Build the stage-storage-discharge relationship — storage by the average-end-area method and outlet discharge by stage — and plot the three curves. The routing input table, not the routing. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: This builds the storage-discharge relationship; it does not route a hydrograph. Pair it with the detention-basin-estimator for the preliminary volume and real routing software (storage-indication / modified Puls) for the final design. Outlet equations here assume free discharge — a submerged outlet behaves differently, and clogging, the usual failure mode, isn't modeled at all.
Stage-storage-discharge
Stage (ft)
Storage (cf)
Storage (ac-ft)
Discharge (cfs)
How the Numbers Are Derived
Average-end-area storage
Between two stages with surface areas A1 and A2 a vertical distance Δh apart, the incremental storage is the conic (average-end-area) volume ΔV = (A1 + A2)/2 · Δh. Cumulative storage sums these from the lowest stage up. On a straight-walled prism (constant area) this returns exactly A · h; on a concave bowl it slightly overestimates — acceptable for planning, and the stage-area survey is the real accuracy limiter, not the method.
Outlet discharge equations
With head h measured above the outlet's invert/crest:
Orifice:Q = Cd · A · √(2g·h), Cd default 0.6, A from the orifice diameter, g = 32.174 ft/s², head taken to the orifice centerline. Rectangular weir:Q = Cw · L · h^1.5, Cw default 3.33. 90° V-notch:Q = 2.49 · h^2.48. Total discharge at a stage is the sum over outlets that are flowing at that stage. (The sibling weir & orifice tool uses Cd 0.61 for a sharp-edged orifice; use your structure's tested coefficient.)
Field notes
This is the input to routing, not routing itself. The storage-indication (modified Puls) method takes this 2S/Δt + Q relationship and marches an inflow hydrograph through it to get the peak stage and outflow.
A submerged outlet transitions from weir to orifice behavior; if downstream tailwater can rise above an outlet, the free-discharge equations here overstate flow. The tool warns when a lower outlet sits below a higher one's invert.
Clogging is the unmodeled failure mode. Trash racks, a minimum orifice size (many agencies floor it near 3 in.), and a factor of safety on the primary outlet are how it's handled — not in the hydraulics.
The stage-area survey is where the accuracy lives. A basin graded off a rough plan and one shot as-built can differ enough to change the routed peak.