A storm drain system is an underground network of pipes, inlets, and structures that collects rainwater from streets, parking lots, and other surfaces and conveys it to a discharge point. Understanding the individual components and how they fit together is essential for anyone involved in drainage design, construction, or maintenance.
How the System Works
When rain falls on a developed surface, water flows across the pavement by gravity, collecting in gutters and low points. Inlets capture this surface flow and direct it into underground pipes. The pipes carry the flow downstream, merging with additional flows at junction structures, until the combined flow reaches an outfall — typically a stream, river, detention pond, or regional drainage facility.
The entire system operates by gravity. No pumps are involved in most storm drain systems (pumping is reserved for special cases like depressed highways and below-grade structures).
Inlets
Inlets are the entry points of the storm drain system — the openings at the surface that allow water to flow underground.
Grate Inlets
Grate inlets sit in the gutter or pavement surface and are covered by a metal grate. Water falls through the grate openings into a structure below.
- Best for: Continuous gutter flow on grades
- Weakness: Susceptible to clogging by leaves and debris
- Key design parameter: Grate length and bar configuration determine interception efficiency
Curb-Opening Inlets
These are vertical openings cut into the curb face, allowing gutter flow to fall or turn into the inlet structure.
- Best for: Flat grades, areas with heavy debris
- Weakness: Less efficient on steep longitudinal slopes (flow overshoots the opening)
- Key design parameter: Opening length determines capacity
Combination Inlets
A combination inlet includes both a grate and a curb opening. The curb opening provides redundancy — when the grate clogs, the curb opening still captures flow.
- Best for: Sag locations (low points) and critical applications
- Standard practice: Most agencies require combination inlets at all sag locations
Area Drains
Area drains are small grate structures placed in low spots of parking lots, plazas, and other flat paved areas. They connect to the storm drain system through short lateral pipes.
Trench Drains
Trench drains (also called channel drains or linear drains) are long, narrow channels with grate covers. They intercept sheet flow across their full length, making them useful at building entrances, parking garage ramps, and loading docks.
Catch Basins and Inlet Structures
Catch Basins
A catch basin is an inlet structure with a sump — a section below the pipe invert that collects sediment, debris, and floatables before they enter the pipe system.
Typical features:
- Sump depth: 2–4 feet below the outlet pipe invert
- Precast concrete or masonry construction
- Grate or combination inlet at the surface
- One or more outlet pipes connecting to the storm drain
The sump must be cleaned periodically (typically annually or more frequently) to maintain capacity and prevent clogged pipes downstream.
Junction Boxes
Junction boxes are underground structures where two or more pipes meet. Unlike catch basins, they may not have a surface inlet — they simply serve as connection points in the pipe network.
Junction boxes are needed:
- Where pipes change direction
- Where pipes change size
- Where two or more pipes merge
- At the upstream end of a long pipe run (for maintenance access)
Pipes
Materials
Storm drain pipes come in several materials, each with different characteristics:
| Material | Manning’s n | Strengths | Common Sizes |
|---|---|---|---|
| Reinforced Concrete (RCP) | 0.012–0.013 | Strength, durability, long life | 12”–144” |
| Corrugated Metal (CMP) | 0.022–0.027 | Light weight, low cost | 12”–120” |
| HDPE (smooth interior) | 0.010–0.012 | Light, corrosion-resistant | 4”–60” |
| PVC | 0.010–0.011 | Smooth, corrosion-resistant | 4”–48” |
| Ductile Iron | 0.012–0.013 | Pressure capability, strength | 4”–64” |
RCP is the most common for municipal storm drains due to its durability and availability in large sizes. HDPE and PVC are gaining popularity for smaller systems (under 36 inches) due to their smoother interior, lighter weight, and resistance to corrosion.
Sizing
Pipes are sized to carry the design flow using Manning’s equation:
Where D is the diameter (ft) and S is the slope (ft/ft).
Key design constraints:
- Minimum diameter: Usually 12 or 15 inches
- Never decrease size downstream — even if the increased slope provides enough capacity, smaller pipes downstream will catch debris from larger pipes upstream
- Minimum velocity: 2.5–3.0 ft/s to prevent sediment buildup
- Maximum velocity: 10–15 ft/s to prevent erosion and pipe damage
Installation
Proper pipe installation is critical for long-term performance:
- Bedding: Pipes rest on prepared bedding material (crushed stone or sand) to distribute loads evenly
- Backfill: Compacted in lifts around and over the pipe to prevent settlement
- Joints: Gasketted or coupled joints prevent infiltration (groundwater leaking in) and exfiltration (stormwater leaking out)
- Minimum cover: Typically 2–3 feet under roadways to handle traffic loads
Manholes
Manholes are vertical access structures that allow inspection, maintenance, and cleaning of the storm drain system.
Standard Features
- Frame and cover: Cast iron, rated for traffic loading (H-20 or HS-20)
- Steps or ladder: For maintenance crew access
- Barrel sections: Precast concrete rings, typically 48-inch inside diameter (larger for bigger pipes)
- Base section: Precast with flow channels (benching) to guide water through the structure
- Cone or flat top: Transitions from the barrel diameter to the frame opening
Placement Requirements
Manholes are required at:
- Changes in pipe direction (horizontal or vertical)
- Changes in pipe size
- Pipe junctions (where two or more pipes meet)
- Maximum spacing intervals: 300–500 feet depending on pipe size and jurisdiction
- Changes in pipe material
Energy Losses
Flow through manholes causes energy losses due to expansion, direction change, and turbulence. These losses are accounted for in the hydraulic grade line analysis using empirical loss coefficients. Benched manholes (where the channel through the base is shaped to guide flow smoothly) have lower losses than flat-bottomed manholes.
Outfalls
The outfall is where the storm drain system discharges. It’s the downstream end of the system and requires careful design.
Headwall and Endwall
A concrete headwall or endwall provides structural support at the pipe outlet and prevents erosion around the pipe. It typically includes wingwalls that flare outward to transition flow from the pipe to the receiving channel.
Energy Dissipation
Concentrated flow from a pipe outlet has significant energy that can erode the receiving channel. Common energy dissipation measures:
- Riprap apron: A layer of loose stone placed at the outlet, sized based on exit velocity
- Stilling basin: A concrete basin that forces a hydraulic jump, dissipating energy before discharge
- Plunge pool: A depression at the outlet that absorbs the falling water’s energy
- Level spreader: Distributes concentrated flow into sheet flow across a broad area
Environmental Considerations
Outfalls to natural water bodies may require:
- Permits: NPDES, Section 404, state/local discharge permits
- Water quality treatment: Sediment basins, bioswales, or manufactured treatment devices upstream of the outfall
- Erosion protection: Both at the immediate discharge point and downstream
- Fish passage: In some jurisdictions, outfalls to fish-bearing streams have specific requirements to avoid creating barriers
System Maintenance
Storm drain systems require ongoing maintenance to perform as designed:
- Inlet cleaning: Remove debris from grates and catch basin sumps (1–2 times per year minimum)
- Pipe inspection: TV camera inspection every 5–10 years to check for damage, root intrusion, and sediment buildup
- Pipe cleaning: Jet cleaning to remove sediment (as needed based on inspection)
- Manhole inspection: Check for structural damage, infiltration, and damaged steps
- Outfall inspection: Check for erosion, scour, and damage to energy dissipation features
References
-
American Society of Civil Engineers. (2017). Gravity sanitary sewer design and construction (ASCE Manual of Practice No. 60). ASCE Press.
-
Federal Highway Administration. (2013). Urban drainage design manual (3rd ed., Hydraulic Engineering Circular No. 22). U.S. Department of Transportation.
-
American Concrete Pipe Association. (2011). Concrete pipe design manual. ACPA.
-
National Corrugated Steel Pipe Association. (2008). Corrugated steel pipe design manual. NCSPA.
Try These Calculators
Put what you've learned into practice with these free calculators.