Event Title

Mid-channel Bar Maintenance on Gravel Bed Rivers with Large Fine Sediment Loads

Presenter Information

Rebecca B. Manners

Location

Ellen Eccles Conference Center

Event Website

https://forestry.usu.edu/htm/video/conferences/restoring-the-west-conference-2014/

Abstract

In the western US, climate change, water development, and the encroachment of non-native species during the past century contributed to changes in channel morphology. Reaches with mid-channel bars or islands narrowed and simplified first, and to a greater degree than other settings on gravel bed rivers with large fine sediment loads. Maintenance of mid-channel bars, and their associated secondary channels on these rivers, requires regular transport of the fine sediment load through the reach. A reduction in the transport capacity, resulting from reduced flows or the expansion of vegetation, for example, results in the accumulation of fine sediment and inevitably leads to secondary channel abandonment (i.e., channel narrowing). Working in the Yampa Canyon in western Colorado, we identify the mechanisms that maintain, or conversely degrade, the mid-channel bar and secondary channel by maintaining (or not) the throughput of fine sediment. A novel observation-based model built from repeat LiDAR and a 2D hydraulic model of three recent flood events, applied to historical topographic, vegetation, and hydrologic conditions, provides a robust series of observations on the processes by which a secondary channel narrowed and filled in with fine sediment during the past 50 years. The streamwise location of the bifurcation point at the upstream end of the bar where the flow divides dictated the relationship between flood magnitude and fine sediment accumulation or evacuation. The elevation and roughness (i.e., vegetation coverage and density) of the mid-channel bar determined the location of the bifurcation during floods with a different recurrence. When the mid-channel bar was low, with little vegetation, floods with a two-year recurrence or less could evacuate fine sediment from the area. As vegetation established and the fine sediment on the mid-channel bar grew vertically and laterally, the bifurcation location for the two-year flood was pushed further upstream. Reduced momentum across the bar resulted in fine sediment deposition, not erosion. As a result, it took larger and larger floods to evacuate fine sediment. Therefore, fine sediment became a more permanent feature across a larger part of the bar, supporting greater new vegetation recruitment. Increasing cyclicity of large and small floods in the latter half of the study period also promoted plant establishment. Fine sediment deposition occurs over a wider range of flows within vegetation than outside. Synthesizing our robust historical reconstruction and observations from other multi-thread settings in the Yampa Canyon, we build a conceptual model of mid-channel bar maintenance that identifies aspects of the flood regime based on valley geometry and channel morphology necessary to maintain multiple channels. This model furthers our fundamental understanding of the multi- thread planform and may more efficiently guide the management of rivers.

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Oct 21st, 9:00 AM Oct 21st, 9:10 AM

Mid-channel Bar Maintenance on Gravel Bed Rivers with Large Fine Sediment Loads

Ellen Eccles Conference Center

In the western US, climate change, water development, and the encroachment of non-native species during the past century contributed to changes in channel morphology. Reaches with mid-channel bars or islands narrowed and simplified first, and to a greater degree than other settings on gravel bed rivers with large fine sediment loads. Maintenance of mid-channel bars, and their associated secondary channels on these rivers, requires regular transport of the fine sediment load through the reach. A reduction in the transport capacity, resulting from reduced flows or the expansion of vegetation, for example, results in the accumulation of fine sediment and inevitably leads to secondary channel abandonment (i.e., channel narrowing). Working in the Yampa Canyon in western Colorado, we identify the mechanisms that maintain, or conversely degrade, the mid-channel bar and secondary channel by maintaining (or not) the throughput of fine sediment. A novel observation-based model built from repeat LiDAR and a 2D hydraulic model of three recent flood events, applied to historical topographic, vegetation, and hydrologic conditions, provides a robust series of observations on the processes by which a secondary channel narrowed and filled in with fine sediment during the past 50 years. The streamwise location of the bifurcation point at the upstream end of the bar where the flow divides dictated the relationship between flood magnitude and fine sediment accumulation or evacuation. The elevation and roughness (i.e., vegetation coverage and density) of the mid-channel bar determined the location of the bifurcation during floods with a different recurrence. When the mid-channel bar was low, with little vegetation, floods with a two-year recurrence or less could evacuate fine sediment from the area. As vegetation established and the fine sediment on the mid-channel bar grew vertically and laterally, the bifurcation location for the two-year flood was pushed further upstream. Reduced momentum across the bar resulted in fine sediment deposition, not erosion. As a result, it took larger and larger floods to evacuate fine sediment. Therefore, fine sediment became a more permanent feature across a larger part of the bar, supporting greater new vegetation recruitment. Increasing cyclicity of large and small floods in the latter half of the study period also promoted plant establishment. Fine sediment deposition occurs over a wider range of flows within vegetation than outside. Synthesizing our robust historical reconstruction and observations from other multi-thread settings in the Yampa Canyon, we build a conceptual model of mid-channel bar maintenance that identifies aspects of the flood regime based on valley geometry and channel morphology necessary to maintain multiple channels. This model furthers our fundamental understanding of the multi- thread planform and may more efficiently guide the management of rivers.

https://digitalcommons.usu.edu/rtw/2014/Posters/11