Cofferdams and small dikes are temporary barriers or dams used to stop or redirect the flow of water from a construction site work area, such as a support column foundation, a shoreline retaining wall location, a trench crossing at a stream or river, or other area adjacent to or within a body of water.
Temporary diversion barrier controls (e.g. cofferdams) allow for “working in the dry” when construction projects are adjacent to or within surface waters. They prevent water from entering work zones on or near waterbodies, such as where excavation, concrete pouring, drilling, or other tasks are being conducted. They function by holding back water and creating a relatively dry area for construction activities. Diversion barrier controls include water exclusion enclosures adjacent to banks, within a river or lake, or water exclusion areas along stream reaches that have been dewatered by damming the upstream channel and creating a bypass ditch or pipe to deliver the diverted flow downstream beyond the work area (e.g., pipe trench location). Diversion barrier controls also prevent work zone sediment from entering the waterbody.
Diversion barrier controls are appropriate for work zones that require dewatering and water exclusion during the construction period. Cofferdams and temporary dikes are almost always removed after work is completed. Floating silt curtains may be used during the installation or use of diversion barrier controls to minimize turbid conditions in the waterbody.
Temporary diversion barrier controls such as cofferdams and smaller dikes or barriers are very site specific. The type and configuration of a barrier is driven by the height and resulting pressure of water to be excluded, the underwater conditions (i.e., soil, rock, etc.), the length of the barrier, the length of time and season of use, and other criteria. Where temporary dewatering and water exclusion can be accomplished, diversion barrier controls provide a means to complete work at underwater locations. Sheet pile cofferdams are often used for larger projects, such as bridge piers. Smaller, easily constructed barriers made of wood, earthen berms, Jersey wall sections, water-filled tubes, sandbags, and other materials can be used for smaller projects. Portable fabric dams can also be used to exclude water while preventing downstream siltation from eroding earthen dams.
Permittees must establish sediment control BMPs on all downgradient perimeters of the site and downgradient areas of the site that drain to any surface water. Diversion barrier controls keep water out of the work zone and sediment out of the waterbody. A floating silt curtain placed in the water is not a sediment control BMP to satisfy item 9.2 except when working on a shoreline or below the waterline. Immediately after the short term construction activity (e.g., installation of rip rap along the shoreline) in that area is complete, permittees must install an upland perimeter control practice if exposed soils still drain to a surface water.
Properly designed and installed cofferdams and other barriers are effective at keeping sediment out of the adjacent waterbody (see table below). Where cofferdams and temporary drainage ditches/channels are used to dewater or divert a stream, river, lake, or other area, special attention is required to ensure that bare soil along the bypass ditch or channel is stabilized. Methods can include lining a small temporary stream bypass channel with plastic, where necessary, or using geotextile to cover exposed bare areas. Careful installation and removal of the barrier and restoration of the work area is required to ensure effectiveness.
Expected performance for temporary diversion barrier controls
Link to this table
|Flow attenuation||Little or no design benefit|
|Runoff volume reduction||Little or no design benefit|
|Soil erosion||Little or no design benefit|
|Sediment control||Primary design benefit|
|Nutrient loading||Secondary design benefit|
|Total suspended solids||Primary design benefit|
|Total phosphorus||Secondary design benefit|
|Heavy metals||Secondary design benefit|
|Floatables||Little or no design benefit|
|Oil and grease||Little or no design benefit|
Temporary diversion barrier control location is typically driven by engineering and operational staging considerations for the overall project (e.g., location of bridge support piers, lakeside armoring, sewer line route, etc.). Whenever possible, install and use temporary barriers during periods of low flow and warmer weather for greater efficiency and effectiveness. Winter and early spring may pose a greater risk of damage to diversions due to ice pressure and ice breakup. Components and materials are selected based on water height/hydraulic pressure, site conditions, longevity requirements, and other relevant factors. Temporary diversion barrier controls should be sited, constructed, and operated so as to cause minimal disruption to the aquatic environment, especially during fish spawning. Note that these projects usually need state, federal, or local permits. For example, placement of cofferdams in regulated waterbodies requires permit coverage from the US Army Corps of Engineers under Section 404 of the Clean Water Act, and a Water Quality Certification from MPCA.
Small barriers (i.e., less than 2 to 3 feet high) can be installed by field personnel based on standard designs and industry practice. Larger barriers require site specific engineering analysis. Design parameters for cofferdams and other water exclusion barriers incorporate the planning considerations noted above, along with safety concerns regarding stability, the power of flowing water at stream and river locations and other water related safety issues. Other design considerations include the following.
Detailed guidance on planning and executing projects within and adjacent to waterbodies can be found in Chapter 3 of the Minnesota DNR’s Best Practices Manual: Best Practices for Meeting DNR General Public Waters Work Permit GP 2004-0001 – Methods for In-Water Construction. In addition, MnDOT Specification 2451.3.A.3.a (Cofferdams; page 282) provides guidance for cofferdam placement and removal.
Due to the proximity to regulated waters, temporary diversion barriers require continuous observation and inspection throughout mobilization, installation, and deconstruction to ensure minimal impacts on nearby waterbodies. Key inspection guidance includes the following.
Maintenance for temporary diversion barriers includes responding to the inspection and monitoring findings listed in the preceding subsection. Particular concerns include turbidity in the adjacent waterbody, erosion and sediment runoff from staging/mobilization areas, and performance of the BMP(s) responsible for sediment removal at the dewatering discharge location. In general, find and eliminate sources of turbidity, or use floating silt curtains to prevent turbid waters from migrating away from the immediate construction area. Clear sediment and debris from dewatering pump intake screens and filters as needed. Clean discharge area filters (e.g., filter fabric, silt fencing, etc.) and remove accumulated sediment before it restricts or consumes half of the effective treatment volume or storage capacity. Replace or clean filter bags as needed.
The following provide examples of maintenance items to inspect (Toronto and Region Conservation Authority).
The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2014.
|Bid Item||Item Description||Units1||Average Price|
|2105.601/00045||Temporary Stream Diversion System||LS||$26,292.50|
|2452.601/00011||Steel Sheet Piling (Temporary)||LS||$107,614.29|
|2452.618/00011||Steel Sheet Piling (Temporary)||SF||$16.75|
1LS = lump sum; SF = square foot
Except where more stringent requirements are presented in this guidance, cofferdams (temporary dikes) shall comply with MnDOT and other state requirements. Primary design references include:
The following is a list of additional resources that are not specific to Minnesota: