Early detection model for the urban stream syndrome using specific stream power and regime theory

Abstract

Unmitigated urbanization frequently alters the flow frequency distribution and changes the source and transport rates of sediments in previously “in regime” channels. These changes may result in increased stream erosion, changes in alluvial materials and degradation of water quality and habitat, conditions called “urban stream syndrome”. Early identification of streams with the potential to develop urban stream syndrome may allow for additional implementation of watershed controls such as Low Impact Development or within stream mitigation (e.g. erosion controls). Regime theory applied to dynamically-stable alluvial channels has identified empirical equations linking width (W), depth (H), slope (S), mean sediment particle size (D50), and the 2-yr return bankfull discharge (Q2). This study examines the application of regime-theory equations to urban streams as a means of identifying channels with urban stream syndrome. To begin the assessment of the applicability of regime equations an extensive database of channel morphology variables for 733 in-regime channels, from North America, Britain and South Asia, was compiled from existing publications. Using this database a Group Method of Data Handling (GMDH) model was developed to predict specific stream power for comparison with observed values to determine whether a channel is in or out of regime. The application of the GMDH model was showcased using regional flow and sediment data to predict channel conditions of several watercourses in Southern Ontario, Canada. The GMDH model reliably identified channels both in and out of regime conditions compared with published in-situ assessments based on Rapid Geomorphic Assessments (RGAs). This study has identified changes in specific stream power (ω) as a reliable early detection metric for occurrence of the urban stream syndrome. The model can inform the strategies to mitigate the urban stream syndrome, including within-channel mitigation and watershed initiatives, including Low Impact Developments (LIDs).