Culvert Capacity

Culvert Capacity is calculated using an equation from USDOT FHA Publication No. FHWA-HIF-12-026, Equation A.3, pg 191.

The equation, expressed in Python, is as follows:

capacity = (
    culvert_area_sqm * math.sqrt(
        culvert_depth_m * (
            (
                head_over_invert / culvert_depth_m
            ) - coefficient_y - coefficient_slope * slope_rr
        ) / coefficient_c
    )
) / si_conv_factor

Variables shown above are as follows:

  • culvert_area_sqm: internal surface area of the culvert
  • head_over_invert: Hydraulic head above the culvert invert, meters
  • culvert_depth_m: Culvert depth. Diameter or dimension b, (height of culvert) meters
  • slope_rr: slope rise/run (meters)
  • coefficient_slope: slope coefficient from FHWA engineering pub HIF12026, appendix A. -0.5, except where inlet is mitered in which case +0.7
  • coefficient_y: coefficient based on shape and material from FHWA engineering pub HIF12026
  • coefficient_c: coefficient based on shape and material from FHWA engineering pub HIF12026
  • si_conv_factor: adjustment factor for units (SI=1.811), defaults to 1.811

The equation returns capacity: capacity in cubic meters / second (m^3/s)

Constants coefficient_c, coefficient_y, coefficient_slope are set based on the culvert entrance shape and material, from FHWA engineering pub HIF12026, appendix A.

In the code

The capacity calculator module in this tool is located in src/drainit/calculators/capacity.py