A weir is a small barrier erected across the stream or river that slightly raises the water level on one side; it’s like a dam but for smaller streams. Because they allow water to pool behind them, weirs provide a steady flow over the top of the dam. Furthermore, weed can also be the crest of a spillway on an enormous embankment dam.
The material weirs can be made of depends on their age and use. A weir can be made of wood, concrete, or a combination of rocks, gravel, and boulders.
The surface over which the water flows in a weir is called the crest. The water that spills over this crest is known as the nappe, simply meaning those molecules of liquid H2O that make it across Enjoy View. But this sort of nappe cannot exist with dams, which prevent the flow over any such structure. If the nappe drops a long way in the air – that is to say, if water elevation rises before running up (weir), it’s called a free discharge weir. However if in such a case water flows with barely an increase of weir, it is called submerged or drowned.
Function of Weir in Hydropower Plant
Together with locks, weirs are used to make a river navigable and ensure it remains flowing evenly. In this case, the weir is set up very much longer than that. It can be structured in a U shape or even diagonal to avoid building it perpendicularly as short and brief as possible. A comparatively longer weir will permit much more water to overflow with only a small increase in the depth of the overflow. In this way, fluctuation in the depth of the river upstream is reduced with changes in flow rate. Thus there is no need for complex designing and using of the lock or irrigation diversion devices.
A weir in hydropower plant serves as a simple method of measuring the flow rate in runs or medium-sized and industrial stream discharges. It is used in large hydropower projects to prevent potential harm as a result of dam development. Since the geometry of the top stage weir is known and water flows over all sides, one may convert the depth of water behind this into a rate. The formula is based on the setting that fluid will pass through the critical depth of its flow regime near to crest of the weir. If the water cannot be carried away from the weir, it will make flow measurement complicated or even impossible. A weir is thus used to maintain the vertical profile of a stream or channel and is then called a grade stabilizer.
An artificial weir will generally increase the amount of dissolved oxygen in water over a crest, and this feature has an effect on local ecology within river systems. A weir that will artificially slow upstream water velocity may increase siltation. Migrating fish may be blocked by the weir. Fish ladders allow fish to pass between the water levels. A water mill shows how the difference in levels of water above and below a weir can provide power.
Also Read: What is Conventional Hydroelectric Plant?
Types of Weir
Here are some of the types of weir in hydropower plants:
- Rectangular weir: This is a typical weir shape with a sharp-crested or narrow crested top edge. It is normally applicable to larger flowing channels.
- Triangular weir: V-notch weir refers to the shape of the weir which is a reverse triangle. This type of weir is particularly suitable for measuring discharge over small flows at greater precision.
- Trapezoidal weir: It is trapezoidal, and serves as a modification of the rectangular weir with greater capacity for the same crest strength. The sides are inclined outwards with a slope of 1:4 (horizontal: vertical).
Also see: What is Hydraulic Head in Hydroelectric Powerplant?