Small scale bank erosion experiments in freezing and thawing conditions

Climatic warming is projected to affect hydrology and change ice-cover periods within river channels, particularly in northern high-latitude regions. These changes will impact sediment transport conditions and longer-term riverine morphology. For example, the duration of the freezing, frozen, thawing and unfrozen periods, may affect river bank erodibility characteristics. However, it is difficult to quantify the combined impact of soil moisture, rates of freezing/thawing, and ambient temperatures on the fluvial bank erosion in addition to altered flow velocity conditions in natural river sites. We therefore present a series of scaled laboratory experiments in a controlled small-scale novel cryolab morphology facility. The flume experiments allow for detection of how these different forcing factors affect riverbank erosion rates. The ultimate goal of the experimental programme is to enhance the process understanding of the sediment transport behaviors in expected future conditions in sub-arctic environments, where the frozen periods are expected to shorten and air temperatures to rise due to climatic change.

 

The experiments presented in this study aimed to detect how the flow velocity, soil moisture content and freezing levels of the bank sediment, affect river bank stability with altered ambient air temperatures. The laboratory experiments were performed using a small-scale Friedkin channel (1945) within a chilled flume system. A suite of experiments were conducted adjusting the ambient air temperature, the water temperature, and the water discharge (flow velocity). As the basis for creating realistic bank characteristics for the experiments, the sediment size, soil moisture and soil temperature parameter values observed in a sub-arctic Pulmanki River during mid-winter, after ice-breakup/before snow-flood peak and non-frozen conditions were used across the experimental set. The sediment bank blocks (2 cm high) were prepared for each of the experimental runs the day before, and kept in the chilled flume room overnight to match ambient temperatures prior to the runs being advanced.

 

Overall ~130 experiments were performed. From each of the experiments the topography was measured before and after the experiment, by taking photos with a semi-automatic Canon camera. Structure from motion methodologies were used to produce surface models, and volumetric change was thus possible to calculate for each experiment. GoPro cameras (HeroBlack 10) were used to film videos of the bank evolution positioned from both nadir and sideways positions, providing linked high-resolution views of the evolving bank morphology. The data was used to detect the bank edge retreat through time. To assess changes in flow structure, buoyant micro-beads were seeded at the beginning and at the end of each experiment, allowing particle tracking velocimetry method to recover and defining the flow velocities of each experiment. Finally, a FLIR A655 thermal camera was used to aid understanding on the thermal transfers between the flow and the banks and the impact this had on morphodynamics.

 

The preliminary results related to the possible links between temperature, moisture, flow velocity and resultant morphodynamics will be presented and the implications for climate change impacts on defrosting landscapes will be discussed.