What This Solves
Calculates how much stormwater a grate inlet or curb opening will intercept from roadway flow, and how much will bypass the inlet.
Best Used When
- You are designing roadway drainage and need to determine inlet spacing
- You need to calculate the interception efficiency of a grate inlet in a gutter
- You are evaluating whether an existing inlet will capture enough flow or if bypassed water is acceptable
Do NOT Use When
- You need to size a catch basin or determine the required sump depth — Use Catch Basin Calculator
- You need to calculate gutter flow depth and spread for pavement drainage design — Use Gutter Flow Calculator
Key Assumptions
- Flow follows FHWA HEC-22 grate inlet interception methodology
- Gutter geometry is uniform approaching the inlet
- Splash-over and frontal flow interception can be estimated from grate length and width
- The inlet is clean and not clogged with debris
- Flow conditions are steady (not pulsing or surging)
Input Quality Notes
Grate efficiency is highly sensitive to grate orientation, bar spacing, and clogging. Consider a clogging factor (reduce effective open area by 50% for conservative design) and verify that the grate bars run perpendicular to flow direction for best efficiency.
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Inlet Interception Calculator
Calculate inlet interception efficiency for roadway drainage systems using FHWA HEC-22 methodology. Determine how much flow is captured by grate, curb-opening, combination, and slotted drain inlets.
Calculate Inlet Interception
For educational purposes only. Not a substitute for professional engineering judgment.
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-50 | 50% | Parallel bars, >3.5" spacing. Good capacity, bicycle hazard. |
| P-30 | 35% | Parallel bars, <3.5" spacing. Bicycle-safe. |
| Curved Vane | 35% | Good efficiency, bicycle-safe, self-cleaning. |
| 45-Degree Tilt Bar | 35% | 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
About Inlet Interception
Inlet interception efficiency is the ratio of flow captured by an inlet to the total approach flow. Understanding interception efficiency is essential for proper inlet spacing and storm drain system design.
Inlet Types
- Grate Inlets - Horizontal openings in the pavement covered by a grate. Capture both frontal flow (directly entering) and side flow (entering from the sides).
- Curb-Opening Inlets - Vertical openings in the curb face. Work well on steep grades where grates may have splash-over.
- Combination Inlets - Both grate and curb opening. Provide redundancy and higher efficiency.
- Slotted Drain Inlets - Continuous slots that intercept flow. Effective for long, low-flow situations.
HEC-22 Methodology
The FHWA Hydraulic Engineering Circular No. 22 (HEC-22) provides standard equations for calculating inlet interception:
- Frontal Flow Efficiency (Rf) - Depends on approach velocity and splash-over velocity
- Side Flow Efficiency (Rs) - Depends on velocity, grate length, and cross slope
- Total Efficiency (E) - E = EoRf + (1-Eo)Rs
On-Grade vs. Sag Inlets
On-Grade Inlets: Located on continuous slopes. Only intercept a fraction of the flow, with the remainder bypassing to downstream inlets.
Sag Inlets: Located at low points (sumps). Must capture 100% of tributary flow since there is no downstream outlet. Analyzed using weir and orifice flow equations.
Design Considerations
- Bicycle Safety - Use grate types with openings that don't trap bicycle tires (P-30, curved vane, tilt bar)
- Debris Clogging - Account for 50% clogging in sag inlet sizing
- Spread Width - Limit spread to maintain traffic safety (typically 6-10 ft)
- Inlet Spacing - Space inlets to limit bypass accumulation
- Minimum Efficiency - Target 70%+ for on-grade inlets, 100% for sag inlets
Resources
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Last verified: February 2026