terrain condition
Mountains
Semi-mountains
Low lands
0.30
0.20
0.10
C2 = Slope of drainage area
above 15%
10 – 15 %
5 – 10 %
2- 5 %
1–2%
0.5
0.4
0.23
0.25
0.2
C3 = Shape of drainage area
When width equals length
When width equals 0.4 of length
When width equals 0.2 of length
0.30
0.20
0.10
Estimation of Design Flood
• Rational Method
• The rational method uses existing rainfall data and
land use in estimating peak runoff from small
drainage areas that are less than 15 km2.
Q  0.278CiA
Estimation of Design Flood
• Rational Method
Runoff Coefficients for the Rational Method
Type Of Drainage Area
Steep, bare rock
Rock, steep but wooded
Plateaus lightly covered, ordinary ground bare
Densely built up areas of cities with metal led roads & paths
Residential areas not densely built up, with metal led roads
Residential areas not densely built up, with unmetalled roads
Clayey soils, stiff and bare
Clayey soils lightly covered
Loam, lightly cultivated or covered
Loam, lightly, largely cultivated
Suburbs with gardens, lawns and macadamized roads
Sandy soil, light growth
Runoff
Coefficient, C
0.90
0.80
0.70
0.70-0.90
0.50-0.70
0.20-0.50
0.60
0.50
0.40
0.30
0.30
0.20
Estimation of Design Flood
• Rational Method
• The rainfall intensity (i) in millimeters per hour for
duration corresponding to the time of
concentration for the catchment area and having
a recurrence period appropriate to the site
conditions, is read from IDF curves.
Estimation of Design Flood
• Rational Method
• Time of concentration is defined as the interval
time in minutes from the beginning of rainfall to the
time when water from the most remote position of
the catchment reaches the outlet.
• Kirkpich formula
Tc  (1 / 52) L
1.155
/H
0.385
Estimation of Design Flood
• Rational Method
R a in fa ll I D F C u rv e s a t H a jja h ; Y e m e n
100 0
2 - Y e a rs
5 - Y e a rs
1 0 -Y e ars
2 5 -Y e ars
I n te n s ity (m m /h r )
5 0 -Y e ars
10 0
1 0 0 -Y e ar s
10
1
1
10
1 00
D u r a tio n (m in u te )
1000
1000 0
Estimation of Design Flood
• SCS Method
• In this method the runoff volume (Q) resulting from a
given rainfall storm (P) is calculated using the
following formula.
( P  0.2S ) 2
Q
P  0.8S
S
25400
 254
CN
Estimation of Design Flood
• Talbot Method
• Design flood can be related to morphological
properties of the catchment. Design flood discharge
(Q),
in m3 /sec, can be calculated using the
following formula
Q  aCA
n
Estimation of Design Flood
• Talbot Method
Runoff coefficients according to catchments characteristics
Catchments characteristics
C
C1 = Terrain condition
Mountains
Semi-mountains
Low lands
0.30
0.20
0.10
C2 = Slope of drainage area
above 15%
10 – 15 %
5 – 10 %
2- 5 %
1–2%
0.5
0.4
0.23
0.25
0.2
C3 = Shape of drainage area
When width equals length
When width equals 0.4 of length
When width equals 0.2 of length
0.30
0.20
0.10
Estimation of Design Flood
• Talbot Method
Formulas according to catchment properties
Catchments area
(sq. km)
Q25 (m3 /sec)
Notes
Qbasic = basic flows derived from
standards figures;
SF = Slope factor for Catchment area
<5
Qbasic * SF
5 – 12.6
0.837 * C * A0.75
C = C1 + C2 + C3
12.6 - 36
4.985 * C * A0.5
C = C1 + C2 + C3
>36
14.232 C A0.4
C = C1 + C2 + C3
Q5 = 0.6 Q25; Q10 = 0.8Q25;
Q50 = 1.2Q25; Q100 = 1.4Q25
Qm  C1 * (0.386* A)
0.894*( 0.386* A) 0.0 4 8
CREAGER’S METHOD
Qm  C1 * (0.386* A)
0.894*( 0.386* A)
0.0 4 8
Where:
Qm
A
C1
=
=
=
Maximum or peak flow for a given return period.
Catchment area (Sq. Km).
Greagers number (max. 130).
Theoretical eq. for runoff est. for Yemen
:)‫المعادالت النظرية الحتساب الجريان السطحي (لليمن‬
•
Jac A.M. van der Gun and others 1995.
R. O.= 0.055*P (mm).
P= precipitation (mm)
•
Arnon 1972
R. O.= 0.6*P*S (mm).
S=Slop
•
Runoff Coefficient Method
V=103C.P.A (m3).
C=Runoff Coefficient
•
Flood Estimation
Q=C.I.A/3.6 (m3/s).
I=Rainfall Intensity (mm/hr)
•
SCS Curve Number Method
F/S = Q/Pe
F=Actual water retention (Pe-Q)
S=Potential Maximum Retention
Q=Actual Runoff
•
Pe=Potential runoff
•
Flow measurements
Flow measurements
Flow measurements
current meters
Current meters