Hydrologic design flow context
Spillway sizing begins with a defensible peak inflow estimate. This calculator supports a quick Rational-method check using catchment area, rainfall intensity, and runoff coefficient. For SI inputs, it applies Q = 0.278 × C × i × A, where i is in mm/hr and A is in km². For larger watersheds, you can bypass the hydrology step and enter a project flow directly. Where possible, validate peak flow with regional regression, gauge records, or hydrologic modeling before final spillway approvals formally.
Catchment area and intensity selection
Area should reflect the contributing drainage boundary and exclude diversions. Convert units carefully: 100 ha equals 1.0 km², and 1 acre equals 0.0040469 km². Rainfall intensity should come from IDF data for a duration near the time of concentration and an adopted return period. Typical preliminary intensities range from 25–150 mm/hr depending on climate and risk.
Runoff coefficient and safety factor
The runoff coefficient C captures losses and imperviousness. Early-stage values often fall between 0.20–0.40 for pervious rural catchments, 0.40–0.70 for mixed development, and 0.70–0.95 for highly impervious areas. A safety factor then converts inflow to design flow, commonly 1.10–1.50 for preliminary checks, but it should match hazard classification and regulatory guidance.
Spillway capacity using a simplified weir model
Capacity is estimated with Q = Cw × L × H3/2, using crest length L and head H. The coefficient Cw is a starting value tied to crest geometry and approach conditions; sharp-crested sections may use about 1.84 (SI) or 3.33 (US), while broader crests can be lower. Final design should review submergence, approach velocity head, and tailwater effects.
Interpreting results and documenting assumptions
A PASS indicates computed capacity meets the selected design flow; CHECK means resize L or allow more head. Use the sizing option to solve for required length or head while holding other parameters fixed. Exported CSV and PDF outputs provide a transparent record of inputs, units, timestamps, and status for internal QA and client submittals.
FAQs
1) What does the PASS or CHECK status indicate?
PASS means the computed spillway capacity is at least the design flow shown. CHECK means capacity is lower, so adjust crest length, allowed head, or coefficient and recalculate using project standards.
2) When should I use the manual design flow option?
Use manual flow when your hydrology comes from a separate model or study, such as unit hydrograph routing, flood-frequency analysis, or agency-provided design discharges. Enter the flow directly and apply a safety factor if required.
3) How do I select rainfall intensity for the Rational method?
Choose intensity from local IDF curves for the selected return period and a duration close to the catchment time of concentration. If Tc is uncertain, test several durations and document the conservative choice.
4) How should I choose the weir coefficient (Cw)?
Start with the preset for your crest type, then replace it with a value from your standard, lab data, or design guide. Cw varies with crest shape, approach depth, contractions, and submergence, so confirm applicability.
5) Does this calculator account for tailwater or submergence?
No. The weir equation here is simplified and assumes free overflow. If tailwater rises near the crest, capacity can drop. Check submergence, approach velocity head, and downstream energy dissipation in final design.
6) Why do the sizing options solve for length or head only?
To keep the sizing step transparent, the calculator holds other parameters constant and solves a single unknown from the same equation. In practice, geometry, allowable headwater, and coefficient selection should be iterated together.