Watershed - Advanced Hydrology - Lecture Slides, Slides of Aeronautical Engineering

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Lecture 2: Watershed and rainfall-runoff

relationship (contd.)

Module 3

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Rainfall-Runoff

 How does runoff occur?

 When rainfall exceeds the infiltration rate at the surface, excess water begins to accumulate as surface storage in small depressions. As depression storage begins to fill, overland flow or sheet flow may begin to occur and this flow is called as “Surface runoff”

 Runoff mainly depends on: Amount of rainfall, soil type, evaporation

capacity and land use

 Amount of rainfall: The runoff is in direct proportion with the rainfall. i.e. as the rainfall increases, the chance of increase in runoff will also increases

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 Interflow or Subsurface Storm Flow: The precipitation that infiltrates the soil surface and move laterally through the upper soil layers until it enters a stream channel.

 Groundwater Flow or Base Flow: The portion of precipitation that percolates downward until it reaches the water table. This water accretion may eventually discharge into the streams if the water table intersects the stream channels of the basin. However, its contribution to stream flow cannot fluctuate rapidly because of its very low flow velocity

 Data collection

 The local flood control agencies are responsible for extensive hydrologic gaging networks within India, and data gathered on an hourly or daily basis can be plotted for a given watershed to relate rainfall to direct runoff for a given year.

Module 3

Rainfall-Runoff Contd….

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• Travel time for open channel flow (Tt)

T t = L/V

where L = length of open channel (ft, m) V = cross-sectional average velocity of flow (ft/s, m/s)

Manning's equation can be used to calculate cross-sectional average velocity of flow in open channels

where V = cross-sectional average velocity (ft/s, m/s) k (^) n = 1.486 for English units and k (^) n = 1.0 for SI units A = cross sectional area of flow (ft^2 , m 2 ) n = Manning coefficient of roughness R = hydraulic radius (ft, m) S = slope of pipe (ft/ft, m/m)

Module 3

Rational Method

Runoff Measurement Contd….

V = k (^) n / n R2/3^ S1/

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Values of Runoff coefficients, C (Chow, 1962)

Module 3

Runoff Measurement Contd….

Rational Method

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Calculation of T c

• T (^) c = ∑T (^) t where Tt is the travel time i.e. the time it takes for water to travel from one location to another in a watershed
• Travel time for sheet flow

where, n = Manning’s roughness coefficient L = Flow length (meters) P 2 = 2-yr, 24-hr rainfall (in.) and S is the hydraulic grade line or land surface

Module 3

Rational Method

Runoff Measurement Contd….

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• Assumptions of rational method

 Steady flow and uniform rainfall rate will produce maximum runoff when all

parts of a watershed are contributing to outflow

 Runoff is assumed to reach a maximum when the rainfall intensity lasts as

long as t (^) c

 Runoff coefficient is assumed constant during a storm event

• Drawbacks of rational method

 The rational method is often used in small urban areas to design drainage

systems and open channels

 For larger watersheds, this process is not suitable since this method is

usually limited to basins less than a few hundred acres in size

Module 3

Rational Method

Runoff Measurement Contd….

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