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Journal of Coastal Research SI 56 337 - 341 ICS2009 (Proceedings) Portugal ISSN 0749-0258 Measurements of Aeolian Mass Flux Distributions on a Fine-Grained Beach: Implications for Grain-Bed Collision Mechanics

S.L. Namikas†, B.O. Bauer‡, B.L. Edwards†, P.A. Hesp†, and Y. Zhu

†Dept. of Geography and Anthropology, Louisiana State University,

Baton Rouge, LA 70803

USA

snamik1@ ‡ Earth and Environmental Sciences & Geography University of British Columbia Okanagan Kelowna, BC V1V 1V7

Canada

ABSTRACT

N AMIKAS, S.L., B AUER, B.O., E DWARDS, B.L., H ESP, P.A., and Z HU, Y., 2009. Measurements of aeolian mass flux distributions on a fine-grained beach: Implications for grain-bed collision mechanics. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 337 – 341. Lisbon, Portugal, ISSN 0749-0258

Saltation is the primary mechanism of sediment transport responsible for supplying sand to coastal dunes. Of the several sub-processes involved in saltation, the collision-rebound process is generally considered the most poorly understood. This study presents measurements of the vertical and horizontal distributions of sand mass flux collected on a fine-grained beach at Mustang Island, Texas, USA, immediately downwind of five test beds containing narrow ranges of pre-sieved sand. Measured mass flux distributions were increasingly stretched in the vertical and horizontal dimensions with increasingly large grain-sizes in the test beds. It is inferred that the effective inertia of the target bed grains imposes a limit on the rebound/launch velocity of saltating grains, such that impact energies in excess of the effective inertia are transferred to the target grains (and possibly adjacent grains) thereby setting them in motion. The characteristic scales of particle trajectories are thus substantively controlled by bed texture.

ADDITIONAL INDEX WORDS:Saltation, Sediment Transport, Bed Texture, Elastic Rebound

INTRODUCTION

The grain-bed collision process is recognized as a critical component of saltation (B AGNOLD, 1941; W HITE and S CHULZ, 1977;W ERNER and H AFF, 1988; M C E WAN, W ILLETTS and R ICE, 1992; N AMIKAS, 2003a; T A and D ONG, 2007). The collision process conditions the kinetic energy level of particles rebounding or splashed from the bed, and thereby largely controls the height and length of saltation hops as well as the subsequent rate of transport (A NDERSON et al., 1991; M C E WAN and W ILLETTS, 1991). Most models of aeolian saltation assume (often implicitly) that particles rebound from the bed in near-elastic fashion, retaining some fixed proportion of their impact energy parameterized by the coefficient of restitution (e.g., B AGNOLD, 1941; A NDERSON and H AFF, 1988; R ICE et al., 1995, 1996; T A and D ONG, 2007).

This conceptualization is intuitively appealing, because many familiar collisions operate in such a manner (e.g., a more energetic downward push on a basketball produces a higher rebound off the floor). However, recent field experiments (N AMIKAS 2003a, 2003b, 2006) have cast doubt upon this conceptualization, and led to the proposal of a new conceptual model of the partitioning of energy during grain-bed collisions in aeolian sedimentary systems. In essence, the proposed model recognizes two collision regimes. In the ‘quasi-elastic’ regime, particles rebound with a roughly fixed proportion of their impact energy (as given by the coefficient of restitution), consistent with the traditional conceptualization of the process. In the ‘inelastic regime’, the unconsolidated nature of potentially movable beach sediments results in quite different collision outcomes that involve bed deformation. The two regimes are separated by a critical impact energy level, akin to a plastic (elastic) limit separating the classic elastic behavior of materials where the stress-strain relationship is linear and reversible, from the plastic-like regime where the material experiences progressive deformation under increased stress.

At collision energy levels below the critical limit, impactors

have insufficient kinetic energy to displace the bed grains and simply bounce off the target grain, in quasi-elastic fashion, retaining a fixed proportion of their impact energy. At collision energy levels above the critical limit, the amount of kinetic energy transferred to the bed grain during the impact exceeds the effective inertia of the bed grain(s), and the bed ‘fails’ or deforms. The ‘deformation’ of the bed takes the form of the target grain (and possibly one or more adjacent grains) being ‘splashed’ into motion and leaving behind small craters or pits. The target grain will be able to reflect back to the impacting grain only an amount of kinetic energy comparable to its effective inertia (which represents the critical impact energy limit). Impact energy above this limit is thus expended in bed deformation (i.e., moving and/or ejecting one or more bed grains), as well as heat dissipation and acoustic energy generation. Thus, the effective inertia of the bed particles (related to grain size, as a first approximatation) acts as a limit on the intial launch energy of rebounding particles, and therefore on the dimensions of saltation trajectories as well as the rate of transport. Earlier studies (N AMIKAS 2003a, 2003b, 2006) indicated that with medium-size, moderately well-sorted sands most collisions fall into this inelastic (plastic-like) regime.

The present study further explores the proposed model. Consideration of the underlying physics suggests that the critical limit on rebound energy (and therefore saltation trajectory dimensions) should be related to the size (mass/inertia) of the bed