Calculate the minimum effective inlet perimeter and maximum center-to-center spacing for drainage inlets on a horizontal or near-horizontal roadway or bridge deck using the same HEC-21 equations as the FHWA Hydraulic Toolbox.
Use only for horizontal or near-horizontal grades
HEC-21 derives this method for zero longitudinal slope. For a grade above 0 but no greater than 0.003, use the result as a flat-deck check alongside the applicable on-grade or vertical-curve procedure. Grades above 0.003 are outside this calculator's scope.
Ready to size the inlet layout
Enter the five-minute storm intensity and flat-deck geometry to calculate both independent FHWA design limits.
For educational purposes only. Not a substitute for professional engineering judgment.
FHWA horizontal bridge procedure
With no sustained longitudinal grade, runoff moves toward each inlet from the midpoint between adjacent inlets. HEC-21 integrates triangular-gutter Manning flow with uniformly distributed Rational-method runoff to obtain the maximum spacing:
Lc = 1312 Sx1.44 T2.11 / (n C i Wp)0.67
The total design flow at an inlet is Q = C i Wp Lc / 43,560. Assuming the inlet behaves as a weir at critical depth, the required effective perimeter is:
P = (C i Wp)0.33 T0.61 / (102.5 Sx0.06 n0.67)
These printed coefficients are US-customary. Metric entries are converted to the source units internally and converted back after calculation, preserving the FHWA result without intermediate rounding.
Applicability and design checks
- The derivation assumes zero longitudinal grade and low-velocity, one-dimensional flow.
- At grades from 0 to 0.003, treat this as a flat-deck check, not a substitute for on-grade analysis.
- The physical inlet must provide at least the effective perimeter after bars and blockage allowances.
- Coordinate inlet locations with deck reinforcement, beams, joints, downspouts, and maintenance access.
- Size the underdeck collection and discharge system separately.
Source
Young, G. K., Walker, S. E., and Chang, F. (1993), Design of Bridge Deck Drainage, HEC-21, FHWA-SA-92-010. See Chapter 9.2 and Appendix B, equations 22a, 24, and 71-77.
Frequently asked questions
When should I use the horizontal-grade inlet calculator?
Use it for bridge decks, causeways, and roadway reaches with zero longitudinal slope. HEC-21 also recommends checking the flat-bridge case when the grade is nearly flat, less than about 0.003 ft/ft. For a nonzero grade, also perform the applicable on-grade or vertical-curve inlet analysis.
What does minimum inlet perimeter mean?
It is the total effective opening perimeter needed to accept flow approaching from the up-station direction, down-station direction, and roadway centerline while the inlet operates as a weir at critical depth. Bars, clogging, or blocked portions are not effective perimeter and need a separate design allowance.
Why does the calculator require a 5-minute rainfall intensity?
HEC-21 assumes a five-minute time of concentration for each inlet on a flat bridge. Enter the five-minute intensity for the selected design return period, not a 24-hour rainfall depth or an intensity for another duration.
Is the reported inlet spacing measured edge to edge?
No. HEC-21 defines the maximum spacing as inlet center to inlet center. Adjust the final layout for structural framing, deck reinforcement, joints, maintenance access, and agency maximum-spacing requirements.
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Last verified: August 2026