Weirs are used in conjunction with locks, to make a river navigable and provide a uniform flow for navigation. In this case, the weir is made significantly longer than the width of the river by making it in a ‘U’ shape or running diagonally instead of a short vertical course. Since the weir is the part where the water overflows, a longer weir allows a lot more water with little increase in overflow depth. This is done to reduce the fluctuation of the upstream depth with the change in river flow rate. Doing so avoids unnecessary complications in designing and using lock or irrigation diversionA weir allows a simple method of measuring the rate of fluid flow in small- to medium-sized streams, or in industrial discharge locations. Since the geometry of the top of the weir is known, and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. The calculation relies on the fact that fluid will pass through the critical depth of the flow regime in the vicinity of the crest of the weir. If water is not carried away from the weir, it can make flow measurement complicated or even impossible. A weir may be used to maintain the vertical profile of a stream or channel, and is then commonly referred to as a grade stabilizer.
A weir will typically increase the oxygen content of the water as it passes over the crest, and hence it can have a detrimental effect on the local ecology of a river system. A weir will artificially reduce the upstream water velocity, which can lead to an increase in siltation. The weir may pose a barrier to migrating fish. Fish ladders provide a way for fish to get between the water levels. Mill ponds provide a water mill with the power it requires, using the difference in water level above and below the weir to provide the necessary energy.
A walkway over the weir is likely to be useful for the removal of floating debris trapped by the weir, or for working staunches and sluices on it as the rate of flow changes. This is sometimes used as a convenient pedestrian crossing point for the river. Even though the water around weirs can often appear relatively calm, they are dangerous places to boat, swim or wade; the circulation patterns on the downstream side can submerge a person indefinitely. devices.
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