Bedload

The influence of snowmelt in semiarid alluvial rivers is determined by the river bed configuration and erosion processes. Moreover, the large amount of available sediment in the floodplains of these rivers limits traditional bedload sampling methodologies and suggests the use of large control volumes and long-term studies to understand erosive dynamics. In this study, bedload monitoring in a large control volume (0.2 hm 3) from 2004 to 2010 was used to study erosive processes of the Guadalfeo River, a semiarid alluvial river in southeastern Spain with high mountain influences and a hydrological regime conditioned by snow dynamics. The methodological approach to testing the performance of different transport models included characterization of the forcing agents (precipitation and snowmelt events), hydraulic configuration of the river, determination of the river bed material (surface and substrate), and one-off measurements for particular events. The results indicate a sediment volume–effective runoff relationship that is consistent with the existence of steep channels with a flashy runoff regime, although the existence of an armored layer derived from snowmelt events greatly controls the transport dynamics, with equivalent fractions of cobble–gravel and sand in the measured bedload. This apparent predominance of near-equal mobility was confirmed by the calibrated hiding function exponent (x ≈ 0.94–0.97). The thresholds for entrainment and transport efficiency are similar to those observed for mountain rivers that exhibit torrential flow, although notable differences can be observed in the dynamics of the processes. The strong performance of the original Meyer-Peter and Müller model (with c = 8) is shown to estimate the total transported volume associated with intense events when the variability of d 50 is included in the analysis. However, this model does not sufficiently capture transport because of moderate events (rain or snowmelt) and thus underestimates bedload on medium and long timescales. The calibrated model of Parker– Klingeman adequately represents erosive dynamics, with τ r50 ⁎ values between 0.03 and 0.041 for moderate and intense events, respectively, and a flow threshold of 14.2 m 3 s −1 that separates these two situations. Nevertheless, these models based on hiding functions present greater dispersion if the variability of diameters related to cobbles and pebbles is considered. For such cases, the Wilcock and Crowe model performed better for both intense and moderate events. The latter is important in the study area, where snowmelt pulses occasionally exceed the critical flow and break the surface layer with significant sediment contributions. These processes differentiate the transport dynamics from other studies and are distinctive characteristics that are more closely related to mountainous influences in semiarid environments where pulses of intense snowmelt are common. Despite its limitations, the proposed methodology, based on a large check-dam, has proven to be adequate and representative for assessment of erosion processes.

The alluvial fill in the Upper Mississippi River Valley (UMV) is a palimpsest of past landscapes, environments, and physical evidence of human life ways. The valley has undergone significant changes in fluvial style during the time humans have occupied its landscapes, including changes in channel pattern, location of depocenters, and sediment lithology. Remnants of late-glacial braidplains that predate human presence in the Upper Midwest occur as sandy terraces and terrace fills. A subsequent major change in alluvial architecture resulted from fundamental changes in seasonal water and sediment input that marked the end of glacial meltwater input into the valley about 12.4 ka (10,500 14C yr B.P.). A shift to net transport of sandy bedload and storage of fine-grained overbank sediment on the floodplain accompanied the change to an island-braided channel pattern at that time. The Holocene channel belt has been significantly narrower, and the zone of sediment storage is reduced relative to that of the late-glacial river. Major Holocene depocenters include alluvial fans and colluvial slopes, floodbasins, natural levees, and fluvial fans at the junction of large tributaries. Climate models, and paleobotanical, and isotopic studies indicate that shifts in large-scale patterns of atmospheric circulation and moisture transport into the mid-continent of North America induced hydrologic and vegetation changes that strongly influenced flood frequency and magnitude, the delivery of sediment from tributary basins, and the evolution of the UMV landscape. Understanding the alluvial architecture of the valley, and the temporal/spatial distribution of biotic environments and processes that have buried, mixed, altered, or destroyed archaeological deposits is essential to develop strategies for sampling the valley for evidence of past human activity and for properly interpreting the archaeological record.

Bedload transport is the sediment load transported in contact with the bed of a river channel. Empirical relations are often used to predict sediment transport rates. However, as rivers are highly variable (e.g. geometries, flow rates), these predictions usually lack accuracy. Consequently, a more detailed comprehension of the physical processes is required. More precisely, we require an improved understanding on how fine sediment inputs to river channels influence sediment transport, channel stability, ecology and stratigraphy. In this context, image processing is a promising technique providing a highly-detailed insight into channel behavior over time. Moreover, it allows the development, and experimental validation, of new models. In order to undertake this, it is necessary to measure important physical quantities such as water depth, slope and their evolution with time. In this work, we propose a new image analysis chain dedicated to the experimental investigation of bedload tra...