Annex 3A: Guidelines for Local Gravel Road Rehabilitation 445 Spacing of check dams depends upon the slope of the ditch (Table 4) Slope Minimum spacing of check dams Less than 6% 15 m 6% to 8% 8 m Greater than 8% 5 m Table 4. Spacing of check dams Lined side drains Lining of side drains with structure should only be used where the turbulent or high velocity flow in excess of 1 m/second cannot be reduced or controlled by other means. Cross drains and culverts Cross drainage structures are used where the road intersects a watercourse. The most common type of cross drainage used for rural roads is concrete pipe culvert. Culverts for streams should be large enough to match the capacity of the channel. The depth of cover between the top of the culvert and the underside of the road pavement, for example, the bottom of the gravel layer should be equivalent to at least half the diameter of the culvert. In order to achieve these two requirements, two or smaller diameter culverts may need to be placed in parallel. The minimum recommended culvert size is 610 mm – smaller diameter culverts are difficult to clean and maintain if they become blocked. The culvert should be aligned with the existing stream channel as far as possible. Where this cannot be achieved, the culvert should be aligned with the center of the channel immediately downstream of the outlet. The grade of the culvert should follow the grade of the existing channel, with a desirable minimum slope of 2%. Additional protection at the outlet will be required and the outlet should be at the same level as the bottom of the existing channel.