The IIHR Dropshaft Solution
by Cornelia F. Mutel
IIHR director Jack Kennedy sat at the large oak table in his office, flanked by four of IIHR’s research engineers: Subhash Jain, Rob Ettema, Jacob Odgaard, and Tatsuaki Nakato. The large number of IIHR consultants was meant to impress the representatives of the Milwaukee Metropolitan Sewerage District (MMSD) who sat across the table. Kennedy wanted the MMSD representatives to know that IIHR was ready to tackle any water problem it was given. It’s true that IIHR had little experience with installing large underground wastewater-storage systems, but no matter. Kennedy believed in IIHR’s ability to devise practical solutions.
Milwaukee and the Clean Water Act
It was the early 1980s. Milwaukee was facing a water problem that plagued many large and growing cities. Their storm-sewer systems, which carried mixed stormwater runoff and sewer water, had functioned well as long as the city remained relatively small. But these systems now were being outgrown and overloaded, a result of rapid population growth and the impermeable pavement that was increasingly covering urban land surfaces. When large rainstorms hit, the cities’ sewers would overflow and spew foulsmelling, polluted water into basements and streets, rivers and lakes. Not only were these “combined sewer overflows” (CSO) untenable, they also violated the mandates of the nation’s 1972 Clean Water Act.
Milwaukee was desperate for a solution. It was clear that some sort of storage system would have to be constructed, something capable of holding large quantities of CSO water during storms until the water could be pumped, gallon by gallon, into the city’s water-treatment plants and safely released into natural waterways. The massive storage system would have to be constructed underneath the bustling city — there was no space for it elsewhere.
But what type of dependable water storage facility could be constructed at a reasonable cost? Chicago, a hundred miles south of Milwaukee, had been fiddling for years with its underground Tunnel and Reservoir Plan (TARP). TARP involved football-field-sized reservoirs interconnected by gigantic conduits, all drilled into bedrock. But TARP had ongoing problems with the system’s dropshafts — the long, wide pipes that fed excessive water from the ground surface down into the reservoirs. Air would become trapped in the down-flowing water and “burp” back upward, throwing manhole covers into the air, or CSO water would surge upward through the shafts and spout into the streets. Entrained air in the downflowing CSO water also could create massive, possibly damaging vibrations. And costly deaeration chambers were necessary for the increasingly-complex underground systems. These and other problems required ongoing solutions, which drove TARP’s cost higher and higher. Milwaukee wanted an affordable system that would work from the start.
Creating the Dropshaft
As the day’s discussions wore on, IIHR’s researchers realized that they would first need to conduct a literature search on dropshaft function, and then perform model studies in IIHR’s laboratories to test whatever dropshaft modifications they proposed. The researchers set to work.
In those pre-Internet days, a literature search meant writing letters to request research papers from a variety of sources. The typewriters were set into motion, and research papers started to appear in IIHR’s mailbox. Within a few months, IIHR had a plan: spiraling the down-flowing water around the outside of the dropshaft, thus creating a twisting vortex flow that avoided TARP’s unstable-air-entrainment problems.
But how could the water be set swirling? Again the researchers searched the literature for alternatives, deciding to try a tangential water inlet described in a Russian paper, which shot the water into the dropshaft at an angle, thus twisting it rapidly around the dropshaft walls. But how would this tangential inlet work for Milwaukee? First small, then large model tests were conducted at IIHR. The tests served as tools in IIHR’s development of a simple descending tangential ramp that was easier to construct and smaller than other vortex-flow inlets. MMSD representatives returned to IIHR to observe the model tests and then started construction.
Keeping Pollution out of Lake Michigan and More
The system was a success: It was relatively inexpensive to construct and it worked as intended. In fact, it worked so well that the design soon was adopted by other large cities. IIHR had, with a single dropshaft project, become an expert on underground CSO storage structures, an achievement that the institute claims proudly to the present day. (See “Modernizing London’s Sewers” for a description of IIHR’s current efforts with London’s underground water storage tunnels.)
Soon IIHR was busy designing alternative dropshafts to meet the specific demands of other large cities. Phoenix needed a system that completely eliminated expensive deaeration chambers; IIHR’s helicoidal ramp, which incorporated twirling vanes into the dropshaft, proved to be a successful alternative. Tokyo, with its extreme space constrictions, wanted to eliminate the tangential water inlet; IIHR did so by modifying the helicoidal ramp, incorporating twirling vanes at both the top and bottom of the dropshaft to avoid air entrainment. Subhash Jain became the go-to person for these projects, a role that he played until he retired in 2003, after which he still returned to IIHR’s labs when specific projects demanded his expertise. He estimates that during his career, he has worked on dropshaft structures for 10 cities.
But his career focus all began more than 30 years ago with Milwaukee, which today claims that its Deep Tunnel underground wastewater storage system has captured and cleaned more than 98 percent of the city’s CSO flow since it started operation, an amount that has kept 100 billion gallons of polluted water out of Lake Michigan. That’s no small achievement for that city, or for IIHR.