2.6.4 BMP S137 - Maintenance of Ditches and Culverts
This BMP applies to the maintenance of all ditches and culverts that are specifically designed for or that can transport stormwater. Common road debris including litter, eroded soil, oils, vegetative particles, and heavy metals can be sources of stormwater pollutants.
Inspect roadside ditches and culverts regularly, as needed, to identify sediment accumulations and localized erosion.
Clean ditches and culverts on a regular basis, as needed. Ditches shall be kept free of rubbish and debris.
Vegetation in ditches often prevents erosion and cleanses runoff waters. Remove vegetation only when flow is blocked or excess sediments have accumulated. Conduct ditch maintenance (seeding and harvesting) in late spring and/or early fall, where possible. Consider leaving segments of undisturbed vegetation to provide natural filtration.
In the area between the edge of the pavement and the bottom of the ditch, commonly known as the “bare earth zone,” use grass vegetation, wherever possible. Vegetation shall be established from the edge of the pavement if possible, or at least from the top of the slope of the ditch.
Diversion ditches on top of cut slopes that are constructed to prevent slope erosion by intercepting stormwater and surface water must be maintained to retain their diversion shape and capability.
Do not leave ditch cleanings on the roadway surfaces. Promptly sweep dirt and debris remaining on the pavement as needed and at the completion of ditch cleaning operations.
Roadside ditch cleanings not contaminated by spills or other releases and not associated with a stormwater treatment system such as a bioswale may be screened to remove litter and separated into soil and vegetative matter (leaves, grass, needles, branches, etc.). The soil fraction may be handled as ‘clean soils’ and the vegetative matter can be composted or disposed of in a municipal waste landfill.
Roadside ditch cleanings contaminated by spills or other releases known or suspected to contain dangerous waste must be handled following the Dangerous Waste Regulations (Chapter 173-303 WAC) unless testing determines it is not dangerous waste. Specific cleanup standards are set forth in the Model Toxics Control Act (Chapter 70.105D RCW) and Regulations (Chapter 173-340 WAC). Disposal shall also comply with the Washington State Department of Ecology Stormwater Management Manual for Western Washington - Appendix IV-B: Management of Street Waste Solids and Liquids.
Inspect culverts on a regular basis for scour or sedimentation at the inlet and outlet, and repair as necessary. Give priority to those culverts conveying perennial and/or salmon-bearing streams and culverts near streams in areas of high sediment load, such as those near subdivisions during construction.
Do not apply fertilizers, herbicides, or pesticides in ditches.
Use temporary erosion and sediment control BMPs or revegetate as necessary to prevent erosion during ditch maintenance.
2.6.4.3 Recommended Additional BMPs
Install biofiltration swales and filter strips to treat roadside runoff wherever practicable and use engineered topsoils wherever necessary to maintain adequate vegetation. These systems can improve infiltration and stormwater pollutant control upstream of roadside ditches. Refer to Volume 4 - Best Management Practices Library of this manual for additional information about biofiltration swales and filter strips.
Additional guidance can be found in the Regional Road Maintenance - Endangered Species Act (ESA) program guidelines.