Inlet Interception Calculator

Enter gutter flow and grate, curb-opening, or combination-inlet geometry to calculate HEC-22 interception efficiency, captured flow, and bypass flow.

Calculate Inlet Interception

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Inlet Configuration

Inlet Type

Type of drainage inlet

Grate Type

Grate configuration affects splash-over behavior

Inlet is in a sag (low point)

Sag inlets use weir/orifice equations instead of interception efficiency

Grate Dimensions

Grate Length

Length in direction of flow

Grate Width

Width perpendicular to flow

Flow Parameters

Approach Flow

cfs

Total gutter flow approaching the inlet

Longitudinal Slope

Street slope in direction of flow (ft/ft)

Cross Slope

Street slope perpendicular to curb (ft/ft)

Reference Spread Width

The calculation reports the HEC-22 spread consistent with approach flow

Manning's n

Pavement roughness (typically 0.013-0.017)

Gutter Depression (optional)

Depressed gutter depth at curb

Depressed Gutter Width

Lateral width W used by the HEC-22 composite-gutter equations

Inlet Interception Overview

Inlet interception efficiency determines what fraction of gutter flow is captured by a drainage inlet. Understanding interception efficiency is critical for sizing and spacing storm drain inlets.

Grate Inlets - Capture frontal and side flow through grate openings
Curb-Opening Inlets - Capture flow through opening in curb face
Combination Inlets - Grate plus curb opening for maximum efficiency
Slotted Drains - Linear slots for continuous flow interception

Grate Type Comparison

Grate Type	Opening Ratio	Characteristics
P-1-7/8	90%	Parallel bars, >3.5" spacing. Good capacity, bicycle hazard.
P-1-1/8	60%	Parallel bars, <3.5" spacing. Bicycle-safe.
Curved Vane	35%	Good efficiency, bicycle-safe, self-cleaning.
45-Degree Tilt Bar	34%	Good capacity, bicycle-safe.
Reticuline	80%	Highest opening ratio, ADA-compliant pattern.

Source: FHWA HEC-22 (2009), Tables 4-4 and 4-5

Design Guidelines

On-Grade Inlets

Efficiency depends on velocity and grate type
Frontal flow captured first, then side flow
Target 85%+ efficiency to limit bypass
Space inlets to handle cumulative bypass

Sag Inlets

Must capture 100% of tributary flow
Weir flow at low depths, orifice at high
Size for clogging factor (50% debris)
Consider combination inlet for redundancy