Case Study

Water In The Hole

American Concrete Pressure Pipe AssociationTABLE WIDTH=80% CELLPADDING=10> What does it take to control the devastating affects of flooding? Where does a watershed community find an area that can effectively mitigate the catastrophic effects of periodic torrential rains?

The loss of life, property and income from area business and tax revenues is an all too certain experience in flood-prone areas. The challenge for Illinois' DuPage County Department of Environmental Concerns, Stormwater Management Division, was to find the funding, the political will, and the engineering expertise to design a system within the geographic and geologic constraints of the Salt Creek Watershed.

In August of 1987, the Salt Creek drainage area of approximately 117 square miles experienced a period of intense rainfall. More than 13 inches fell in four days, and an additional 9.4 inches of runoff from Chicago's O'Hare Airport was recorded over 24 hours. It was a record that exceeded the previous severe storm events of 1954, 1957, and 1972, and shy of the 18-in. deluge of July, 1996 in nearby Aurora, (in the same county but in a different drainage basin). The event of 1987 produced a flow in Salt Creek equaling 3,400 cfs at maximum flood stage, according to estimates.

It was reported that the flood caused over $200,000,000 in property damage. Responding to the flood and the resulting losses, the Illinois Legislature acted to allow the five counties surrounding Cook County to develop stormwater management programs. With the subsequent development of regional ordinances and the funds appropriated to support their plan, the County of DuPage formed a Stormwater Committee in 1987. This Committee, in turn, created the Stormwater Management Division of the County.

The consulting firm of <%=company%> was retained by the County to evaluate and ultimately design a system of flood control. It subcontracted part of the design and feasibility of the system to Rust Environment and Infrastructure, Inc. and Woodward-Clyde Consultants. The conceptual design for the Lower Salt Creek Watershed Plan was to construct five flood control projects within the basin. The search for a high capacity facility within the basin ultimately focused on the Elmhurst Quarry, a limestone quarry adjacent to Salt Creek. This property covered nearly 70 acres with an excavated quarry depth of 200 feet below grade, and a flood storage capacity of 8,300 acre feet.

Challenging the engineering team was the need to design a structure that would effectively divert flood flows from Salt Creek into the quarry. The hydraulic modeling of the drainage area was developed by the use of an FEQ (full equation) hydraulic analysis computer model. The dynamic, continuous simulation computer model computed the run-off and flood stages for the basin. Hydrologic information was derived from an extensive data base developed over the past 40 years. The FEQ model ultimately established the critical flood stage elevations, and aided in the economic analysis of the diversion control structure.

As designed the hydraulic diversion structure featured a fixed-side overflow weir measuring 140 ft long at its crest with an adjustable 80-ft long stop-log weir. The stop-log feature allows for flexible weir height control when future upstream reservoirs will control the flood stage elevations of Salt Creek. The only other mechanical feature of the structure is a 7-ft x 7-ft sluice gate, set below the weir crest. This gate will control not only the diversion of the upstream rainfall events but also those flows requiring diversion due to storm events downstream of its location.

Given the degree of sophistication and complexity of its design, it was agreed that a 1:20 scale model would be constructed. The featured components not only included those previously described, but also 1,400 ft of Salt Creek, a blasted rock tunnel beneath the six-lane Illinois Route 83, and a vortex drop shaft. The shaft combined in a single structure, the features of a baffled drop inlet and an air vent. The high velocity drainage vortex creates a water-free area in its center that vents or draws out the air, preventing air blockage to the drainage flow.

The quarry was previously excavated into two lobes, an East Lobe and a West Lobe. These were separated by a high wall and joined together by a tall, narrow tunnel, situated beneath West Avenue.

The high wall was also used to support a pump station on its west face. Modifications to the moderately-fractured rock barrier were necessary to provide adequate bearing capacity and stability. The face was stabilized with a matrix of rock bolts in combination with shotcrete by Baker/Mellon Stuart Construction, which had to come up with a novel platform for the specialized rock-bolt equipment and personnel. The platform was designed to be suspended by a crane located at the top of the high wall, and proved to be an effective and safe method for drilling and rock-bolting the wall.

Four vertical pumps were installed at this pumping station, which had to be capable of de-watering the quarry at a rate of 200 acre-feet/day (100 cfs) from the West and East Lobes. These sump pumps convey the water up the face of the West Avenue high wall to the headworks structure, then pump it through over 1,900 ft of 48-in.-diameter pre-stressed concrete embedded cylinder pipe (PCCP)), manufactured by <%=company%> to AWWA Standard C-301. The 48-in. PCCP outlet conduit was designed to convey stormwater back to Salt Creek in flood-stage controlled amounts.

Construction of this project was bid in four separate contracts as follows:

Contract 1: Vortex Drop Shaft and Tunnel - McNally Tunneling Corporation Contract 2: Inlet Works and Access Decline and Ventilation Shaft Bulkheads - Baker/Mellon Stuart Contract 3: West Lobe Modifications - T. W Edmier Contract 4: Pump Station - Baker/Mellon Stuart

Construction on these projects began in March of 1993. Called the "Elmhurst Quarry Flood Control Project," it is one of the largest, non-federally funded, off-line reservoirs in the United States. The total combined flood water storage system design includes the Meacham Grove Reservoir (600 acrefeet), Wood Dale-Itasca Reservoir (1,775 acre-feet) and Westwood Creek Reservoir (100 acre-feet). The Elmhurst Quarry has proven to be the most economical flood control facility in DuPage County, Illinois, at a cost of about $7,500 per acre-foot of storage.

References: 1: "Elmhurst Quarry Flood Control Project," a paper presented to the ASCE Water Resources Planning and Management Division, Division Annual Conference, May 23, 1994.
2: "Elmhurst Quarry Flood Control Project, A Conceptual Design Report," Prepared by <%=company%> in Association with Rust Environment and Infrastructure and Woodward-Clyde Consultants.

Editor's Note: This article appeared in the Fall 1996 issue of Concrete Pressure Pipe Digest, a publication of the <%=company%>. It was contributed by Tommy Thomas, a sales engineer on the staff of <%=company%>., South Beloit, IL, a member of the association.

Edited by Ian Lisk