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You are here : eLibrary : IAHR World Congress Proceedings : 35th IAHR Congress - Chengdu (2013) : THEME 5 - FLUVIAL HYDRAULICS AND RIVER MANAGEMENT : Armour Layer Development and Destruction: An Investigation into the Effectiveness of Beach Raking
Armour Layer Development and Destruction: An Investigation into the Effectiveness of Beach Raking
Author : Joseph Chaloner Warman and Heide Friedrich
This study aims to determine the effectiveness of a novel river management initiative, termed beach raking. Beach raking is a process which involves the manual breakup of the armour layer on the surface of river beaches, using a tractor, and custom-built ?ripping? blades. There is anecdotal evidence to suggest this process allows the smaller, more frequent flood events, to entrain sediment which would normally be protected by the armour layer. A laboratory study, based at The University of Auckland, was conducted to quantify the effectiveness of this process. Gravel-bed topography data were obtained using an acoustic depth profiler. Topography measurements, at a resolution of 1 x 1.225mm mapped the bed surface, and provided bed elevation data for the purposes of statistical characterisation. The armoured, raked and post-flooded bed surfaces were characterised in accordance with Goring et al. (1998), Aberle and Nikora (2006) and Coleman et al. (2011), to describe surface response to flow and relative bed composition. The sediment, which was eroded from each bed, was collected post flooding, and was subsequently weighed and graded. The response of each bed to flood could be compared through the interpretation of changes to bed-elevation statistics, and confirmed via comparison of the eroded masses and grain size distributions. Armoured beds developed during this study can be described through the increase of standard deviation, and skewness with increasing armouring flow rate. The increase in standard deviation represents greater variation of the bed surface, due to the relative protrusion of the larger grains (representative of bed roughness). The increase in skewness can be attributed to the accumulation of coarse grains on the bed surface, while smaller grains fill the interstitial spaces between, creating a bed which is stable, and resistant to movement. Once raking had destroyed the armour layer, the raked beds where characterised by larger standard deviation and a negative shift in skewness. The larger standard deviation is a representation of the inherent vertical variation introduced to the bed surface, through the crests and furrows of the raking. The skewness shifting negative is an important development in this study. It illustrated that the process of raking destroying the armour layer can be characterised through changes in statistics. Therefore, characterisation of the final raked bed surface will give an accurate indication of the effect of subjecting a raked bed to flooding, The raked bed showed a significant change in surface character, and a greater mass of eroded material. The raked bed eroded between 30 and 40% more material than the non-raked bed, and the d of the eroded material from the raked bed was 50 consistently higher. The raked bed surface appears to coarsen beyond the level that could be achieved though the armouring flow rate alone, due to the successive removal of fine material from the surface. This study illustrates that the process of breaking the armour layer on a gravel-bed surface, allows for sediment movement to occur across a bed which previously would have remained stable.
File Size : 479,284 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : 35th IAHR Congress - Chengdu (2013)
Date Published : 18/07/2016
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