Median/Ditch Drop-Inlet Calculator

Analyze HEC-22 median and ditch drop inlets on grade or in sag locations, including trapezoidal-channel hydraulics, grate interception, bypass, clogging, and downstream channel-block height.

Input Parameters

Analysis Configuration

Choose the inlet location and known channel quantity

Inlet Location

Known Channel Quantity

Design Flow

cfs

Flow approaching the drop inlet

Trapezoidal Ditch

HEC-22 computes the unknown flow or normal depth with Manning's equation

Bottom Width

Side Slope

H:1V

Horizontal distance for one unit of vertical rise

Longitudinal Slope

ft/ft

Enter as a decimal, for example 0.03 for 3%

Manning's n

Don't know this value? Look it up

Drop-Inlet Grate

Dimensions are measured parallel and perpendicular to approach flow

Grate Type

Grate Length

Dimension parallel to flow

Grate Width

Dimension perpendicular to flow

Downstream Channel Block

Optionally size a berm for 100% interception

Compute required channel-block height Checks both inlet control depth and approach specific energy, as documented by the FHWA Toolbox.

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.