Once the design discharge and allowable headwater are determined and the culvert alignment and slope decided upon, an efficient culvert size may be found through the use of nomographs as shown in “Hydraulic Design of Highway Culverts,” Hydraulic Design Series 5, FHWA.
An example of an inlet control nomograph is shown in Fig. 5.14. Since the structure size is not known, the design is an iterative process. To use the chart, the trial culvert size and inlet configuration, design discharge, allowable headwater depth, culvert length, and culvert slope must be known. The relationship of the inlet control headwater to the diameter or height of the culvert (HW/D) is read directly from the inlet control nomograph by extending a line from the culvert size scale (left scale) through the discharge/bottom width ratio scale (center scale) to the scale of the headwater depth in terms of height (right scale). The inlet control headwater equals this value multiplied by D. For example, assume a 5-ft X 2-ft box culvert with the design storm, Q, equal to 75 ft3/s. The nomograph shows that HW/D = 1.75, 1.90, or 2.05, depending upon the inlet configuration. By multiplying these values by the box rise of 2 ft, the corresponding headwater depths are found to be 3.5 ft, 3.8 ft, and 4.1 ft, respectively. (Note: To use the chart with SI units, first convert from SI units to U. S. Customary units—1 ft = 0.305 m, 1ft3/s = 0.0283 m3/s).
