Coastal, River, & Wetland Restoration
- Mill Creek at Center Hill Landfill
the challengeThe Bioengineering Group was requested by the City of Cincinnati, Office of Environmental Management to identify a cost-effective, environmentally sensitive bank stabilization plan for a portion of Mill Creek to halt erosion threatening to uncover waste materials contained in the Center Hill Landfill. The eroding bank was approximately 27 feet high and 600 feet long with clay strata overlain by highly permeable, loosely compacted sandy and gravelly soils mixed with landfill materials creating a zone of intense seepage at the interface. Both leachate and solid waste were entering the river due to ongoing bank erosion of this highly urbanized river. The site is immediately above a concrete lined flood control channel and within the limits of a US Army Corps of Engineers flood control project area.
the interdisciplinary approach The Bioengineering Group performed an investigation of site geomorphic and hydrologic parameters, assessed existing geotechnical data, and identified appropriate measures, providing construction documents, and overseeing the bidding and construction process. Bioengineering treatments were selected because of their ability to aid stabilization, assist in leachate extraction, and enhance wildlife habitat, water quality, and aesthetics. The bioengineering design features the use of live poles integrated into a riprap toe, brush layers, geogrid lifts, live staking, and a brush fascine. An innovative design for incorporating vegetation into a leachate collection system used phreatophytic (highly water consuming) species to intercept and extract leachate from a gravel collection trench. Vegetation and geosynthetic materials function to stabilize the over-steepened bank, with trees and shrubs adding shade along the stream corridor. The chosen treatments are highly resilient to flashy peak flows, urban floatable debris, and potential future hydrologic impacts due to increasing development within the watershed.
the resultsDespite the novelty of the techniques in this geographic region, the project was completed on time and within budget. The regulatory community, local citizens groups, and municipal engineers were all extremely pleased with the design and its performance. Additionally, our design offered a savings of 30% over a different bioengineering design by others, while being favored for ecological integrity and stability. Accurate cost estimating and thorough construction supervision helped achieve the financial goals. After more than five years of highly effective performance, the community opted to expand the treated area to cover two miles of riverbank upstream and downstream of this site, incorporating a public greenway to allow recreational access to what has become a stable, beautiful, and biologically diverse riparian corridor.