Roadways: Ditches

Ditches

Primary Guidance Ensure ditches are of sufficient depth to provide positive flow and adequate freeboard For roadway ditches, also provide enough freeboard to prevent seepage under the pavement or through the sub-base When pavement edge drains are necessary, ensure ditches are of sufficient depth to accommodate the locations of the drains above the water surface elevation for the pavement drainage design Meet the fore slope and back slope requirements Provide appropriate slopes necessitated by clear zone principles and/or soil conditions Select ditch linings that prevent erosion, promote vegetation establishment, and avoid increases in stormwater management treatment needs Avoid causing new or exacerbating existing adverse impacts to adjacent properties

Discussion Control surface runoff from sidewalks, pavement, and the surrounding area and convey flow to a stable outfall in a non-erosive manner. The safety effect of slope combinations for occupied vehicle traversing roadside ditches is an important consideration. Channel slope combinations may be selected to produce a cross-section that is safely traversable by errant vehicles. Consider the cost of right-of-way and long-term maintenance when selecting roadside slopes. Ensure roadside ditch depths are sufficient to convey runoff without saturating the subgrade, pavement base, and pavement. Water depth that may be appropriate in ditches, particularly on flat channel slopes, is dependent on the soil characteristics. Ditch maximum longitudinal grade is based on tolerable vegetation or other ditch lining velocity, soil type shear, and sedimentation minimization. Roadside ditch channel grade need not follow the adjacent road grade, particularly if the road is flat. Not only can the channel depth and width be varied to meet different runoff quantities, longitudinal slopes, lining types, and the distance between discharge points, but the lateral distance between the channel and pavement edge may also be varied. Take care to avoid abrupt roadway section changes that produce a roadside environment discontinuity and violates driver expectancy. Take care to also avoid major channel grade breaks that may cause unnecessary scour or silt deposition. Ensure ditches maintain adequate freeboard as not to contribute to saturation of the roadway base. All ditches require some form of lining to promote vegetative establishment and/or to protect against erosion. At a minimum, all ditches will use type ‘A’ soil stabilization matting. Concrete ditch linings are discouraged as they will require additional stormwater management treatment. Design ditch linings according to HEC-15.

See Also

• Practical Design Implementation Guidance