Governing Culvert Headwater (HDS-5)

This runs both circular checks and reports the governing headwater and which control wins. INLET control (the inlet size and edge set the ponding) and OUTLET control (barrel length, roughness, and tailwater set it) are computed independently per FHWA HDS-5 (FHWA-HIF-12-026), and the ACTUAL design headwater is the larger. 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 36 in concrete pipe, square-edge headwall, 100 ft on a 1% slope (n 0.012) passing 50 cfs with a 2 ft tailwater is inlet-controlled at 3.99 ft (HW/D 1.33); the same barrel as corrugated metal (n 0.024) on a flatter 0.5% slope, 150 ft long, under a 4 ft tailwater flips to outlet control at 7.27 ft (HW/D 2.42) - the reason both checks are required. If HW/D climbs above about 1.5, check the allowable headwater against the roadway/overtopping elevation. Runs both the inlet-control and the outlet-control check. HW is measured above the inlet invert; circular barrels only; a box culvert is separate. 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). HW_inlet from HDS-5 Appendix A inlet-control equations; HW_outlet from the Chapter 3 full-flow energy equation HW = H + ho - So L. The control type is whichever headwater is greater. HW is measured above the inlet invert.

FHWA HDS-5, Hydraulic Design of Highway Culverts, 3rd ed. (FHWA-HIF-12-026, 2012), the 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 culvert-inlet-control and culvert-outlet-control computes.

HDS-5 is a public-domain FHWA publication (FHWA-HIF-12-026); the inlet- and outlet-control procedures are reproduced from it. Diameter, 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: diameter_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
• Inlet control HDS-5 Appendix A weir/orifice equations with the Table A.1 constantsHDS-5 Appendix A (culvert-inlet-control)
• Outlet control / scope full-flow HW = H + ho - So L; circular barrels only (a box uses the box tiles)HDS-5 Chapter 3 (culvert-outlet-control)