A check dam is a special type of engineering measure for soil conservation in erodible areas.
It is a dam constructed across the waterway to control erosion by reducing the velocity of the flow.
It retains flood water and traps sediment.
It consists of an embankment and spillway. Some check dam consists of the only embankment.
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 dam is generally used in concentrated flow areas but not in-stream or channel. It can either be the permanent or temporary barrier that prevents erosion and promote sedimentation by slowing flow velocity.
2. Functions of the Check Dam
A check dam is 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 the stability of banks.
3. Design Consideration of Check Dam
Design consideration includes the following points:-
b)The spillway has to be large enough with sufficient free-board to take the 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)It should be made at a straight and firm stream bed. It should not be made on a curve and junction of gullies or streams.
d)The foundation of the check dams 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 the 2 sides is too high, 1 or more sides have to be selected in between so that structural height will be limited.
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:
h=Height of check dam up to the notch
So=Existing bed slope in percentage
Se=Establishing slope of the bed in percentage
The number of check dams required is determined from the following formula:
number of check dams =(a-b)/H
d’=horizontal distance between first and last dam
4. Design – Example
We have provided this design example in PDF Format for your ease. Download it and grab the knowledge.
5. Hydrological Aspect
The rational formula is the most commonly used method for the estimate of runoff.
The peak rate of runoff is calculated by using the following rational formula;
Q=Rate of runoff
C=Catchment coefficient or the coefficient of runoff
I=Intensity of runoff
Some advantages of this dam are:
a. It provides an irrigation facility.
b. It reduces flow velocity and helps in the protection of river banks.
c. Economical than other dam types.
d. It is simple to construct the check dam. So, no requirement for advanced machines and technologies.
e. It helps in flood control.
f. It facilitates the production of electricity on a small scale.
Some disadvantages of this dam are:
a. It causes a rise in the groundwater table.
b. Affects aquatic lives.
c. It has a limited life span and strength.
|Read Also: Earthen Dam|