Drainage

Getting water off and under the road: runoff and water-quality treatment on the hydrology side, and gutters, inlets, storm drains, culverts, energy dissipation, detention and open channels on the hydraulics side. It carries 8–12 of the 80 questions on the Transportation exam.

What the problems look like

Calculation and design reasoning from a plan, profile or hydrograph. Recurring shapes — rational-method peak flow with weighted runoff coefficients and time of concentration; IDF reads; infiltration (Green-Ampt, Horton); detention storage from a hydrograph; pollutant mixing at an outfall; gutter spread and inlet spacing; storm-drain sizing and partial-flow depth; ditch capacity by Manning; and culvert headwater, outlet velocity and inlet depression under FHWA HDS-5. Treatment practices — filter strips, swales, bioretention, extended detention — appear as identification items.

By exam day you should be able to —

• compute tc along the governing path and a rational-method peak with composite C;
• size a detention volume from inflow and allowed release;
• find the inlet spacing that holds gutter spread;
• size a storm drain for full flow and find depth at partial flow;
• run a culvert in inlet and outlet control and take the governing headwater;
• find outlet velocity — from normal depth in inlet control, and from critical depth, tailwater or full barrel in outlet control.

Where people lose points

• Time of concentration: tc is the travel time along the path that takes longest — not necessarily the longest path, and not the sum of parallel paths; an agency minimum is a floor on tc, not a time to add to it.
• Weighting: a composite C is the area-weighted ΣCA/ΣA, not a plain average; pollutant concentrations mix by runoff (C × A), not by area; count only the area that actually drains to the point.
• Detention: storage is the area between the inflow hydrograph and the release rate, and it keeps growing until inflow drops back to that rate — not just until the peak.
• Geometry: wetted perimeter follows the sloping sides and stops at the free surface; a full pipe's R is D/4. Take slope from inverts, not rims, and round pipe sizes up.
• Culvert control: barrel roughness and tailwater matter only in outlet control; in inlet control the entrance governs. The design headwater is the higher of the two.
• Outlet control details: start the grade line at the higher of the tailwater and (dc + D)/2; use the entrance-loss coefficient for the actual inlet; D is the box's rise, not its span.

Have these at your fingertips

The rational method, runoff-coefficient and sheet-flow relations, Manning and pipe relations, the hydraulic-elements graph — and where they live in the Handbook. FHWA HDS-5 (3rd ed., 2012) for culverts and the Green Book for gutter-spread criteria are the same editions before and after April 2027; know which HDS-5 chart fits each inlet type.

Going deeper

• Roadside and Cross-Section Design — cross slopes and ditches
• Vertical Design — sag curves, where inlets go

Part of the Transportation exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.