Analysis of the effect of changing discharge on channel morphology and instream uses in a Braided River, Ohau River, New Zealand

Abstract

Constant discharges of 26.5, 56.7, 105,240, and 507 m3s -1 were released down the Ohau River from Ohau A power station, and measurements of water depths and mean velocities made along cross sections in a braided section of the channel. Frequency distributions of water depth and velocity are presented both singly and jointly for each discharge; the methods used provide much more information on the changing character of the fiver than is provided by conventional hydraulic geometry relations. As discharge increases, existing channels become wider, deeper, and faster and frequently merge to become a single larger channel. However, additional channels are generated with the same characteristics as those existing at lower discharges, and the total number of channels at a cross-section remains constant. Hence the increase of water surface area tends to be by addition of faster, deeper water to a constant area of shallow, slow water (whose location in the river bed changes). This process has the major implication for instream uses such as salmonid spawning in that the area suitable for these uses may remain constant over a wide range of flows. In some respects, then, the braided river is morphologically more stable than a single-thread river. -26.5 ß ß ell ß ß ß ß ß ß ß ß ß ß ß ß . ß ß ß ß © ß ß ß _ ß ' '14 9-16..... '25 ß <1 4-9 16-25 thousand m 2 --105 ß , e ß © ß . ß ß ß ß 0 ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß _ _ .... 56.7 ß ß eOe. ß _ ß ß ß el O ß ß ß ß ß _ ß ß ß O ß ß . ß ß ß ß _ . ß _ _ ß ß -240 ß eeO©. ß --_ ß Oe oil. ß ß --_ ß ß ß ß ß 0 ß ß ß ß --_ ß oOoO ß ß ß ß ß _ _ --_ ß --_ ß --_

Issues in characterizing heterogeneity and connectivity in non-multiGaussian media

“…1a-c). It is built from an aerial photograph displaying braided channels in the Ohau River, New Zealand [30]. According to the classification of natural rivers from Rosgen [32], the architecture displayed on this photograph belongs to the type D: braided channels.…”

Flood Hydraulics and Impacts on Invasive Vegetation in a Braided River Floodplain, New Zealand

“…The rate of increase was slightly greater for the average velocity across the floodplain (from 1.1 to 1.5 m/s). The greatest average velocity of >3.7m/s in the main channel during the largest flood is slightly higher than values of approximately 3 m/s measured by Mosley (1982) in the Ohau River during the approximate 50-year flood. Measured velocities in several other South Island braided rivers for more typical flows ranged from 0.1 to 1.7 m/s (Mosley, 1983).…”

“…Surprisingly little information on measured inundation areas (or depths and velocities) during floods is available for braided rivers in New Zealand. Mosley (1982) analysed relationships between increasing channel widths with discharge, including during a flow released from hydropower operations approximating the 50-year flood, in the slightly larger Ohau River (immediately north of the Ahuriri), and for a more frequent range of flows in several other South Island rivers (Mosley, 1983). The relationships were nonlinear and cross sectional area of water increased at the greatest rate.…”

“…In some cases, any relevant gauged or observed flow, velocity or level data collected in a river or nearby/similar rivers can be used to the extent possible, or qualitatively to compare to model output and evaluate results. Therefore, in this study any available data collected during high flows or results from other modelling studies in the Ahuriri or similar rivers (Mosley, 1982(Mosley, , 1983Nicholas, 2003;Duncan & Bind, 2009) were used for comparison and evaluation.…”

Response of sandhill crane (Grus canadensis) riverine roosting habitat to changes in stage and sandbar morphology

“…The colour infrared imagery obtained after the crane migration season, 15 June 2005, was registered ( Figure 4F) and the area of the exposed sandbars was found to be only somewhat smaller, 125 535 versus 135 270 m 2 , but in a similar configuration from that digitized using the thermography collected on 25 March 2005 ( Figure 4E) Mosley (1982) who noted that increases in discharge in the braided Ohau River in New Zealand were accompanied by addition of faster, deeper water to a constant area of shallow, slow water. A layer of annual vegetation was also present on the sandbars in the Platte River at the time the colour infrared imagery was collected and provided the sandbars some resistance to erosion.…”