231.6 Roadside Ditches
| Key Points |
231.6.1 Introduction
231.6.1.1 Ditch Slopes
The primary purpose for the roadside ditch is the control of surface drainage from the pavement and surrounding area. An open-channel ditch, cut into the natural terrain along the roadside, is the most economical method to produce a drainage channel. The most desirable channel, from the standpoint of hydraulic efficiency, is one with steep sides. However, limitations on soil stability and roadside safety (clear zone principles) require flatter slopes. The effect of slope combinations on the safety of a vehicle traversing them is an important consideration. Slope combinations for channels can be selected to produce a cross-section that can be safely traversed by an errant vehicle. Additionally, the cost of right of way needs to be considered when selecting combinations of slopes for the roadside.
The depth of the roadside channel must be sufficient to remove surface water without saturation of the subgrade including the pavement base. The depth of water that can be tolerated in the channel particularly on flat channel slopes, depends on the soil characteristics. In regions with severe winter climates, channel sideslopes of 1V:5H or 1V:6H are preferable to reduce snow drifts.
231.6.1.2 Ditch Grades
The minimum desirable grade for a roadside channel is based on the drainage velocities needed to avoid sedimentation. The maximum desirable grade for roadside channels is based on a tolerable velocity for vegetation and shear on soil types. The channel grade does not have to follow the grade of the adjacent roadbed, particularly if the roadbed is flat. Not only can the depth and width of the channel be varied to meet different quantities of runoff, slopes of channel, types of lining, and the distance between discharge points, but the lateral distance between the channel and the edge of traveled way can also be varied. Care should be taken to avoid abrupt changes in the roadway section that produces a discontinuity of the roadside environment and violates driver expectancy. Care should also be taken to avoid major breaks in channel grade that could cause unnecessary scour or silt deposition.
231.6.1.3 Construction and Maintenance Operations
Maintenance and construction core team members will provide input on the ditch cross section, e.g., an 8 ft. (2.4 m) flat bottom ditch may be desirable for maintenance equipment to remove fallen rock.