The Basis of Design for Rehabilitation of the Underwood Creek Channel and Floodplain, an Urban Stream in Milwaukee, Wisconsin

Underwood Creek, a primary tributary of the Menomonee River, flows for about eight miles through a watershed that is 84% developed with a variety of residential, commercial, industrial and institutional land uses. In an effort to address local flooding, Underwood Creek was drastically altered in the 1970s and 1980s through floodplain filling, channel widening and realignment, and installation of concrete streambed lining along most of its length. Channel alterations successfully reduced peak water levels in some areas, but contributed to flooding conditions along portions of the Menomonee River. In addition, the modifications entirely disrupted channel and floodplain functions. Aquatic habitats were completely eradicated during installation of the concrete lining and associated filling within the floodplain. Plant communities along the riparian corridor are dominated by early successional and non-native species, typical of those found in disturbed conditions.As part of a comprehensive plan to restore watercourses within the watershed, the Milwaukee Metropolitan Sewerage District developed plans to rehabilitate 6,600 feet of Underwood Creek by removing the concrete channel lining where feasible, creating a more natural rock-lined channel with riffle and pool sequences, reestablishing a functional floodplain by removing fill adjacent to the channel, and reestablishing plant communities within the newly created floodplain. Phase 1 of this effort, involving 2,400 feet of stream and associated floodplain, was constructed in 2009. The restored wetland plant communities provide compensatory mitigation for wetland impacts associated with the project as well as other flood management projects in the region. In addition to the requirement to provide in-kind compensation for wetland losses, habitat was to be provided for the Butler's gartersnake (Thamnophis butleri), a Wisconsin Threatened species that occupies the project area and utilizes the floodplain and portions of an abutting upland mesic forest. Upstream passage for native Northern pike (Esox lucius) and a range of habitat for introduced steelhead (Salmo salar) were also required.One of the most critical design elements involved determination of channel geometry. The Underwood Creek hydrology is typical of highly urbanized watersheds: very low base flows, more frequent moderate flows with short duration peaks, and extreme high-magnitude flood flows. Channel dimensions and this range of seasonal flows together dictate the frequency, duration and water depth of floodplain inundation. Inundation characteristics affect the composition, diversity and health of the riparian plant community that will reestablish on the restored floodplain. Simultaneously, the amount of water in the channel affects aquatic habitat, water quality, and whether the stream appears natural.Natural channel design typically relies on an understanding of effective discharge, regional curves for bankfull discharge and reference reaches in order to determine appropriate channel dimensions. Since most of Underwood Creek was concrete lined, sediment continuity did not drive channel design, nor were reference reaches available for consideration. Regional bankfull geometry curves were not applicable because they represented disturbed rather than desired conditions. The rehabilitated Underwood Creek channel design was instead based on a characterization of individual flow events as they affect both floodplain inundation and stream flow.