A check dam is a small, low dam constructed in a gully or other watercourse to decrease the velocity of streamflow, for minimizing channel scour, and promoting the deposition of eroded material.
Functions of Check Dam
- To reduce the velocity of water to a non-erodible one, thus trap the silt going to the lake or reservoir
- To prevent erosion by not allowing water to run on long slopes
- To reduce the water depth by widening the riverbed
- To raise the bed level to such an extent that it can support the unstable side slopes
- To store water and increase its infiltration and percolation in the soil
- To collect sufficient soil and water to promote plant growth
- To prevent bank erosion
Types Of Check Dam
Check dams are classified based on their durability and the material used for their construction. Based on the durability, the check dams may be temporary such as brushwood, poles, or loose stone, or permanent such as gabion, masonry, or reinforced. Following are the 5 different types of check dams:
- Brushwood check dam
- Log check dam
- Loose stone check dam
- Boulder check dam
- Gabion check dam
Brushwood Check dam
Brushwood check dams are made of wooden poles and brushes. They are the least permanent of all types, but they are cheap and can be made by farmers with locally available materials.
They are suitable for small gullies of 1 to 2 meters in depth. Generally, two types of brushwood check dams are made either with a single row of poles or with a double row of poles.
They are temporary structures and should not be used to treat ongoing problems such as concentrated runoff. They can be employed with land-use change such as reforestation or improved range management until vegetative and slope treatment measures become effective.
- The sides of the gully are cut to a slope of 1: 1
- The size of poles is 1.5 to 2 meters in length and 8 to 12 centimeters in diameter
- Any tree or shrub species can be used however, sprouting poles from Salix, simali(vitex), and poplar are most preferred
- A trench of about 15 cm. deep is made across the gully, and the wooden poles are driven to a depth of about 1/3 to ½ of the pole length, and at a spacing of about half a meter
- The tops of the poles in the middle are made lower than the sides to form a notch of the required size to accommodate the maximum runoff
- The flexible branches of trees (Salix, poplar, vitex, etc.) and shrubs are woven between wooden poles driven into the ground till a dam of the required height is obtained
- The ends of inter-linked materials should enter at least 30 cm into the sides of the gully
- Brushwood check dams are constructed when the soil in the gully is saturated or during the early rainy season
- The brush used preferably should have been left on
- If non-sprouting species (pine, Alnus, as poles, and bamboo strips as interlinked materials) are used, brushwood check dams can be constructed during any season
- The back of the brushwood check dams must be filled up with soil
- A brushwood apron of about 1.5 times the height of the check dam is necessary to protect the channel from scouring.
Log check dams
- Made of logs and posts which are placed across the gully
- Also can be built using planks, heavy boards
- Its main objective is to hold fine and coarse material carried by flowing water in the gully
- They are used to stabilize incipient small and branch gullies not longer than 100m and with a catchment area less than 2 hectares
Loose stone check dams
- Relatively small rocks/stones are placed across the gully
- The objective of this dam is to control channel erosion along the gully bed
- They are used to stabilize the incipient and small gullies and branch gullies of a continuous gully
- The length of the gully channel is not more than 100m and the gully catchment area is 2 ha or less
- They are constructed where rocks are available abundantly
- The stone check dams are most commonly used in the gully control works in developing countries like Nepal, India, and Pakistan.
Construction and Design Specification of loose stone check dams
- The sides of the gully are cut to a slope of 1: 1
- The maximum effective height of the dam is 1m and the foundation depth is at least 0.5m
- The thickness of the spillway level is 0.5 to 0.7m
- The inclination of its D/S face is 20% i.e 1/5
- U/S face is generally vertical
Boulder Check Dam
- Boulder Check dams are the same as loose stone check dams, except here big boulders are used.
- If properly made they are almost like permanent structures like gabions, masonry, and concrete structures
The use of the above-mentioned dimension does not need the stability test against overturning, collapsing, and sliding. However, the size of the spillway needs to be computed accordingly to the maximum discharge of the gully catchment area.
Gabion Check Dam
The gabion check dams are constructed with large hexagonal or square wire crates (Gabion boxes) filled with stones. they are preferable where big boulders are not available.
They have some significant advantages over solid structures such as Masonry and Concrete:
- Flexible: Gabions bend without breaking, and in contrast to concrete or masonry with mortar do not crack.
- Permeable: Gabion structures are permeable and do not need an extra drainage system.
- Economical: Usually they are cheaper to construct than other solid engineering structures.