Check Dam

1.1:-Introduction

            A check dam is a special type of engineering measure for spoil conservation in erodible areas. A check dam consists of embankment and spillway. Some check dam consists of the only embankment.

               Check dam retains flood water and traps sediment. At the beginning of construction,  a water body would be formed behind the check dam, gradually the water body will be filled by sediment deposit. The area formed by the deposit is usually used as farmland because it plays an important role in the grain production in that region.

      Check dams are generally used in concentrated flow areas but not in-stream or channel. Check dams can either be permanent or temporary barriers that prevent erosion and promote sedimentation by slowing flow velocity.

1.2:-Function of check dams

                Check dams are pre-designed and constructed to meet the following concept:-

1)To reduce the velocity of the water.

2)Raise the bed level and reduce the slopes in a gully by silting up and trapping the silt.

3)Support the unstable side slopes.

4)Promote the water percolation in the soil and conserve water for plant growth and also for stability banks.

1.3:-Design consideration of check dams

                    Design consideration of check dams includes the following points:-

1)Site selection

a)The site should be wide enough to provide high runoff.

b)The spillway has to be large enough with sufficient freeboard to take expected maximum runoff. (concrete mortared rock dam must not exceed 8 m in height whereas the log crib dam must not exceed 2 m. The  spacing of a dam for 2 m height in a 20-degree channel will be 12 m.)

c)A check dam should be made at straight and firm stream bed and dam. It should not be made on a curve and junction of gullies or streams.

d)The foundation of check dam should be rest on base rock.

e)The selection of the site is also restricted by the height of the structure. If the elevation difference between 2 sides is too high, 1 or more side have to be selected in between so, that structural height will be limited.

2)The spacing of check dams

            The spacing of check dams should be so placed that, the line joining the top of the lower check dam and the bottom of the successive upper check dam gives the gradient. This gradient will give a non-erosive velocity of flow. This gradient is known as a compensation gradient. For general practice, the compensation gradient is taken as 3 to 5% of the slope . The horizontal distance between successive check dams is given by

                           

                 b=h*100/(So-Se)

 

where,

            b=spacing between two check dams

            h=Height of check dam up to the notch

            So=Existing bed slope in percentage

            Se=Establishing slope of the bed in percentage

3)Number of check dams

          The number of check dam required is determined from the following formula

                  number of check dam =(a-b)/H

where,

           a=total vertical distance between first and last check dam

           b=Se*d’/100

           d’=horizontal distance between first and last check dam

           H=average height of check dam

1.4:-Hydrological aspects of check dams

Runoff estimate

            The rational formula is the most commonly used method of the estimate of runoff. The peak rate of runoff is calculated by using the following rational formula.

                             Q=CIA/360

Where,

            Q=Rate of runoff

            C=Catchment coefficient or the coefficient of runoff

            I=Intensity of runoff

            A=Catchment area

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