Macroturbulent coherent structures in an ice‐covered river flow using a pulse‐coherent acoustic Doppler profiler

Kasvi

Kämäri

et al. 2017

The effects of ice cover on flow characteristics in meandering rivers are still not completely understood. Here, we quantify the effects of ice cover on flow velocity, the vertical and spatial flow distribution, and helical flow structure. Comparison with open‐channel low flow conditions is performed. An acoustic doppler current profiler (ADCP) is used to measure flow from up to three meander bends, depending on the year, in a small sandy meandering subarctic river (Pulmanki River) during two consecutive ice‐covered winters (2014 and 2015). Under ice, flow velocities and discharges were predominantly slower than during the preceding autumn open‐channel conditions. Velocity distribution was almost opposite to theoretical expectations. Under ice, velocities reduced when entering deeper water downstream of the apex in each meander bend. When entering the next bend, velocities increased again together with the shallower depths. The surface velocities were predominantly greater than bottom/riverbed velocities during open‐channel flow. The situation was the opposite in ice‐covered conditions, and the maximum velocities occurred in the middle layers of the water columns. High‐velocity core (HVC) locations varied under ice between consecutive cross‐sections. Whereas in ice‐free conditions the HVC was located next to the inner bank at the upstream cross‐sections, the HVC moved towards the outer bank around the apex and again followed the thalweg in the downstream cross‐sections. Two stacked counter‐rotating helical flow cells occurred under ice around the apex of symmetric and asymmetric bends: next to the outer bank, top‐ and bottom‐layer flows were towards the opposite direction to the middle layer flow. In the following winter, no clear counter‐rotating helical flow cells occurred due to the shallower depths and frictional disturbance by the ice cover. Most probably the flow depth was a limiting factor for the ice‐covered helical flow circulation, similarly, the shallow depths hinder secondary flow in open‐channel conditions. Copyright © 2016 John Wiley & Sons, Ltd.

Section: Study Sitesupporting

confidence: 71%

“…The study site has similarities to the study area of Demers et al . (, ). In particular, the width of the river is similar to that of Demers et al .…”

Section: Introductionmentioning

confidence: 97%

“…Only two recent studies have used the newest acoustic doppler current profiler (ADCP) for studying winter flow in a small natural meandering river (Demers et al ., , ). Demers et al .…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

The effects of ice cover on flow characteristics in a subarctic meandering river

Kasvi

Kämäri

et al. 2017

“…Even though ice-covered flow has been studied since the beginning of the twentieth century (Barrows and Horton, 1907), so far mainly laboratory studies of icecovered flow and sediment transport have been published (Sayre and Song, 1979;Lau and Krishnappan, 1985;Urroz and Ettema, 1994;Ettema, 1994, 1996;Smith and Ettema, 1995;Ettema et al, 2000;Wang et al, 2008;Sui et al, 2010). Only three recent field studies (Demers et al, 2011(Demers et al, , 2013Lotsari et al, 2017) have analysed the effects of ice on flow in meandering rivers, despite the fact that concerns regarding the insufficient understanding of the processes under ice-covered conditions have existed for decades (Ettema, 2002;Turcotte et al, 2011). In particular, some processes have only been known conceptually (e.g.…”

Section: Introductionmentioning

confidence: 99%

Spatial variation of flow characteristics in a subarctic meandering river in ice‐covered and open‐channel conditions: A 2D hydrodynamic modelling approach

Tarsa

Kämäri

et al. 2019

To be able to understand year‐round river channel evolution both at present and in the future, the spatial variation of the flow characteristics and their sediment transport capabilities under ice cover need to be detected. As the measurements done through cross‐sectional drill holes cover only a small portion of the river channel area, the numerical simulations give insight into the wider spatial horizontal variation of the flow characteristics. Therefore, we simulate the ice‐covered flow with a hydrodynamic two‐dimensional (2D) model in a meandering subarctic river (Pulmanki River, Finland) in mid‐winter conditions and compare them to the pre‐winter open‐channel low flow situation. Based on the simulations, which are calibrated with reference measurements, we aim to detect (1) how ice‐covered mid‐winter flow characteristics vary spatially and (2) the erosion and sedimentation potential of the ice‐covered flow compared to open‐channel conditions. The 2D hydrodynamic model replicated the observed flow characteristics in both open‐channel and ice‐covered conditions. During both seasons, the greatest erosional forces locate in the shallow sections. The narrow, freely flowing channel area found in mid‐winter cause the main differences in the spatial flow variation between seasons. Despite the causes of the horizontal recirculating flow structures being similar in both seasons, the structures formed in different locations depended on whether the river was open or ice covered. The critical thresholds for particle entrainment are exceeded more often in open‐channel conditions than during ice‐covered flow. The results indicate spatially extensive sediment transport in open‐channel conditions, but that the spatial variability and differences in depositional and erosional locations increase in ice‐covered conditions. Asymmetrical bends and straight reaches erode throughout the year, whereas symmetrical, smaller bends mainly erode in open‐channel conditions and are prone to deposition in winter. The long ice‐covered season can greatly affect the annual morphology of the submerged channel. © 2019 John Wiley & Sons, Ltd.

Sub‐arctic river bank dynamics and driving processes during the open‐channel flow period

Hackney

Salmela

et al. 2020

There is growing concern that rapidly changing climate in high latitudes may generate significant geomorphological changes that could mobilise floodplain sediments and carbon; however detailed investigations into the bank erosion process regimes of high latitude rivers remain lacking. Here we employ a combination of thermal and RGB colour time‐lapse photos in concert with water level, flow characteristics, bank sediment moisture and temperature, and topographical data to analyse river bank dynamics during the open‐channel flow period (the period from the rise of the spring snowmelt flood until the autumn low flow period) for a subarctic river in northern Finland (Pulmanki River). We show how variations of bank sediment temperature and moisture affect bank erosion rates and locations, how bank collapses relate to fluvial processes, and elucidate the seasonal variations and interlinkages between the different driving processes. We find that areas with high levels of groundwater content and loose sand layers were the most prone areas for bank erosion. Groundwater seeping caused continuous erosion throughout the study period, whereas erosion by flowing river water occurred during the peak of snowmelt flood. However, erosion also occurred during the falling phase of the spring flood, mainly due to mass failures. The rising phase of the spring flood therefore did not affect the river bank as much as its peak or receding phases. This is explained because the bank is resistant to erosion due to the prevalence of still frozen and drier sediments at the beginning of the spring flood. Overall, most bank erosion and deposition occurrences were observed during the low flow period after the spring flood. This highlights that spring melt, while often delivering the highest discharges, may not be the main driver of bank erosion in sub‐arctic meandering rivers. © 2019 John Wiley & Sons, Ltd.