Department of Civil Engineering
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Browsing Department of Civil Engineering by browse.metadata.advisor "Bosman, Adele"
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- ItemAnalysis of the Hydrodynamics of rectangular plunging Jets and the subsequent scouring in broken-up Rock Beds(Stellenbosch : Stellenbosch University, 2016-03) Calitz, Jacques; Bosman, Adele; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The scour mechanisms of rock are highly variable, equally so the geotechnical and hydrodynamic conditions of each study area, which complicate the prediction of scouring. Cases such as Kariba Dam and Ricobayo Dam are examples where, due to inadequate scour prediction methods, scouring caused major damage to the downstream riverbed. Rock scouring due to plunging jets is thus an important area of study. Various studies have been done to predict the scour hole depth and its extent using physical laboratory models and their subsequent empirical formulas. A classification method, the Erodibility Index method (EIM) by Annandale (1995), was also developed to give an indication of the depth of scouring. Physically based methods, such as the Comprehensive Scour Method (CSM) including the Quasi-steady impulsion method (QSI) by Bollaert (2002 and 2014), tried to incorporate several of the rock scour mechanisms, to determine both the depth and extent of the scour hole. The current study focused on using a 1:40 physical laboratory model to ascertain the applicability of using PVC blocks to replicate rock blocks and the subsequent scouring thereof. The drop height, as well as the tailwater, varied between the different tests. The PVC blocks were able to replicate the scour hole to a relatively good extent and could sustain steep slopes replicating the repose angle of rock. The subsequent scour holes from the physical laboratory models were compared to the depths calculated using empirical formulas and the classification method by Annandale (1995) (EIM). The methods overestimated the depths, and the EIM was found to be very sensitive to both the hydrodynamic and geotechnical boundary conditions. The physically based method proposed by Bollaert (2002) (CSM) overestimated the scour hole extent, as compared to the physical laboratory model scour hole, but the scour hole profile (shape and depth), was in agreement. The use of Computation Fluid Dynamics (CFD) to simulate hydraulic problems has become more viable, due to advancements in computational power. The hydrodynamic characteristics of the jet in the air and in the plunge pool was modelled in 2D using FLUENT, as the scour mechanisms of rock cannot presently be modelled using commercial CFD codes. The hydrodynamic conditions in the air were modelled with good comparison to both that of the physical laboratory model, as well as the calculated conditions of Stellenbosch University https://scholar.sun.ac.za -iiithe jet in the air, while the hydrodynamic conditions (velocity and pressure) in the plunge pool were overestimated in comparison to current methods available, as used in the EIM as well as the CSM and QSI methods, due to possible flow confinement and deflection effects. In conclusion, the study firstly confirmed the applicability of using PVC blocks to model scouring due to plunging jets in rock beds, and secondly it established the use of current scour prediction methods in validating small scale scour hole profiles (shape and depth). The use of 2D CFD modelling, in predicting the hydrodynamic conditions of the plunging jet in the air and plunge pool, was also introduced with relative success.
- ItemCulvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study(Stellenbosch : Stellenbosch University, 2020-03) Brooks, Johannes Andreas; Bosman, Adele; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Culverts are widely used as drainage structures to allow streams to traverse a roadway in a controlled manner. Due to the reduced flow area of a culvert, culverts can be seen as a constriction in a stream. Interference with the normal flow conditions caused by a culvert in a stream can lead to boulder deposition at the culverts. Mountainous areas have been identified as locations where the potential for boulder blockages are higher due to the hydraulically steep bed slope. Limiting the study to the Western Cape specified the boulder type that was tested, thus, naturally rounded boulders were considered. The occurrence of blockages caused by boulder accumulation at culvert sites motivated the study. Boulders will settle at the culvert entrance which reduces the available flow area through the culvert. A reduced flow area increases the risk of flooding, and in extreme cases, the roadway could be washed away. The objective of the study was therefore to investigate boulder blockages at culverts and to develop a modified culvert inlet design to mitigate boulder deposition at a culvert. Achieving the objective comprised of field research and a physical hydraulic model in the Hydraulics Laboratory of Stellenbosch University. The required slope for the incipient motion of 1 m diameter boulders was determined to be 1:25 in the laboratory setting at a prototype discharge of 28.53 m³/s. The laboratory flume had a width of 12 m (prototype). A rectangular 5 m × 2 m (B × D) culvert was selected for the experimental tests and model development. Experimental tests were conducted at a scale of 1:16 using Froude scale similitude. The inlet of the culvert was identified as the location where boulders would generally settle. A modified inlet was proposed to streamline the flow through the culvert barrel and to prevent boulder deposition in the barrel of the culvert. Three culvert inlet layouts were developed as a desktop study, of which two of the inlet layouts were tested as physical models, namely the tapered and compound tapered inlet. The tapered inlet model featured an inlet with a side-wall contraction and slope depression, with a 1:10 (V × H) slope, to increase the flow velocity through the culvert. The tapered inlet produced unstable flow conditions with a shock wave forming in the inlet just upstream of the barrel inlet. A compound tapered model was developed, featuring a side wall taper of 1:4 and a bed slope taper of 1:9.6 (V × H) to increase the flow velocity. A control point was created by the taper upstream, effectively moving the control of the barrel upstream. Experimental results indicated that the flow depth through the culvert is reduced, increasing the flow velocity. The self-scouring velocity prevented boulders from settling near the inlet of the compound tapered culvert. Boulders that settled upstream of the culvert inlet, settled out upstream of the new control section. Therefore, the compound tapered culvert inlet layout can effectively mitigate boulder deposition near the inlet and inside the culvert barrel. A two- and three-cell compound tapered model was tested. The two-cell model performed in a comparable manner to that of the single-cell model. The three-cell model caused boulders to deposit at the inlet since the upstream control point was not designed as a flow control point. The contraction between the upstream flow and the inlet lip was not sufficient to control the flow. In conclusion, the compound tapered models proved to mitigate boulder deposition at culvert entrances, if designed correctly. Design guidelines were developed in designing culvert inlets to mitigate boulder deposition.
- ItemDesign guidelines for multi-stage outlet structures situated in stormwater attenuation facilities of residential developments in South Africa: physical model investigation(Stellenbosch : Stellenbosch University, 2016-12) Myburgh, Marisa; Bosman, Adele; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Flood attenuation controls are becoming a topic of interest and are more frequently being used within urban areas of South Africa, as local authorities bring into effect stormwater policies and legislation. Another reason for the interest is the increase of urban development, which increases the impervious area within a watershed, which in return increases the run-off. This could have detrimental effects on the morphology of rivers and streams due to erosion. Literature also points out that, due to effects of climate change, the future flow, for a return period corresponding to a similar pre-development period, could increase, and stormwater ponds will then be under designed. This scenario would increase the storage volume required for detention ponds which, due to spatial constrictions, would by then be difficult or impossible to increase. The importance of accurate calculation of future discharge from a multi-stage outlet thus becomes critical. Attenuation facilities in the past were designed to control only a single recurrence interval (RI) design storm, such as the 50-year RI storm event. However, various metropolitan municipalities now instruct developers to implement on-site flood control structures, which must be capable of controlling run-off for a full range of design flows. Previous research reports concluded that multi-stage outlet structures were more effective at mimicking the pre-development flow during a range of storm events than a single outlet structure. The aim of this research was to evaluate the hydraulic performance of a multi-stage outlet structure and to determine the optimal range of recurrence interval storms which the multi-stage outlet structure was capable of meeting while providing the pre-development flow rates. A 1:3 scaled physical model was constructed in order to verify that thee theoretical equations and design guidelines recommended in literature accurately calculate the flow through multi-stage outlet structures. Six different configurations of multi-stage outlet structures were tested in the hydraulic laboratory of Stellenbosch University to evaluate the control of discharge for a wider range of scenarios. The multi-stage outlet models were designed to control the flow to pre-development peaks for inland and coastal regions receiving either 400 mm, 700 mm or 1000 mm of mean annual precipitation. It became evident from the physical model test results that multi-stage outlets consisting of discharge devices sized to control four of the RI storms (2- , 10-, 50- and 100-year), were sufficient to control all six (2- , 5-, 10-, 20- 50- and 100-year) RI storms. Thus, individual control devices were not required to control the intermediate RI storms (5- and 20-year storms), as the 2- and 10-year devices would control the outflow at the corresponding 5- and 20-year water surface elevations. Thus, designing discharge control devices to control the 2- and 10-year recurrence interval storms would shorten the iterative design process of the multi-stage outlet structure. The experimental data indicated that corrections are required to be applied to the discharge coefficient for the low flow orifice. If the value of the actual discharge coefficient is higher than the equation discharge coefficient (typically 0.61 for rectangular orifices), the outflow from the multi-stage outlet could exceed the design criteria. The experimental data was used to further develop the spreadsheet-based model and Visual Studio program for practitioners to use when determining the discharge from multi-stage outlet structures. It can be concluded that the multi-stage outlet structures were effective at mimicking the pre-development flow during a full range of storm events for inland and coastal regions. The design of multi-stage outlet structures could, therefore, help to prevent erosion of the water bodies to which they discharge.
- ItemDetermination of the critical incipient failure conditions for angular riprap dumped on wide & steep trapezoidal channels(Stellenbosch : Stellenbosch University, 2019-04) Appolus, Michael; Bosman, Adele; 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.
- ItemEstimating sanitary sewer pipeline infrastructure from basic characteristics of a service zone(Stellenbosch : Stellenbosch University, 2021-03) Winter, Jessica May; Loubser, Carlo; Bosman, Adele; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The standard detailed design and cost estimation for a sewer network involves considerable time and financial investment. There are,however, many cases where a rapid assessment of the sewer infrastructure or related costs associated with a service zone might be required. Accordingly, there have been numerous approaches to rapid sewer infrastructure assessment in the literature, ranging from the automated generation of entire sewer network plans to direct cost estimation methods. Yet,to date, no widelyavailable tool has been developed thatcan be applied to reliably estimate the expected sewer pipeline infrastructure associated with aservice zone in South Africa.The main aim of this study was therefore to develop a method for estimating the sewer pipeline infrastructure required for a service zone, based on limited information, thatcould be applied toboth existing and future developments. In order toachieve the stated aim, a database of South African sewer network data was used in the development of three major study outcomes. StudyOutcomeI involved developing multiple linear regression models for estimating the total sewer pipeline length for a service zone using only basic service zone characteristics. StudyOutcomeII involved determining the average pipeline diameter distributions for different types of service zones, by which the total pipeline length could be disaggregatedinto lengths per diameter. StudyOutcomeIII involved determining the average number of manholes per kilometre of sewer pipeline for different types of service zones, by which the total number of manholes for a service zone could be determined.To satisfy Study Outcome I, models were developedfor nine different categories of land use and area sizeusing weighted least squares regression. The modelsallowed for estimation ofthe total pipeline length as a function ofthree variables, namelythe service zone area size, relief (in terms of the difference between the mean elevation and the expected elevation of the network endpoint), and the density of contributing users (in terms of the number of unit hydrographs per hectare). The model strengths ranged fromvery good to moderate, with average percentage errors in the order of 10–35%.To satisfy Study Outcome II, pipeline diameter distributions were developed for 17 different categories of land use, area size and relief, which showed that the proportion of pipes with diameter ≤160 mm was always at least 90% for residential service zones,and at least 70% for non-residential service zones. To satisfy Study Outcome III, the average number of manholes per kilometre of pipeline was determined for six different categories of land use and area size,which yieldedan average manhole distribution in the order of 20 manholes/km.Combined, the three study outcomes form an infrastructure estimation tool that enablesreasonably reliableestimation of the sewer pipeline length perapproximatediameter and the number of manholes associated with a service zone, applicable to service zones on a development scale smaller than 450 hectares.
- ItemIncipient motion of Armorflex articulating concrete blocks on steep slopes(Stellenbosch : Stellenbosch University, 2019-12) Delport, Kobus; Basson, G. R.; Bosman, Adele; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Armorflex is an articulating concrete block erosion protection measure that has been used as an alternative to riprap (dumped rock) for many years. Even though extensive research and hydraulic testing have been conducted on Armorflex, the principal constraint on the use of concrete blocks has been the lack of information on prototype performance. Furthermore, there are no standards for Armorflex or articulating concrete block revetments in SANS. The aim of this study is to improve the understanding of the critical flow conditions under which Armorflex blocks are lifted up and removed by flowing water in open channel flow applications. Armorflex 140 and Armorflex 180 blocks are studied in particular. Liu’s theory (1957) of incipient motion is of primary interest and is applied in an attempt to define the point where block movement is initiated. Scaled laboratory tests were conducted to determine whether Liu’s theory holds for Armorflex blocks. For particle Reynolds numbers between 11025 and 131397, the results from the study indicate respective Movability Numbers of 0.249 and 0.220 for Armorflex 140 and 180 installed on bed slopes. Dimensionless stability factors of 1.47 and 1.33 can respectively be applied to Armorflex 140 and 180 blocks installed on side slopes. The results from the laboratory tests were compared with the manufacturer design guidelines of Technicrete (2016) and Contech Construction Products inc. (Armortec Incorporated, 1981). Technicrete (2016) provides a maximum desired slope of 1V:1.5H and limiting flow velocities only, while Contech Construction Products inc. (Armortec Incorporated, 1981) includes flow velocities and hydraulic radius at varying bed slopes as limiting parameters. The results proposed that Technicrete’s respective limiting flow velocities of 3.5 m/s and 5.5 m/s for Armorflex 140 and 180 blocks may be an overestimation for blocks installed on bed slopes. On side slopes, however, failure was observed at flow velocities similar to the limits stated by Technicrete. Comparing the laboratory findings to the limiting velocity guideline of Contech Construction Products inc. (Armortec Incorporated, 1981), no block failures were achieved at flow velocities lower than the design guideline velocities. According to incipient motion theory, however, flow velocity is not a suitable parameter for defining incipient motion. Therefore, the limiting flow velocity guidelines of Technicrete (2016) and Contech Construction Products inc. (Armortec Incorporated, 1981) alone cannot be used to design Armorflex lined structures in practice. Instead, this thesis recommends the use of Liu’s Movability Number to determine the point of incipient motion of Armorflex. The results from the study were used to develop a Microsoft Excel model for the safe design of Armorflex-lined drainage channels. The Movability Numbers of Armorflex blocks obtained in this study are greater than the Movability Numbers recommended by researchers for riprap (dumped rock) and Renomattresses. Unlike riprap and Reno-mattresses, Armorflex has no particles smaller than the design weight that can be washed away by forces of flowing water, undermining the larger particles. The Movability Numbers presented in this thesis are also greater than Rooseboom & van Vuuren's (2013) recommended Movability Number of 0.12 for articulating concrete blocks.
- ItemPhysical Modelling Investigation of Rock Scour Extent Due to a Plunging Jet for Typical High Head Dams(Stellenbosch : Stellenbosch University, 2016-03) Umumararungu, Marie Grace; Bosman, Adele; Stellenbosch Universiity. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Hydraulic structures can release high-velocity plunging jets, which can result in scouring of the rock downstream of the structure. The extent of the scour hole must be predicted and analysed to ensure the safety of hydraulic structures. Many researches had developed empirical or semi-empirical formulae based on physical or prototype assessment. These formulae have limitations in estimating the scour hole shape, i.e. determining the pressure propagation in fissure or joints of rock and the velocity of the jet as it travels through the plunge pool. A 1:40 scaled physical hydraulic model was constructed to determine the scour hole geometry in rock material. The main objective of the study was to determine the scour hole geometry formed in rock as a result of to an impinging jet by using a physical model and analysing the factors that cause scouring. The tests were conducted in two parts, namely the assessment of the scour hole geometry and secondly the measurement of the pressures inside the rock joints and the air concentration in the plunge pool. In this research, the Erodibility Index developed by Annandale(1995) was used to quantify the relative ability of rock to resist the scour capacity of water. The Comprehensive Scour Model developed by Bollaert (2002; 2010; 2012) was also used to evaluate the ultimate scour depth. The physical model data was obtained in the laboratory at Stellenbosch University, South Africa, and were compared to the predicted results based on scour prediction methods found in the literature, namely the Comprehensive Scour model (Dynamic Impulsion and Quasi Steady Impulsion methods) and the Erodibility Index method. There was a reasonable difference between the physical model results and the prediction method results. The differences could be readily attributed to limitations of the laboratory (available pumping capacity, physical model scale and the density of the PVC blocks used for replicating rock blocks was lower than that of rock).
- ItemProtection of river embankments downstream of low-level river crossings using stepped chute energy dissipation structures(Stellenbosch : Stellenbosch University, 2019-04) Cloete, Pierre Lourens; Bosman, Adele; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Low-level river crossings (LLRCs) have been used as an economic means of access for lower order roads. LLRCs typically provide openings underneath the prepared surface to allow for passing flow but additionally allow for overtopping flow. However, when the LLRC overflows, the portion of flow on the approach roads tends to accelerate and discharges on the embankment directly downstream of it. This causes erosion of the downstream embankment, exposing the approach road foundations and damaging the structure. Following such a case where ten LLRCs in the Eastern Cape sustained tremendous damage, the use of a stepped chute energy dissipator (rather than the traditional means of erosion protection with riprap) was considered to protect the downstream embankments. No formal design guidelines for the use of a stepped chute energy dissipator downstream of a LLRC were available. A 1:15 scale hydraulic model of an LLRC with a stepped chute energy dissipator was designed and constructed at the Hydraulic Laboratory of Stellenbosch University to verify anticipated hydraulic operation and to identify unforeseen potential phenomena. Two model configurations were considered. The first configuration included a chute sidewall to contain approach road overflow on the stepped chute. Flow from the road formed a nappe that impinged on the chute steps and formed a hydraulic jump on each step. Flow was then diverted down the chute, and steps in the direction of the chute further dissipated energy while returning the flow to the main channel. The second model configuration omitted the chute sidewall and discharged approach road overflow onto riprap, placed directly downstream of the chute. Flow from the approach road formed a nappe that impinged on the chute steps. The formation of hydraulic jumps on each step was, however, not as effective as with the first model configuration and instances of supercritical discharge onto the riprap were noted. Pressures of the nappe cavity (forming from the approach road onto the chute steps) were found to be sub atmospheric. However, the magnitude of the sub-atmospheric conditions (less than 1 m below atmosphere) was not enough to cause a collapsed/clinging nappe and was also found to be within the cavitation threshold (7 m below atmosphere). Ventilation of the nappe due to negative pressure indicated that an increase in overflow discharge caused an increase in the air requirement. The addition of air vents however, did not have a marked effect on the nappe pressure or the flow profiles on the stepped chute in either one of the model configurations. The use of a stepped chute energy dissipator to prevent erosion of downstream LLRC embankments is a viable option, particularly in cases where large rock for riprap revetments are not available. The first model configuration (stepped chute with sidewall) is a more favourable option as the approach road overflow is contained on the stepped chute and diverted back to the main channel in a controlled manner. A regression analysis was performed on the model variables of model setup 1 and the relationships can be used to determine the width, height and length of the chute steps.
- ItemSanitation in South Africa(Stellenbosch : Stellenbosch University, 2021-03) Benecke, Nicholas; Loubser, Carlo; Bosman, Adele; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The state of South African infrastructures is in decline, yet the degree of decline is neither well quantified nor well understo od. Specific to the infra structura l field of sanitation in South Africa, a framework of data analysis and uncertainty analysis was developed in order that logical arguments could be deduced. Due to South African specific conditions which have radically altered the social and infrastructural systems of the country, a holistic coverage of this topic does not exist in literature. Through presentation and analysis of macro-patterns of decay in the nation’s most efficacious sanitation system (Cape Town), coupled with a technical analysis of a key macro-pattern (mechanics of sedimentation) a concise argument is presented in an attempt to define an otherwise indistinct problem. This argument was formulated through case study analyses in combination with an exhaustive uncertainty analysis of the theory of the mechanics of sedimentation. Through the application of Systems Thinking and Theory as well as complex analytics, this research posits that sanitation systems in South Africa are experiencing heavy duress and are likely failing to deliver requisite efficacy.