Site Isolation, Maintaining Downstream Flow and Dewatering

To prevent drying up downstream habitats on a stream or river, you must maintain one hundred per cent of the downstream flow. Ensure you have a plan and equipment ready to do this. If you will be using a diversion channel to move water around the site, you must build it before you place the coffer dam, and you should line it with rock and a geotextile fabric or heavy poly plastic to prevent scour and erosion.

If bypassing water using pumps, the pumps must be sized to handle flows, including potential high-flow situations such as rainfall or runoff events. If using pumps, you must check them to ensure flows are not cut off if pumps should fail. A backup pump should be set up and ready to go in the event the main pump fails. It is not advisable to conduct work that will require isolation and maintaining flow during spring melt, as stream flows are highly variable at this time of year and it is difficult to maintain flow.

• Pump intakes must not disturb the lake bottom/streambed. You must screen pump intakes used in fish-bearing watercourses to prevent entrainment and impingement of fish. Properly sized screens will prevent fish from being sucked into the pump or against the screen. For more information on screening requirements, please see Fisheries and Oceans Canada’s (DFO) Fish Screen Guidelines.
• Water re-entering the watercourse from the pumping work must be clear of sediment (equal or better water clarity than the receiving water). You should release the water into a well-vegetated or an armoured area to prevent any scouring or erosion to the bed and bank

Fish Salvage

Once you isolate the site from the main water, and before starting any in-water work such as dredging, excavation or dewatering of the site, you must ensure fish are properly removed from the isolated work area. Once a salvage has been completed, fish should not be able to re-enter the isolated site. Fisheries biologists typically perform fish removal using appropriate equipment. A Fish Salvage Permit must be obtained from the Saskatchewan Ministry of Environment (MoE) before handling or capturing of any fish. You can obtain a permit from MoE (fisheries.permits@gov.sk.ca), or the fisheries staff working with you will have already obtained the permit. To protect fish during sensitive life stages (such as spawning), in-water work and the use of site isolation is not typically allowed during the Restricted Activity Timing Window .

Once the coffer dams are in place and water is being bypassed around the site, and the fish salvage has been completed to the required standards, you can begin dewatering your work area. Often water being pumped out of the isolated work site will contain suspended sediment; you will need to remove this sediment from the water before water can re-enter the watercourse. You will need to ensure the sediment-laden water (i.e. dirty water) is discharged into a vegetated area or into a settling basin large enough to allow the sediment to settle out. Vegetated areas can help settle out fine sediment and these areas are less likely to scour and erode from the discharge.

• Protect all pump discharge areas from scour and erosion with rock, geotextile fabric or some other energy-dissipating device. Pumped water must not cause erosion or further introduce sediment into the watercourse. Remove the energy-dissipating material when works are complete.
• Monitor pumps and downstream flows. Sediment-laden water (i.e. dirty water) from the work site must not re-enter the watercourse.

Note: if water continues to enter your isolated site, dewatering may need to continue as you are working. This will depend on the nature of your site. Upon completion of your work in the isolated area, you must remove all equipment and cofferdam materials.

• In flowing water, remove the downstream isolation berm first and then slowly remove the upstream dam, allowing water to re-enter the channel previously isolated. You must completely remove dams from the watercourse and return the bank and bottom to pre-disturbance conditions.
• To prevent water from rushing into the work site, consider pumping water into the isolated work area to equalize the pressure on both sides of the coffer dam before breaching the coffer dam.

Note: if water continues to enter your isolated site, dewatering may need to continue as you are working. This will depend on the nature of your site.

Upon completion of your work in the isolated area, you must remove all equipment and cofferdam materials.

• In flowing water, remove the downstream isolation berm first and then slowly remove the upstream dam, allowing water to re-enter the channel previously isolated. You must completely remove dams from the watercourse and return the bank and bottom to pre-disturbance conditions.
• To prevent water from rushing into the work site, consider pumping water into the isolated work area to equalize the pressure on both sides of the coffer dam before breaching the coffer dam.

For some projects, it may be acceptable to do the work without dewatering the work area, but still isolating the work area from the rest of the water body. This is often done using floating or staked sediment barriers to contain suspended sediment within the work area. It is important that the measures are designed for your expected site conditions (e.g., depth of water, wave action) and installed according to the manufacturer’s directions and best management practices. The sediment barriers must also be routinely inspected to ensure they are functioning as intended. Once the project is complete, sediment barriers must remain in place long enough (often several days) to allow suspended sediment to settle to the bottom of the water body before removal.