Check dams, also called ditch checks, dikes, wattles, etc., are temporary or permanent linear structures placed perpendicular to concentrated flows such as in drainage ditches, channels, and swales to reduce flow velocities and prevent channel down-cutting. Some sediment trapping may occur during low flows. Check dam materials may include rock, fiber logs (e.g., wattles), triangular sediment dikes, sand bags, and other materials or prefabricated systems. Straw/hay bales and silt fences should not be used for check dam applications, as they are not intended for concentrated flow areas.
Check dams are not intended as a replacement for proper ditch/channel stabilization (e.g., erosion blanket or turf mat over seed, use of rip-rap, etc.). Check dams help reduce ditch and channel velocities, prevent erosion, and trap small amounts of sediment by intercepting flow along a ditch or channel. The disruption in flow direction and speed creates low velocity areas on the upgradient side of the check dam, causing deposition of heavier sediment particles and resulting in reduced scour potential (i.e., lateral and vertical erosion). Under low-flow conditions, water ponds behind the structure and then slowly drains through, infiltrates, or evaporates. Under high-flow conditions, water flows over and/or through the structure. The main function of a check dam is to decrease velocity, not to collect sediment, although sediment capture and increased infiltration is an added benefit. Check dams are not a suitable substitute for major perimeter sediment trapping measures, and can be easily washed away by high ditch/channel flows if they are not designed or installed properly.
Check dams may be used in ditches or channels to reduce erosion and trap sediment. Check dams are relatively inexpensive and easy to install. They are not approved for use in regulated waterbodies (i.e., Waters-of-the-State) without permit coverage from the U.S. Army Corps of Engineers under Section 404 of the Clean Water Act. MPCA water quality certification requirements also apply.
Check dams are used to regulate flow velocities, reduce scour erosion, and trap small quantities of sediment along higher-risk ditches and channels that have slopes greater than 10 percent and soil types conducive to erosion (i.e., sandy/silty soils). They are appropriate for both temporary and permanent ditches and swales. While most flatter and shorter channels (i.e., slope < 3%, length < 200 feet) generally do not need check dams if they are stabilized immediately after construction (i.e., with sod, or seed and the appropriate rolled erosion control product), longer and steeper ditches can benefit from check dam installations. When evaluating the use of check dams for a particular site, consider the following:
Check dams are part of the overall stabilization system for a ditch or channel, and their use is driven by site conditions – specifically, channel slope, length, soil type, and flow velocities. Section IV.B.4 of the 2013 MPCA Construction Stormwater General Permit states that the permittee(s) “must stabilize the normal wetted perimeter of any temporary or permanent drainage ditch or swale that drains water from any portion of the construction site, or diverts water around the site, within 200 lineal feet from the property edge, or from the point of discharge into any surface water. Stabilization of the last 200 lineal feet must be completed within 24 hours after connecting to a surface water or property edge.”
In addition, the permittee(s) must “complete stabilization of the remaining portions of any temporary or permanent ditches or swales within 14 calendar days after connecting to a surface water or property edge and construction in that portion of the ditch has temporarily or permanently ceased.” Check dams should be used in conjunction with seed, mulch, blankets, mats, and/or other stabilization measures to help meet these requirements.
Check dams are moderately effective in trapping sediment, and highly effective in preventing downcutting in a ditch or channel when used with appropriate rolled erosion control products installed over seed. They generally provide relatively fair-to-good removal of coarse and medium-sized sediment from runoff; however, most fine silt and clay particles will pass through the voids on these structures. Their primary benefit lies in preventing erosion prior to seed germination and growth in vegetated ditches and channels. Table 7 summarizes expected performance for an array of typical water quantity and quality target constituents for check dams.
Expected performance for check dams
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Water Quantity | |
---|---|
Flow attenuation | Secondary design benefit |
Runoff volume reduction | Secondary design benefit |
Water Quantity | |
Pollution prevention | |
Erosion prevention (i.e., for channel banks) | Primary design benefit |
Sediment control | Secondary design benefit |
Nutrient loading | Secondary design benefit |
Pollutant removal | |
Total suspended solids | Secondary design benefit |
Total phosphorus | Secondary design benefit |
Heavy metals | Secondary design benefit |
Floatables | Secondary design benefit |
Oil and grease | Secondary design benefit |
Check dams are rarely effective in steep channels (i.e., more than 10% slope), and are easily dislodged by high ditch/channel flow velocities if they are not designed, sized, and installed properly. Common causes of failure include:
Planning guidelines and material selection for check dams are driven by site considerations (e.g., ditch/channel slope, length, flow velocities, soils) and the longevity desired. In general, ditch/channel slope should not exceed 10 percent (otherwise, a drop structure should be considered), the drainage area should not exceed 10 acres, and flow velocities should not exceed 12 feet per second for a 10-year, 24-hour storm frequency.
When installing drainage ditches or channels, plan to stabilize them immediately after construction, as required. Un-vegetated/unarmored ditches and channels erode quickly on contact with flowing water – especially when they are long and steep. Include check dams as part of the ditch erosion prevention and sediment control system when ditches are longer than 200 feet and steeper than 3 percent. Wide, flat swales (i.e., < 1% slope, > 4 foot width) can also benefit from check dam installations when used as temporary linear sediment traps during the construction phase.
Check dam types vary by composition and installation approach. Fiber logs, filled bags, and other check dams can include items made of straw, wood fiber, compost, wood slash, soil, sand, aggregate, riprap, and specialty products. These specific material compositions of various check dams largely determine its longevity once installed in the field. Spacing of the check dams within the ditch or channel will vary in accordance with slope and soil type (see Table 8 for example spacing).Some check dam types, like decomposable fiber logs, typically do not require removal since they can be left in the ditch or channel to deteriorate and add organic matter to further support vegetation establishment. Where appropriate, rock check dams can be spread out along ditch bottoms after the channel is vegetated as long as disturbance from equipment to do is minimized or stabilized o immediately afterwards. Manufactured sediment dikes and rock bag check dams must be removed after stabilization, prior to closing out the project permit.
General installation guidelines for all check dams include the following:
Example check dam types spacing slope applications and longevity
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Check Dam Type | Spacing for Various Ditch Slopes (feet)1 | Slope Applications | Longevity | |||
---|---|---|---|---|---|---|
Up to 2% | 3-5% | 6-9% | 10-15% | |||
Mixed size rock (2 ft height) | 100 | 67-40 | 33-22 | 20-13 | Up to 15% | > 2 years |
Rock bags (16 inch height) | 75 | 50-30 | 25-17 | 15-10 | Up to 15% | Up to 1 year |
Rock bags (10 inch height) | 42 | 28-17 | 14-9 | 8-6 | Up to 15% | Up to 1 year |
Triangular sediment dike (10 inch height) | 42 | 28-17 | 14-9 | 8-6 | Up to 15% | 1 to 2 years |
Fiber log – wood / mulch (10 inch) | 42 | 28-17 | 14-9 | 8-6 | Up to 15% | 1 to 2 years |
Fiber log – straw (10 inch) | 42 | 28-17 | 14-9 | NA | Up to 10% | Up to 6 months |
1 Note: reduce spacing intervals for highly erodible soils.
Rock/sand bags are manufactured from durable, weather resistant tightly woven geotextile fabric material sufficient to prevent leakage of the filler material. They are relatively low cost and easy to install, move, replace, and reuse (if not damaged). Rock/sand bag check dams are a good short-term solution where concentrated flows are causing erosion, and can also be used to divert and slow the velocity of small flows. Biofilter bags may also be used when firmly staked to the ground. Additional rock/sand bag considerations include:
• Use fabric or net bags, with one-inch stone. • Fill bags only three-fourths full of rock, to reduce gaps in the check dam. • Construct check dams 1.5 to 2 feet high, with side slopes no steeper that 2H:1V. • Rock/sand bags are not appropriate on steep slope applications or where water velocities or volumes are high. • They can easily be damaged by construction equipment and are generally only effective for a few months. • The ends of the bags should be tightly abutted and overlapped to direct flow away from bag joints.
For example spacing and other details, refer to Table 8 above.
Triangular sediment/silt dikes are pre-fabricated triangular shaped blocks typically made of foam or other flexible, lightweight material and covered with geotextile fabric. The geotextile extends from the bottom of the dike to provide aprons on the upslope and downslope sides, and the dike is anchored by trenching and stapling the aprons. They will form a check dam when laid in the channel. Material, section length, and weights vary among manufacturers, but they are all designed *to be lightweight and relatively easy to install and maintain. Consider the following when using triangular dikes: