Properly designed and mounted wires provide the most precise flow measurement. However, incorrect setting and procedure can result in significant errors in the calculation of discharge. In order to ensure accurate calculation performance, the following steps are mandatory for the use of weirs.
The weirs should be set at the bottom of a long pool sufficiently wide and deep with a steady flow at speeds of less than 15cm/sec.
Baffles can be placed in a weir pond to minimise velocity.
The wall of the weir must be vertical.
The middle line of the wire should be parallel to the direction of flow.
The crest of the weir ought to be level such that the water flowing into it would be at the same width at both points around the crest.
Notch should be a normal form and should have a rigid and straight tip.
The weir crest is to be above the bottom of the approach channel.
The crest of the weir should be set high enough to allow the water to fall freely below the weir.
The depth of the water flow over the rectangular weir should not be less than 5 cm and not more than 2/3 crest distance.
The scale or gauge used to measure the head should be about four times the approximate head. Zero of magnitude should be precisely at the same degree as the crest of the weir.
A weir must be situated in a balanced section of the river, where even the river is unwilling to alter direction.
The weir needs to be installed high enough to meet the command specifications. During high flooding, the river could overwhelm its banks and reverse its course. The position with firm, well-defined banks must therefore be chosen for the construction of the weir.
The site must have nice bed requirements, including such rock outcrops, where appropriate.
Rectangular weirs: These weirs have a rectangular opening and are the most common type of weir. They are used in a wide range of applications and are easy to construct.
Triangular weirs: These weirs have a triangular opening and are commonly used in irrigation systems. They are also used in water treatment plants and in other applications where a moderate flow rate is required.
Trapezoidal weirs: These weirs have a trapezoidal opening and are commonly used in large-scale irrigation systems. They are also used in industrial applications where a high flow rate is required.
Cipoletti weirs: These weirs have a curved opening and are used in applications where a high flow rate is required. They are commonly used in large-scale irrigation systems and in hydroelectric power plants.
Q = Discharge (volume per unit time, usually in cubic feet per second or cubic meters per second)
C = Discharge coefficient, which depends on the geometry of the weir and the velocity distribution in the approach flow (typically ranges between 0.55 and 0.65)
L = Length of the weir crest
H = Head of water above the crest of the weir, also known as the "weir height" or "head of water on the weir"
The Classification of Weir According to Construction Material Is
Timber Weirs: These weirs are constructed from wood or timber. They are commonly used in small streams or channels, where the flow rate is not very high.
Masonry Weirs: These weirs are constructed from stones, bricks, or concrete blocks. They are more durable than timber weirs and can be used in larger rivers or channels.
Steel Weirs: These weirs are constructed from steel plates or beams. They are typically used in large rivers or channels with high flow rates
Earthen Weirs: These weirs are constructed from soil or earth. They are simple to construct and can be used in smaller streams or channels.
Rock-fill Weirs: These weirs are constructed by placing rocks or boulders on top of each other to form a weir. They are commonly used in mountainous regions where rock is abundant.