HR: 10:20h
AN: H42B-01    [Abstracts]
TI: Effects Of Changing Flow Regime, Sediment Supply, And Riparian Vegetation On The Morphology Of The Braided Waitaki River, New Zealand
AU: * Hicks, M
EM: m.hicks@niwa.co.nz
AF: NIWA, PO Box 8602, Christchurch, 8001 New Zealand
AU: Coulthard, T
EM: t.coulthard@aber.ac.uk
AF: Institute of Geography and Earth Sciences, University of Wales Aberystwyth, Aberystwyth, SY23DB United Kingdom
AU: Walsh, J
EM: j.walsh@niwa.co.nz
AF: NIWA, PO Box 8602, Christchurch, 8001 New Zealand
AB: The Waitaki River is a large gravel-bed river (mean discharge $\sim$ 380 cumecs) draining the eastern slopes of the Southern Alps, South Island, New Zealand. The 60-km long coastal reach is broad and braided and has, since 1937, experienced a damped flood-flow regime and reduced sediment supply as a result of hydroelectric power development in the upper catchment. Sediment budget estimates indicate that the hydro dams have reduced the bed-material supply to the Lower Waitaki River by approximately 50%, while the mean annual flood discharge of the lower river has been reduced by 18%. Over the same time frame, the braidplain has become more vegetated, its active width has narrowed (from $\sim$ 2000 m to 500 m) and is now only prevented from further narrowing by human intervention. The invading vegetation species are largely exotic, and their establishment was catalysed by the regulated flow regime. Furthermore, the intensity of braiding has decreased, a dominant thalweg has appeared, channel locations have tended to become more stable, and surface armouring has generally increased. However, only the upper part of the reach has clearly degraded. The character of the change has varied downstream according to the relative effects of reduced flow and sediment supply as tributaries join the mainstem. Reduced gravel supply to the coast is also expected, although no clear signal has yet appeared in erosion rates of the adjacent coast. Potential for further flow regime change in the Waitaki catchment has led to the need to predict future river morphology, as this underpins the character of in-stream and riparian habitat. This is required over a 10-100 year time frame for the full span of lower river. Tools that offer a deterministic solution over these time and space scales are limited. A 2-d cellular model, that incorporates the influence of riparian vegetation on bank strength and flow resistance, has been used to simulate bedload transport and morphologic change in the river under historical and future scenarios of river flow regime and vegetation control. The model results are evaluated.
DE: 1815 Erosion and sedimentation
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting