Water and Sediment Control Basins

Water and sediment control basins are used as a method to minimize soil erosion. They are effective in preventing bank and gully erosion by collecting and storing sediment along with other waterborne debris. Generally, they prevent downstream erosion that can result from concentrated flows. Sediment control basins are recognized as an effective Best Management Practice to enhance and improve water quality. They are often referred to as "bench-terraces" or "floodwater storages for smaller drainage areas".

Water and sediment control basins consist of a berm, a ponding area and a pipe or tile outlet. The berm is constructed across the slope of the land to provide a temporary storage area for the surface runoff. Soil is piled and used to direct water to a stabilized location.

The ponded water is slowly discharged through an underground outlet system. This eliminates erosive actions occurring further down the slope and prevents damage to crop or other vegetation. A grassed waterway, diversion or drop inlet may also be used in combination with the water and sediment control basin as an effective soil conservation system.

The ponding process slows the water flow, allowing sediment and debris to settle out before it reaches the drop inlet. The larger the storage basin the more time available to enhance the proportion of settled sediment. This prevents the undesirable deposition of silt on low lands or critical areas. Water and sediment control basins reduce pollution by collecting silt, fine sand, wastes and other debris.

(1) Narrow base berm system
The narrow based berm system is the most economical as it usually takes less material to construct. These berms can be used in areas where the land slope is greater than 6%. The berm generally has a top width of approximately 1.2 m (4 feet). The structure has a back slope of 2:1 and a front slope of 3:1 as illustrated in figure 1. This type of berm has steep side slopes, thus it is not workable by farm machinery.

(2) Bench-terrace basin
These steep back slope or bench-terrace berms are used where the average land slope is 6% or less. The berm consists of a side slopes of 8:1 and 2: 1. The front 8:1 side slope is workable by farm machinery. They are commonly used in conservation systems which use a series of Bench-terraces to collect and discharge runoff.

1. Determine the peak flow rate and the total incoming volume of runoff for a 24 hour duration storm having a one in 10 year recurrence frequency. A rule of thumb is that 50 m 3 (0.04 acre-ft) of storage is required for each acre of the drainage area. These conditions are all dependent on the watershed's characteristics. However, the temporary storage should have sufficient capacity to accommodate all the runoff from a 2 year frequency - 24 hour duration storm.
2. The storage capacity of the sediment basin should take into consideration a reduction in volume as a result of accumulated sediment . Also a minimum freeboard of 0.3 m (1 ft.) should be added to the design height of berm to prevent over topping Soil erosion prediction models such as USLE are commonly used to estimate the annual soil loss.
3. The outlet and spillway dimensions are generally sized based on peak flow and the basin's volume. Generally, the outlet pipe is sized so that the ponding time is limited to less than 24 hours, or depending on the crop tolerance to flooding.
4. Berm side slopes in Sandy Soils should not exceed 1:3 and 1:2 in Clay Soils. If berms are being cropped the location of the pipe inlet has to take into consideration the size of the farm equipment.
5. Where multiple berms or collection basins are connected to a common outlet tile the pressure and velocities may become excessive. Tile sizing and materials must consider discharge capacity, type of flow control, and the intake rate. Intake rates can be controlled by incorporating orifice control plates or specially selected intake structures.
6. The location of surface and subsurface drainage outlets must take into consideration landscape features and adjacent property boundaries. Permission must be obtained by downstream owners and in situations which may impact on highways or within local municipalities. All outlets must be well protected and designed according to the NBDARD Subsurface Drainage Standards.
7. Emergency spillways are used to prevent water from over topping the berm. Some common overflow structures include rockchutes and grassed waterways.
8. In areas where rodents are a problem, wire mesh can be installed several inches below the finished soil surface to reduce potential berm damage.

Site Selection:
Water and sediment control basins should be located in an area that will minimize interference with existing farm practices. The surface drainage patterns in the area must be known, contour maps are useful. Soil characteristics and the depth of soil layers are often required to determine the site's potential.

Sediment control basins should never be built on potentially unstable soils. Thus, site location is an important factor in determining the longevity of the structure. For example, all precautions should be taken when locating a berm at the top edge of an existing gully. It may become highly susceptible to future settlement and erosion of the berm.

The earth used for the construction of a berm should consist of at least 10% clay and be stone free. The clay content creates an impermeable membrane for the settling pond and minimizes seepage. If the soil characteristics of the earth are inadequate, it may be advisable to transport suitable soil to the site.

Construction techniques:
Ideally, the proposed pond area would provide the earth required for berm construction. If the amount of fill from the pond area is insufficient , fill from an alternate source will be needed. The available fill on site should be managed so that the topsoil is excavated and stockpiled for later use. The subsoil is then used in the berm construction process. The topsoil is then replaced on the compacted subsoil layer of the newly constructed berm and seeded appropriately. A minimum layer of 15 cm (6") of topsoil is required to support vegetative growth.

The berm should be constructed similar to a dam except instead of holding water it holds back soil. All subsurface outlets and standpipe inlets should be installed prior to berm construction.

The process of berm construction should be accompanied by proper grade control to maintain a uniform, level structure. A bulldozer or earth scraper may be used to transport and reshape the earth. The fill is placed in 15 cm (6") layers and compacted. The height of the completed berm should account for settlement, retention capacity and freeboard.

To avoid soil compaction of the surrounding crop land by heavy equipment during the construction, of these water and sediment control basins, the soil should be in a dry and workable condition. It is critical that the construction occur in the summer or early fall to allow for the establishment of an adequate vegetative cover on the berm. This practice will reduce the risk of rill and gully erosion.

It is more economical to fix a potential problem before a major runoff event than to endure the consequences which may result from the problem.

1. The berm should be routinely inspected for cracks and settlement.
2. Both inlets and outlets should be inspected for obstructions such as debris and silt and these should be removed. The ponding capacity should be monitored and routinely cleaned as required to minimize the excessive build up of sediment.
3. The emergency spillway may need maintenance after a large storm to maintain proper elevation. If this becomes a frequent practice an improvement or upgrading of the system's capacity may be required.
4. The vegetation of the berm must be maintained with proper fertilization, lime application and annual mowing

Sediment basins are considered as a practical Best Management Practice used to control soil erosion and prevent the degradation of water quality by protecting watercourses from excessive runoff and sediment from agricultural land and/or barnyard runoff.

Through minimizing the amount of sediment and other waterborne debris in watercourses, water quality can be enhanced. Siltation is minimized and the productive layer of soil is protected. By improving your land base the capital and aesthetic value will be enhanced. Best Management Practices have proven to be an effective means of reducing soil erosion.