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Abstract: Rates and the spatial extent of near-channel floodplain sedimentation on regulated and unregulated reaches in two upland rivers in central Vermont, U.S.A. are measured using the short-lived fallout radionuclide 210Pb. We find consistent profiles of 210Pb inventories across all sites; inventories are low immediately next to the channel, increase to a peak value as the inundation frequency decreases and then asymptotically diminish with distance from the channel to the equilibrium inventory associated with atmospheric deposition alone. We infer from our data that flow regulation has impacted sediment deposition to floodplains below the dam; total sediment deposition is less and it is constrained to a narrower band immediately along the active channel. Flow regulation does not appear, however, to impact the general form of the 210Pb inventory profile, suggesting a uniformity of process across regulated and unregulated floodplains. [Copyright &y& Elsevier]

Abstract: Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) 7Be (t 1/2 =53days) and 210Pbex (t 1/2 =22.3years). We placed mesh cylinders filled with 7Be-free sediment into a stream bed to quantify the capture of 7Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410km2, and core depths ranged from 19 to 77cm. The 210Pbex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, 7Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to >300days in unregulated rivers. Cores from a regulated river are notably different—subsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment. These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function. [Copyright &y& Elsevier]

Abstract: We demonstrate that alluvial channel response to regulation depends on the characteristic mode of sediment transport in a channel and, thus, for a channel in supply-transport equilibrium, on the caliber of sediment mantling the channel bed, as well as changes in characteristic water discharge and sediment load. We find that regulated, coarse gravel- and cobble-bed alluvial rivers dominated by bedload transport are predicted and observed to preserve the value of a dimensionless ratio of characteristic water discharge, channel slope, width, and size of sedimentary particles mantling the bed associated with the critical conditions required for the onset of significant sediment transport. The covariant geomorphic adjustment of such rivers to regulation is independent of the degree of sediment impoundment, although the nature and rate of changes in any specific characteristic appear to be sensitive to changes in sediment supply. In contrast, covariant geomorphic changes in sand- and fine gravel-bed alluvial rivers dominated by weak suspension transport are predicted and observed to be dependent on the degree of sediment impoundment and commensurate with the change in the sediment concentration of characteristic, formative flows. In both cases, specific geomorphic characteristics of alluvial rivers exhibit varied responses to regulation in different rivers, but across all rivers a systematic covariation in channel slope, width, and grain size is observed that, for a given mode of sediment transport, preserves a characteristic value of the Shields number. [ABSTRACT FROM AUTHOR]

Abstract: We explore the novel technique of using short-lived fallout radionuclides to date fluvial surfaces with the goal of determining lateral migration rates. Specifically, we use the fallout radionuclide 210Pb (half-life t 1/2 ~22.3years) to determine the lateral migration rates of meandering reaches on the Winooski River, VT, the upper Connecticut River, NH, and the Genesee River, NY. We find that, particularly near the channel, 210Pbex inventories are affected by the initial 210Pbex inventories of freshly deposited sediment (inheritance). Inheritance differs at each site, with lower inheritance occurring at sites with larger upstream drainage areas. After accounting for inheritance, 210Pbex inventories in surface sediments along a transect orthogonal to the channel yield migration rates of 0.7, 3.1, and 4.7m/y for the Winooski, Connecticut, and Genesee Rivers, respectively. These rates agree well with values derived independently from historical aerial photography. Variations in 210Pbex inventories along the transects also provide details of the channel migration histories not evident from the limited available aerial photographs, including evidence for both nearly constant linear and episodic channel migrations. Results suggest the broad applicability and added value of our approach for quantifying decadal-scale variations in fluvial surface ages. [Copyright &y& Elsevier]

Abstract: Compressively loaded ice deformed within the brittle regime fails by axial splitting under no confinement and by shear faulting under moderate confinement. This paper investigates the transition between these two failure modes using a novel very low-confinement loading device. It is found that, with increasing confinement, the orientation of the macroscopic failure plane rotates rapidly away from the direction of maximum compression until the confining stress reaches ∼1% of the maximum compressive stress, and then, under higher confinement, remains within the range of orientations generally attributed to Coulombic faulting. Quantitative modeling of the effect of confinement on wing crack growth and on the distribution of excess shear stress ahead of the wing crack tip results in predictions of the macroscopic fault orientation in good agreement with experimental observations. [Copyright &y& Elsevier]

Abstract: Systematic experiments on laboratory-grown polycrystalline granular ice and columnar S2 ice loaded triaxially under a high degree of confinement at T =−10°C to T =−40°C at applied strain rates to show the mechanical response and microstructural evolution leading to terminal failure. Terminal failure is characterized by a sudden brittle-like drop in load, localized heating and the development of a narrow shear band, consisting of recrystallized grains, oriented on a plane of maximum shear. This mode of failure, termed plastic (P) faulting, is consistent with the idea of adiabatic heating leading to localized mechanical instability and shear deformation. The microstructural state of the material, including the development of dynamic recrystallization and any prior loading history, does not have a significant affect on the character of shear localization or the levels of deformation required to generate P-faulting. [Copyright &y& Elsevier]

Abstract: Biaxial compressive experiments of terminal and post-terminal failure have been performed at −3 and −10°C on columnar-grained, S2 freshwater ice proportionally loaded across the columns under moderate confinement where Coulombic faulting limits the terminal strength. The results show that the internal friction coefficient that governs terminal failure is closely similar to the coefficient of static friction across the fault. An examination of the literature reveals that granite exhibits the same relationship. When the internal friction coefficients measured here are used to evaluate the recently developed comb-crack mechanism for brittle compressive failure under low confinement, the agreement between theory and experiment is better than first found. [Copyright &y& Elsevier]