A check dam is a specialized structure built across a channel, drainage ditch, or swale to manage the movement of water. It is a relatively small, often temporary barrier designed to slow down the flow rate of runoff. These structures function primarily as flow regulators, interrupting the natural path of water to mitigate its destructive power. By controlling the speed of the water, check dams help reduce the energy of the flow and promote the settling of suspended materials.
Core Function: How Check Dams Slow Water and Trap Sediment
A check dam reduces the velocity of water flowing through a channel. When water encounters the barrier, the cross-sectional area of the flow increases, which significantly lowers the speed of the water. This reduction in flow velocity correlates to a decrease in the stream’s energy, which is required for effective erosion control.
The diminished energy means the water loses its capacity to carry its suspended load, forcing particles to settle out. Sediment drops to the channel floor immediately upstream of the structure. This process of deposition builds up the channel bed, effectively stabilizing the streambank by reducing the downward scouring action.
Over time, a series of check dams creates a cascading “step-pool” system within the channel. This terraced structure helps maintain the longitudinal profile of the stream, preventing the downward cutting or deepening of the streambed known as headcutting.
The pooling of water behind the barrier also allows for increased infiltration into the surrounding soil. This localized saturation can slightly raise the water table immediately adjacent to the channel. A higher water table can support healthier, more resilient vegetation along the banks, further contributing to long-term channel stability and reducing runoff volume.
Types and Materials Used in Check Dam Construction
Check dams are broadly classified based on their expected lifespan and the materials used in their construction. Temporary check dams are intended for short-term use, often during construction projects, to manage runoff until permanent vegetation is established. These structures are typically easily removed or designed to degrade naturally over time after they have served their purpose.
Common materials for temporary use include straw bales, which are secured and staked into the channel to filter runoff and slow the flow. Sandbags or proprietary fabric barriers, often filled with gravel or local soil, provide a more robust, non-degradable option for managing moderate flow volumes. The choice often depends on the required durability and the expected duration of the erosion hazard at the site.
Permanent and semi-permanent check dams are constructed using much harder, more resilient materials to withstand repeated high-velocity flows. Structures made of rock or stone are dry-stacked or mortared to provide long-term stability against erosive forces. Log check dams utilize timber secured across the channel, which is a common practice in forested environments.
Gabion baskets, which are wire cages filled with rock, offer flexibility and strength for semi-permanent installations in larger channels with higher flow potential. The selection of material is highly dependent on factors like material availability, the sheer volume of water expected, and the desired lifespan of the structure.
Appropriate Locations for Installation
Check dams are most effective when installed in areas experiencing concentrated flow and high levels of erosion. They are specifically targeted for use in ephemeral streams, which are channels that only flow immediately after rainfall events, and in deeply incised gullies. Steeply graded drainage ditches, particularly along roadways or construction sites, are also prime candidates for installation.
The slope of the channel is a primary determinant of whether a check dam system is appropriate and how it must be designed. Channels with gradients between 2% and 10% often benefit most from these structures, as the flow energy increases substantially with slope. In channels steeper than 10%, more robust materials and closer spacing are necessary to manage the higher kinetic energy of the water.
Proper spacing between individual check dams is paramount for the system’s overall success and structural integrity. The general rule for placement is to ensure the top of the downstream structure is level with the toe of the next dam upstream. This configuration ensures that the ponded water behind one dam protects the base of the dam above it from erosive forces.
The size of the drainage area feeding the channel also dictates the height and strength of the structures required. Structures in larger drainage areas must be significantly taller and more robust to handle the increased volume of water and potential debris. Incorrect spacing or undersized dams can lead to the structure being quickly overwhelmed, causing failure and accelerating erosion.