Check dams are commonly used for energy dissipation and/or sedimentation control in streams. They are constructed of boulders, sand, timbers or gabions covered with crushed rock or other earthen materials. One important design consideration is the hazard to downstream property posed by dam failure. Proper design and construction techniques will reduce this hazard.
Check dams constructed of earth fill are shown in Figure 16. These check dams are constructed of crushed limestone and other soil materials. The core is typically smaller sized crushed material enclosed by courser, large sized rock. Depending on the intent, a check dam may be porous and act as a filter or covered with clay, making it impervious. Both pervious and impervious check dams can contain outlet and overflow structures. Drop structures and weirs are used with either type of check dams.
Figure 16 Alternative Check Dam Designs
Porous Check Dam
Porous check dams are primarily used for trapping sediment. Sediment is trapped upstream of the check dam while the storm water runoff percolates through the voids of the crushed limestone used in the dam's construction. On steeply sloped channels, porous check dams are also used for energy dissipation. Check dams designed for trapping sediment can be used temporarily during construction or permanently in watersheds subject to erosion and channel degradation.
Rock check dams have large void spaces. Depending on the level of sediment trapping required, smaller crushed limestone and filter fabric are added to trap more sediment. Gabions, when used as a check dam, are strictly used for energy dissipation and flow control. Because of the large sized rock used, a gabion check dam traps very little sediment.
Impervious Check Dam
Impervious check dams are primarily used for energy dissipation and water level control. Erosion and scour are controlled in steep channels by installing a number of check dams, thus, effectively reducing the velocity and energy of the stream. Check dams used in series change the flow characteristics of a stream by providing backwater ponding upstream of each dam.
A check dam is placed in the path of a stream of storm water runoff containing the potential to cause scour and erosion. Potential is identified as an adequate volume of water at a sufficient velocity to cause damage. The location of the check dam is dependent on surrounding topography and size of the drainage basin. The dam must be placed in a location allowing it to function and be accessible for maintenance. As the stream of water meets the check dam, it is slowed and released at a designed rate. Storm water in excess of the dam's capacity flows over the dam. The size of the drainage basin identifies the design capacity. Topography surrounding the dam's location determines the practical capacity available for storage. Dams with a design capacity greater than practical capacity will over-top more frequently. A dam that is over-topped will suffer degradation and require frequent maintenance.
The Natural Resource Conservation Service or local County Engineer can provide assistance in designing check dams. A field inspection of a potential location will confirm the site's practical capacity to store storm water.
Multiple small check dams (Figure 17) are more effective than one or two large check dams. Smaller dams are less obtrusive and create small settling pools that collect sediment and alter storm water flow efficiently. Over time, these small pools create a natural setting by attracting native plant species and wildlife.
Figure 17 Check Dams