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Modeling Braided River Morphodynamics

The River Feshie is a major development site for a new morphodynamic model for braided river evolution. Braided rivers like the Feshie are complex and dynamic systems, marked by a continually shifting network of channels diverging and converging around bars. Several factors, including high rates of sediment supply, high channel slope, and readily erodible banks can lead to a braided channel planform. Due in large part to (and perhaps in spite of) their dynamic nature, braided streams comprise vital habitat to numerous fish and aquatic organisms. Additionally, the continually-shifting network of channels makes designing roads, bridges, and other infrastructure in these areas quite difficult.

This project seeks to better predict the response of braided rivers, like the Feshie, to changes in flooding regimes and sediment supply by combining the unprecedented high-resolution survey data that we have collected from a multi-year field campaign with a new simulation model that will allow predictive experimentation over tens to hundreds of years. The project addresses a deficiency in current predictive river modeling capability by developing a model many researchers have called for over the past two decades. We have a pilot version of this model working and we will extend, refine, and rigorously test this model before making it available to the broader research and river management communities. The insight gained from our planned experiments, as well as our plan to make the new modeling tool available, may open up new avenues of experimentation and discovery.

One of the major shortcomings of current morphodynamic models for gravel-bed rivers is their inability ot capture the 'middle ground' of modeling; that is, we are currently unable to adequately predict channel evolution at bar-scale (e.g. meters) over decadal to centennial timescales. This is because currently-available models are largely either too complex to run over meaningful timescales, or too simplified to produce realistic outputs. The model under development addresses these shortcomings by simplifying  the way that sediment is routed through channel reaches by mobilizing particles according to predefined or field-measured path-length distributions, which describe the downstream transport distances of sediment during floods. These path length distributions can take many forms, including exponential decay or heavy-tailed distributions, gaussian distirubtions, or a series of peaks corresponding to depositional locations on a downstream series of bars.

Several examples of path-length distributions based on empirically or theoretically

The importance of using a path-length distribution for sediment transport lies in our ability to subsequently simplify the model's computational requirements. As such, we are able to employ a high-resolution two-dimensional hydraulic model (e.g. Hydro2de or Delft2D) to delineate flowpaths along which sediment is mobilized. Once a threshold stress for particle entrainment is reached, sediment is mobilized via a chosen path-length distribution, deposited, and a new channel bed surface is computed via a simplified Exner equation.

Conceptual diagram of morphodynamic model workflow. Note that the timestep of the model is one flood, or mobilizing event.

As they become available, model code, topographic datasets, and model outputs will be posted here.

The true importance of this model lies in its ability to make societally-relevant predictions of channel evolution over the timescales inherent to channel change brought about by shifts in factors which determine water or sediment delivery to channels. Braided rivers represent some of the most energetic and rapidly evolving environments and are frequently hotspots for biodiversity and serve as vital habitat for salmon as well as other highly valued species. Braided rivers were historically much more common, but engineering and river management have worked to discourage braiding. Increasingly, efforts to restore rivers are recognizing the importance of sometimes promoting braiding. It is likely that climate-change-induced shifts in flooding and the amount of sediment (e.g., gravel and sand) supplied from upstream could push many rivers back to a braided state. Despite the importance of braided rivers, our ability to predict their behavior and understand their function is immature at best. This project aims to advance both. Aside from the scientific value of the research, the project can help provide guidance to river managers and restoration practitioners charged with working with and/or restoring braided rivers. These professionals are seek reasonable quantitative predictions for managing braided rivers. The project broadens the participation of underrepresented groups through initiatives involving research opportunities for undergraduates and new experiences for K-12 students. The project leverages a variety of existing resources (e.g., field data, computational resources, outreach programs, web portals) by pursuing collaborations with funded efforts of other organizations, some supported by NSF. Finally, the modeling approaches developed as part of this research could help advance simulation modeling efforts in other fields interested in forecasting climate change impacts and/or environmental management.

Funding for this project is generously provided by the National Science Foundation via the Geomorphology and Land Use Dynamics Directorate.