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Weirs Generally, any flow obstruction that causes water to rise to flow over it, but used exclusively for intentional obstructions Uses include flow measurement (sharp-crested weirs) and control of water surface profile, e.g., by inducing super-critical flow (broad- crested weirs)

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Definition Diagram and Analysis of Sharp-Crested Weirs Drawdown at crest is typically ~0.15 H

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Nappe entrains air underneath it and can collapse onto the downstream side of the weir; following analysis assumes that this does not occur (can ventilate this area to assure an air space)

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Consider thin layer dh in plane of weir in a rectangular channel. Assuming V 2 >>V o 2 and negligible frictional headloss, V=(2gh) 0.5, so (replacing L by b): where H C is the distance from the crest of the weir to the water surface at that location.

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Measurements of the water surface elevation above the weir crest are typically made upstream of the weir, where the water surface has not been significantly affected, and this value of H is used instead of H C in the calculation. To account for this and other approximations, an empirical coefficient is added to the equation: C w can be approximated by 0.611+0.075(H/P w ) and is typically in the range 0.64-0.70.

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Other Common Shapes for Sharp-Crested Weirs V-notch weir useful if low flows are of interest, since they could cling to the plate of a rectangular weir. For V-notch weirs: Contracted weir V-notch or Triangular weir Weir can totally block part of the channel width, in which case the channel is contracted. C w,V-notch typically ~0.6

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Streamlined or Broad-Crested Weirs

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Water Profiles (CEE 477)

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