1. Flood and Runoff estimation methods
Indirect Methods
(Equations)
Direct Methods
(measurements)

2. Indirect Methods Talbot’s CN (SCS) Rational Unit Hydrograph
Manning’s Formula
Equation Creager’s

3. Direct Methods
Current meter
Staff gauge
Crest stage gauge

4. SCS Method
SCS Method
In this method the runoff volume (Q) resulting from a given rainfall storm (P) is calculated using the following formula.

5. 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

6. Talbot Method Talbot Method Catchments characteristics C
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

7. 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.

8. Estimation of Design Flood
Rational Method
Runoff Coefficients for the Rational Method
Type Of Drainage Area
Runoff Coefficient, C
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
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20

9. 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.

10. 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

11. Estimation of Design Flood
Rational Method

12. 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.

13. 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

14. 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

15. Estimation of Design Flood
Talbot Method
Formulas according to catchment properties
Catchments area (sq. km)
Q25 (m3 /sec)
Notes
< 5
Qbasic * SF
Qbasic = basic flows derived from standards figures;
SF = Slope factor for Catchment area
5 – 12.6
0.837 * C * A0.75
C = C1 + C2 + C3
4.985 * C * A0.5
>36
C A0.4
Q5 = 0.6 Q25; Q10 = 0.8Q25;
Q50 = 1.2Q25; Q100 = 1.4Q25

16. CREAGER’S METHOD Where:
Qm = Maximum or peak flow for a given return period.
A = Catchment area (Sq. Km).
C1 = Greagers number (max. 130).

17. Theoretical eq. for runoff est. for Yemen
المعادلات النظرية لاحتساب 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

18. Flow measurements

19. Flow measurements

20. Flow measurements current meters

21. Current meters