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Maximum flow rate Weir loading rate

Weir Loading Rate

Weir loading rate is the calculation of how much water is being pushed into the launder weirs.

Launder weirs are installed at the basin or tank outlet to provide a uniform collection and flow-on of settled water. If the flow through the weir becomes too high then floc may be carried over in the settled water.

Weirs are usually designed for a maximum flow rate (per metre of length) based on optimum sedimentation conditions. Comparing actual weir loading rates to the design rates gives a quick idea of whether the weirs are overloaded and contributing to any water quality problems.

Weir loading rate = surface loading x weir length
                                                      tank surface area


Example

Calculate the weir loading rate when the surface loading for the tank is 1.98kL per hr per m 2 and the weir length is 30m and the surface area of the tank is 200m2.

Weir loading rate = 1.98 kL per m2 x 30 m
                                     200 m2

Weir loading rate = 0.297 kL per hr per m

Note the units used here. We could also give the answer in minutes instead of hours by multiplying by 60.

Weir loading rate = 0.297 x 60 kL per hr per m
Weir loading rate = 1.782 kL per minute per metre

If the weir loading rate is too high, there may be surging of water across the weir. Very long weirs may need a sawtooth edge (called a v-notch) to avoid uneven flow across the weir.

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