Storm Drainage Design: From Hydrology to Pipe Sizing

Designing a storm drainage system is a step-by-step process that starts with understanding how much runoff your site generates and ends with a pipe network that conveys that runoff safely to a discharge point. This guide walks through the complete design process used by practicing engineers.

The Design Process Overview

A storm drainage design follows these phases:

1. Hydrology — Determine design flows at each point in the system
2. Layout — Locate inlets, pipes, and manholes
3. Pipe sizing — Size each pipe segment for its design flow
4. Hydraulic grade line check — Verify the system has adequate capacity
5. Outlet protection — Design the discharge point to prevent erosion

Each phase builds on the previous one. Shortcuts in early phases compound into problems in later phases.

Phase 1: Hydrology

Design Storm Selection

The first decision is the design storm frequency — the storm return period the system must handle without flooding. Common standards:

Rational Method

For sites under 200 acres (the vast majority of site-scale projects), the Rational Method is the standard approach:

Where:

• Q = peak flow (cfs)
• C = runoff coefficient (0 to 1.0)
• i = rainfall intensity for duration = Tc (in/hr)
• A = drainage area (acres)

The Rational Method gives you the peak flow rate at each inlet location. Each inlet’s contributing area and composite runoff coefficient determines its design flow.

Calculate peak flows →

Determining Rainfall Intensity

Rainfall intensity depends on the time of concentration (Tc) — the time for water to flow from the most distant point in the drainage area to the inlet. The assumption is that the critical storm duration equals Tc.

For small developed areas:

• Sheet flow over pavement: Use NRCS sheet flow equation (limited to 300 ft)
• Shallow concentrated flow: Use NRCS velocity charts
• Channel/pipe flow: Use Manning’s equation

Most small site drainage areas have Tc values of 5–15 minutes. Many jurisdictions set a minimum Tc of 5 minutes to avoid unrealistically high intensities.

Calculate time of concentration →

Phase 2: System Layout

Inlet Placement

Inlets are placed where water collects or where flow must be intercepted:

• Sag points (low points in the road profile) — Always place an inlet here. Water has nowhere else to go.
• Intersections — Prevent flow from crossing pedestrian paths
• Upgrade of crosswalks and driveways — Intercept flow before it crosses
• Maximum spread limits — Place inlets wherever gutter spread exceeds allowable width
• Property line/building entrances — Prevent flow from entering private property or buildings

Pipe Routing

Storm drain pipes connect inlets and run to the outfall. Layout guidelines:

• Follow roadway alignments and easements
• Maintain minimum cover (typically 2–3 ft under roadways)
• Route pipes downhill — gravity is your friend
• Avoid conflicts with existing utilities (water, sewer, gas, electric)
• Place manholes at direction changes, slope changes, pipe size changes, and at maximum 300–500 ft intervals

Catch Basin vs. Inlet

A catch basin includes a sump below the pipe invert that traps sediment and debris before they enter the pipe system. Standard catch basins have 2–4 feet of sump depth. A simple inlet connects directly to the pipe without a sump.

Size catch basins →

Phase 3: Pipe Sizing

Accumulating Flows

Starting from the most upstream inlet, add the design flow from each inlet’s contributing area as you move downstream. At each pipe segment:

Design flow = Upstream pipe flow + Inlet design flow

For the Rational Method, you don’t simply add peak flows because each subarea may have a different Tc. Instead, you use the longest Tc to the design point and recalculate the total flow with the corresponding (lower) intensity but larger total area.

Manning’s Equation for Pipes

Size pipes using Manning’s equation for circular sections flowing full:

Which simplifies to:

Where D is the pipe diameter in feet and S is the pipe slope (ft/ft).

Size storm drain pipes →

Design Criteria

Standard pipe design criteria:

Pipe Sizing Procedure

For each pipe segment:

1. Determine the design flow (accumulated from upstream)
2. Set the pipe slope (usually matching the ground slope or the minimum for velocity)
3. Select a pipe diameter that provides adequate capacity
4. Check velocity at design flow — adjust slope or diameter if outside limits
5. Standard pipe sizes: 12, 15, 18, 21, 24, 27, 30, 36, 42, 48 inches (and larger)

Pipes should never decrease in size in the downstream direction, even if the slope increases enough to carry the flow in a smaller pipe. Debris that passes through upstream pipes must pass through downstream pipes.

Phase 4: Hydraulic Grade Line Analysis

The HGL analysis is the final verification of system capacity. It accounts for all energy losses in the system, not just pipe friction.

Energy Losses

Pipe friction: Calculated from Manning’s equation along each pipe segment.

Junction losses: At each manhole or junction box, energy is lost due to flow expansion, direction change, and merging flows. Loss coefficients depend on the junction geometry:

The junction loss is: h = K × V²/2g

Entrance/exit losses: Where flow enters or exits the pipe system.

HGL Check

Starting at the outfall (known tailwater elevation), work upstream through the system calculating the HGL at each junction. The HGL must remain below the ground surface at every point. If the HGL exceeds the ground surface, the system surcharges — water backs out of inlets and floods the surface.

If the HGL is too high, remedies include:

• Increasing pipe sizes (reduces friction losses)
• Increasing pipe slopes (reduces the HGL slope)
• Reducing junction losses (using benched manholes, smooth transitions)
• Adding parallel relief pipes

Phase 5: Outlet Protection

Where the storm drain discharges, the concentrated flow can cause severe erosion. The outlet velocity determines the type of protection needed:

The outlet must also comply with any permit requirements for the receiving water body. Discharge permits may restrict flow rates, require water quality treatment, or mandate specific outfall configurations.

Putting It All Together

Storm drainage design is systematic — each step informs the next. The most common mistakes happen when designers skip steps or make shortcuts:

• Skipping the HGL check and discovering surcharging during construction
• Using minimum pipe sizes everywhere without checking velocity
• Ignoring tailwater effects at the outfall
• Not accounting for bypass flow from upstream inlets

A complete design package includes plan and profile drawings showing pipe sizes, slopes, invert elevations, rim elevations, and the computed HGL.

References

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

2. American Society of Civil Engineers. (2017). Gravity sanitary sewer design and construction (ASCE Manual of Practice No. 60). ASCE Press.

3. American Iron and Steel Institute. (1999). Modern sewer design (4th ed.). AISI.

4. Natural Resources Conservation Service. (1986). Urban hydrology for small watersheds (Technical Release 55). U.S. Department of Agriculture.