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Chapter 3
Designing for Environmental Stewardship in Construction & Maintenance
3.8. Drainage Ditches, Berms, Dikes, and Swales

Ditches, berms, dikes and swales are temporary or permanent measures used to intercept and direct surface runoff to an overside/slope drain or stabilized watercourse, away from the road. Several variations of ditch treatments include raised curbs, berms, vegetated, rock-lined, and lead out ditches. Raised curbs, or berms, are used to prevent water from entering or exiting the roadway. Numerous lead out ditches are typically installed to remove water impounded by the curbs and release it where water can be handled appropriately, with minimal resource damage.

Ditch treatments can be used on roadsides, trails, parking areas, urban and rural settings, and any other place where managing runoff is important. A scoured, entrenched roadside ditch and the presence of gullying on the downslope side are indicators that a ditch treatment may be necessary. Ditches, berms, dikes and swales are usually implemented for the following purposes:

  • To convey flow around maintenance activities.
  • To divert flow away from maintenance stockpiles.
  • At the top of slopes to divert run-on from adjacent slopes and areas.
  • At bottom and mid-slope locations to intercept sheet flow and convey concentrated flows.
  • At other locations to convey runoff to overside/drains, stabilized watercourses, stormwater drainage system inlets (catch basins), pipes and channels.
  • To intercept runoff from paved surfaces.
  • Along roadways and facilities subject to flood drainage.

Vegetated ditches are ditches with vegetation to reduce water velocities, and erosion control grass mixtures are typically used to vegetate ditches. Vegetated ditches help improve the quality of stormwater that runs off a highway by slowing water velocities and trapping sediment, metals, nutrients, petroleum products, pesticides, bacteria and other contaminants.

Lead out ditches are built to carry water away from the roadway, onto grassed or forested areas, allowing infiltration and dispersion of water. Rock or stone-lined ditches reduce velocities and capture sediment in the interstices. Maintenance cleaning is required when trash or debris have accumulated.

Temporary diversion dikes are intended to divert overland sheet flow to a stabilized outlet or a sediment trapping facility during establishment of permanent stabilization on sloping, disturbed areas. 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. This practice is considered an economical one because it uses material available on the site and can usually be constructed with equipment needed for site grading. The useful life of the practice can be extended by stabilizing the dike with vegetation.

Design and Construction Considerations and Practices for Ditches, Dikes, and Swales

Caltrans has an excellent Fact Sheet on Earth Dikes/Drainage Swales and Lined Ditches available on-line, which provides implementation guidance, drawings, specifications, and maintenance considerations. The EPA and Florida DOT have online information on permanent diversions and diversions. Diversions are preferable to other types of constructed stormwater conveyance systems because they more closely simulate natural flow patterns and characteristics. Flow velocities are generally kept to a minimum. Recommended environmental stewardship practices for implementation of ditches, berms, drains, swales, and diversions include: [N]

Evaluate risks due to erosion, overtopping, flow backups or washout.

Consider outlet protection where localized scour is anticipated.

  • Examine the site for run-on from off-site sources.
  • Conveyances should be lined if high flow velocity is anticipated. Consider use of riprap, engineering fabric, asphalt concrete, or concrete.
  • Establish adequate vegetation as soon as possible after installation of a diversion.
  • Stabilize the drainage area above the diversion so that sediment will not enter and accumulate in the diversion channel.
  • Diversions should be constructed before clearing and grading operations begin. If used to protect a flat, exposed area, a diversion might be constructed as a dike or berm. Berms made of gravel or stone can be crossed by construction equipment.
  • Diversions should have stabilized outlets which will convey concentrated runoff without erosion. Acceptable outlets include paved flumes, stormwater conveyance channels, outlet protection, and level spreaders. Outlets should be constructed and stabilized prior to the operation of the diversion.
  • Disturbed areas draining into the diversion should be seeded and mulched prior to or at the time the diversion is constructed.
  • Permanent diversions should include a filter strip of close growing grass maintained above the channel. The width of the filter strip, measured from the center of the channel, should be one-half the channel width plus 15 feet (4.5 m).
  • Unless otherwise stabilized, the ridge and channel should be seeded and mulched within 15 days of installation in accordance with permanent seeding .
  • If the diversion dike is going to remain in place for longer than 30 days, it is very important that it be established with temporary or permanent vegetation.
  • The slope behind the dike is also an important consideration. If the channel slope is less than or equal to 2 percent, stabilization may not be required. If the slope is greater than 2 percent, the channel should be stabilized in accordance with BMPs for stormwater conveyance channels.
  • Whenever feasible, the dike should be built before construction begins on the project.
  • The dike should be adequately compacted to prevent failure.
  • The dike should be located to minimize damages by construction operations and traffic.

Slope Diversions for Intercepting Stormwater

Diversions are channels constructed across a slope with a supporting ridge on the lower side, to reduce slope length and to intercept and divert stormwater runoff to stabilized outlets at non-erosive velocities. On moderately sloping areas, they may be placed at intervals to trap and divert sheet flow before it has a chance to concentrate and cause rill and gully erosion. Diversions may be placed at the top of cut or fill slopes to keep runoff from upland drainage areas off the slope. They can also be used to protect structures, parking lots, adjacent properties, and other special areas from flooding.

Diversions are often constructed:

  • Where runoff from higher areas may damage property, cause erosion, or interfere with the establishment of vegetation on lower areas.
  • Where surface and/or shallow subsurface flow is damaging upland slopes.
  • Where the slope length needs to be reduced to minimize soil loss.
  • Below stabilized or protected areas. They should not be used below high sediment producing areas unless land treatment practices or structural measures, designed to prevent damaging accumulations of sediment in the channels, are installed with or before the diversions.
  • Diversions should not be placed on slopes greater than fifteen percent.

Level Spreaders

Level Spreaders are outlets for dikes and diversions consisting of an excavated depression constructed at zero grade across a slope whereby concentrated runoff may be discharged at non-erosive velocities onto an undisturbed area stabilized by existing vegetation. Their purpose is to convert concentrated runoff to sheet flow and release it onto an area stabilized by existing vegetation. This practice applies only in those situations where the spreader can be constructed on undisturbed soil and the area below the level lip is stabilized by natural vegetation. The water should not be allowed to reconcentrate after release.

  • Level spreaders should be constructed on undisturbed soil (not fill material).
  • The entrance to the spreader should be shaped in such a manner as to insure that runoff enters directly onto the 0 percent channel.
  • The level lip should be constructed on zero percent grade to insure uniform spreading of storm runoff.
  • The released runoff should outlet onto undisturbed stabilized areas in sheet flow and not be allowed to reconcentrate below the structure.

 

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Table of Contents
 
Chapter 3
Designing for Environmental Stewardship in Construction & Maintenance
3.1 Beyond Mitigation: Projects to Achieve Environmental Goals
3.2 Context Sensitive Design/Solutions
3.3 Avoiding Impacts to Historic Sites
3.4 Designing to Accommodate Wildlife, Habitat Connectivity, and Safe Crossings
3.5 Culverts and Fish Passage
3.6 Stream Restoration and Bioengineering
3.7 Design Guidance for Stormwater and Erosion & Sedimentation Control
3.8 Drainage Ditches, Berms, Dikes, and Swales
3.9 Design for Sustainable, Low Maintenance Roadsides
3.10 Designing to Reduce Snow, Ice, and Chemical Accumulation
3.11 Designing to Minimize Air Quality Problems
3.12 Design and Specification for Recycling
3.13 Designing to Minimize Noise
3.14 Lighting Control/Minimization
3.15 Design for Sustainability and Energy Conservation
3.16 Safety Rest Areas, Traveler Services, and Parking Area Design
   
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