6.22_Diversion Dike (Perimeter Protection)

Practice Standards and Specifications
6.22
P D —i
Definition A dike or dike and channel constructed along the perimeter of a disturbed
construction area.
Purpose To prevent storm runoff from entering the work area, or to prevent sediment -
laden runoff from leaving the construction site.
Conditions Where Diversion dikes may be located at the upslope side of a construction site to
prevent surface runoff from entering the disturbed area or at the downslope
Practice Applies side of the work area to divert sediment -laden runoff to on -site sediment traps
or basins. Diversion dikes do not usually encircle the entire area.
The upslope dike can improve working conditions at the construction site and
prevent erosion. The downslope dike assures that sediment -laden runoff will
not leave the site without treatment.
Planning A diversion dike is a special application of a temporary or permanent
diversion. It differs from other diversions in that the location and grade are
Considerations usually fixed, and the cross section and stabilization requirements are based on
the existing grade of the work boundary. Hence, the design cross section may
vary significantly throughout the length. Give special care to avoid erosive
velocities in steep areas. Identify areas where sedimentation will occur since
they are often subject to overtopping.
Immediately vegetate diversion dikes after construction, but make sure channel
flow area is stabilized during construction. Exercise caution in diverting flow
to be certain that the diverted water is released through a stable outlet and
that the flow will not cause flood damage. Diversion dikes may be either
temporary or permanent depending on site conditions (Figure 6.22a).
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Figure 6.22a Perimeter dikes prevent surface runoff from entering construction sites.
6.22.1

0
Design Criteria Drainage area-5 acres or less.
Capacity —consistent with the hazard involved and design life and with a 10
year peak runoff minimum.
Velocity —See Table 8.05a, Appendix 8.05.
Dike design— side slope: 2:1 or flatter
3:1 or flatter where vehicles must cross
width: 2.0 feet minimum top width
height: 1.5 feet minimum
freeboard: 0.5 feet minimum
settlement: 10% of total fill height minimum
Channel design— shape: parabolic, trapezoidal, or V-shaped
side slope: 2:1 or flatter
3:1 or flatter where vehicles must cross
stabilization: based on velocity by reaches
Grade —Dependent on site topography. Channel should have positive grade.
Outlet —Divert sediment -laden water into a temporary sediment trap or
sediment basin. Runoff from undisturbed areas should empty into an outlet
protection device such as a level spreader or riprap outlet structure unless well
stabilized natural outlets exist.
Construction 1. Remove and properly dispose of all trees, brush, stumps, and other
Specifications
objectionable material. Fill and compact, to natural ground level or above, all
ditches and gullies that will be crossed by machinery.
2. Disk the base of the dike before placing fill.
3. Ensure that the constructed cross section meets all design requirements.
4. Compact the dike by tracking with construction equipment.
5. Ensure that the top of the dike is not lower at any point than the design
elevation plus the specified settlement after it has been compacted.
6. Leave sufficient area along the dike to permit machine re -grading and
cleanout.
7. Immediately seed and mulch the dike after its construction, and stabilize the
flow portion in accordance with design requirements.
Maintenance Inspect diversion dikes once a week and after every rainfall. Immediately
remove sediment from the flow area and repair the dike.
Check outlets, and make timely repairs as needed to avoid gully formation.
When the area above the temporary diversion dike is permanently stabilized,
remove the dike, and fill and stabilize the channel to blend with the natural
surface.
6.22.2

Practice Standards and Specifications
References Surface Stabilization
6.10, Temporary Seeding
6.11, Permanent Seeding
6.14, Mulching
Outlet Protection
6.40, Level Spreader
6.41, Outlet Stabilization Structure
Sediment Traps and Barriers
6.60, Temporary Sediment Trap
6.61, Sediment Basin
Appendix
8.05, Design of Stable Channels and Diversions
6.22.3