Temporary Diversion Method 2 of 3
Engineered River Diversion
Temporary Diversion Methods
Second paper in a three-part series.
Design, Selection and Type
Based on uses and design considerations, NCS Fluid Handling Systems design engineering can aid in the determination of the most suitable type of diversion. Working close with the Clients representatives, key equipment specialists, the NCS engineers and designers gather TDM sizing parameters that include: tributary area, imperviousness, Factors of Safety (FoS) and seasonal sizing coefficient.
Using the applicable sizing methodology and performing necessary calculations, NCS engineers determine temporary diversion design flow.
NCS engineers and designers along with this collaborative client team consider:
Approximated Project Duration (as an example)
|
Long Duration |
Short Duration |
Interim Duration |
|
3 months or greater |
Less than 1 month |
Between 1-3 months |
|
IE: Capital Improvement Projects (CIPS), Traditional Land Development |
IE: Repairs and Maintenance |
IE: Small projects, Repairs and Maintenance |
Types of TDMs
A Channel Diversion may divert smaller streams for construction or river access – such as dams, detention basins or pipeline crossings.
For short duration projects with low baseflows and/or when there is limited space, a Pumped Diversion or Piped Diversion may be used and possibly incorporate a Channel Diversion.
To confine flow for any size of stream, a Berm or Coffer dam may be appropriate.
NCS Engineers and System operational team can be contacted to discuss options. Regardless, it is recommended at a minimum the design steps laid out below are reviewed for each TDM.
Channel Diversion:
- Determine temporary diversion design flow rate using sizing methodology.
- On existing and proposed site conditions, determine slow. Based on lining material, determine maximum permissible velocities. Pay attention to diversion channel entrance bends, transitions and downstream return to stream where scour forces require greater protection. Because of applicable projects being a short duration, it is not long enough to establish adequate vegetative lining but must be designed to be stable enough for the design flow.
- Determine the channel geometry and check the capacity.
- Determine depth of flow. Provide a minimum of 0.5 foot of freeboard above the design water service elevation.
Berm or Coffer Dam
- For coffer dams or berms that are intended to isolate a portion of the stream from the work area steps 1- 4 should be applied to the “wet” side of the coffer dam or berm.
- Fish migration, handling or relocation must be reviewed and considered as part of a responsible plan.
- Determine temporary diversion design flow rate using sizing methodology.
- Based on existing and proposed site conditions determine channel slope.
- Perform initial channel sizing calculations. Based on lining material, determine maximum permissible velocities. Pay attention to diversion channel entrance bends, transitions and downstream return to stream sections. Environmental consideration to preserve native vegetative and topography may need to be designed into project plan as well to ensure area is stable enough for the design flow. This stability criterion applies to the stream-side of berms when berms are used to isolate a work area within a stream.
- Kinetic energy walls may be required to be installed prior to temporary berms or water dams.
- Determine the channel geometry and check the capacity.
Pumped Diversion
- Determine temporary diversion design flow rate using sizing methodology.
- A backup pump (or pumps) with capacity equal to or greater than the diversion design flow rate should be on site and in good working order always.
Piped Diversion
- Determine temporary diversion design flow rate using sizing methodology.
- To accommodate the design flow by sizing the pipe using no more than 75 percent of the pipe full flow capacity for contingency.