Determination of the critical incipient failure conditions for angular riprap dumped on wide & steep trapezoidal channels

Appolus, Michael (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

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.

AFRIKAANSE OPSOMMING: Die hoofoogmerk van hierdie verhandeling is om die kritiese MN vas te stel wat die beginweieringtoetande van hoekige stortklip, gestort op breë en steil trapesoïedkanale, bepaal. Altesaam 32 fisiese hidrouliesemodeltoetse is in drie toetsreekse uitgevoer. Sewe toetse is in Toetsreeks een uitgevoer, terwyl 15 toetse in Toetsreeks twee en 10 toetse in Toetsreeks drie uitgevoer is. Die toetse is uitgevoer deur vloeitempo’s geleidelik oor die hidrouliese model te verhoog om die vloeitempo, wat die begin van klipstorting in ’n spesifieke hidrouliesemodel-opset bewerkstellig, vas te tel en op te teken. Weiering is gedefinieer as die vloeitempo wat ’n beduidende beweging van stortklippe van minder as en gelyk aan D50 bewerkstellig het. Gegrond op die fisiese modeltoetse in hierdie verhandeling is daar bevind dat, vir klipstorting op die bedding van ’n relatief breë trapesoïedkanaal (bodembreedte-tot-D50-verhouding van 16 tot 31) en steil beddinghellings (van 0.333-0.5), die kritiese MN-waarde, wat die beginweieringtoestande vir hierdie steil beddinghellings bepaal, 0.12 is, met ’n oorskrydingswaarskynlikheid van 95%. Hierdie MN-waarde stem ooreen met Rooseboom (1992) se MN-kriteria van 0.12. Verder is daar bevind die MN vir die bepaling van die kritiese beginweieringtoestand van klipstorting op ’n 0,4-steil sywalhelling is 0.227, met ’n oorskrydingswaarskynlikheid van 95%. Gebaseer op die HEC-RAS-program se numeriese simulasies van die bestendigetoestandvloei in die fisiese modeltoetsreeks wat vir hierdie verhandeling uitgevoer is, is daar bevind die HEC-RAS-program oorskat die werklike beginweiering-MN. Die HEC-RAS-program oorskat die kritiese beginweiering-MN van die steil bedding en die steil sywal met ’n kritiese faktor van onderskeidelik 1.91 en 1.35. Gevolglik word dié twee faktore aanbeveel as die MN-aanpassingsfaktore (die steilbedding- en sywal-MN moet tot MN-waardes van onderskeidelik 0.12 en 0.277 aangepas word) vir die bepaling van die beginweiering van ’n spesifieke D50-rotsgrootte wanneer die HEC-RAS-bestendigetoestandvloeiontleding gebruik word. Laastens is die toepaslikheid van hierdie studiebevindings beperk tot stortklip wat in reguit trapesoïeddwarssnitkanale met steil beddings, wat van 0.333 tot 0.5 wissel en met sywalhellings van 0.4, gestort word. Die skaal van die hidroulies-fisiese model wat in die studie gebruik is, is relatief groot gekies, dus 1:15 om die skaaleffekte van die model te verminder. Die model D50-grootte was 0.038 m en 0.075 m, wat prototipe-klipgroottes met D50 tussen onderskeidelik 0.57 m en 1.125 m verteenwoordig. Die resultate van die studie is dus slegs geldig vir die ontwerp van prototipe D50-klipgrootte tussen klipgrootte van 0.57 m en 1.125 m. Veral van belang is dat die beddingbodembreedte-tot-D50-verhouding tussen 16-31 moet wees.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106238
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