Testing a Watershed-Scale Stream Power Index Tool for Erosion Risk Assessment in an Urban River

Successful watershed planning requires a watershed-scale approach to the assessment of erosion hazards. Stream power, or the rate of energy expenditure of flowing water, is a driver of river erosion and morphologic change. Stream Power Index for Networks (SPIN) is a newly developed geographic information system (GIS)-based toolbox that uses simple inputs to calculate total stream power, specific stream power, and their change between past, present, and future land-use conditions. We tested the ability of SPIN to identify areas of erosion risk by comparing its outputs against an independent database of erosion mitigation sites in an urban river. Objectives were to (1) validate the channel slope and threshold particle size calculated by the toolbox against data collected from field sites, (2) assess the utility of the toolbox outputs for interpreting spatial patterns of erosion control structure construction and maintenance, and (3) evaluate how the toolbox can be used to support or enhance current erosion monitoring techniques. Channel slope and threshold substrate size values calculated by SPIN matched those measured in the watershed. Both total and specific stream power increased between the rural (preurban) and urban land-use conditions, and maps generated by the toolbox provided a visual assessment of areas of expected erosion risk. The density of erosion control structures increased with total and specific stream power, and threshold stream power ratio values were identified above which erosion control structures were widespread. Stream power indices were also related to the condition of erosion control structures when defined as the distance between the channel erosion and the infrastructure. The SPIN toolbox makes a novel contribution to erosion hazard assessment at a watershed scale. In combination with existing local monitoring and assessment techniques, this method should improve decision making around erosion mitigation strategies in urban rivers.