Tunnicliffe, JBrierley, GReeve, Michelle2016-06-2720162016http://hdl.handle.net/2292/29177Full text is available to authenticated members of The University of Auckland only.Gravel raking is a gravel-bed river management technique that uses a tractor to drag large metal ripping blades across exposed channel bars, mechanically disrupting the coarse armour layer. Anecdotal and laboratory evidence have indicated gravel raking is effective, but at present there have been no published field studies to substantiate this. In this work, the impacts of gravel raking were characterised along the 129 km Tukituki River (Hawke’s Bay), where 51 km has been systematically raked since 2003. This was achieved through assessment of (1) longitudinal and (2) reach-scale surface grain size and channel morphological change patterns. Longitudinal changes in grain size character were determined by sampling surface gravel (≥250 rocks) at the coarsest depositional locales on the channel bed at 31 sites. Gravel volume, channel area, and minimum channel bed level changes were calculated at 171 cross-sections, both before (1990-2003) and after (2003-2015) raking began. Channel width changes were characterised at each cross-section from 1996-1999 to 2015. At 15 reaches, drone-based imagery and Structure-from-Motion techniques were used to generate high resolution Digital Surface Models (DSMs) of 0.20 m pixel-1. Reach-scale impacts of gravel raking were determined with 2006/2010 to 2015 Digital Elevation Models of Difference (DoDs), channel bed roughness, and grain size data which were calculated using Geomorphic Change Detection software. Analysis of cross-section changes indicated a 270% shift in longitudinal sediment storage, from a total sediment gain of 108,662 m3 yr-1 (1990-2003) to total sediment loss of 182,587 m3 yr-1 (2003-2015). This provided strong evidence that gravel raking promotes marked changes in sediment transport capacity along the Tukituki River. Weak downstream fining trends were found for the median and coarse grain size fractions (0.001 to 0.002 km-1). This suppressed fining was thought to occur because the longitudinal profile in the middle to lower reaches is relatively uniform, and because it is likely that gravel raking promotes continuous withdrawal of material from a relatively uniform subsurface parent distribution, in a similar manner to a degrading channel bed. At the reach-scale, raking initially reduced the D16 grain size percentile (by at least 1.1 ), but nine months after raking the D16 had coarsened beyond that of unraked beds (by 0.5 ). The D50 and D84 for unraked and raked beds changed little until a 1-in-12 year flood in September 2015 initiated substantial fining of grain size distributions and reduced channel bed roughness. Episodes of deposition/erosion and aggradation/degradation did not vary systematically downstream, both before and after gravel raking began. Periods of aggradation or deposition, however, were typically followed with degradation or erosion. In addition, after gravel raking began, channel width changed little upstream and considerably downstream of 70 km from the headwaters, likely due to coarse sediment supply from the Waipawa tributary at 63 km, and the transition from raked to unraked reaches at 69 km from the headwaters. Furthermore, cross-sections from raked reaches showed a greater magnitude and diversity of change than unraked reaches. Bank weakening, grain size fining, and bedrock outcrops initiated increases in the relative channel width (channel width/maximum depth) of 2 to 43 m m-1 after gravel raking began. More research is required to fully understand the temporal changes which occur in the year after each gravel raking episode, since raking effectiveness is reliant on consequent spatial and temporal grain size textural changes. Future monitoring is essential to ensure the benefits of raking continue to outweigh the financial and environmental cost. Keywords: gravel raking, surface grain size, morphological change, bed mobility, river management, Tukituki River, Hawke’s Bay.Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher.Restricted Item. Available to authenticated members of The University of Auckland.https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htmhttp://creativecommons.org/licenses/by-nc-sa/3.0/nz/ThesisCopyright: The AuthorQ112926334