Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by Author "Appolus, Michael"

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    Determination of the critical incipient failure conditions for angular riprap dumped on wide & steep trapezoidal channels
    (Stellenbosch : Stellenbosch University, 2019-04) Appolus, Michael; Bosman, Adèle ; Basson, G. R.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The main objective of this thesis was to determine the critical MN that defines the incipient failure conditions of angular riprap dumped on wide and steep trapezoidal channels. A total of 32 physical hydraulic model tests were performed in three test series. There were 7 tests performed for Test series one, 15 tests performed on Test series two and 10 tests were performed on Test series three. The tests were executed by gradually increasing flow rates over the hydraulic model to enable establishment and recording of the flow rate that induced incipience of riprap for a specific hydraulic model setup. Failure was defined as the flow rate that instigated a significant movement of riprap stones less and equal to D50. Based on the physical model tests of this thesis it was found that for the riprap on the bed of a relatively wide trapezoidal channel (bottom width to D50 ratio of 16 to 31) and steep bed slopes (of 0.333-0.5), the critical MN value defining the incipient failure conditions for these steep bed slopes was 0.12 with an exceedance probability of 95%. This MN value is in good agreement with Rooseboom’s (1992) MN criteria of 0.12. In addition, the MN for defining the critical incipient failure condition of riprap on a 0.4 steep side bank slope was found to be 0.227, with an exceedance probability of 95%. Based on the HEC-RAS steady state flow numerical simulations of the physical model tests series performed in this thesis, it was found that HEC-RAS overestimates the actual incipient failure MN. HEC-RAS overestimated the critical incipient failure MN of the steep bed and steep side bank by a critical factor of 1.91 and 1.35, respectively. As a result, the two factors were recommended as the MN adjustment factors (the steep bed and side bank MN must be adjusted to MN values of 0.12 and 0.227, respectively) for defining the incipient failure of a specific D50 rock size when using HEC-RAS steady state flow analysis. Lastly, the applicability of the findings of this study are limited to riprap dumped in straight trapezoidal cross-sectional channels with steep beds ranging from 0.333 to 0.5 and with side bank slopes of 0.4. The scale of the hydraulic physical model used in the investigation was selected relatively large i.e. 1:15 to minimize model scale effects. The model D50 size was 0.038 m and 0.075 m which represent prototype stone sizes with D50 between 0.57 m and 1.125 m respectively. The results of the study are therefore only valid for the design of prototype D50 stone size between stone 0.57 m and 1.125 m. Most importantly, the bed bottom width to D50 ratio needs to be between 16-31.