The West Lents floodplain restoration project includes reversal of significant historic channel manipulations in Johnson Creek over a 36-acre region in highly developed southeast Portland. Key to success on this project was working with City staff to co-develop restoration criteria and alternatives.
Alternatives focused on aquatic habitat, water quality, and flow attenuation to reduce localized flooding using varying degrees of a Stage 0/ 8 floodplain measures. Alternatives were modeled using HEC-RAS 2-D to examine both flood risk reduction and habitat features. Key outcomes of the analyses were that significant reduction in peak flood elevations of up to two feet could be achieved via a Stage 8 design, and the flood reduction benefits extended upstream several thousand feet.
W2r also conducted a geomorphic assessment of the creek extending well beyond the project limits. It showed the geomorphic regime of upper Johnson Creek is dramatically different than that of the lower creek, which is much more dynamic (counter-intuitively). This helped to recognize that restoration approaches that work well in the upper reach will not work the same way in the lower reach, which in turn helped explain why prior efforts by the City had not fully achieved restoration objectives.
