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HydroCAD Stormwater Modeling - Since 1986

Broad-crested Weir

Sample Weir #1

The following photograph shows a dam on an old mill pond.  The dam is constructed of stone blocks with a wooden spillway.  The dam is located in Chocorua, New Hampshire, a small village that was previously known as Tamworth Iron Works when the mill was operating.  Note the gate valve (far left) used to supply the penstock, and the unusual wave pattern in the falling water.

Modeling as a sharp-crested weir

The spillway is constructed of wooden planks set at about a 45 degree incline, resulting in a relatively sharp crest.  This suggests that the sharp-crested weir equation would be appropriate for modeling the stage-discharge relationship.

Note that the flow is relatively even along the length of the spillway, right up to the stone blocks at each end.  This indicates the absence of end contractions that can result from lateral flow at the spillway.  In this case the dam wall forms an approach channel that ensures unidirectional flow as the water approaches the spillway.  This might not hold true at higher heads, since the effective length of the "approach channel" is reduced in relation to the flow velocity.

Modeling as a broad-crested weir

Many questions arise about the proper application of the broad-crested weir equation.  In general, the broad-crested weir capability is useful only in cases where the discharge coefficient varies with head.  If the coefficient is relatively constant, nothing is gained by using the (more complex) broad-crested weir technique.

To determine the potential variation in the discharge coefficient, consult the table of weir coefficients on page 165 of the HydroCAD-5 Owner's Manual.  Note that some weir profiles exhibit significant coefficient variations, while others are relatively constant under varying head.

How much does it matter?

A final consideration is to evaluate the sensitivity of the overall model to the weir techniques being considered.  For example, some ponds may display only a minor change in peak water level, even with a significant change in the weir coefficient, and the peak discharge may change even less.

It is therefore suggested that some type of sensitivity analysis be performed before extensive effort is invested in exact determination of weir parameters.  In some cases, a broad-crested weir with accurate coefficients may be required, while in others, the basic sharp-crested weir equation may suffice.

For a discussion of compound weirs, see weir example #2.

 

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