Erosion and Sediment Control actions (I)

Check Dam


When
· To stabilize constructed and existing flow corridors when flow is anticipated to exceed the erosive velocity.
· To control sediment in a stream in conjunction with a sediment sump.

Why
· To reduce water velocity minimizing erosion in flow corridors and channels.
· To temporarily protect vegetation during early stages of growth or permanently to reduce flow velocities.

Where
· Within and across an existing or constructed flow corridor.

Scheduling
· Around Year.

How

1. Configure check dams to site specific conditions. Utilize an engineer as necessary to determine the notched center dimensions and spacing between check dams based on channel slope, flow length, discharge, flow velocity, and soil type. Permanent check dams should be designed to pass, at a minimum, a 10‑year, 24‑hour storm at non-erosive velocity.
2. Permanent check dams should be constructed of clean rock placed on geo-textile fabric which has been toed in a minimum of 3 inches. Ninety percent of the rock should range between 2 to 4 inches for slopes less than 2 percent and 3 to 12 inches for steeper grades. The rock size should be large enough to stay in place during anticipated flows. When larger rock is used, place smaller aggregate immediately upstream to filter sediment and improve efficiency.
3. Temporary check dams that will experience low flow conditions can utilize pea-stone or gravel filled bags instead of rock over geotextile fabric. New commercially available technologies include prefabricated check dams that are effective and sometimes reusable.
4. When not engineered but used in series, the toe of the upstream check dam should be set at the same elevation as lowest point in the top of the downstream check dam.
5. The side slopes of the check dam should be 2 horizontal to 1 vertical or flatter or equivalent to the existing streambank slopes.
6. The middle of the dam should be a minimum of 9 inches lower than the outer edges, allowing flow to go over the depression in the center as opposed to around the sides where it could erode the banks.
7. The outer edges should be keyed into adjacent banks and extend to an elevation above the anticipated flow depth to prevent washouts.
8. Sediment sumps should be used upstream of check dams when working in sandy soils when excessive amounts of sediment is expected to accumulate.
9. Riprap should be placed immediately below the check dam to help dissipate the energy of the water flowing over the dam. In areas of higher velocities energy dissipation may be needed downstream of the check dam to prevent undercutting.
10. Temporary check dams should be constructed to handle the anticipated flow and sediment load until the site is stabilized. Aggregate filled bags are easier to remove than a rock check dam and the aggregate can usually be spread along the channel bottom when the check dam is removed. Aggregate meeting the gradation requirements of 6A is recommended; use nothing finer than pea-stone.




Maintenance
· Inspect check dams following each runoff event to ensure there is no piping under the structure or around the banks until the flow corridor has been stabilized.
· Initiate identified repair needs as soon as possible following inspection.
· Remove and properly dispose of sediment when it accumulates to 1/2 the check dam height. Spread sediment in an upland area and seed immediately.
· In some instances clogged stone must be cleaned to remain effective.
· Inspect downstream structures to ensure they have not been damaged or clogged with displaced rock or stone.
· After flow corridor or channel has stabilized remove accumulated sediment from behind the check dam. If check dam is temporary, remove check dam and then stabilize the area.



Limitations
· Check dams greater than two feet in depth at the center may seriously impact the flow characteristics of the flow corridor or channel and should not be used.
· Removal of rock check dams is labor intensive and expensive.
· Does not remove suspended clay and silt, therefore polymers may be needed.

Indian standard (IS) for Design of Dam

Indian standard (IS) for Design of Dam

1. Criteria for Design of Solid Gravity Dams- I.S. 6512-1984.
2. Guidelines for fixing spillway capacity –I.S. 11223-1985.
3. Criteria for Earthquake resistant design of structures I.S. 1893-1984.
4. Code of practice for stability analysis for earth dams- I.S. 7894-1975.

The Indian Standard IS: 11223-1985 “ Guidelines for fixing spillway capacity” gives the criteria for inflow design flood as under:
The seismic zone together with appropriate coefficients for use in such analysis is given in IS: 1983-84 “Criteria for Earthquake Resistant Design of Structures”.

The various design condition of analysis along with the minimum values of factors of safety to be aimed at and use of type of shear strength for each condition of analysis is given in I.S.:7894-1975 Code of Practice for stability analysis of earth dams .
IS:1893-1984. Criteria for earthquake resistant design of structures (3rd rev.).
Till specific reliable procedures become available for assessment of ice pressure, it may be provided for at the rate of 250 kpa applied to the face of the dam (I.S..6512-1984).
I.S. 1893-1984 “Criteria for Earthquake resistant design of structures” or similar Method.
As per Indian Standard IS:6512-1984 the compressive strength of concrete and masonry
The IS : 6512-1984 states that the factor of safety against sliding may be calculated on the basis of partial factor of safety in respect of friction (FQ) and partial factor of safety in respect of cohesion (Fc)

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