The impact of particle shape on the angle friction and implicaitons for sediment dynamics at a...
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Earth Surf. Dynam., 2, 469–480, 2014 www.earth-surf-dynam.net/2/469/2014/ doi:10.5194/esurf-2-469-2014 © Author(s) 2014. CC Attribution 3.0 License. The impact of particle shape on the angle of internal friction and the implications for sediment dynamics at a steep, mixed sand–gravel beach N. Stark 1,* , A. E. Hay 1 , R. Cheel 1 , and C. B. Lake 2 1 Dalhousie University, Department of Oceanography, Halifax, Canada 2 Dalhousie University, Department of Civil Engineering, Halifax, Canada * now at: Virginia Tech, Department of Civil and Environmental Engineering, Blacksburg, VA, USA Correspondence to: N. Stark ([email protected]) Received: 14 October 2013 – Published in Earth Surf. Dynam. Discuss.: 10 December 2013 Revised: 18 July 2014 – Accepted: 24 July 2014 – Published: 27 August 2014 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 ≥ 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. 1 Introduction Subaqueous sediment dynamics play a major role in coast- line, river and lake development, as well as scour around submerged structures, and coastal hazards such as subma- rine landslides (Kuehl et al., 1996; Simons and ¸ Sentürk, 1992; Bradley and Stolt, 2006; Masson et al., 2006). De- spite the widespread interest and ongoing research in this field, the complex system consisting of the many diverse factors governing subaqueous sediment dynamics and beach dynamics (hydrodynamics, morphology and sediment prop- erties) is still far from being fully understood. 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 friction angle depends on grain size, sorting, density, particle arrangement, and particle shape (Schanz and Vermeer, 1996; Das, 1990). In particular, the importance of particle shape with regard to the friction angle and initiation of subaqueous Published by Copernicus Publications on behalf of the European Geosciences Union.
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Paper on angle friction of mixed sand-gravel beach
Transcript of The impact of particle shape on the angle friction and implicaitons for sediment dynamics at a...
Earth Surf. Dynam., 2, 469480, 2014www.earth-surf-dynam.net/2/469/2014/doi:10.5194/esurf-2-469-2014 Author(s) 2014. CC Attribution 3.0 License.The impact of particle shape on the angle of internalfriction and the implications for sediment dynamicsat a steep, mixed sandgravel beachN. Stark1,*, A. E. Hay1, R. Cheel1, and C. B. Lake21Dalhousie University, Department of Oceanography, Halifax, Canada2Dalhousie University, Department of Civil Engineering, Halifax, Canada*now at: Virginia Tech, Department of Civil and Environmental Engineering, Blacksburg, VA, USACorrespondence to: N. Stark ([email protected])Received: 14 October 2013 Published in Earth Surf. Dynam. Discuss.: 10 December 2013Revised: 18 July 2014 Accepted: 24 July 2014 Published: 27 August 2014Abstract. The impact of particle shape on the angle of internal friction, and the resulting impact on beachsediment dynamics, is still poorly understood. In areas characterized by sediments of specic shape, particularlynon-rounded particles, this can lead to large departures from the expected sediment dynamics. The steep slope(1 : 10) of the mixed sandgravel 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
