Canoe Chute Designed for Chicago River
5/27/2009 12:00:00 PM
By Joyce Mast
The Chicago River North Branch Dam, located at the confluence of the Upper North Branch of the Chicago River and the North Shore Channel and bordered on the north by a football stadium and on the south by River Park, is owned and maintained by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) and operated by the Chicago Park District. Like more than 90 percent of the two million dams in the United States and 1,300 in Illinois, it is a low-head dam.
Low-head dams have been called "drowning machines" because of the powerful currents around them. Proposed solutions to the danger they pose have included removal, limiting access, and design modifications, but competing interests make the solutions anything but clear-cut. Thanks to a CEE study funded by the MWRDGC and led by Professor Marcelo H. García, the Chicago River North Branch will be the site of a newly designed structure optimized to improve safety for boaters while addressing ecological concerns.
The small size of low-head dams belies the severity of danger around them. Water flowing over the drop forms a hole, or hydraulic, at the base that traps and re-circulates anything that floats. A victim caught in the backwash will be carried to the face of the dam where the water pouring over it will wash him under to a point downstream called the boil. There, roiling water from below surfaces and moves either downstream or back toward the dam. Air bubbles mix in the water, decreasing its buoyancy by one-third. The victim has a hard time staying afloat, even with a life jacket. Once caught, he struggles to the surface, where the backwash again carries him to the face of the dam, continuing the cycle.
The area immediately below the dam may be scoured out, creating a hole that adds to the force of the recirculation. During periods of high water and heavy rains, the backwash current problems get worse as the reach of the backwash current is extended downstream and debris washed into the river becomes lodged at the dam making even more currents. Rescuing trapped individuals is dangerous and often unsuccessful.
One approach to the dam problem is to remove it–to the dismay of those who enjoy fishing near the dam and hearing the "waterfall" sound and the relief of ecologists who have shown that low-head dams limit the upstream distribution of fish species. Another is to create "exclusion zones" that restrict paddling access 300 feet above a dam to 50 feet downstream. One such proposal, by Rep. Tom Cross (R-Oswego), is in response to the drownings three years ago of a kayaker and two men who tried to save him in in the dam at Yorkville on the Fox River. The Illinois Paddling Council claims the exclusion zones would encompass many portages paddlers use to walk around dams and argue that the measure would make Illinois’ rivers essentially off-limits, according to an April 14 story in the Chicago Tribune. Then there is the problem of enforcing the rule.
A third approach is to modify the dam to accommodate a fish passage with the goal of enhancing fish communities. In 2006 the Friends of the Chicago River (FOCR) commissioned a study by a Chicago environmental engineering firm to assess habitat and fish communities along the North Branch. Fish species commonly found in Illinois streams tend to avoid very high velocity water, making it unlikely they would swim up traditional fish passages used on high-gradient waterways found, for example, in the Pacific Northwest. The FOCR had wanted the dam removed, but the structure serves to control the grade (bottom elevation) of the stream. Taking out the dam could cause upstream degradation of the riverbed, followed by stream bank erosion. And because of the challenges inherent in designing a dedicated fish passage, a multi-use structure was deemed more appropriate.
A combined fish-passage/canoe chute at the North Branch Dam site would allow local fish communities to safely pass upstream and recreational canoes and kayaks downstream. In addition, it would improve stream ecology in the Chicago River system by improving habitat conditions and connecting the Lower and Upper reaches of the North Branch. The additional turbulence generated as the flow cascades over the canoe chute drops would preserve the "waterfall" sound and increase the amount of dissolved oxygen in the river, improving water quality, particularly during the summer months when dissolved oxygen is low in the waterways. This last aspect motivated the MWRD to fund the CEE study to design the structure.
First, the MWRD surveyed the channel, the bank areas, and the river flow. These data were converted to cross-sections and a concrete basin 1/20th scale model built by CEE research staff in the Ven Te Chow Hydrosystems Laboratory. Specifications were based on earlier research within CEE sponsored by the Illinois Department of Natural Resources. Graduate research assistant Christiana Barnas (BS 07) has taken flow velocity measurements in the model for the last two semesters as part of an independent study project. Data are being processed by academic staff assistant for the CEE hydro lab, Andrew R. Waratuke (BS 97, MS 99).
Jorge Abad (MS 02, PhD 08), a post doctoral researcher who worked on this project and will soon be an assistant professor at the University of Pittsburgh, is the first author of a paper showing the latest design of the canoe chute/fish passageway in which the fish swim up both sides of the chute. A series of vertical baffles with offset openings from one to the next allows a sinuous flow path that decreases flow velocities within the fish passageways and provides relatively quiet areas for the fish to swim. His paper was presented in Kansas City, Mo., in May at a meeting of the Environmental and Water Resources Institute of the American Society of Civil Engineers.
Although the design is still being finalized, the structure is expected to be implemented at the Chicago River North Branch Dam within the year, García says, offering a solution for safety that will please recreationists and ecologists alike.
Photos, top to bottom:
1. CEE student Christiana Barnas takes measurements in the canoe chute/fish passage model, built to 1/20th scale in the Hydrosystems Laboratory.
2. A dam can act as a "drowning machine," as shown in this illustration by Joyce Mast.
3. Proposed placement of the canoe chute/fish passage. There are four drops in the chute, each about one foot.
4. Computer simulation image by Jorge Abad of the latest design of the structure.