Abstract
Abstract. The impact of particle shape on the angle of internal friction, and the resulting impact on beach sediment dynamics, is still poorly understood. In areas characterized by sediments of specific shape, particularly non-rounded particles, this can lead to large departures from the expected sediment dynamics. The steep slope (1 : 10) of the mixed sand–gravel beach at Advocate Harbour is stable in large-scale morphology over decades, despite a high tidal range of 10 m or more, and intense shore-break action during storms. The Advocate sand (d < 2 mm) was found to have an elliptic, plate-like shape (Corey Shape Index, CSI ≈ 0.2–0.6). High angles of internal friction of this material were determined using direct shear, ranging from φ ≈ 41 to 49°, while the round to angular gravel was characterized as φ = 33°. The addition of 25% of the elliptic plate-like sand-sized material to the gravel led to an immediate increase in friction angle to φ = 38°. Furthermore, re-organization of the particles occurred during shearing, characterized by a short phase of settling and compaction, followed by a pronounced strong dilatory behavior and an accompanying strong increase of resistance to shear and, thus, shear stress. Long-term shearing (24 h) using a ring shear apparatus led to destruction of the particles without re-compaction. Finally, submerged particle mobilization was simulated using a tilted tray submerged in a water-filled tank. Despite a smooth tray surface, particle motion was not initiated until reaching tray tilt angles of 31° and more, being &amp;geq;7° steeper than for motion initiation of the gravel mixtures. In conclusion, geotechnical laboratory experiments quantified the important impact of the elliptic, plate-like shape of Advocate Beach sand on the angles of internal friction of both pure sand and sand–gravel mixtures. The resulting effect on initiation of particle motion was confirmed in tilting tray experiments. This makes it a vivid example of how particle shape can contribute to the stabilization of the beach face.
Highlights
Subaqueous sediment dynamics play a major role in coastline, river and lake development, as well as scour around submerged structures, and coastal hazards such as submarine landslides (Kuehl et al, 1996; Simons and Sentürk, 1992; Bradley and Stolt, 2006; Masson et al, 2006)
Three different types of laboratory tests were performed to assess the behavior of sediments from Advocate Beach during shear: geotechnical direct shear tests and ring shear tests, as well as simple physical simulations of sediment transport initiation
With regard to the research objectives of this study, the following conclusions can be drawn. (i) It was found that the geotechnical laboratory methods offer important insight into the soil mechanical processes under shear stress and sediment resistance to shear, with potentially important implications for the sediment dynamical behavior at the beach face. (ii) it was proposed that the flat, elliptic shape of the Advocate sand undergoes a specific process of particle re-arrangement and alignment that results in a significant increase in angles of internal friction
Summary
Subaqueous sediment dynamics play a major role in coastline, river and lake development, as well as scour around submerged structures, and coastal hazards such as submarine landslides (Kuehl et al, 1996; Simons and Sentürk, 1992; Bradley and Stolt, 2006; Masson et al, 2006). Focusing on the sediment properties, the friction angle is known to be a major factor controlling the critical shear stress required to initiate particle motion (Middleton and Southard, 1984; Bagnold, 1988; Kirchner et al, 1990; Soulsby, 1997). The importance of particle shape with regard to the friction angle and initiation of subaqueous. N. Stark et al.: Impact of particle shape on friction angle sediment motion was pointed out by Kirchner et al (1990) for river sediments. There is still a certain lack of data pertaining to the impact of particle shape – non-rounded shapes – on subaqueous sediment dynamics and beach morphodynamics
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