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A Baffling Solution to a Complex Problem Involving Sewage Drop Structures

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Abstract:

Dropping sewage from near surface collection systems into deep tunnels involves a number of challenges, including the solids management, energy dissipation, odor control, air entrainment, surge control, and access for personnel and equipment. The use of drop structures to handle large flow volumes will become more prevalent over the next several years due to the large number of CSO tunnel projects on the horizon. Numerous types of drop structures have been used to drop sewage, with varying degrees of success. Throughout its collection system, the Northeast Ohio Regional Sewer District (NEORSD) has successfully used several types of drop structures, including H-4 type vortex, drill drops, jet drop, simple, and a little-known structure referred to as the baffle drop structure, also known as a cascade drop structure. Since the NEORSD has over one hundred baffle drop structures in its collection system (some of which have been operational for decades), the baffle drop is proven and effective. Until cently, however, baffle drops had been used for smaller flow rates, and the design of baffle drop structures was based upon engineering judgment and prior experience.

The specific site circumstances on a recent NEORSD project suggested that a baffle drop may be suitable. However, the flow rates at this site are larger than the flows at other baffle drop structure sites. The NEORSD needed to be sure a baffle drop at this site was designed and constructed for optimal efficiency and minimal cost.

The NEORSD chose to model the baffle drop structure for this site. Two methods were identified for evaluating the baffle drop structure: numerical modeling and physical modeling. Due to the highly complex three-dimensional flows in a baffle drop structure, physical modelingwas chosen.

Initial baffle drop structure dimensions (e.g., overall height, baffle spacing, shaft diameter, and inlet and outlet configurations) were estimated using basic hydraulic calculations and steady state assumptions. A 1:12.5 Froude scale hydraulic model was built and tested using a variety of flow rates and baffle spacings. The results verified that the baffle drop structure is an effective method for dropping large amounts of flow from multiple sources into deep tunnels, compared to other drop structure types. Qualitatively, baffle drops seem to perform better than other types of drop structures with respect to outlet air entrainment. The results indicate that the baffle drop structure is well-suited for this project. Furthermore, the modeling also established the fundamental equations and methodology for designing baffle drop structures for other flows, in other locations, and for different drop heights.

Keywords: CSO; Drop structure; Froude model; access structure; air bubble; air entrainment; baffle drop; cascade drop; combined sewer overflow; drop manhole; energy dissipation; multiple inlet drop; sewage; tunnel

Document Type: Research Article

DOI: https://doi.org/10.2175/193864710798216279

Publication date: 2010-01-01

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