11 - Diversion Dams / Forest Road Drainage Installation Practices in the Pacific Northwest

Forest Road Drainage Installation Practices
in the Pacific Northwest

11 - Diversion Dams	Downstream Dams, Upstream Dams

Diversion of a stream can be achieved with a diversion ditch, a gravity system, or a "pump-around". A diversion ditch simply carries the water around the site of the drainage structure to be built, allowing the installation to take place in the dry. For pump-arounds and gravity systems, dams have to capture all the water in the natural stream channel. The water is then moved by gravity or pumps to below the drainage structure to dry up the channel for "instream" work. Develop and follow a step-by-step stream diversion plan.

Downstream Dam

Build the downstream dam from hay bales wrapped in filter cloth, just below your work site, at a wide spot in the creek (low velocity). Stay as close as possible to the drainage structure to be installed, but leave enough room to work safely. The lower the stream gradient, the further downstream the dam has to be built, otherwise backed up water will saturate your work site. Weigh hay bales down with rocks, and support downstream side of bales with rocks also.

The dam's primary purpose is to intercept minor sediment or suspended particles generated during the installation of the crossing structure if the channel is not completely dry. The dam is also very useful when water is permitted to re-enter the stream channel after the installation. Start up pump(s) as soon as silty water builds up, and pump to a depression in the vicinity that will not allow any erosion. Maintain the downstream dam as long as possible or until water runs clear. In the picture above, note that silty water generated during the crossing installation is pumped from the left side of the hay bale dam onto the forest floor nearby. The polyethylene pipes supply clean water from above the work site to fish below the dam.

Upstream Dam

Build dams with sandbags, from logs or lumber reinforced by a tarp, or with a combination of the above. With the downstream failsafe dam in place to remove minor sediment, one scoop of the excavator bucket can help create a good sized catch basin for the upstream dam.

To capture the entire flow of the stream, a water-tight dam is essential. It can be difficult to obtain a tight seal along the streambank or in cobbly stream channels. From your sediment control kit, fill triple garbage bags (60cm x 90cm, or 2ft x 3ft) to 1/4 with sand or gravel, then fold the front of each bag upstream to seal the dam. Use sandbags to secure the intake of the diversion pipe (500 mm or 20" or smaller polyethylene pipe) for gravity diversions. Allow enough length of this pipe to maintain adequate working space. Calculate water flows to have enough hoses and pumps (spares!) and gas for "pump-arounds", but avoid using pumps for jobs which can't be completed in one shift. Fish removal is mandatory for the dewatering of channels.

11 - Diversion Dams