The Influence of Lateral Confinement Upon the Downstream Gradation in Grain Size of the Lower Ngaruroro River, New Zealand

ABSTRACT

Systematic grain size measurements were recorded for all geomorphic units in 41 cross sections spaced 400- 800 m apart along the lower course of the Ngaruroro River. Although the surveyed bed slope has a uniform grade through this reach, valley width imposed by terrace margins increased markedly, with a notable increase 6 km down-reach. Mean grain size and D95 diminution coefficients of 0.052 and 0.073 were determined for the 19 km study reach. Marked differences in downstream fining trends were evident for the zones upstream and downstream of 6 km, with significant fining upstream of 6 km and more subdued fining downstream of 6 km (D95 diminution coefficients of 0.109 and 0.017 respectively). The increase in active channel area associated with the increase in valley width is considered to mark a process control shift from competence-limited to capacity-limited conditions. Selective entrainment rather than abrasion is inferred to be the primary mechanism for downstream fining. Lateral confinement also affects patterns of geomorphic units. The competence-limited environment (upstream 6 km) comprises stabilised bars, stabilised gravel sheets and gravel sheets while the capacity-limited environment (downstream of 6 km) is characterised by significant lateral variability of gravel sheets, high flow chutes, ridges, ramps, platforms, stabilised bars and stabilised gravel sheets. Findings from this study highlight the importance of lateral confinement as a control on river processes and forms.

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