A depth of 18 inches is usually sufficient, but it may have to be deeper if water flows in from hillsides. The distance between cross culverts or ditch outlets influence depth. Inspection will indicate if depth is adequate, or if culverts or outlets should be installed. Ditches are often constructed too deep. Lining is more difficult, and the added depth increases ditch erosion.
Deep ditches can create a safety hazard to motorists, especially on roads with narrow shoulders.
Ditches are necessary to carry water off and away from roads and must be regularly maintained. They must have the proper shape, slope, and lining. Water flow, road surface sediment, and growing vegetation attack these features. UNH Technology Transfer Center. A Series of Quick Guides for New Hampshire Towns.