Geologic Controls on Erosion Mechanism on the Alaska Beaufort Coast

Two prominent arctic coastal erosion mechanisms affect the coastal bluffs along the North Slope of Alaska. These include the niche erosion/block collapse mechanism and the bluff face thaw/slump mechanism. The niche erosion/block collapse erosion mechanism is dominant where there are few coarse sediments in the coastal bluffs, the elevation of the beach below the bluff is low, and there is frequent contact between the sea and the base of the bluff. In contrast, the bluff face thaw/slump mechanism is dominant where significant amounts of coarse sediment are present, the elevation of the beach is high, and contact between the sea and the bluff is infrequent. We show that a single geologic parameter, coarse sediment areal density, is predictive of the dominant erosion mechanism and is somewhat predictive of coastal erosion rates. The coarse sediment areal density is the dry mass (g) of coarse sediment (sand and gravel) per horizontal area (cm2) in the coastal bluff. It accounts for bluff height and the density of coarse material in the bluff. When the areal density exceeds 120 g cm−2, the bluff face thaw/slump mechanism is dominant. When the areal density is below 80 g cm−2, niche erosion/block collapse is dominant. Coarse sediment areal density also controls the coastal erosion rate to some extent. For the sites studied and using erosion rates for the 1980–2000 period, when the sediment areal density exceeds 120 g cm−2, the average erosion rate is low or 0.34 ± 0.92 m/yr. For sediment areal density values less than 80 g cm−2, the average erosion rate is higher or 2.1 ± 1.5 m/yr.

Critical Gaps in Shoreline Plastics Pollution Research

Shoreline surveys are an accessible and common method for monitoring plastic pollution in aquatic environments. Their results are critical to well-informed pollution mitigation efforts. Here, we show that three environmental variables: (1) coarse sediment, (2) accumulations of organic material, and (3) snow and ice are dramatically underrepresented by existing shoreline plastic pollution research efforts. We reviewed 361 published shoreline surveys, encompassing 3,284 sample sites, and found that only 4% of sites included coarse sediment, only one study described sampling organic material for plastic, and only 2.5% of sites are sampled in the presence of ice or snow. The relative absence of these environmental variables may stem from the tailoring of shoreline survey guidelines to a narrow range of shoreline environments. These three features influence plastic deposition and retention on shorelines, and their underrepresentation signals a need to recalibrate research efforts towards better methodological reporting, and regional representation and relevance.

Yield Stress Model for Natural Debris Flows in Presence of Fine and Coarse–Grained Sediments

When dealing with natural geo–hazards, it is important to understand the influence of sediment sorting on debris flows. The presence of coarse fraction is one of the aspects which affects the rheological behaviour of natural viscous granular fluid mixtures. In this paper, experiments on reconstituted debris flow mixtures with different coarse–to–fine sediment ratios are considered. Such mixtures behave just as non–Newtonian yield stress fluids and their rheological behaviour is largely affected by the presence of coarse fraction. Experimental results demonstrate that yield stress is very sensitive not only to bulk sediment concentration but also to coarse sediment fraction. A novel yield stress model is presented. It accounts for an empirical grading function depending on the coarse–to–fine grain content. The yield stress model performed satisfactorily in comparison with the experiments, showing that it is almost independent of the coarse–to–fine grain fraction in case of dominant coarse sediment content.

Width control on event scale bedload dynamics in bedrock-confined channels

<p>In mixed bedrock-alluvial rivers, the response of the system to a flood event can be affected by a number of factors, including coarse sediment availability in the channel, sediment supply from the hillslopes and upstream, flood sequencing, and coarse sediment grain size distribution. However, the impact of along-stream changes in channel width on bedload transport dynamics remains largely unexplored. We combine field data, theory, and numerical modeling to address this gap. Observations from two flood events in the Daan River gorge in western Taiwan suggest that coarse sediment evacuation and re-deposition can cause intra-flood changes of up to several meters in channel bed elevation that are distinct from measured before/after bed changes. We hypothesize that this could be related to the abrupt change in width between the 1 km long bedrock gorge and the river upstream and downstream. An analysis of the theoretical relationships between discharge, channel width, and bedload transport capacity shows that for a given slope, narrow channels transport bedload more efficiently than wide ones at low discharges, while wider channels are more efficient at high discharges. We used the model sedFlow to explore this effect, running a random sequence of floods through a channel with a narrow gorge section bounded upstream and downstream by wider reaches. Channel response to imposed floods is complex, as high and low discharges drive different spatial patterns of erosion and deposition, and the channel may experience both of these regimes during the peak and recession periods of each flood. Our modeling suggests that width differences alone can drive substantial variations in sediment flux and bed response, without the need for variations in sediment supply or mobility. Further, the deposition or erosion that takes place within a flood is often not reflected in the before/after changes to the bed, and this disconnect increases with increasing flood size.</p>

Particle-size distribution of transported sediment in the Pisha sandstone slope under the influence of complex erosion

<p>Coarse sediment of the Yellow River in the complex erosion area of the Pisha sandstone region of the Ordos Plateau is deposited on the downstream riverbed, posing a threat to the flood control safety of the river. The study of sediment particles in this erosion process can deepen the understanding of the erosion process, provide a theoretical basis for the establishment of an erosion prediction model, reveal the internal law of composite erosion, and guide the planning and design of soil and water conservation and the allocation of soil and water conservation measures. In this study, complex erosion indoor tests were carried out through the artificial rainfall-wind-freezing-thawing cycle solid model. The enrichment rate (ER), fractal dimension, and median diameter (d<sub>50</sub>) of soil particles were used to quantify the size distribution characteristics of sediment particles under different erosion dynamics. The coarse sediment was first transported in the process of soil erosion because of the special texture and terrain characteristics of Pisha sandstone soil. Moreover, the degree of heterogeneity of sediment under complex erosion was larger than that under water erosion. The effect of wind could aggravate the instability of the erosion dynamic system. Under the combined action of freezing-thawing, wind, and water, the particle size composition changed greatly, and the erosion energy was extremely unstable. The effect of complex erosion created conditions for the coarse sediment transportation. Under the freezing-thawing-wind-water combined action, the particle size of eroded sediment was the coarsest, and that of water erosion was the smallest. We concluded that the reason why the Pisha sandstone area has become the core area of the concentrated source of coarse sediment in the Yellow River is related not only to the special nature of the Pisha sandstone soil itself but also to the effect of complex erosion.</p>

Validation of an Experimental Procedure to Determine Bedload Transport Rates in Steep Channels with Coarse Sediment

The current study presents an experimental procedure used to determine bedload sediment transport rates in channels with high gradients and coarse sediment. With the aim to validate the procedure for further investigations, laboratory experiments were performed to calculate bedload transport rates. The experiments were performed in a laboratory tilting flume with slopes ranging from 3% to 5%. The sediment particles were uniform in shape (spheres). The experiments were divided into four cases based on sediment size. Three cases of uniform sizes of 10 mm, 15 mm and 25 mm and a case with a grain size distribution formed with the uniform particle sizes were considered. From the experimental results a mathematical bedload transport model was obtained through multiple linear regression. The experimental model was compared with equations presented in the literature obtained for gravel bed rivers. The experimental results agree with some of the models presented in the literature. The closest agreement was seen with models developed for steep slopes especially for the highest slopes considered in the present study. Therefore, it can be concluded that the methodology used can be replicated for the study of bedload transport rates of channels with high gradients and coarse sediment particles to study more general cases of this process such as sediments with non-uniform shapes and sizes. However, a simplified model is proposed to estimate bedload transport rates for slopes up to 5%.