Spatio-temporal development of frazil ice at a sub-arctic meandering river – a case study based on close range remote sensing and numerical modelling

The knowledge of supercooling and related frazil ice phenomena in rivers is critical for purposes of flow control, operation of hydraulic works and estimation of the conveyance capacity of the channels, in particular, at rivers having long and severe winters. Frazil ice is occurring when air temperatures are varying above and below zero degrees, and there is a drop in temperature overnight. However, the spatial analyses of the frazil ice and where it is anchored in riverbed are rare, especially in meandering sandy river systems, as it has been difficult to measure them in detail, such as based on aerial image data sets, together with reference flow measurements during consecutive days. Our hypothesis is that the frazil ice will attach to the river bottom in low velocity areas around bends and across slope reductions and in areas where the channel constricts.

 

The aim of the study is to detect the development of the spatial distribution of anchored frazil ice in a sandy-gravelly, meandering sub-arctic river. The work is based on field measurements done in the autumn freezing season of mid-October 2021. The data includes UAV-based orthophotos from a four days period when frazil ice anchored in the riverbed of the meandering Pulmankijoki river, in northern Finland. The flow velocities, derived from close-range remote sensing and Acoustic Doppler Current Profiler data, are used for detecting the causes of the anchoring of the frazil ice. Hydrodynamic modelling is performed to further gain information about the hydraulic characteristics within the regions of the anchored ice. The preliminary results are presented and discussed. The results indicate that the anchored frazil ice varied day to day during the four days period, covering about 5 % to 9 % of the channel area. The frazil ice advanced downstream, and spread throughout the channel, even although during the first day it was denser in the straight reach when compared to the meandering sections. The surface flow velocities were reduced during the course of time along with the increasing rim ice development.