Bedform patterns in nearshore sands

Wilson, Douglas James (1996) Bedform patterns in nearshore sands. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (10MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

In this thesis a self-organization mechanism is shown to reproduce the observed patterns of ripples in nearshore sands under waves. When viewed from above, these ripples assume a variety of patterns, including linear, brick pattern, oblique crossed sets, and lunate forms. Experimental and modeling evidence combine to demonstrate that the factors controlling bedform pattern include wave height and wave asymmetry, and the mechanisms of formation include self-organization in which the shape of the bed modifies the trajectories of those sand particles which are in motion such that a specific bed pattern is reinforced. In particular, the formation of crossed sets of ripples oblique to the incident wave direction is difficult to explain except by the self-organization mechanism described in this work, hence these crossed ripple sets are the principal focus here. -- Previous field observations have associated these different patterns with depth and hence distance from shore, but little quantitative work has been done due to difficulties in making observations. In this study difficulties in making quantitative field observations were overcome by using a remotely operated, bottom mounted rotating head sidescan sonar system to image the bottom bedforms, and electromagnetic current meters and pressure sensors mounted on the same frame to quantify the wave forcing. Experimental results from a field experiment at Burley Beach, Ontario and wave tank results from two experiments at the National Research Council wave flume in Ottawa are presented. A simple computer simulation is developed to establish mechanisms of formation of the ripple pattern types observed. -- Cross-ripples appeared in the field study at the rise and decline of each storm under similar wave conditions each time. In the wave flume cross-ripples were observed for two different sand grain sizes under regular waves, grouping waves, and irregular waves. Thus cross-ripples are formed under a range of sand sizes and wave conditions. The computer model produces cross-ripples when there is asymmetry in the forcing under sufficiently energetic wave conditions. This is consistent with the field and laboratory results. There are no length scales of forcing in the model which correspond to the length scales of the bedforms produced. The sum of these observations supports the theory that cross-ripples form as a result of the self-organization mechanism.

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