Seismic Monitoring of a Subarctic River: Seasonal Variations in Hydraulics, Sediment Transport, and Ice Dynamics

L E Polvi,J M Turowski,L Lind,E Lotsari,M Dietze

doi:10.1029/2019jf005333

Abstract

AbstractHigh‐latitude rivers are commonly covered by ice for up to one third of the year. Our understanding of the effects of ice on channel morphodynamics and bedload transport is hindered by the difficulties of sensing through the ice and dangers of field work on thin ice or during ice break‐up. To avoid this drawback, we used seismic signals to interpret processes and quantify water and sediment fluxes. Our objective was to determine seasonal differences in hydraulics and bedload sediment transport under ice‐covered versus open‐channel flow conditions using a small seismic network and to provide a first‐order estimation of sediment flux in a Fennoscandian river. Our study reach was on a straight, low‐gradient section of the Sävar River in northern Sweden. Interpretations of seismic signals, from a station 40 m away from the river, and inverted physical models of river stage and bedload flux indicate clear seasonal differences between ice‐covered and open‐channel flow conditions. Diurnal cycles in seismic signals reflecting turbulence and sediment transport are evident directly after ice break‐up. Analysis of seismic signals of ice‐cracking support our visual interpretation of ice break‐up timing and the main ice break‐up mechanism as thermal rather than mechanical. Assuming the bulk of sediment moves during ice break‐up and the snowmelt flood, we calculate a minimum annual sediment flux of 56.2 ± 0.7 t/km2, which drastically reduces the uncertainty from previous estimates (0–50 t/km2) that exclude ice‐covered or ice break‐up periods.

Highlights

Insufficient information on the spatial and temporal variation of the processes under ice‐covered conditions limits our understanding of subarctic river dynamics (Ettema, 2002; Lotsari et al, 2019; Turcotte et al, 2011), including estimation of stage‐discharge relationships, sediment transport, ice‐cover formation, and channel‐thalweg alignment (Ettema, 2002; Lotsari et al, 2019; Turcotte et al, 2011)
By applying a model inversion approach, we provide a proof of concept for seismic measurements of sediment transport and hydraulics in ice‐covered conditions and during ice break‐up
Approach and Hypotheses Within our larger goal of calculating cumulative sediment flux, we aim to address several additional research questions related to hydraulics and sediment transport in ice‐covered rivers: (1) How do seismic signatures of hydraulics and bedload sediment transport differ between closed and open‐channel flow conditions? (2) How distinct are the average patterns of fluvial processes at high temporal resolution, including possible diurnal cycles, under ice‐covered and open‐channel flow conditions? (3) Can we detect the seismic signature of the ice break‐up and to which extent does it show properties of mechanical versus thermal processes?

Summary

Introduction

Insufficient information on the spatial and temporal variation of the processes under ice‐covered conditions limits our understanding of subarctic river dynamics (Ettema, 2002; Lotsari et al, 2019; Turcotte et al, 2011), including estimation of stage‐discharge relationships, sediment transport, ice‐cover formation, and channel‐thalweg alignment (Ettema, 2002; Lotsari et al, 2019; Turcotte et al, 2011). Larger morphologic change (i.e., movement of boulders up to ~2 m in diameter) was detected on a gravel/boulder‐bed reach of an arctic Finnish river as a result of ice break‐up than possible by sediment transport during the snowmelt flood, based on the critical shear stress criterion used for ice‐free channels (Lotsari, Wang, et al, 2015). Between these two end‐member modes of ice break‐up, it is possible to have ice cracking as a result of increased temperatures; this can lead to downstream transport of ice blocks with associated sediment, as is common during mechanical break‐up. Researchers commonly consider any fracturing of the ice, even if it has already been subjected to thermal decay, to indicate mechanical breakup (Beltaos, 1997)

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