River-ice effects on bank erosion along the middle segment of the Susitna river, Alaska
Introduction
Observed effects of river ice on channel-bank erosion and channel alignment have been reported but few studies provide data on the extent that river ice erodes banks along river reaches. Descriptions exist of ice-run gouging and abrasion during ice-break up events (e.g., Marusenko, 1956; Smith, 1980; Beltaos, 1995; Zabilanksy et al., 2002; Prowse and J.M., 2003; Ettema and Kempema, 2012; Beltaos, 2018a; Beltaos et al., 2018; Baker et al., 2020). However, few studies have attempted to quantify the extent to which ice-related erosion of banks affects channel morphologies, such as bends (e.g., Ashton and Bredthauer, 1986). Osada et al. (2020), conversely, investigated how channel morphology affects ice conveyance, and found that morphology plays complex roles in this regard.
A broad question remains unanswered: Do river channels in boreal (subarctic) environments exhibit an ice-scoured morphology not explained by typical fluvial models? Several rivers in subarctic environments display channel morphologies typical of fluvial environments (Bray, 1982), other rivers have enlarged (widened) channels (e.g., Smith, 1979; Smith, 1980; Boucher, 2008; Boucher et al., 2009; Boucher et al., 2012). Many channels, though, exhibit equivocal effects of ice on channel morphology (e.g., Uunila and Church, 2015). Inherent in this broader question is defining the extent and frequency with which ice-induced processes cause bank erosion. The answers to these questions usually are specific to river reaches and watersheds (e.g., Best et al., 2005; Boucher, 2008).
To quantify the effects of ice on bank erosion, especially at the reach scale, is a complex undertaking, often complicated by the remoteness of river reaches in northern locations and the size of large rivers. The generation of datasets throughout the annual stages of river-ice, including ice formation, presence and breakup is typically costly and dangerous. However, an opportunity to quantify such erosion effects arose with the wide-ranging studies conducted of the Susitna River, Alaska, which was being considered as the site for a large hydroelectric project that included construction of a 215 m-high, hydroelectric dam (AEA, 2012). Baseline environmental studies were initially performed in the 1980s (Alaska Power Authority [APA], 1984) then more recently by the Alaska Energy Authority (AEA) in the period from 2011 to 2015. The studies produced an extensive, publicly available dataset regarding the behavior of the Susitna River during ice-covered and ice-free periods.
This paper presents the findings of a study that used the 2011–2015 dataset from the Susitna's Middle River segment (defined below) to quantify the extent to which bank erosion along the river is driven by river-ice or open-water fluvial processes. Though all the river's banks along the segment displayed signs of ice disturbance, the extent to which river ice influenced the alignment of the existing channel and drove bank erosion was unknown and unquantified. The study's objectives, therefore, include characterizing the ice processes causing the bank erosion, and quantifying when, annually, the majority of bank erosion occurs: during conditions of substantial open-water flow or, when ice floes and rubble are moving during the breakup of a channel's ice cover. The data suggest that negligible bank erosion occurs during ice-cover formation early in winter.
Section snippets
The Middle Susitna River and the study reaches
The Susitna River originates in the Alaska Range and is sub-divided into four large-scale geomorphic segments as it flows approximately 580 km to Cook Inlet near Anchorage, Alaska (Fig. 1). The river's most downstream, southerly geomorphic segments, including the Lower and Middle Rivers, do not flow through permafrost terrain. The Middle River extends 135 km downstream from the proposed dam site to the Susitna River confluence with two major tributaries (the Chulitna and Talkeetna Rivers) that
Investigation method
Two different reaches selected for investigation were monitored over two one-year periods, 2011–2012 and 2012–2013. The investigation comprised three parts:
- 1.
Observation of bank condition and erosion processes. On-the-ground bank observations between 2013 and 2014, and aerial-reconnaissance videos and photographs from 2011 through 2013 were analyzed to characterize dominant bank-erosion processes;
- 2.
Quantification of bank erosion. A time sequence of aerial photography was analyzed in GIS over two,
Results and discussion of erosion data
Erosion trends varied by study reach. The multi-channel reach (6), in comparison to the predominantly single channel reach (7), had an increased number of locations with lateral bank retreat (50 locations in Reach 6 compared to 13 locations in Reach 7 between 2012 and 2013), and an increased magnitude of total area eroded (5700 m2/km/yr in Reach 6 compared to 1400 m2/km/yr in Reach 7 between 2012 and 2013). Consistent with trends reported in the literature on ice jams (e.g., Beltaos, 1995), the
Conclusions
The two, one-year periods of observation along two reaches of the Middle Susitna River led to the following conclusions about the bank-erosion effects of river ice:
- 1.
Most bank erosion and consequent bank retreat occurred between 2012 and 2013, which was marked by the dynamic breakup of the river's ice cover. Minimal bank erosion and retreat occurred between 2011 and 2012, when the Middle River's ice cover disintegrated thermally.
- 2.
Over half of the erosion that occurred between 2012 and 2013, 61% of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Field data utilized as part of this study were collected as part of the Susitna-Watana Hydroelectric Project (FERC Project No. 14241) and funded by Alaska Energy Authority. All data are publicly available at the Susitna-Watana Hydroelectric GIS portal. The cumulative datasets, collected between 2011 and 2015 for each study, are posted at //gis.suhydro.org/reports/cumulative-suwa-data
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