15.2 Storm Drain Inlets and Minor Systems

Minor Storm Drainage Systems

A minor drainage system is the network that collects and conveys runoff from ordinary design storms: pavement cross slope, curb and gutter, swales, inlets, manholes, laterals, storm sewer pipes, and small culverts. A major drainage system is the overflow route used when the minor system is exceeded, clogged, or surcharged. On the PE Civil WRE exam, inlet questions sit between Project Sitework, Hydrology, and Hydraulics - Open Channel.

The first decision is what water reaches the inlet. For small paved or developed areas, the common PE method is the Rational Method, Q = C I A, where Q is peak flow in cubic feet per second, C is runoff coefficient, I is rainfall intensity in inches per hour, and A is drainage area in acres. Rainfall intensity should match the selected return period and the time of concentration, not an arbitrary duration.

Common Inlet Conditions

Inlet capacity is the intercepted flow, not necessarily the full approach flow. If 7 cfs approaches an on-grade inlet and the inlet captures 5 cfs, then 2 cfs bypasses downstream. The next inlet receives its own local runoff plus the upstream bypass. This is a common source of missed PE points.

Node-by-Node Calculation Workflow

1. Delineate tributary area. Use proposed grades, not just property lines. Pavement high points, ridges, swales, and curbs define where runoff travels.
2. Choose runoff parameters. Select C based on surface type and development, then find I from intensity-duration-frequency information using the governing time of concentration.
3. Compute local peak flow. For Rational Method problems, Q = C I A. Keep acres and inches per hour if using the standard cfs form.
4. Add upstream bypass. At each downstream inlet, total approach flow equals local runoff plus bypass from upstream inlets.
5. Check inlet capture and spread. Compare approach flow to inlet capacity and any allowable street spread or ponding depth.
6. Carry captured flow to the pipe. Captured flow enters the storm sewer; bypass remains on the surface.
7. Check pipe hydraulics. Use Manning equation for gravity storm sewer capacity, then check velocity, cover, hydraulic grade line, tailwater, and junction losses where relevant.

Quick Inlet Example

A 1.6 acre paved area drains to an on-grade inlet. Use C = 0.90 and I = 4.2 in/hr. The peak approach flow is:

Q = C I A = 0.90 x 4.2 x 1.6 = 6.05 cfs

If the inlet captures 4.8 cfs at the design spread, the bypass is 6.05 - 4.8 = 1.25 cfs. If the next inlet has 3.4 cfs of local runoff, its approach flow is 3.4 + 1.25 = 4.65 cfs before its own capture check.

Pipe and Structure Checks

A storm sewer that can convey the arithmetic sum of inlet flows may still fail if the downstream tailwater is high or if the hydraulic grade line rises above allowable elevations. Manholes and junction boxes add losses where pipes turn, merge, or change size. Minimum cover matters for traffic loading and constructability. Excessive velocity may require outlet protection, while very low velocity can encourage sediment deposition.

For PE WRE, avoid treating inlets as magic drains. The surface system, inlet geometry, and pipe system interact. The most defensible answer follows the water from pavement to gutter to inlet to pipe to outlet, while checking what happens when water is not captured.