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Abstract

The reach of the Clark Fork River just west of Missoula is quite dynamic, and due to erosion and deposition of sediments can shift quite rapidly during floods. During spring runoff in 2012 the main channel of the Clark Fork River about 1.3 miles west of the Reserve Street Bridge scoured through the point bar (a type of gravel or sand bar) on the inside of a meander bend and created a new channel. This event is called a chute cutoff and is one of two main ways that rivers cut through their banks to create new channels. Understanding of the mechanics driving these processes is limited, hindering predictive capabilities. In order to study this event I am conducting a case study using topography surveys in the reach from 2009, 2011, and 2015 to create digital elevation models to examine sediment removal and deposition from 2009 to 2015. This will be aided by the use of aerial photos from the USDA from 2005 to 2015 and analysis of flow data from USGS gaging stations. In addition I will use the Hec-Ras program to model the conditions likely to produce a chute cutoff event. Investigations to date suggest that the cutoff event occurred primarily due to a decrease in the radius of curvature of the proceeding bend causing increased erosion at the base of the point bar, a lack of vegetation on the bar since the last scour event, and a shortening of the length necessary to cut through the point bar. Understanding how these events occur and the mechanics behind them can help to inform managers working to improve the geomorphic and ecological health of rivers while minimizing economic risks such as lateral migrations of the river which can erode property and destroy infrastructure.

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Physical Sciences

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Apr 15th, 10:00 AM Apr 15th, 10:20 AM

Chute Cutoffs and alluvial point bar morphodynamics: How rivers move over time

The reach of the Clark Fork River just west of Missoula is quite dynamic, and due to erosion and deposition of sediments can shift quite rapidly during floods. During spring runoff in 2012 the main channel of the Clark Fork River about 1.3 miles west of the Reserve Street Bridge scoured through the point bar (a type of gravel or sand bar) on the inside of a meander bend and created a new channel. This event is called a chute cutoff and is one of two main ways that rivers cut through their banks to create new channels. Understanding of the mechanics driving these processes is limited, hindering predictive capabilities. In order to study this event I am conducting a case study using topography surveys in the reach from 2009, 2011, and 2015 to create digital elevation models to examine sediment removal and deposition from 2009 to 2015. This will be aided by the use of aerial photos from the USDA from 2005 to 2015 and analysis of flow data from USGS gaging stations. In addition I will use the Hec-Ras program to model the conditions likely to produce a chute cutoff event. Investigations to date suggest that the cutoff event occurred primarily due to a decrease in the radius of curvature of the proceeding bend causing increased erosion at the base of the point bar, a lack of vegetation on the bar since the last scour event, and a shortening of the length necessary to cut through the point bar. Understanding how these events occur and the mechanics behind them can help to inform managers working to improve the geomorphic and ecological health of rivers while minimizing economic risks such as lateral migrations of the river which can erode property and destroy infrastructure.