Hydraulics–Open Channel

Flow with a free surface: uniform flow in channels, grade lines and energy dissipation at drops and culvert outlets, storm drainage from gutter to outfall, and the behavior of sub- and supercritical flow. It carries 7–11 of the 80 questions on the Water Resources & Environmental exam.

What the problems look like

A mix of calculation and reasoning about flow regime. Recurring shapes — Manning capacity or normal depth for a trapezoid, a compound section with overbanks, or a multi-cell channel; critical depth, Froude number and alternate depths at gates, rises and contractions; a hydraulic jump below a drop or culvert (sequent depth, energy loss); culvert headwater under inlet or outlet control; gutter spread and inlet flow; and storm-sewer HGLs worked up from the outfall. Profiles and water-surface classification (M1, M2, S2…) show up as conceptual items.

By exam day you should be able to —

• compute A, P and R_H for any section, and apply Manning in USCS (1.486) and SI (1.0);
• split a flooded section into subsections with their own n;
• find critical depth, Fr and alternate depths from specific energy, and predict what a rise or contraction does in each regime;
• compute a jump's sequent depth and energy loss, and tell where tailwater puts it;
• check culvert headwater and pick the governing control;
• size a storm sewer and run its HGL upstream from the tailwater.

Where people lose points

• Hydraulic radius: the wetted perimeter stops at the free surface but includes interior walls; the wide-channel shortcut R ≈ y fails on a narrow section; a full pipe's R_H is D/4, not D/2.
• Units: the Manning constant is 1.486 in USCS and 1.0 in SI; g changes with them.
• Critical depth: in a rectangular channel use unit discharge — y_c = (q²/g)^(1/3), with q = Q/b, not Q; any other shape solves Q²/g = A³/T.
• Froude number is v/√(g·A/T) — hydraulic depth (area over top width), not hydraulic radius, and no 2g.
• Regime reversal: subcritical and supercritical flow respond oppositely to rises and contractions; the energy diagram tells you which limb you're on.
• Jumps: the relation gives a ratio y₂/y₁, and the head loss is (y₂ − y₁)³/(4y₁y₂), not the jump height.
• Grade lines: start a storm-sewer HGL at the tailwater when the outlet is submerged, and keep the HGL and EGL separate — surcharge is judged by the water surface.

Have these at your fingertips

Manning's n tables, the specific-energy and jump relations, and gutter-flow and culvert relations — plus where they live in the Handbook. Culvert design (FHWA HDS-5) and energy dissipators (FHWA HEC-14) join the supplied design standards only from the April 2027 exam; before then, work from the Handbook.

Going deeper

• WRE design standards: the April 2027 change
• Hydrology — the runoff that feeds the inlets
• Hydraulics–Closed Conduit

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