Allowable headwater

The allowable maximum headwater when passing the design discharge must be measured from the bottom of the channel. For arch pipes, the maximum allowable headwater must be based on the rise of the pipe, and the pipe size category must be the equivalent round pipe size. For multiple pipe installations, the pipe diameter used to calculate the allowable headwater must be the diameter of the largest pipe. Tailwater resulting from downstream conditions, either natural or manmade, must be accounted for in the determination of the crossing's capacity and the resulting headwater. Additional guidance is provided in the North Dakota department of transportation design manual. If a crossing results in less than one-half foot [15.24 centimeters] of headloss when passing the appropriate design discharge, this section does not apply. Streambed Slope (feet/mile) Pipe Size Allowable Headwater <5 24" - 54" pipe diameter + 2 feet ≥ 60" 1.5 pipe diameters 5 to 10 24" - 36" pipe diameter + 2 feet 42" - 54" 1.5 pipe diameters ≥ 60" 2 pipe diameters >10 ≥ 24" 2 pipe diameters