Faster than an urban stream: Evaluating morphodynamics in urban channel reaches that have undergone form-based channel enhancement

Abstract

Urban streams are frequently the focus of channel enhancement efforts due to extensive degradation, failing infrastructure and land use change caused by urban development. Following construction, short-term monitoring is a standard step in evaluating channel stability and amelioration success. However, monitoring activities rarely fully capture adjustment mechanisms, as geomorphic adjustment can take several years to observe and characterize. This study assessed the performance and adjustment processes observed within six form-based channel enhancements projects, in a heavily urbanized area along Highland Creek, Toronto, Ontario, Canada. Bankfull flow conditions occurred multiple times in a single season as a function of reduced stormwater retention in the basin and as such the beds and banks experience high shear stresses. Rapid geomorphic assessments were conducted along the enhanced reaches, and over 200 design features (e.g., pools, riffles, rock weirs) were surveyed to analyze stability and evaluate design feature conditions following the various channel enhancement projects that have occurred. Stability indices for each reach indicate that most of the enhanced reaches showed signs of on-going adjustment 15-years post-construction, with features along banks and within grade control structures showing the most change. For example, the observed change through this system in a short period of time demonstrates that urban streams are changing at rates faster than streams in areas with less urban land use. This study informs mid- to long term stability assessment and furthers our understanding of active adjustment processes following construction in challenging urban landscapes.