Mallard Lake Forest Preserve Channel Restoration & Stabilization
The scope of services provided by WBK Engineering, LLC included topographic survey, wetland delineation, a stream geomorphology assessment, a hydraulic study, alternatives analysis, permitting submittals and coordination, preparation of construction documents, and (under separate contract) construction observation.
The project site had a long history of anthropogenic modification before the project began. Aerial photographs revealed that the natural alignment of the WBDR passed through the center of what is today the landfill. At some point the river was diverted around the existing landfill to begin operations. The river upstream of the project was also realigned and channelized with other development. The result of this history was a channel characterized by steep banks and very little floodplain. The geomorphic and hydraulic assessment indicated that this has been a significant contributor to the bank erosion. High groundwater was also identified as another potential contributing factor.
The project sought to stabilize the landfill slope through two primary means: realignment of the channel to provide physical separation between the channel and the landfill and lining the banks with a graded aggregate filter. The realigned channel was designed to imitate natural geomorphic conditions and create opportunities for hydraulic connectivity between the channel and the over-bank area and provide an ecological functional lift. The realigned reach included river enhancements such as riffle structures, pools, and native plants.
The river was lined with the graded aggregate filter where existing infrastructure made realignment impractical. Constraints included existing infrastructure in the form of a sanitary force main and water main located in the bank, a school parking lot and the retaining wall supporting it, and the limits of FPDDC property. The filter was designed to provide scour protection while allowing the shallow groundwater to pass through to the channel, alleviating pore pressure in the banks.
In addition to the physical conditions, the project was complicated by an effective regulatory hydraulic model that contained an erroneous cross-section on the site, resulting in an inappropriately low base flood elevation. WBK worked with IDNR and FEMA to correct the existing flood study model and obtain a conditional letter of map revision.
WBK also worked to obtain permits from the US Army Corps of Engineers and DuPage County for floodplain, wetlands, and waters impacts. The project was successfully constructed in 2018 and 2019
In 2015 the West Branch DuPage River, which runs along the toe of the Mallard Lake North landfill, was experiencing erosion which if left unchecked had the potential to expose the landfill cap and buried waste. The FPDDC sought to proactively arrest erosion and mitigate threats to the stability of the landfill and, where practical, naturalize and enhance the ecological function of the WBDR. WBK was hired to investigate the cause of the erosion, assess potential methods of stabilizing the river, and permit and develop plans for proposed improvements.