Stream Power Based Approaches to Watershed Management
Wednesday June 28 10:45am-12:05pm
Leads: Sally Betts (Credit Valley Conservation) and Christina Bright (Toronto and Region Conservation Authority)
The lack of reliable watershed-scale sediment transport models represents an ongoing and consequential knowledge gap in our understanding of watershed systems. Stream power represents the rate of energy expenditure along a watercourse. It has a direct relationship with sediment transport which can be used to understand sediment dynamics across a range of spatial and temporal scales. In this session, the focus will be on using stream power to understand erosion potential and change in erosion potential in light of climate and land use change to inform erosion management at a watershed scale.
Keywords: Stream power, Erosion risk, Sediment transport processes, Watershed management
Following the conference, presentations that have been made available will be linked here.
Pamela E. Tetford and Joseph R. Desloges
Department of Geography, University of Toronto, Toronto, ON Canada
Discharge variability via climate change has been strongly implicated as a key factoring in explaining spatial variations in river channel geometry. Stream competence and stream power drive sediment mobility and quantity that are moderated by channel boundary characteristics. Changing land use can strongly influence the “natural” hydrologic and sediment regimes. The purpose of this study is to: (i) develop a reliable downstream hydraulic geometry relationship in terms of flood frequency and magnitude for 207 hydrometric gauge stations in S. Ontario, (ii) compare the hydrologically derived flow estimates to channel morphological capacity while considering upstream land use and (iii) test the derived hydraulic geometry relations using three sample watersheds that represent land use classes of urbanized, urbanizing/agricultural and natural/rural. We found that smaller drainage areas (< 100 km2) generate greater geometrically derived estimates of bankfull discharge than the hydrometrically derived Q2 values. Channels impacted by high levels of land use disturbance show significantly lower width to depth ratios than channels with moderate or low disturbance. This research is important because it quantifies the formative bankfull stage (i.e., Q2) to better understand the link between a self-forming channel’s ‘natural’ hydrogeomorphic response and land use. Using the test watersheds, channel response to gradual urbanization results in less channel widening when the channel is permitted to self-adjust and increased sinuosity in response to variations in stream power. For comparative land use conversion studies, it is important to account for the inaugural riparian vegetation as an indicator of existing bank strength to ensure that the impact of the pre-existing land use is consistent across all survey areas. For channel restoration or realignment, a tree-lined channel of deep-rooted trees resist channel migration more than opportunistic pioneer species. Channels that are realigned in opposition to natural fluvial processes will tend to migrate back toward their original form.
Sally-Beth Betts and Chris McKie
Credit Valley Conservation, Mississauga, Canada
Accounting for sediment in rivers is important, especially in the context of a changing climate, and this is rarely done in a meaningful way. The purpose of this study is to investigate sediment transport patterns at the watershed scale to support the 2020-2024 Credit River Watershed Plan. Stream power is a particularly useful tool because it is a process-based assessment that considers the energy available in a watercourse in relation to material present in that system. It is also a practical method given the availability of LIDAR DEMs and GIS software which provide a powerful basis to model the required input parameters.
Credit Valley Conservation (CVC) has completed phase one of an ongoing stream power mapping project. All 4th order and above watercourses across CVC’s jurisdiction have been mapped for specific stream power, stream power gradient and excess stream power. Specific stream power is an expression of potential sediment transport. Excess stream power incorporates the critical stream power to entrain median grain size bed material for a more complete sediment transport index. The stream power gradient can help to interpret the potential for erosion and aggradation.
CVC is continuing to expand, update and refine the stream power model. Specific areas of focus are sediment transport to identify areas prone to erosion and aggradation and potential uses for fisheries management and restoration. As part of the Watershed Plan, CVC will be re-running the stream power model for various future land use and climate change scenarios. Stream power is an incredibly powerful tool in watershed management, and we plan to use it to help us identify imbalances in our watershed. This tool will allow us to evaluate existing conditions as well as to predict how resilient our rivers will be in resisting the impacts of climate change and urbanization.
Christina Bright1, Priyanka Hire2, Elli Papangelakis2, David Gingerich1, Bruce MacVicar3 and Peter Ashmore4
1Toronto and Region Conservation Authority, Vaughan, Canada
2McMaster University, Hamilton, Canada
3University of Waterloo, Waterloo, Canada
4University of Western Ontario, London, Canada
The watershed planning process helps characterize the current conditions of the watershed, assesses how the watershed will respond to potential future land use changes and climate change, and identifies measures to protect, enhance, and restore watershed health. One of the components that is assessed through this process is the natural hazards – erosion component. Typically, this component is assessed based on results from established monitoring sites which are used as surrogates to comment on the overall health of the watershed. In TRCA’s recent watershed planning exercises, in addition to using data from our monitoring sites, TRCA has used a larger scale approach of assessing the changes to the stream power (a measure of the energy of flowing water) in the channels across the entire watershed. Stream power indices were calculated using a GIS-based tool named Stream Power Index for Networks (SPIN) developed by researchers at University of Waterloo and University of Western Ontario. This tool has been used for the characterization of erosion potential in the Etobicoke Creek and Humber River watersheds. Furthermore, the SPIN tool will also be used to assess erosion risk as part of the future scenario analysis for the Humber River Watershed Plan.
In this presentation, we will introduce the SPIN tool and discuss the process of characterization of erosion potential. We will also present the results of the characterization study for both the Etobicoke Creek and Humber River Watershed Plans, and will conclude with recommendations for how erosion potential can be assessed in other jurisdictions.
Priyanka Hire1, Thiruni Thirimanne2, Peter Ashmore3, Kimisha Ghunowa4, Bruce MacVicar5, and Elli Papangelakis1
1McMaster University, Hamilton, Canada
2University of Waterloo, Waterloo, Canada
3University of Western Ontario, London, Canada
4Credit Valley Conservation Authority, Mississauga, Canada
A watershed-scale approach is required to successfully manage increasing erosion risk in urban watersheds and mitigate its negative effects on infrastructure and river systems. Stream power – the rate of energy expenditure of flowing water per unit length – has been shown to be related to river erosion, channel instability and morphologic changes. Stream Power Index for Networks (SPIN) is a GIS-based toolbox that calculates the total and specific stream power (stream power per channel width) along a river network based on a digital elevation model (DEM), land-use information, and HEC-RAS data. The tool is being used to support decision-making by conservation authorities and municipalities in Ontario. SPIN has been incorporated into Credit Valley Conservation’s Risk and Return on Investment Tool (RROIT) to estimate damage costs due to erosion and is being used to inform updates to Official Watershed Plans for two watersheds in the jurisdiction of Toronto and Region Conservation Authority. The current research is supported by the National Research Council’s Climate Resilient Built Environment (CRBE) program.
In this presentation, we 1) outline how SPIN works to calculate stream power metrics at the watershed scale and 2) provide an overview of recent SPIN improvements and new capabilities. Case studies in Southern Ontario show that SPIN yields reasonable results, with modelled channel slope and threshold substrate sizes comparable to those measured in the field, and stream power metrics that correlate with urban development and infrastructure erosion. Recent improvements in the algorithms have increased the processing speed and flexibility of the code by making it fully open-source in Python (SPINpy). Future steps include i) further comparison of SPIN results to field measurements of river morphology and erosion indices and ii) improvements to discharge estimation algorithms to better account for natural and engineered green infrastructure. These initiatives will help to develop nature-based solutions for river management and restoration as part of future land-use and climate scenario testing.