Governing Box Culvert Headwater (HDS-5)

Runs both the inlet- and the outlet-control check and reports the governing value and which control wins, per FHWA HDS-5 (FHWA-HIF-12-026). INLET control (the inlet size and edge set the ponding) and OUTLET control (barrel length, roughness, and tailwater set it) are computed independently. Inlet control usually governs on steep barrels with a good entrance; outlet control takes over on long, rough, or flat barrels and under a high tailwater. A 6 x 4 ft box, 30-75 deg wingwall flares, 100 ft on a 1% slope (n 0.012) passing 150 cfs with a 2 ft tailwater is inlet-controlled at 4.34 ft (HW/D 1.08); the same box on a flatter 0.4% slope, 200 ft long, rougher (n 0.02), with 45 deg bevels and a 4 ft tailwater flips to outlet control at 5.03 ft (HW/D 1.26). If HW/D climbs above about 1.5, check the allowable headwater against the roadway/overtopping elevation. Runs both box control checks; D is the box RISE. HW is measured above the inlet invert. Public-domain FHWA reference; the HDS-5 nomographs carry about +/-10%, and the engineer of record and the DOT drainage manual govern.

HW_design = max(HW_inlet, HW_outlet) for a box. HW_inlet from HDS-5 Appendix A box inlet-control equations; HW_outlet from the Chapter 3 full-flow energy equation HW = H + ho - So L with box geometry. The control type is whichever headwater is greater. D = box rise; HW is measured above the inlet invert.

FHWA HDS-5, Hydraulic Design of Highway Culverts, 3rd ed. (FHWA-HIF-12-026, 2012), the box inlet-control (Appendix A) and outlet-control (Chapter 3) procedures run together, by name; the design headwater is the greater of the two per the HDS-5 procedure. Delegates to the verified box-culvert-inlet-control and box-culvert-outlet-control computes.

HDS-5 is a public-domain FHWA publication (FHWA-HIF-12-026); the box inlet- and outlet-control procedures are reproduced from it. Span, rise, discharge, slope, length, Manning n, tailwater, and the inlet configuration are the user's inputs.

Estimate. AHJ and licensed professional govern.

Field names used by the API: span_in, rise_in, flow_cfs, slope, length_ft, manning_n, tw_ft, config, governing_hw_ft, inlet_hw_ft, outlet_hw_ft, hw_over_d

• Governing headwater HW_design = max(HW_inlet, HW_outlet); control = whichever is largerHDS-5 inlet/outlet control procedure
• Box inlet control HDS-5 Appendix A box equations (Form 1 flares / Form 2 headwalls) with Table A.1 constantsHDS-5 Appendix A (box-culvert-inlet-control)
• Box outlet control / scope full-flow HW = H + ho - So L, A = span x rise; box barrels (a circular pipe uses culvert-headwater)HDS-5 Chapter 3 (box-culvert-outlet-control)