AECOM simulations using FLOW-3D put the design of a combined sewer overflow tunnel-connector on the fast track to meet EPA requirements.
Sewer systems don’t seem very exciting unless they serve as action-movie get-away routes. Most likely you don’t even think about them until torrential rain brings water up to curb-level. Unfortunately, the sewer systems beneath more than 770 older American cities create pollution problems during heavy storms. These particular older designs used cost-effective single-style pipes for both sewage and storm runoff, sending the combined sewage overflow (CSO) into rivers and lakes in an approach that predates modern health/environmental concerns and knowledge.
In 1994, the U.S. Environmental Protection Agency (EPA) issued a policy requiring the affected municipalities, mostly in the Northeast and Great lakes regions, to reduce or eliminate CSO-related problems (a policy that became law as part of the Clean Water Act of 2000). Indianapolis, being one of those cities where even a light rain storm can cause raw sewage backup and overflow, knew it needed to work fast – in major construction terms – to address the problem by the required 2025 deadline.
Indianapolis called upon AECOM, an international design firm, to design the first of three deep rock storage tunnels that Citizens Energy Group is building. This system, running a combined 25 miles, includes a massive below-ground pumping station and interconnecting structures that vertically drop CSO down from the existing sewer lines. For the first tunnel, three large drop structures would divert CSO into the storage tunnels for subsequent treatment after the rainfall subsided.
To tackle the project, AECOM chose FLOW-3D to simulate the behavior of multiple possible drop-structure designs, minimizing the need for rework on the one physical model budgeted to be built and evaluated. The test results were so spot-on, matching predicted to measured values, that no redesign was necessary; in addition, AECOM now routinely uses CFD simulations as the first step for its hydraulic tunnel design work.
Large Scale Project on a Tight Delivery Schedule
The 20thcentury construction of sewage treatments plants brought new awareness to what could and should be done with residential, commercial and environmental runoff outflow. During normal operation, CSO discharge goes directly to treatment plants and all is well. Unfortunately, during major storms, to avoid over-capacity problems at those plants, built-in relief structures called regulators still discharge excess flow into nearby water bodies. These discharges carry a range of pollutants from oil and grease to pesticides and wildlife waste.
In an encouraging sign of success, new CSO separation, storage and treatment facilities – whose construction began in the early 1990s – have already made a 67% improvement against the effects of pollution, but much work remains to be done. For Indianapolis, that effort began in 2008 when the City of Indianapolis’ Department of Public Works prepared a CSO long-term control plan. The core of their “storage and transport” approach, which will hold the overflow until the normal treatment plants can handle it, is called the Indianapolis Tunnel Storage System, or DigIndy.
The first phase of this system is a $180 million project called the Deep Rock Tunnel Connector (DRTC). DRTC is a seven-mile long, 18-foot diameter underground tunnel that will rework the flow path of three existing Indianapolis sewer-to-river outflow connections (Fig. 1). The goal is to safely redirect excess rainfall runoff away from these relief outlets into massive tunnels, via drop structures between the existing sewers and new tunnels, and holding it until it can be pumped to a treatment plant for post-storm treatment.
At an average depth of 250 feet below ground surface, the DRTC is designed to minimize disruption to the neighborhoods above both during construction and ultimate operation. But the size and complexity of the project added urgency to AECOM’s task: design and evaluate possible drop structures for each of the three locations, finishing 60% of the structures’ designs in seven months.
The intent of such structures is to deliver the sewage flow from the standard city sewage system to the deep storage tunnel while avoiding both efficiency losses (slow-down or back-up) and long-term structural damage that can occur if the size and shape of each section isn’t carefully matched to the volume and velocity of the incoming flow.
Consultant Ryan Edison, an AECOM senior technical specialist, knew the contract’s scheduling requirement would limit any physical build-and-test activity to just one model for validation only. Having used FLOW-3D flow simulation software for 15 years on other major construction projects, he was confident that its ability to predict turbulence, overtopping and energy dissipation would be well suited to the design project. Moreover, the software’s options for running multiple what-if scenarios allowed him to minimize the risk of having to redo the design details – a critical benefit given the ripple effect of any changes on a project with multiple parallel construction tracks.
In spite of the tight time constraints, Edison was particularly pleased with the challenge because of an unusual opportunity: creating the drop-structure design with CFD as well as following up with a physical study. “Because these are such big structures,” he says, “there are not many of them built, and they’re usually just done either with physical models or with hand-calculations. CFD’s not really been used.”
For the DRTC project, he would first test the computer design against simulated operational conditions. Edison used FLOW-3D, a software package AECOM had previously employed for its ability to model three-dimensional, transient, turbulent flow conditions, its uniquely accurate free-surface-tracking algorithms and its ability to model different design geometries without changing the computational mesh for each design.
Armed with simulation data, Edison would then compare those results with operational data from a 1:10-scale physical model tested at the University of Iowa’s IIHR facility. (The latter was originally called Iowa Institute of Hydraulic Research, but is now known as IIHR—Hydroscience & Engineering, reflecting the group’s multidisciplinary scope).
Zeroing in on the Drop-Structure Challenge
The most restrictive DRTC site geometry occurs at the regulator designated CSO 008. This location requires a vertical run of more than 150 feet to connect the existing CSO regulator (at about 75 feet below grade) with the new 18-foot-diameter collection tunnel far below. With each drop structure costing $7 million or more, project managers were eager to lower the chances of needing expensive and time-consuming redesigns after the physical model was built.
Historically, drop structures are designed on paper as adaptations of previous projects then built as scale-models; testing alone can take six months or more. In this accelerated project, AECOM’s initial task, beginning in fall 2009, was to choose between two standard concepts: a baffle-plunge (cascading) style and a tangential vortex version, both designed to slow down and control the often 35 mph storm waters. Hand calculations plus initial CFD analyses with FLOW-3D determined general structure diameters and components sizes, which AECOM used to evaluate constructability and cost considerations.
Given the site requirements at CSO 008 and the cost efficiency, the city and AECOM chose the tangential vortex drop structure. The core element of this design is a vertical tube (drop shaft) fed by a tapered (widening) approach channel that directs the flow first into an annular jet, then creates a vortex-induced spiral flow pattern down the shaft wall. This controlled descent slows down and safely handles flows that will reach more than 300 million gallons per day (mgd). Avoiding potentially destructive turbulence in the storage tunnel is the key goal, so preconditioning the drop-shaft flow is at the crux of the design.
The structure itself consists of six major parts: 1) the approach channel (coming from several existing sewer tunnels), 2) a rectangular transitional taper channel that widens out and delivers the horizontal flow to a vertical drop shaft, 3) the drop shaft itself which must control the flow downwards to the tunnel, 4) a de-aeration chamber (that redirects the flow to the horizontal direction and reduces air-entrainment), 5) a vertical air-vent that removes entrained air from the drop and keeps the air core of the dropping fluid open, and 6) a pipe (adit) that connects the de-aeration chamber with the storage tunnel chamber (Fig. 2).
Design and CFD Analysis
While the general vortex design is widely accepted, each of the drop structures had to be properly sized to the Indianapolis topology to ensure optimal tangential flow characteristics. In particular, AECOM’s plan for CFD evaluation of possible designs had three goals: determine if the site-specific constraint of limiting the combined approach and taper channels to short lengths was acceptable and did not create excessively turbulent conditions in the approach; validate that stable flow conditions exist in the taper channel; and assess the flow stability over a range of flow conditions. A logical reference point was the well-known and documented system called the Milwaukee Inline Storage Project (constructed after the 1993 outbreak of cryptosporidiosis from the city’s drinking water).
Edison based the initial design on the Milwaukee drop-structure design designated H-4, scaled to the DRTC project dimensions, including a basic drop length of 166 feet, and set up a FLOW-3D analysis specifying the volume flow rate, walls, symmetry and other initial parameters.
What we found with CFD, he notes, is that with Milwaukee, if you used this design, it wasn’t working well for our application. FLOW-3D was showing that, so we used CFD to do some variants and come up with our own modified design.
Modifications included using a wider approach channel, wider taper, and/or a deeper taper depth; Edison says it was extremely fast to set up each variation in FLOW-3D to go through an optimization process (Fig. 3, 4 and 5). The progression of improvement was encouraging; the high level of detail of the simulated results even convinced him to add a baffling plate at the base of the drop shaft to improve scour (erosion) protection and decrease turbulence where the vertical flow transitions to horizontal.
The FLOW-3D output behavior for the ninth design variation, V9, which widened the approach section, exhibited good flow stability at all flow volume levels up through 300 mgd, with no hydraulic jumps and good Froude numbers (a dimensionless quantity used to indicate the influence of gravity on fluid motion), so AECOM selected it for physical testing and validation starting in February 2010 (Fig. 6). The plan was to do further CFD and optimization based on the Iowa lab’s test results.
Regarding the dimensional parameters determined in V9, Edison said, “We took that design to Iowa and built it, based on CFD, and it worked perfectly. The staff (at IIHR) told us this is the first time they’ve actually just set something up and it just worked – they didn’t have anything that they said to change.” Measured data included water surface elevation within the drop shaft connecting structures, quantification of air entrainment in the adit, and air flow up the vent shaft. Photos of the vortex development as the flow increased showed good rotation and attachment to the shaft wall all the way to the de-aeration chamber (Fig. 7).
Edison traveled several times to the test facility for follow-up; due to the physical model’s working correctly right from the start, he had time to expand the test program. “What was fun is that we then explored some things that we were curious about, like moving the vent, so we had time to intellectually play with it.” Being ahead of schedule, Edison was able to use the remaining project time to investigate the hydraulics within the de-aeration chamber and the adit.
AECOM wrapped up its overall work on the DRTC in July 2010. In March 2013, excavation began on the 18-ft-diameter tunnel, and all three CSO drop structures (with the other two designed by CFD only) are now under construction.
In Edison’s opinion, civil engineering on the whole has been slow to adopt CFD. For proof, he described what he saw on his first trip to the IIHR facility, the so-called “graveyard” of old tangential vortex models, “They were dismantled and rebuilt because they worked poorly and IIHR had to redo them.” He, however, is sold on the use of simulation for hydraulic designs.
Summing up the DRTC effort, Edison said, “It was really kind of fun. I learned more about where physical modeling was needed and when so, for certain instances, you actually can just do a pure CFD-based design. A lot of the DRTC work is the proof of that. The physical model really wasn’t needed, but it gave the validation and reduced risk. It was incredible to get both those things done on a project.”
This article first appeared in WaterWorld Magazine.