Temporary Diversion Method 1 of 3
Temporary Diversion Methods
First paper in a three-part series.
Any construction activities to take place in a stream, along the banks, or beneath an active channel require rerouting water or restricting flows to a designated portion of the stream channel. Construction activities of detention ponds, dams, in-stream grade control structures, utility installation and other activities, including maintenance, require water quality protection. To protect water quality in and around construction by waterways, Temporary Diversion Methods (TDMs) are used. TDMs include temporary diversion channels, pump arounds, piped diversions, coffer dams and other similar practices.
Berms or coffer dams constructed within the stream are used to confine flow to one side while work progresses on the dry side of the berm, are more suitable for larger streams. Whereas, for smaller streams or construction of detention basins or dams, the TDM may be to divert the entire waterway.
A pump and/or bypass system may serve as a TDM for low baseflows on a short duration, typically less than a month for construction. Regardless of TDM used, construction should be scheduled for drier times of the year, depending on your region and progress quickly to reduce exceeding TDM capacity. Construction timing and duration are primary considerations to determine design flow most appropriate, to not consider these variables would result in inflated project costs, land disturbances including increased erosion and little to no water quality benefit. If a TDM is used, it should be on a per project, site-specific basis.
However, for short term projects (typically maintenance of utilities and stream crossings, minor repairs to outfalls and eroded banks) constructed during drier times of the year, installation of TDM may cause greater disturbance than the actual construction taking place. Alternatively, winter months when ground is frozen, as is the case in the energy sector related projects in Western Canada, may be the only time of the year providing access to the site, and so the TDM will be adapted to the weather and climate. Just because the ground is frozen, a creek or river may not be.
Therefore, many factors should be considered to define if and/or which TDM to use.
- Special consideration of water quality and aquatic life of waterway
- Size of stream, anticipated flow rates and tributary watershed areas should be considered, especially to larger streams with large tributary and high flow rates.
- Realistic project duration estimates based on the time of the year the activity is occurring.
- Is there adequate space to construct TDM?
- Is surrounding land highly congested or does it have limited Right of Way?
- Will the greater environmental impact be the construction of TDM or completing project without TDM? This applies to short duration, small scale maintenance such as bank erosion repair, drop structure or pond maintenance, outfall improvements or repairs and other limited construction activities.
- Considerations of consequences based on difference of Baseflow vs. peak flows; Seasonal, irrigation flow increase and/or weather (storm runoff / flash floods) exceeding diversion capacity and/or diversion failure