Weirs are structures consisting of an obstruction, such as a dam or bulkhead, placed across the open channel with a specially shaped opening or notch. The weir causes an increase in the water level, or head, which is measured upstream of the structure. Common weir constructions are the rectangular weir, the triangular or v-notch weir and the broad-crested weir. Weirs can be circular or rectangular in shape. Rectangular weirs and triangular or v-notch weirs are often used in water supply and wastewater and sewage systems. A function that is quantified as it pertains to weirs is the weir overflow rate. The calculation of weir overflow rate is important in detecting high velocities near the weir, which adversely affect the efficiency of the sedimentation process. When excessively high velocities occur, the settling solids are pulled over the weirs and into the effluent troughs. Weirs improve the flocculation process by creating additional turbulence for mixing. The effluent end of the floc basin needs to offer a smooth and gradual transition into the sedimentation basin. Weirs also collect water from the sedimentation basin and control the uniformity of flow.
Let’s look at a couple of weir overflow rate problems.
Based on the plant conditions provided in the two problems below, determine the weir overflow rate for each problem.
Weir length = 250 feet
Flow rate = 3,500,000 gpd
Weir diameter = 45 feet
Flow rate = 2.56 MGD
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