| Most studies of strath terrace formation have been mainly descriptive and thus have limited use; therefore, we present a new conceptual model for strath terrace formation based on physical process and the equilibrium hydraulic geometry of natural rivers.; In Chapter 2, the prevailing genetic hypotheses for fluvial strath terraces are reviewed and critiqued. A genetic model consisting of three stages is proposed based on probability analyses, theoretical analyses, experimental flume studies, and fieldwork. Stream networks derived from Digital Elevation Models (DEM) are analyzed and show that channels of a composite alluvial/bedrock perimeter, hereafter called mixed-bank channels, have a higher probability to flow against the bedrock sidewall relative to their alluvial counterparts. We believe that this tendency for mixed-bank channels to flow against its bedrock sidewall arises because channels with a bedrock wall maximize their bedload transport per unit water discharge. This conjecture is tested by our flume studies, where experimental channels, under constant discharge, slope, and uniform grain size, evolved to width-depth ratio that maximized their bedload transport rate. Our flume studies and current experimental work show sedimentation mechanics that result from secondary circulation patterns, erode an inner channel next to the bedrock wall.; Our model of strath formation is supported by fieldwork (Chapter 2 and Chapter 3) that focuses on terrace genesis in both the Huangarua and Yuba Rivers, where straths can be shown to have been cut when increased sediment influx caused channel aggradation and eventual stream planation. Experimental flume results (Chapter 2) have shown that hydraulic geometry is determined not only by flow discharge, but also by channel shape and channel roughness, particularly in the case of a composite alluvial/bedrock perimeter. A relationship between maximum sediment transport rate and channel shape has been developed through both theoretical and experimental methods. The maximum sediment transport rate per unit stream power occurs at a ratio of width to depth that is neither too narrow to limit the surface area over which bedload can be moved, nor too wide to significantly reduce the bed shear stress. Probability analyses using high-resolution digital elevation data and field mapping of the Huangarua River, New Zealand and the Yuba River, California, support a fundamental assumption in our model and extend the experimental data into the realm of natural rivers. (Abstract shortened by UMI.)... |
