Roadway Drainage Systems: Inlets, Gutters, and Storm Sewers

Roadway drainage is one of the most critical elements of highway and street design. Water on the road surface creates hydroplaning hazards, reduces visibility, and accelerates pavement deterioration. A well-designed drainage system captures runoff efficiently and conveys it safely away from the roadway.

The Roadway Drainage System

A complete roadway drainage system consists of three interconnected components:

1. Gutters — Triangular channels formed by the pavement cross-slope and the curb that collect and convey surface runoff along the edge of the roadway.
2. Inlets — Openings in the gutter or pavement surface that capture flow and direct it into the subsurface storm drain.
3. Storm drains — Underground pipes that convey the captured runoff to an outfall (stream, detention facility, or larger conveyance).

The design of each component depends on the others. Gutter capacity determines where inlets are needed; inlet efficiency determines how much flow enters the storm drain; and storm drain capacity must handle the accumulated flow from all upstream inlets.

Gutter Flow Analysis

Gutter Geometry

Most curbed streets use a uniform cross-slope (typically 1.5–4%) that creates a triangular gutter section. Manning’s equation for a triangular section gives:

Where:

• Q = gutter flow (cfs)
• n = Manning’s roughness (typically 0.016 for asphalt)
• Sx = cross-slope (ft/ft)
• SL = longitudinal slope (ft/ft)
• T = spread (width of flow measured from curb face, ft)

Calculate gutter flow →

Spread Criteria

The spread (T) is the key design parameter for gutter flow. It represents how far water extends into the travel lanes. Design standards limit spread based on road classification:

When the calculated spread exceeds the allowable limit, an inlet must be placed to intercept the flow and reduce the spread.

Composite Gutters

Many streets have a depressed gutter section (a steeper cross-slope near the curb, typically 8–12%) that increases capacity near the curb. The composite section is analyzed by splitting the flow into the depressed portion and the pavement portion, then summing.

Inlet Types and Selection

Grate Inlets

Grate inlets are openings in the gutter covered by a metal grate. They intercept flow by allowing water to fall through the grate openings.

Advantages: Good bicycle safety (with proper bar orientation), effective on continuous grades Disadvantages: Can clog with debris, reduced efficiency at high gutter velocities, may not intercept flow at the curb face

Common grate types defined in HEC-22:

• P-50 — Parallel bar grate (50% open area)
• P-30 — Parallel bar grate (30% open area)
• Curved vane — Curved bars that direct flow into the opening
• Reticuline — Mesh pattern, bicycle-safe

Curb-Opening Inlets

Curb-opening inlets are vertical openings in the curb face. They intercept flow that runs along the curb line.

Advantages: Not prone to clogging by debris, do not interfere with traffic Disadvantages: Less effective on steep grades (flow shoots past the opening), require longer openings for equivalent capacity

The length of curb opening required for 100% interception is:

In practice, inlets are often shorter than LT, resulting in partial interception. The efficiency of a shorter inlet is:

Combination Inlets

Combination inlets use both a grate and a curb opening. They provide the highest interception capacity and the best debris handling — the curb opening continues to intercept flow even when the grate is partially clogged.

Calculate inlet interception →

Slotted Drain Inlets

Slotted drains are continuous linear openings in the pavement, typically used where wide, uniform interception is needed (parking garage entrances, highway medians, airport runways).

Inlet Spacing

Inlet locations are determined by working along the roadway from the high point (where gutter flow begins) downstream:

Step 1: Calculate the gutter flow at each point along the road using the Rational Method (Q = CiA) with the contributing drainage area.

Step 2: At each point, calculate the resulting spread using the gutter flow equation.

Step 3: Where the spread exceeds the allowable limit, place an inlet.

Step 4: Calculate the inlet’s interception capacity. The bypass flow (flow not captured) continues downstream to the next inlet.

Step 5: Repeat, accumulating carryover flow, until reaching the system outfall.

Sag Inlet Design

Sag inlets are the most critical inlets in the system because all approaching flow converges at the low point. They are typically designed with:

• Combination inlets (grate + curb opening) for redundancy
• Flanking inlets on each side of the sag as backup
• Head-based analysis (the ponded depth determines capacity)

For a grate inlet at a sag, the capacity under weir flow conditions (shallow ponding) is:

Where P is the perimeter of the grate and d is the depth of water. At greater depths, the grate transitions to orifice flow and capacity is governed by the grate open area.

Storm Drain Design

Pipe Sizing

Storm drain pipes are sized using Manning’s equation for pipe flow. The pipe must convey the design flow while maintaining:

• Minimum velocity: 2.5–3.0 ft/s to prevent sediment deposition
• Maximum velocity: 10–15 ft/s to prevent erosion and pipe damage
• Minimum diameter: Typically 12 or 15 inches (varies by jurisdiction)
• Maximum d/D ratio: Most agencies require the design flow depth to be no more than 80% of the pipe diameter, maintaining open channel flow conditions

Size storm drain pipes →

Hydraulic Grade Line

The hydraulic grade line (HGL) analysis is the most important check in storm drain design. It verifies that the system has adequate capacity by calculating the water surface elevation at each point in the system, accounting for:

• Pipe friction losses (Manning’s equation)
• Junction losses at manholes (energy and momentum methods)
• Entrance and exit losses
• Bend losses

The HGL must remain below the ground surface (or below the gutter invert at inlet locations) to prevent surcharging and surface flooding.

System Layout

Storm drain layout follows practical guidelines:

• Pipes should follow roadway alignments where possible
• Manholes are required at changes in direction, slope, pipe size, and at maximum spacing intervals (typically 300–500 ft)
• Minimum cover depends on traffic loading (typically 2–3 ft under roadways)
• Pipe slopes should generally match the road profile to minimize excavation

Design Standards

The primary reference for roadway drainage design in the United States is FHWA HEC-22 (Urban Drainage Design Manual), now in its third edition. This document provides:

• Gutter flow equations and nomographs
• Inlet interception capacity formulas for all inlet types
• Storm drain design procedures including HGL analysis
• Pavement drainage criteria

State DOTs typically adopt HEC-22 procedures with state-specific modifications for design storms, spread criteria, and material standards.

References

1. Federal Highway Administration. (2013). Urban drainage design manual (3rd ed., Hydraulic Engineering Circular No. 22). U.S. Department of Transportation.

2. Federal Highway Administration. (2009). Bridge scour and stream instability countermeasures (Hydraulic Engineering Circular No. 23). U.S. Department of Transportation.

3. American Association of State Highway and Transportation Officials. (2014). Drainage manual. AASHTO.

4. Brown, S. A., Stein, S. M., & Warner, J. C. (2001). Urban drainage design manual (2nd ed., Hydraulic Engineering Circular No. 22). FHWA.