Wisconsin Administrative Code
Department of Natural Resources
NR 300-399 - Environmental Protection - Water Regulation
Chapter NR 320 - Bridges And Culverts In Or Over Navigable Waterways
Section NR 320.07 - Culvert area calculation, culvert sizing, and culvert length determination

(1) CALCULATION OF REQUIRED CULVERT AREA. To determine the required culvert area, 3 measurements shall be made: channel width of the stream in feet at the ordinary high water mark (W1), channel width of the stream in feet at the stream bottom (W2), and the height in feet of the high water above the stream bottom (H) (see diagram). These 3 measurements shall be made at each of 3 locations or transects along the stream: the location of the proposed crossing, 100 feet upstream from the crossing, and 100 feet downstream from the crossing. The individual measurements of W1, W2 and H shall be averaged to derive the final W1, W2 and H values. The required culvert area is then calculated with the following equation:

Required Culvert area (square feet) = H x (W1 + W 2)

(2) CULVERT SIZING. Once the required culvert area has been determined, the applicant can identify the proper culvert size by reviewing the following tables. The minimum size culvert needed to pass flows must have an area that is as much or more than the required culvert area. For example, the recommended culvert size for a culvert area calculation of 8.50 square feet would be 42".

Table 1. Round culvert diameters and corresponding culvert area.

Note: To find the area of any round culvert, use the following equation:

Culvert diameter (in.) divided by 2 = _____ divided by 12 = _____ (radius in ft.)

Radius x Radius x 3.14 = _________ (area of culvert in sq. ft.)

Table 2. Pipe arch dimensions and corresponding culvert area.

(3) CULVERT LENGTH DETERMINATION. The proper culvert length is determined by the following calculation: