The interplay between sediment supply (S s ), sedimentation rate (S r ), and the frequency of channel avulsion (A f ) exerts a primary control on alluvial architecture. In order to investigate the effect of sediment supply on avulsion frequency, four Froude-scale model experiments of an aggrading braided river were undertaken in which the magnitude of S s was progressively increased over an eightfold range. The value of A f increases at a rate slower than the increase in S s , contrary to the trend previously reported by Bryant, Falk, and Paola in their experimental study on alluvial-fan dynamics. These results suggest that the relationship between A f and S s is dependent upon bed slope and that the response of A f to an increase in S s in unconfined braided rivers may be different than that on steep alluvial fans.

We propose and test a conceptual framework for evaluating the relative timing of different types of sedimentary indicators of tectonism in alluvial foreland basin settings. We take the first occurrence of a detrital grain from a newly exposed source‐area lithology to provide the best indicator of the onset of tectonic uplift in the source area. Source‐area unroofing may lag behind initial uplift because of the type, thickness and structure of rocks in the uplifted mountain block, drainage patterns and climate. However, once exposed, advective transport disperses grains quickly throughout fluvial systems. Because of increased subsidence rate from thrust belt loading, an increase in sedimentation rate begins coincident with tectonic load emplacement within the flexural half‐width of the basin. However, farther out into the basin increased sedimentation rates lag behind the composition signal because of time lags associated with propagation of the thrust load and attendant sediment loads into the basin. The progradation of syntectonic gravel lags behind all of these signals as a direct function of the relative proportion of gravel fraction within transported sediment and rates and geometry of subsidence, which selectively traps the coarsest grain‐size fractions in the most proximal parts of the basin. We demonstrate this signal attenuation in the syntectonic Horta–Gandesa alluvial system (late Eocene–Oligocene), exposed along the southeast margin of the Ebro Basin, Spain. The results demonstrate that: (1) the time spans between the compositional signal and the progradation of the gravel front can be geologically significant, on the order of more than a million years within as little as 20 km of the thrust front; and (2) time lags between the signals increase with distance away from the deformation front. No lag time was observed between the first appearance of a new clast composition and the arrival of gravel front when the thrust front was within a few tens of metres from the depositional site. In contrast, the time lag was 0.5–1 Myr when the thrust front was about 5–6 km away and it increased to >1 Myr when the deformation front was about 8 km away. At the most extreme position, when the thrust front was 15–20 km away, the gravel front never reached the study area.

Experimental modelling of the braided Ashburton River, Canterbury Plains, New Zealand, has allowed investigation of the relationship between the frequency of channel avulsion (Af), the duration of time that the braidplain is occupied by flow, the spatial pattern of braidplain sedimentation and how these respond to a change in sediment supply (Ss). Model results demonstrate a strong, positive relationship between Ss and Af and that there is no downstream change in Af over the short braidplain distances (100 m) modelled herein. Although Af is strongly dependent on Ss, the degree of channel switching does not influence the rate, or spatial pattern, of braidplain sedimentation. All experiments used a single, central input for water and sediment, and the channels occupied the centre of the alluvial basin for a longer period of time than the margins for all sediment supply rates and distances downstream. Despite this spatio-temporal pattern in flow occupancy, braidplain sedimentation rates were nearly uniform both downstream and across the basin, and increased at approximately the same rate as increases in Ss. Consequently, less frequent, and possibly short-lived, flows at the braidplain margins deposited and preserved more sediment per unit time than at the centre of the basin where flow occupancy was higher. An approximate order of magnitude change in sediment supply resulted in only a factor of two change in bed slope, probably due to an increase in channelization and sediment throughput. This result suggests that linear diffusion models are unlikely to be applicable in aggrading multi-thread rivers.\ud \u

Rowan, A. V., Plummer, M. A., Brocklehurst, S. H., Jones, M. A., Schultz, D. M. (2013). Drainage capture and discharge variations driven by glaciation in the Southern Alps, New Zealand. Geology, 41 (2), 199-202.Sediment flux in proglacial fluvial settings is primarily controlled by discharge, which usually varies predictably over a glacial?interglacial cycle. However, glaciers can flow against the topographic gradient to cross drainage divides, reshaping fluvial drainage networks and dramatically altering discharge. In turn, these variations in discharge will be recorded by proglacial stratigraphy. Glacial-drainage capture often occurs in alpine environments where ice caps straddle range divides, and more subtly where shallow drainage divides cross valley floors. We investigate discharge variations resulting from glacial-drainage capture over the past 40 k.y. for the adjacent Ashburton, Rangitata, and Rakaia basins in the Southern Alps, New Zealand. Although glacial-drainage capture has previously been inferred in the range, our numerical glacier model provides the first quantitative demonstration that this process drives larger variations in discharge for a longer duration than those that occur due to climate change alone. During the Last Glacial Maximum, the effective drainage area of the Ashburton catchment increased to 160% of the interglacial value with drainage capture, driving an increase in discharge exceeding that resulting from glacier recession. Glacial-drainage capture is distinct from traditional (base level?driven) drainage capture and is often unrecognized in proglacial deposits, complicating interpretation of the sedimentary record of climate change.Peer reviewe

Engine durability tests are used by manufacturers to demonstrate engine life and minimum performance when subjected to doses of test dusts, often Arizona Road Dust. Grain size distributions are chosen to replicate what enters the engine; less attention is paid to other properties such as composition and shape. We demonstrate here the differences in the probability of interaction of a particle of a given particle Reynolds number on to a vane if particle shape, vane geometry, and flow Reynolds number are varied and discuss why the traditional definition of Stokes number is inadequate for predicting the likelihood of interaction in these flows. We develop a new generalized Stokes number for nozzle guide vanes and demonstrate its use through application to 2D sections of the General Electric E3 nozzle guide vane. The new Stokes number is used to develop a reduced-order probability curve to predict the interaction efficiency of spherical and nonspherical particles, independent of flow conditions and vane geometry. We show that assuming spherical particles instead of more realistic sphericity of 0.75 can lead to as much as 25% difference in the probability of interaction at Stokes numbers of around unity. Finally, we use a hypothetical size distribution to demonstrate the application of the model to predict the total mass fraction of dust interaction with a nozzle guide vane at design point conditions and highlight the potential difference in the accumulation factor between spherical and nonspherical particles.