In order to combat low dissolved oxygen levels that were often observed in area waterways, the Metropolitan Water Reclamation District of Greater Chicago (MWRD) built five Side-stream Elevated Pool Aeration (SEPA) stations along the Calumet River system in the early 1990s. The stations were designed to withdraw water from the nearby river, lift it 12-15 ft, and allow it to pass over a series of waterfall cascades, thereby aerating the water before releasing it back into the river.
Since their construction, the stations have been performing well, measurably increasing dissolved oxygen levels in the waterways. One unanticipated problem is the occurance of persistant siltation within the pumping station itself and in the pools located upstream of the cascades. The siltation is a major maintenance problem for MWRD, causing the growth of aquatic plants in the pools that must be removed periodically. The help of the Ven Te Chow Hydrosystems lab was sought for the Siltation Aleviation study for SEPA station #3 located in Blue Island, Illinois. The role of the lab was to help quantify the primary causes of siltation within the station as well as to assist in the development of a design solution to the problem.
The first phase of the project, quantifying the cause of the siltation, was completed in several steps. First, water/suspended sediment samples were collected at various points in the station during different events
- Dry weather (no precipitation for 72-hours preceeding sample collection)
- Wetweather (1-inch of rainfall within the 12-hours preceeding sample collection)
- 20-minutes prior to barge passage
- 20-minutes after barge passage
- 60-minutes after barge passage
The purpose of these samples was to determine the environmental/navigation conditions that resulted in the suspended sediment being drawn into the station. From these samples, suspended sediment concentration, particle grain-size distributions, and particle settling velocity were determined. This data could then be used to determine the flow velocities that would be necessary to move the sediment through the station without it settling or to scour out any sediment that had already deposited.
Field velocity measurement in the pump forebay and at the discharge weirs were collected to determine flow patterns in the pumping station in order to determine areas of low velocity that would be much more prone to sediment deposition. Additionally, numerical modelling of the pump intake channel, pump forebays, and the discharge channel was performed to determine the dominant flow patterns to aid in predicting areas that would be most susceptible to sedimentation.
The second phase of the project was to assist in developing a design solution to the siltation problem. In order to complete this task, a numerical model of the pumping station was developed using the commercially available program FLOW-3D (Flow Science, Inc). After verifying the numerical modeling results with the field velocity measurements, several design alternatives were investigated using the numerical model. These alternatives included:
- filling the stagnant areas in the outlet structure and concentrating flow discharge near the center on the structure
- filling the bottom of the pools with a sloped concrete surface, creating a sheet-flow type aeration, and
- filling stagnant areas of the weir pools