The role of dynamic ice-breakup on bank erosion and lateral migration of the Middle Susitna River, Alaska
Date
2018
Authors
Vandermause, Renee A., author
Ettema, Robert, advisor
Zevenbergen, Lyle, advisor
Rathburn, Sara, committee member
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Abstract
Rivers in northern, boreal regions experience frigid winters and ice formation that influence several geomorphic processes including bank erosion and lateral channel migration. Not only are the ice-driven processes complex and highly variable in time and space, but they are difficult to observe due to the logistical challenges of conducting fieldwork when the rivers are frozen in the winter and during ice breakup in the spring. Yet, characterizing and quantifying the processes that drive bank erosion, whether during the summer open-water period or when ice is in the channel, is important for predicting channel dynamics in the boreal rivers where there is a mixed ice-fluvial regime. Of particular importance is understanding the erosional processes that form or maintain complex channel and riparian systems which in turn provide diverse aquatic habitat for a range of salmonid species. This study capitalized on an extensive archive of field data and observations, of which the author was involved, to quantify the extent to which channel change is driven by ice and fluvial processes on the Susitna River, a large gravel-cobble bed river in south-central Alaska. As bank erosion is a key element of channel change, this study focused on bank erosion along the middle reach of Susitna River located downstream of a proposed dam site. Using aerial photography and videography over two one-year periods, 2011 to 2012 (included a thermal ice-breakup) and 2012 to 2013 (included a dynamic ice-breakup), the study identified short-term erosion rates for distinct geomorphic reaches, determined the amount of erosion by the type of geomorphic surface, and quantified when the most bank erosion occurred annually; whether during the open-water season or when ice was in the channel, particularly during ice-breakup. The aerial imagery was supplemented by observations and data collected along the Middle Susitna River over two field seasons. The study also used a synthesis of observations and 2-D depth averaged fluvial modeling using SRH-2D to characterize processes that drive bank erosion. This study found that the majority of bank erosion, 54 to 61 percent by sub-reach, occurs or is initiated over a short period of time during dynamic breakup of the river's ice cover. The dominant erosion process is attributable to the combination of relatively high-water discharge and the presence of ice floes and ice rubble. Vegetated bars and terrace margins were the most susceptible to bank erosion, notably by impacting ice floes. Erosion of banks adjoining floodplain surfaces, partly protected by vegetation rootmats and by shear walls of smaller ice rubble, accounted for less overall erosion than vegetated bars or terraces. Wide sub-reaches with multiple channels were prone to ice-jamming, diversion of flow conveying ice into side channels, and localized pockets of subsequent bank erosion. Bank erosion occurred less in predominantly single-channel reaches than in the predominantly multi-channel reaches. Rates of bank erosion along the Middle Susitna River are relatively low in comparison to bank erosion rates measured along gravel-bed rivers generally. This may partly be explained by the low frequency of dynamic, ice-breakup events that initiate large-scale bank erosion. It may also be due to protective effects of ice that inhibit bank erosion such as formation of gravel-cobble pavements at bank toes, or protective effects from vegetation such as root-reinforced top of bank layers that slump over cantilevered banks effectively providing vegetated rip-rap to an exposed bank face.
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Subject
boreal river
erosion
banks
ice
breakup