Morphological Analysis of Main and Chute Channel Discharge, Scroll Spacing, and Migration Rates Along the Kantishna River, Alaska
English, G., Sentz, C., Chamberlin, E.,Trop, J.

Channel migration rate in meandering rivers is influenced by hydraulic geometry, bank strength, and bend curvature, and this lateral movement allows channels to deposit scroll bars, curved ridges of sediment that form on the inner bank. In some river systems, chute channels also migrate and deposit scroll bars, making a natural laboratory for testing the role of local channel size on channel lateral migration and scroll bar deposition. Here we investigate main and chute channel migration rates and the spacing of their respective scroll bars at two sites on the Kantishna River, a sinuous, chute-dominated, meandering river in the Alaskan subarctic, to test the hypothesis that scroll bar spacing directly correlates with lateral migration rate and channel discharge in both main and chute channels.
We collected discharge and bathymetry data from the main channel (MC) and chute channel (CC) of two bends on the Kantishna using a Teledyne RiverPro ADCP with a GNSS Hemisphere, and completed field mapping of scroll bar spacing using a laser rangefinder. For the same two bends, we calculated annual channel migration rates from 1984-2020 using satellite images from Google Earth Engine processed with RivMap, a MATLAB package that measures changes in channel centerlines.
Preliminary results show ADCP-measured discharges of 144m3/s and 191m3/s for sites 1 and 2 MCs and 28m3/s to 31m3/s for the respective CC, showing that these MCs have 5x the flow of CCs. Based on image analysis, MC width is 146m and 163m at sites 1 and 2 respectively while associated CC width is 86m and 102m. MC mean annual migration rates are 4.1m/yr and 4.9m/yr for sites 1 and 2 respectively, compared to 5.6m/yr and 13.3m/yr for CCs. Field measurements show average scroll bar spacings of 5.4m and 13.6m for MCs at sites 1 and 2, and CC scrolls have similar average spacings of 7.6m and 14m at the same sites. Additionally, the year of maximum migration rate differs for MC and CCs at both sites.
Overall, MC and CC migration rates and scroll bar spacings are very similar within both bends, despite CCs having 5x lower discharges. This suggests that the scroll spacing is correlated with local migration rate, not the local channel discharges. Therefore, an alternative variable such as bifurcation angle or local slope may be more important for migration rate and scroll bar spacing along each reach.

The Effects of Pins on Force Chains in a Granular System: A Simulation

Granular media are large collections of disordered macroscopic particles interacting via dissipative forces. We use molecular dynamics simulations to study a two-dimensional, 50:50 binary mixture of purely repulsive harmonic disks of radii 1:1.4. By freezing the top and bottom walls of particles we shear the system at a constant rate and apply dissipative interactions depending on relative velocity. We study how force chains are influenced by the addition of fixed minuscule disks of radius 0.004 placed on a square lattice. We study the forces Fij between particles i and j. We will present distributions P(Fij) both for the complete system as well as for layers. We also study the system both near the jamming transition at p = .00025 as well as slightly above the transition at p = 0.001. We compare the distributions in steady state during the shear.

Investigating the Microscale Mechanics of Actomyosin Networks

Biological cells exhibit a myriad of complex nonlinear responses to stress or strain, namely stress stiffening, softening, elastic recovery, and plastic deformation depending on the nature of the stress applied. Actin, a vital cytoskeleton protein, plays a key role in maintaining cellular stability, enabling motion, facilitating replication, and powering muscle contraction. Its filaments are entangled and crosslinked, which gives the cytoskeleton its viscoelastic response and modulates various mechanically-driven processes regulated by actin-binding proteins such as myosin. This important molecular protein-Myosin, generates contractile forces by exerting tension on actin filaments in opposing directions which creates elongated tail-to-tail thick filaments through ATP hydrolysis. The process generates forces at the piconewton level. The forces generated are crucial for orchestrating localized pulling forces during fundamental cellular processes such as division, migration, and muscle contraction. Despite the growing evidence entailing myosin-II as a key part in such microscale mechanics, the mechanical behavior/response of the network crosslinked by myosin-II is poorly understood. Beyond its role in introducing network contractility, the precise threshold concentration of myosin-to-actin and ATP ratio, to which myosin acts as a crosslinker, remains elusive.
Our project aimed to develop a series of in vitro reconstituted actomyosin networks by tuning the myosin-to-actin ratios to examine the mechanical properties and structural reorganization of these networks. This approach discerned the chemical environments conducive to actomyosin network formation, provided insight into how different ratios of myosin-to-actin affects the network’s structure and mechanical properties, and determined how the relaxation time scale depends on the concentration of myosin.