Control Section Lab

Scenario

The reach

Discharge cfs

Friction, Manning n

SmoothRough

Throat width, C4 ft (30 = no contraction)

Sill height at J3 ft

Chute slope, C6 ft/ft (0.001 = no break)

Outfall at OUT1

Tailwater stage ft above outfall invertUsed when the outfall is FIXED.

Overlays

What the reference solves

z₁ + y₁ + V₁²/2g = z₂ + y₂ + V₂²/2g + Δx·(S_f1+S_f2)/2

S_f = (n V / 1.486 R^2/3)² · y_c = (Q²/g b²)^1/3

No subcritical root ⇒ critical depth: a control.

Jump where M_sup < M_sub, M = Q²/(g b y) + b y²/2

Hydraulic control

What to notice

Candidate controls

Key depths

SWMM5 options for this run

NORMAL_FLOW_LIMITED

BOTH is the 5.2.4 default (project.c line 850).

INERTIAL_DAMPING

PARTIAL is the default: σ falls from 1 at Fr 0.5 to 0 at Fr 1.

Split each conduit into

More, shorter conduits: the usual cure. Watch the steadiness verdict.

Throat entry loss Kentry, C4

Throat exit loss Kexit, C4

The run

The page runs the stock EPA SWMM 5.2.4 source, compiled to WebAssembly, on the same eight conduits: steady inflow at J1, three hours of dynamic wave, and the state at the end. Depths shown are the final step; the bars are the range over the last half hour.

Node depths, SWMM5 against the reference

Reference depth is taken on the downstream side of each node; where a width change makes the two sides differ, the upstream-side depth is shown in brackets. SWMM5 carries one level per node.

Flow classification from the SWMM5 report

The rules in the code (EPA SWMM 5.2.4, verbatim)

InfoWorks 1D solver, documented defaults

Lower Froude numberInertia terms start being phased out above this.

Upper Froude numberInertia terms dropped above this. The help warns that values above 1.0 may cause instabilities.

Time weighting θ (simulation)0.65

Space step20 × min(b, h)

Space step limits0.5 – 100 m

Min nodes per conduit5

Computational nodes in this reach

No ICM engine runs in this page. The violet line is the steady profile that ICM's documented rules imply for this reach: free outfall at min(y_n, y_c), inertia phased out between the two Froude numbers (linear assumed), one level per node. Treat it as a prediction to test. Export the .inp, import it into an InfoWorks network in ICM, and run with defaults.

Why damped inertia erases the control

Steady flow in a prismatic channel obeys dy/dx = (S₀ − S_f) / (1 − σ Fr²). With full inertia (σ = 1) the denominator passes through zero at Fr = 1, and that zero is what forces critical depth at a control. Both engines damp σ toward zero as Fr approaches 1, so the denominator never reaches zero and no control can form from the momentum equation alone. Controls then come only from explicit rules: the outfall, a free fall at an offset drop, the normal-flow limit.

Rule by rule

What the engine actually does here

Seven measured findings on this reach. The numbers below were recorded when the page was built; the button re-runs every case in your browser with the same engine, so no figure on this tab can go stale silently.