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title: "Broad-crested Weir Coefficients"
canonical: "https://help.innovyze.com/space/xps/19662708/Broad-crested%20Weir%20Coefficients"
format: markdown
---
**Bridge Link roadway overtopping** can be represented as a Broad-crested weir with the following coefficient attributes based on the **FHWA Design Manual.**

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![image](media://7d8f92ca-686b-4553-a7d4-6c3d98e033b5)

***Figure 1: Roadway overtopping***

  
Roadway overtopping will begin when the headwater rises to the elevation of the roadway (Figure 1). The overtopping will usually occur at the low point of a sag vertical curve on the roadway. The flow will be similar to flow over a broad crested weir. Flow coefficients for flow overtopping roadway embankments are found in HDS No. 1, Hydraulics of Bridge Waterways, as well as in documentation of HY-7, the Bridge Waterways Analysis Model. Curves from the Bridge Waterways Analysis Model reference are shown in Figure 2 below. Figure 2-A is for deep ovetopping, Figure 2-B is for shallow overtopping, and Figure 2-C is a correction factor for downstream submergence. The equation below defines the flow across the roadway.

Q<sub>0</sub>=C<sub>d</sub>LHW<sub>r</sub><sup>1.5 </sup>

Where: 

Q<sub>0</sub> = the overtopping flow rate in m<sup>3</sup>/s (ft<sup>3</sup>/s)

C<sub>d</sub> = the overtopping discharge coefficient (for use in SI units, see note below)

L = the length of the roadway crest, in m or ft

HW<sub>r </sub>= the upstream depth, measured from the roadway crest to the water surface upstream of the weir drawdown, in m or ft.

> ⚠️ C<sub>d</sub> is determined from Figure 2, and other English unit research must be corrected by a factor of 0.552 [C<sub>d </sub>(SI) = 0.552 (C<sub>d</sub> English)]


![image](media://c9c0bb82-c905-44e1-818f-d8222ec48752)

***Figure 2 - English Discharge Coefficients for Roadway Overtopping***

  
Other Bridge Link Data is discussed on the [Bridge Link](https://help-innovyze.atlassian.net/wiki/spaces/xps/pages/19661513/Links#Links-BridgeLink) Help topic section.