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**Flood and Runoff estimation methods**

Indirect Methods (Equations) Direct Methods (measurements)

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**Indirect Methods Talbot’s CN (SCS) Rational Unit Hydrograph**

Manning’s Formula Equation Creager’s

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Direct Methods Current meter Staff gauge Crest stage gauge

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

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**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

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**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

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**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.

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**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

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**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.

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**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

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**Estimation of Design Flood**

Rational Method

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**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.

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**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

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**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

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**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

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**CREAGER’S METHOD Where:**

Qm = Maximum or peak flow for a given return period. A = Catchment area (Sq. Km). C1 = Greagers number (max. 130).

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**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

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Flow measurements

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Flow measurements

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**Flow measurements current meters**

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Current meters

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Hydrology & Water Resources Engineering

Hydrology & Water Resources Engineering

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