Influence of a Dip Plate on the Discharge of Water over a Sharp Crested Rectangular Weir.

Dip plates (sometimes referred to as scum plates) are used with increased frequency upstream of weirs in drainage systems in order to separate sewage from clean, rain (storm) water and in streams and canals to protect the weir structure from floating debris. However, there is a marked lack of information on the effect that introduction of these structures has on the discharge over a weir. The present investigation was initiated in an attempt to rectify that situation. The problem was tackled both experimentally and analytically. Apparatus used for the experimental part of the investigation consisted of a suitably modified laboratory test channel 24 inches (609. 6 mm) wide, in which additional measuring stations were introduced. Modifications of channel geometry consisted of designing and building a weir dip plate assembly which enabled the distance between the two plates to be varied. False side plates were also introduced and suitable adjustable fittings designed so that the ratio of channel/weir width could be pre-set as required. Tests were carried out at three values of that ratio; these were 1/1, 1. 6/1 and 2. 4/1. Water surface level was measured at various points both upstream and downstream of the dip plate. Measurements of flow velocities were also carried out. In positions where the velocities were small and the use of pitot tubes resulted in inadmissibly low readings of manometers, a specially designed velocity meter based on the principle of drag on a sphere was used. Altogether 107 calibrations of the weir with the upstream head ranging from zero to 7 inches were carried out. The distance between weir and dip plates was varied from ____ inch to 5 inches. The depth of immersion of the dip plate varied from the bottom edge of the plate level with the sill of the weir to 4 inches below it. The width of the weir was 10 inches (254 mm). In addition to depth and velocity measurements, the pattern of flow both on the surface and within the body of the fluid was observed. Visualisation of surface currents was done by sprinkling coloured perspex filings on to the water surface. Deep currents could be seen by observing the motion of small polystyrene beads dropped into the flow. Further indication of the flow pattern was obtained from tracings and photographs of motion of the beads in a narrow glass sided flow visualisation flume. As a first approach to the analytical solution of the problem of discharge over a weir provided with a dip plate, a unidimensional approximation was applied. The flow through dip plate - weir configuration was considered to be equivalent to a flow which first passes through a drowned orifice and then over a weir. The head downstream of the orifice (formed by the dip plate) acts at the same time as the upstream head of the weir. Bearing in mind that the rate of flow is the same through both devices, a relationship determining the value of the head downstream of the dip plate in terms of the upstream head and the channel - dip plate - weir configuration is established (equation 3. 12). Substitution of this expression into the conventional equation for a rectangular weir gives a prediction of a rate of flow over the weir (equation 3. 13). The flows calculated using the analytically deduced equation and based on the assumption of the coefficient of discharge for the weir, equal to unity, are compared with the experimentally obtained results. The correlation between the two sets of results is such that it only requires an introduction of a constant value coefficient of discharge (independent of the variation of upstream head) in order for the calculated values to fall within the band of uncertainty of experimental results. The values of the coefficient, dependent on the dip plate - weir plate configuration, are all found to fall numerically in the range accepted (BS 3680) for the coefficients of discharge for sharp crested rectangular weirs. The variation in the value of coefficient is observed and discussed. The investigation thus resulted in providing a method of prediction of the rate of flow over a rectangular sharp crested weir fitted with a dip plate. It has also indicated a method of approach to any similar problem. In addition, the change in a flow pattern resulting from the introduction of a dip plate was studied and discussed