Hydraulic Design Tool

Broad Crested Weir Calculator

Enter the weir geometry and flood discharges. The calculator follows the same workbook-style solving approach and prepares rating curves plus DFL/CFL water-level summaries.

–Cd range
–Ho/L range
–Max flood Q

Input Parameters

Reference geometry and flood values are preloaded and editable.

Use L as the plan overflow width / weir length across the crest. It is not the side-section profile length of the weir body.

Input Guide

The figure separates the plan width input from the side-section level inputs and calculated outputs.

Broad crested weir input guide Plan view showing the weir length L used in calculation and side section showing crest level, upstream apron elevation, upstream water level, head over crest, and discharge direction. Broad Crested Weir Input Guide Plan view: L used in calculation L Overflow width / weir length plan width across the crest Side section: level inputs and calculated outputs Upstream side Downstream side Ho calculated depth above crest Flow direction U/S WL calculated output Crest level input U/S apron EL input crest top Q Flood Q is entered in the table. U/S water level and Cd are calculated for each discharge. Use plan width L from the plan view. Do not use the side-section profile length of the weir body.
L is the plan overflow width / weir length across the crest.
Crest level and U/S apron elevation are vertical levels using the same datum.
Water level and depth above crest are calculated outputs for each discharge.

Flood Discharge Table

Editable return periods for the final summary.

Return PeriodFlood Q (m³/s)
The calculator solves the required upstream water level automatically for each flood discharge. Cd is calculated row-by-row from the head-ratio relationship and applied in the solution.