A.T. Margevicius (AECOM, USA), C. Wan (Jacobs, NZ)
There are numerous publications that discuss different types of sewage drop structures. This paper provides an update on the state-of-the-practice, as well as a comparison of four different types of sewage drop structures.
Dropping sewage from near-surface collection systems into deep tunnels involves several challenges, including air entrainment, energy dissipation, surge control, and maintenance access. The use of four specific types of drop structures to handle large flows is becoming more prevalent. Numerous types of drop structures have been used to convey sewage down to tunnels, with varying degrees of success. Several recent projects have demonstrated that not all drop structure types perform equally well. While numerous models (both mathematical and physical) have been used to analyse drop structures, there is relatively little data that compares the performance of each type. Much of the previous work has focused primarily on hydraulics, and comparatively little emphasis has been placed on the pneumatics of drop structures. Both full-scale prototypes and laboratory-scale models suggest that the flow regimes in drop structures can be complex and variable. Further, the true nature of twophase flow, which occurs in many drop structures, is not well understood or modelled in many instances. Using both qualitative and quasi-quantitative criteria, this paper compares the relative performance of four types of sewage drop structures:
• The plunge drop,
• The helicoidal ramp,
• The vortex drop, and
• The cascade (baffle) drop.
In comparing the four types of drop structures, the paper draws on several case studies and lessons learned from others. This effort had several goals:
• To compare the relative performance of four different types of sewage drop structures.
• To describe the major factors influencing the air and water flow rates and pressures.
• To document the results of several case studies on the how certain drop structure features played a role in determining the selection of a drop structure.