Check dam
A check dam is a small, sometimes temporary, dam constructed across a swale, drainage ditch, or waterway to counteract erosion by reducing water flow velocity. Check dams themselves are not a type of new technology; rather, they are an ancient technique dating from the second century AD. Check dams are typically, though not always, implemented in a system of several dams situated at regular intervals across the area of interest.
Types of Check Dams
There are two main categories of check dams. They are:1) Closed type check dams. Closed-type check dams have been used commonly for hundreds of years, and are constructed without openings to block water and sediment flow. However, closed-type check dams may disrupt natural ecosystems through impeding the natural flow of water and sediment.
2) Open-type check dams. Open type check dams are likewise constructed across a waterway, but have openings to allow for water and sediment to flow more freely. Openings can be created by either implementing spaced trusses or beams into the design of the dam. The distance between these openings, based on whether trusses or beams were used in construction, can be designed to achieve a desired rate of sediment passthrough at the check dam. Furthermore, open-type check dams can be constructed to be modifiable, granting communities or governments the ability to better manage sediment and water flow by adding or removing modular sections of the dam.
Function
A check dam placed in the ditch, swale, or channel interrupts the flow of water and flattens the gradient of the channel, thereby reducing the velocity. In turn, this obstruction induces infiltration and reduces erosion. They can be used not only to slow flow velocity but also to distribute flows across a swale to avoid preferential paths and guide flows toward vegetation. Although some sedimentation may result behind the dam, check dams do not primarily function as sediment-trapping devices.For instance, on the Graliwdo River in Ethiopia, an increase of hydraulic roughness by check dams and water transmission losses in deposited sediments is responsible for the delay of runoff to reach the lower part of the river channels. The reduction of peak runoff discharge was larger in the river segment with check dams and vegetation than in segment without treatment. Reduction of total runoff volume was also larger in the river with check dams than in the untreated river. The implementation of check dams combined with vegetation reduced peak flow discharge and total runoff volume as large parts of runoff infiltrated in the sediments deposited behind the check dams. As gully check dams are implemented in a large areas of northern Ethiopia, this contributes to groundwater recharge and increased river base flow.
Applications
Grade control mechanism
Check dams have traditionally been implemented in two environments: across channel bottoms and on hilly slopes. Check dams are used primarily to control water velocity, conserve soil, and improve land. They are used when other flow-control practices, such as lining the channel or creating bioswales, are impractical. Accordingly, they are commonly used in degrading temporary channels, in which permanent stabilization is impractical and infeasible in terms of resource allocation and funding due to the short life period. They are also used when construction delays and weather conditions prevent timely installation of other erosion control practices. This is typically seen during the construction process of large-scale permanent dams or erosion control. As such, check dams serve as temporary grade-control mechanisms along waterways until resolute stabilization is established or along permanent swales that need protection prior to installation of a non-erodible lining.Water quality control mechanism
Many check dams tend to form stream pools. Under low-flow circumstances, water either infiltrates into the ground, evaporates, or seeps through or under the dam. Under high flow – flood – conditions, water flows over or through the structure. Coarse and medium-grained sediment from runoff tends to be deposited behind check dams, while finer grains flow through. Floating garbage is also trapped by check dams, increasing their effectiveness as water quality control measures. In addition to overall water quality, check dams have positive effects on biodiversity both in the water and in the surrounding area. In a study of check dams in the Andes Mountains, check dams were found to be effective in this way by both increasing the mass of organic matter within trapped sediments and the amount of macroinvertebrate individuals found near the dam. These trapped sediments also work as carbon capture. However, it is important to note that these effects may only apply in conditions or environments that are similar to mountainous streams. Additionally, riparian vegetation has been found to increase in the presence of check dams, in turn increasing biodiversity and stabilizing the land with root systems. This has the effect of further reducing erosion, which can contribute to the check dam's overall effectiveness in protecting against disaster. Check dams may be implemented with bioswales to manage stormwater runoff, and those structures together have been shown to be effective in facilitating runoff drainage into the surrounding soil.Arid regions
In arid areas, check dams are often built to increase groundwater recharge in a process called managed aquifer recharge. Winter runoff thus can be stored in aquifers, from which the water can be withdrawn during the dry season for irrigation, livestock watering, and drinking water. This is particularly useful for small settlements located far from a large urban center as check dams require less reliance on machinery, funding, or advanced knowledge compared to large-scale dam implementation. Additionally, local civilians can easily monitor and maintain these check dams to ensure optimal groundwater recharge. In one such study in India, it was found that check dams were highly effective when used and maintained for managed aquifer recharge, and the water collected by the dam was used to support around 16 percent of agricultural activity in the surrounding communities being served by the aquifers.Check dams can be used in combination with limans to stop and collect surface runoff water.
Mountainous regions
As a strategy to stabilize mountain streams, the construction of check dams has a long tradition in many mountainous regions dating back to the 19th century in Europe. Steep slopes impede access by heavy construction machinery to mountain streams, so check dams have been built in place of larger dams. Because the typical high slope causes high flow velocity, a terraced system of multiple closely spaced check dams is typically necessary to reduce velocity and thereby counteract erosion. Such consolidation check dams, built in terraces, attempt to prevent both headward and downward cutting into channel beds while also stabilizing adjacent hill slopes. They are further used to mitigate flood and debris flow hazards.Case studies validate the effectiveness of these check dam systems. On the Guerbe River in Switzerland, where a series of over 100 check dams had been implemented to control landslides and other natural debris flow disasters, it was found that the check dams reduced the amount of sediment being carried by the river. The dams did so by flattening out the riverbed and retaining an initial amount of sediment, although the main impact came from the gradient change. However, the benefits of these dams are often erased in the event of a failure, which releases trapped sediment and destabilizes both the riverbed and surrounding land, increasing the risk of disaster.