Drop structure

Abstract: Drop structures used during wastewater collection and treatment are sources for volatile organic compound (VOC) emissions. To assist in the reduction of such emissions, pilot-scale experiments were completed using municipal wastewater to study the effects of drop height, liquid flow rate, and tailwater depth on oxygen transfer, and to evaluate the effects of the same parameters on the stripping of 10 VOC tracers. Results were used to develop predictive models for oxygen and VOC transfer. Oxygen uptake at the pilot drop structure suggests that the drop height is the most important parameter influencing oxygen uptake at enclosed drop structures. Tailwater depth had little effect on oxygen transfer at the drop structure. Stripping of VOCs at drop structures was seen to be a strong function of Henry's law coefficient. This sensitivity was related to gas-phase resistance in mass-transfer and/or VOC accumulation in the air bubbles. Incorporating gas-phase resistance and an appropriate a factor for wastewater into the model allowed the prediction of VOC deficit ratios and estimation of VOC stripping at drop structures for both clean water and wastewater.

Abstract: A theory is developed that is shown to give good prediction of the head-discharge relationships for two designs of vortex drop structures. The first design is the widely used spiral guide wall design, and it is shown that control is shared between the critical section at the top of the shaft and the vortex inducing asymmetry of the guide wall inlet design. A second simplified and more space saving design, called the vertical slot vortex drop structure, is also analyzed. It is shown that, although the flow conditions and analysis are quite different, the resulting head-discharge relationship is quite similar to the spiral guide wall design. For both designs the head-discharge relationship is shown to be almost linear and the theoretical and measured discharges are in close agreement. The proposed vertical slot design produces a stable air core, similar to the spiral guide wall design, and consequently the flow is free from risk of surging to the pipe-full flow condition. The compactness of the new design is especially suitable for underground installations.

Abstract: A baffle-drop structure is a flow conveyance structure that can be used for transport of urban storm water down to underground storage tunnels. The water cascades down the structure from baffle to baffle and plunges into a pool at the bottom from where it is conveyed to the tunnel through an adit. The structure has been used successfully in a limited number of urban drainage schemes. However, its hydraulics and air entrainment characteristics are not fully understood. Using a series of laboratory experiments, an analysis has been tested and validated that may be used for design. The analysis provides a dimensionless relationship between key variables, including design discharge, shaft diameter, baffle spacing, and position of a vertical wall dividing the shaft in a dry and a wet portion. Using this relationship the shaft can be designed to maintain atmospheric pressure throughout its height with little or no air being entrained into the tunnel.