Analyze a grate inlet placed in a trapezoidal median or roadside ditch using the same HEC-22 procedure implemented by the FHWA Hydraulic Toolbox. Calculate on-grade capture and bypass, sag ponding under weir or orifice control, clogging effects, and downstream berm height.
FHWA Median/Ditch Inlet Method
For an inlet on grade, HEC-22 divides the approach discharge into frontal and side flow, applies grate splash-over efficiency, and reports intercepted and bypass flow. In a sag, the calculator solves both weir and orifice equations and uses the larger required depth.
A downstream block can force complete interception. Its height is checked against the ponding depth, approach specific energy, bed drop to the block, and selected freeboard.
For educational purposes only. Not a substitute for professional engineering judgment.
FHWA calculation sequence
- Solve Manning's equation for normal depth or discharge in the trapezoidal approach ditch.
- For an inlet on grade, divide the approach flow into frontal and side-flow portions, apply the selected grate's splash-over curve, and compute interception efficiency.
- For a sag or downstream channel block, reduce perimeter and clear area for clogging, solve both the grate-weir and grate-orifice equations, and use the larger required depth.
- Where a block is selected, compare the hydraulic depth with approach specific energy, account for bed drop to the block, and add freeboard.
The method follows FHWA HEC-22 Sections 4.4.4, 4.4.5, and 4.4.7 and the Hydraulic Toolbox User's Manual Section 13. It analyzes surface interception; verify the receiving structure and outlet-pipe capacity separately.
Frequently asked questions
What is a median or ditch drop inlet?
It is a grate inlet placed flush with the bottom of a median, roadside, or similar open ditch. The FHWA method estimates how much on-grade flow enters the grate and how much bypasses it, or the ponding depth needed to capture all flow at a sag.
How does the calculator handle a sag inlet?
It computes the depth required by both the HEC-22 grate-weir and grate-orifice equations. Following current FHWA guidance, the greater of those two depths governs so capacity is not overstated in the transition between controls.
What does the clogging percentage change?
For a sag or an on-grade inlet with a downstream block, clogging reduces both the grate effective perimeter used for weir flow and its clear opening area used for orifice flow. The FHWA Toolbox does not enable clogging for an ordinary unblocked on-grade analysis.
How is the downstream channel-block height calculated?
The minimum height is the greater of the weir-or-orifice ponding requirement and the approach-flow specific energy, adjusted for the channel bed drop between the grate and the block. The selected freeboard is then added to obtain the recommended height.
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Last verified: August 2026