Screen a Small Crossing with the Rational Method and Manning Flow

1. Estimate peak runoff

Use Q = CiA / 360, with dimensionless runoff coefficient C, intensity i in mm/h, area A in hectares and Q in m³/s. Assume C = 0.55, i = 75 mm/h and A = 16 ha. The result is 1.833 m³/s. If intensity increases to 85 mm/h, the result is 2.078 m³/s.

Verify the delineated area, design recurrence and concentration time before selecting an intensity. The site’s small-catchment Rational screen excludes areas of 50 ha and above. See ERA Drainage Design Manual (2013), Chapter 5 §5.7.

2. Compare full-barrel capacities

Use Qfull = (A/n)(A/P)^(2/3)√S with section area A in m², wetted perimeter P in metres, roughness n and slope S in m/m. For a full circular barrel, A = πD²/4 and P = πD. A full closed box includes its roof in P. See ERA Drainage Design Manual (2013), Chapter 6 §6.12.2.

At S = 0.006 m/m and n = 0.013, the screened capacities for 900, 1,000 and 1,200 mm pipes are 1.402, 1.857 and 3.020 m³/s. The smallest listed pipe that clears 1.833 m³/s is 1,000 mm, with a demand-to-capacity ratio of about 0.987. At 2.078 m³/s the first listed candidate is 1,200 mm.

Use entrance geometry, barrel length and inverts, upstream limits and tailwater for a separate inlet and outlet control analysis. Check overtopping, debris and scour for the site. FHWA Hydraulic Design of Highway Culverts (HDS 5)

3. Check a separate roadside ditch

Assume 0.60 m bottom width, side slopes of 2 horizontal to 1 vertical, n = 0.030 and S = 0.008 m/m. At a depth of 0.45 m, flow area is 0.675 m², wetted perimeter is 2.612 m and top width is 2.40 m. Manning flow is about 0.8164 m³/s.

In the tool use C = 0.35, i = 70 mm/h and A = 12 ha. A demand of 0.8167 m³/s gives a displayed normal depth of 0.450 m. The normal-depth calculation assumes steady uniform flow. Check available depth, lining, freeboard and transitions against the project criteria.