Local scour at cylindrical bridge piers in both uniform and non-uniform cohesionless sediments was investigated experimentally. The aim of the study was to improve understanding of local scour around bridge piers with sediment transport. Three empirical functions which relate the equilibrium depth of scour with approach velocity, flow depth and sediment size were obtained for uniform sediments. The effects of armouring and sediment sizes were also investigated for non-uniform sediments. The experimental results for the variation of equilibrium scour depth (normalised with the pier diameter) with approach velocity show that the equilibrium scour depth reaches a maximum at the threshold condition of the bed sediment. Above the threshold velocity, the scour depth first decreases and then increases again with increasing velocity to a maximum at the transition flat bed condition. At still higher velocities, the equilibrium scour depth decreases due to the formation of antidunes. Lesser scour depths are recorded with ripple forming sediment at threshold conditions because the bed associated with a ripple forming sediment is unable to remain planar. In live-bed conditions, the effect of rippling diminishes for increasing velocity and becomes negligible for UO/UOC > 2. The experimental results for the variation of equilibrium scour depth with flow depth show that the trend for live-bed scour of increasing scour depth with increasing YO/D until a maximum influence of YO/D is reached, is similar to that for clear water scour as shown by Ettema (1980). A flow depth adjustment factor, K(YO/D), which is related to YO/D with D/d50 as the third parameter is presented which indicates to a designer the sequence of estimation of the effect of flow depth on the equilibrium depth of scour. The effect of sediment size on the eguilibrium scour depth is presented in terms of the relative size of pier to sediment, D/d50. A family of curves, at various values of UO/UOC, which relate dav/D and D/d50 for live-bed scour was obtained. The curves show that the equilibrium scour depth increases almost linearly for increasing values of D/d50 until it reaches the value of D/d50 = 50 after which the scour depth becomes independent of D/d50. A similar trend was obtained by Ettema (1980) for clear water scour. For design purposes, the data for large values of YO/D are presented in terms of a sediment adjustment factor, K(D/d), which is shown to be independent of the flow velocity. Both flow depth and sediment size functions include results by Shen et al (1966), Ettema (1980), Chee (1982), and the present study. Armouring and sediment size play an important role in reducinq the equilibrium scour depth for non-uniform sediments. The latter is particularly significant in laboratory experiments where the size of the pier is generally small relative to the size of the coarse particles in non-uniform sediments. Experiments were conducted under dynamic equilibrium conditions where there is continuous sediment input from upstream of the scour hole such that at equilibrium, the amount of sediment entering the bridge site is equal to that leaving. Both the effects of armouring and sediment size diminish for increasing velocity. At high velocity where all the sediment particles are mobile, the non-uniform sediment behaves like a uniform sediment. Hence, armouring does not occur and the equivalent size used for sediment adjustment is based on the d50 size of the sediment bed. At low velocity, armouring at the base of the scour hole is prominent and adjustment of D/d is based on the d90 size of the original mixture. An alternative condition can exist in natural rivers in contrast to the dynamic equilibrium conditions simulated in this study. This is where the upstream river is armoured such that there is little or no sediment input to the scour hole. It is postulated that, in this case, the equilibrium scour depth can approach the maximum equilibrium scour depth for clear water conditions (i.e. dav/D + 2.3) when the approach velocity is equal to the critical velocity of the non-uniform sediment. Finally, a design flow chart is presented for estimation of the equilibrium depth of local scour for design purposes. In live-bed scour where bed features are present, the results show that half the height of the bed features can be added to the estimated equilibrium scour depth.
| Identifer | oai:union.ndltd.org:ADTP/247546 |
| Date | January 1984 |
| Creators | Chiew, Yee Meng |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
Page generated in 0.0014 seconds