14.5 Reviewing Drainage Plans & Evaluating Design Alternatives

1. Review as a Distinct Skill

The Drainage Systems content area contains three tasks: preparing a drainage plan and profile, designing and selecting components, and Review Drainage Plans (e.g., evaluate existing design or design alternatives). The third is tested separately because reviewing requires a different discipline than designing: you are looking for the condition that cannot be true, not for the solution you would have chosen.

2. The Drainage Review Sequence

Step 1 — System continuity

Trace the entire network from the most upstream inlet to the outfall.

• Does every inlet connect to a downstream structure?
• Does every pipe segment have a structure at each end?
• Does the system terminate at a legal and capable outfall — a public storm main with capacity, a receiving watercourse, or a designed basin?
• Does any branch dead-end?

The outfall is the most commonly missed item. A beautifully detailed network that discharges into a neighbor's yard, into a system with no remaining capacity, or onto an unarmored slope has no outfall.

Step 2 — Drainage area accounting

• Do the delineated sub-areas sum to the total contributing area, including offsite area that drains through?
• Are runoff coefficients or curve numbers consistent with the proposed land cover, not the existing?
• Is each area assigned to the structure that actually receives it?

Unassigned area is the classic hydrologic error: a 0.4-acre strip that belongs to no sub-area still generates runoff, and it arrives somewhere.

Step 3 — Invert and profile verification

This is where review finds fatal errors:

Pipe size may increase or stay constant downstream; it may never decrease. A downstream reduction creates a bottleneck, surcharges the upstream structures, and can blow manhole lids.

Crown matching. When pipe size increases at a structure, matching the crowns (rather than the inverts) prevents the smaller upstream pipe from backing up into the larger one. This lowers the upstream invert and must be carried through the profile.

Step 4 — Capacity and velocity

• Is each segment sized for the required design storm, with the design storm stated?
• Is velocity within the acceptable band?

Minimum slope requirements exist to achieve the self-cleansing velocity, which is why a flat pipe fails even when its capacity calculation passes.

Step 5 — Structure spacing, inlet type, and sag conditions

• Is structure spacing within the jurisdiction's maximum for cleaning access (commonly 300 to 400 feet)?
• Is there a structure at every change in size, slope, direction, or material?
• Is inlet type matched to condition? Curb inlets stay clear of debris and suit continuous-grade streets; grate inlets capture more flow per foot but clog; combination inlets are used where both capture and clogging resistance are needed.
• Are sag (low point) inlets sized with a clogging factor?

A sag inlet is the critical review item. On a continuous grade, flow bypassing a clogged inlet simply continues to the next one. In a sag there is no next inlet — water rises until it finds an overland path, which may be a doorway. Sag conditions require a clogging factor in sizing plus either a flanking inlet or a designed overland relief route.

Step 6 — Outfall condition and erosion

• Is energy dissipation provided (riprap apron, stilling basin, plunge pool) sized for the outlet velocity?
• Is the receiving channel stable at the discharge velocity?
• Is the outfall above the receiving water's normal level, or has tailwater been analyzed?
• Are downstream property rights and drainage easements in place?

Step 7 — Interdisciplinary and code checks

• Do storm structures conflict with trees, utilities, footings, or the accessible route?
• Are grates in pedestrian areas compliant — openings not permitting passage of a 1/2-inch sphere, with elongated openings oriented perpendicular to the dominant direction of travel?
• Are structures outside the accessible route where possible, and flush where not?

3. Evaluating Drainage Alternatives

When comparing surface, subsurface, and hybrid approaches, use a consistent set:

Standard evaluation logic. Surface conveyance is preferred where land is available, because it is cheaper, provides treatment and attenuation, and fails visibly. Piped conveyance is used where the surface is needed for program, where grades cannot achieve a swale, or where velocity would exceed a vegetated lining. Most real projects are hybrids: LID cells and swales manage frequent small storms and water quality, while a piped system conveys the larger design storm safely.

The maintenance question decides more alternatives than the hydraulics do. An owner with no vegetation maintenance capability will not sustain a bioswale network, and a bioswale that is not maintained becomes a mowed ditch with none of the treatment benefit it was credited for. Evaluate against the owner's actual operations capability, not the ideal one.

4. Exam Traps & Pitfalls

1. Not tracing to the outfall. A system without a legal, capable outfall is not a system.
2. Accepting a downstream pipe size reduction. Sizes may hold or increase downstream, never decrease.
3. Checking capacity but not velocity. Below about 2.5 ft/s the pipe silts; above about 10 ft/s it abrades.
4. Treating a sag inlet like an on-grade inlet. A sag has no bypass path and needs a clogging factor plus relief.
5. Matching inverts where the pipe size increases. Match crowns so the smaller pipe does not back up.
6. Ignoring offsite contributing area. Area that drains through the site is part of the system.
7. Selecting an alternative the owner cannot maintain. Maintenance capability is a design criterion.