Field observations and flume experiments on the nature of comet marks

Details Field observations and flume experiments on the nature of comet marks Field observations and flume experiments on the nature of comet marks

Apr 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980sedg...26..233w&link_type=abstract

Sedimentary Geology, Volume 26, Issue 1-3, p. 233-262.

Mathematics

Logic

14

Scientific paper

Comet marks are defined as obstacle-induced, long erosional strips occurring on current-affected sea bottoms. They are mainly observed on side-scan sonographs due to the acoustical back-scattering contrast between the erosional zone and the surrounding bed. Their detailed morphology has been observed in diving. Comet marks are found typically in shallow seas where a thin layer of fine sand occurs on residual sediments. Flow obstacles are provided frequently by boulders as relicts from glacial deposits. Distribution patterns suggest that the arrangement of different types of comet marks might correspond to current intensities. For such an application as ‘geological current meters’, however, it is necessary to know more about the conditions of their formation. A flume experiment was designed to study the flow character in the wake of obstacles and their effects on a mobile bed. This led to the following conclusions:The velocity distribution in the wake of obstacles corresponds to a paired helical motion and an increased bottom stress with a component directed symmetrically outward.This ‘sweeping effect’ leads to erosion by carrying sediment particles out of a parabolically widening wake zone.‘Ideally shaped’ parabolic comet marks develop over a wide range of flow velocities and when the wake pattern is not disturbed by turbulent motions arising from adjacent areas. In natural environments, these conditions occur on hard bottoms with a thin veneer of mobile sand, and no morphological grouping related to flow conditions will be possible.A morphological grouping due to flow conditions is possible in contrast to these ‘veneer conditions’ when a disturbing turbulence field, belonging to moving small-scale ripples on the surrounding bed, acts in dynamical competition with the wake flow turbulence. Under these conditions a morphological grouping seems to be possible; (a) within a certain range, the flow rate controls the invasion rate of the ripple field into the comet tail zone and thereby the length and shape of the comet marks; (b) a sudden increase of the comet marks tails to extreme lengths takes place when the flow velocity is approaching the upper limit of conditions for ripple stability, because the decreasing bed roughness causes a reduction of the disturbing ripple field turbulence.An excavation zone behind the obstacles can be obtained also under conditions below the critical shear stress for erosion when an obstacle-triggered ripple field had developed. In natural environments, however, this mechanism probably does not play an important role.

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