Diversion, Temporary
Definition
A ridge or channel, or a combination ridge and channel, constructed across sloping land; or at the top or bottom of a steep slope used to convey runoff water.
Purpose
- To reduce slope lengths, break up concentration of runoff, and move water to stable outlets at a non-erosive velocity.
- To protect work areas from upslope runoff.
- To divert sediment-laden water to an appropriate sediment-trapping facility.
Where Practice Applies
This practice applies to construction areas where runoff can be diverted and disposed of properly to control erosion, sedimentation, or flood damage. Specific locations and conditions include:
- Above disturbed existing slopes, and above cut or fill slopes to prevent runoff over the slope;
- Across unprotected slopes, as slope breaks, to reduce slope length;
- Below slopes to divert excess runoff to stabilized outlets;
- Where needed to divert sediment-laden water to sediment traps;
- At or near the perimeter of the construction area to keep sediment from leaving the site;
- Above disturbed areas before stabilization to prevent erosion and maintain acceptable working conditions.
- Where active construction activities make the use of a permanent diversion unfeasible.
Temporary diversions may also serve as sediment traps when the site has been over excavated on a flat grade. They may also be used in conjunction with a sediment fence.
Advantages
Diversions are among the most effective and least costly practices for controlling erosion and sedimentation.
Planning Considerations
- A temporary diversion is intended to divert overland sheet flow to a stabilized outlet or a sediment trapping facility during establishment of permanent stabilization on a sloping disturbed area. When used at the top of a slope, the structure protects exposed slopes by keeping upland runoff away. When used at the base of a slope, the structure protects adjacent and downstream areas by diverting sediment-laden runoff to a sediment trapping facility.
- If the diversion is going to remain in place for longer than 15 days, it should be stabilized with temporary or permanent vegetation.
- It is important that diversions are properly designed, constructed and maintained since they concentrate water flow and increase erosion potential. Particular care must be taken in planning diversion grades. Too much slope can result in erosion in the diversion channel or at the outlet. A change of slope from steeper grade to flatter may cause deposition to occur. The deposition reduces carrying capacity and may cause overtopping and failure.
- Frequent inspection and timely maintenance are essential to proper functioning.
- Sufficient area must be available to construct and properly maintain diversions. It is usually less costly to excavate a channel and form a ridge or dike on the downhill side with the spoil than to build diversions by other methods. Where space is limited, it may be necessary to build the ridge by hauling in dike fill material or using a sediment fence to divert the flow. Use gravel to form the diversion dike where vehicles must cross frequently.
- Temporary diversions may be planned to function one year or more, or they may be constructed anew at the end of each days grading operation to protect new fill.
- Temporary diversions may serve as in-place sediment traps if overexcavated 1 to 2 feet and placed on a nearly flat grade. The dike serves to divert water as the stage increases. A combination silt fence and channel in which fill from the channel is used to stabilize the fence can trap sediment and divert runoff simultaneously.
- Wherever feasible, build and stabilize diversions and outlets before initiating other land-disturbing activities.
Design Criteria
Temporary diversions must be planned to be stable throughout their useful life and meet criteria given below. Otherwise, they should be designed as permanent diversions.
Drainage area
Not more than three acres.
Capacity
Peak runoff from 10-year storm.
Minimum cross section
| Top Width | Height Side | Slopes |
| 0 ft. | 1.5 ft. | 4:1 |
| 4 ft. | 1.5 ft. | 2:1 |
Grade
The grade may be variable depending upon the topography and must have a positive grade to the outlet. The maximum channel grade should be limited to 1.0 percent.
Spacing
The maximum spacing of diversions on side slopes or graded rights-of-way should be no greater than the following:
| Land Slope (%) | Spacing (ft.) |
| 1 or less | 300 |
| 2 | 200 |
| 3-5 | 150 |
| 5 or greater | 100 |
- Diverted runoff should outlet onto a stabilized area, into a properly designed waterway, grade stabilization structure or sediment trapping facility.
- Diversions that are to serve longer than 30 working days should be seeded and mulched as soon as they are constructed to preserve dike height and reduce maintenance.
Maintenance
- Inspect temporary diversions once a week and after every rainfall.
- Damage caused by construction traffic or other activity should be repaired before the end of each working day.
- Immediately remove sediment from the flow area and repair the diversion ridge.
- Check outlets carefully and make timely repairs as needed.
- When the area protected has been permanently stabilized, remove the ridge and the channel to blend with the natural ground level, and appropriately stabilize it.
References
North Carolina Department of Environment, Health, and Natural Resources, Erosion and Sediment Control Field Manual, Raleigh, NC, February 1991.
North Carolina Sediment Control Commission, Erosion and Sediment Control Planning and Design Manual, Raleigh, NC, September, 1988.
Washington State Department of Ecology, Stormwater Management Manual for the Puget Sound Basin, Olympia, WA, February, 1992.