River-ice effects on bank erosion along the middle segment of the Susitna river, Alaska

Abstract

This paper presents findings quantifying the extent to which river ice impacted and eroded banks along the middle segment of the Susitna River, Alaska. Analyses of aerial photography (taken over two, one-year periods) established bank-erosion rates for two reaches of differing morphology (multi-channel; and, single channel). Reconnaissance videos were used to establish the timing of erosion within each one-year period. Whereas the first winter ended with a thermal breakup of the river's ice cover, the second winter ended with the dynamic breakup of the ice cover in both reaches. The relatively low flow rate and weak ice associated with thermal breakup caused no large-scale erosion in the reaches, while the higher flow rates and stronger ice associated with dynamic breakup initiated large-scale bank erosion; up to 61% of bank erosion over a one-year period. Single year erosion values ranged from 10 to 30% of the historical erosion over two 30-yr periods. Analysis of the erosion and ice-scar data along the Middle River suggest the recurrence interval for dynamic ice-breakup is about 5–13 years. During dynamic breakup, ice-laden flows abraded banks, and directly impacted trees and shrubs. Thicker ice covers and ice jam development compounded ice-induced bank erosion in the multi-channel reach compared to the single channel reach.