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Impact of surface roughness on flow dynamics over ogee spillways
Thesis (MEng)--Stellenbosch University, 2025.
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| Language: | English |
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Stellenbosch : Stellenbosch University
2025
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| _version_ | 1867613844819410944 |
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| access_status_str | Open Access |
| author | Groenewald, Janneke |
| author2 | Bosman, Adèle |
| author_browse | Bosman, Adèle Groenewald, Janneke |
| author_facet | Bosman, Adèle Groenewald, Janneke |
| author_sort | Groenewald, Janneke |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (MEng)--Stellenbosch University, 2025. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/134626 |
| institution | Stellenbosch University (South Africa) |
| language | English |
| last_indexed | 2026-06-10T12:42:35.472Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2025 |
| publishDateRange | 2025 |
| publishDateSort | 2025 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/134626 Impact of surface roughness on flow dynamics over ogee spillways Groenewald, Janneke Bosman, Adèle Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. Spillways -- Design and construction Computational fluid dynamics Turbulence Surface roughness Open channel flow Thesis (MEng)--Stellenbosch University, 2025. Groenewald, J. 2025. Impact of Surface Roughness on Flow Dynamics over Ogee Spillways. Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/41c37863-a7cc-4cc1-b55b-c7a3254d4790 ENGLISH ABSTRACT: Spillways are essential hydraulic structures that protect dams from overtopping by safely conveying excess water downstream. Among these, the ogee spillway is a widely implemented structure due to its hydraulic efficiency and stability. Despite their proven effectiveness, the long-term performance of ogee spillways can deteriorate due to surface degradation over time, leading to increased surface roughness. This can significantly affect flow behaviour, introducing risks such as cavitation, air entrainment, and flow separation – each of which may compromise structural integrity. The research is motivated by the need to better understand how operational wear and environmental exposure impact spillway safety and effectiveness. As surface roughness increases, flow conditions can shift unpredictably, leading to elevated risks of structural damage. This research addresses the impact of increased surface roughness, due to wear and ageing, on the hydraulic performance and safety of ogee spillways. A combined experimental and numerical research design was used. This research investigates the influence of uniform surface roughness heights on the hydraulic performance of an ungated, concrete ogee spillway with a standard United States Bureau of Reclamation (USBR) crest profile. It excludes adjacent energy dissipating structures, focusing solely on flow behaviour along the spillway face. Flow behaviour was analysed using a physical model tested in a laboratory flume and a numerical model developed in FLOW-3D HYDRO. Both were tested under five discharges, namely 0.072 m³/s, 0.105 m³/s, 0.222 m³/s, 0.447 m³/s, and 0.503 m³/s, and five roughness heights, smooth (0 mm), 0.15 mm, 0.6 mm, 0.9 mm and 1.18 mm. The numerical model was calibrated against the experimental results to ensure accuracy. Both model results were analysed using multiple linear regression and revealed that surface roughness influenced hydraulic behaviour across the ogee spillway profile. Although many observed changes were statistically significant at selected locations, the overall hydraulic impact was minor – suggesting that while roughness, as a proxy for prolonged, natural surface degradation, should be considered in the context of long-term maintenance and material degradation, its influence is not substantial enough to warrant prioritisation in initial design compared with discharge and other governing parameters. Increased roughness consistently reduced velocities in both models, reaching up to a 17% velocity decrease in the numerical model. Both the physical and numerical models showed that discharge decreased pressures upstream and increased downstream. Roughness generally increased pressures, but this increase was statistically significant only near the spillway toe, and the absolute change was very small (≈0.02% along the profile). Increased roughness generally raised the cavitation index (s), reducing cavitation risk. However, this protective effect is statistically significant only at selected mid-chute locations. Notably, the cavitation index never fell below the risk threshold of 0.02, only reaching a minimum of 8.79 near the toe region. Flow separation risk is greatest near the crest under increased roughness and lower Reynolds numbers, but decreases downstream where higher 𝑅𝑒 values, stronger turbulence, and bulk acceleration help the flow remain attached. However, 𝑅𝑒 remained above values typically associated with laminar or transitional separation risk, reaching a minimum 𝑅𝑒 of ≈10 219. Air entrainment behaviour varied between the models. In the numerical model, the inception point moved downstream with increasing discharge, and higher roughness enhanced aeration, consistent with the literature. In contrast, the physical model showed reduced aeration with increased roughness and the highest aeration near the crest, indicating that the physical response warrants further investigation. However, it must be emphasised that the physical model’s air entrainment data were subject to significant measurement uncertainty due to probe submergence limitations, local surface disturbances, and scale effects. These results were therefore used qualitatively and excluded from numerical model calibration and validation. The trends reported from the numerical model are thus considered more reliable for interpreting aeration behaviour. Overall, the findings highlight that roughness is not merely a sign of deterioration but can, in some contexts, offer hydraulic advantages when strategically applied. AFRIKAANSE OPSOMMING: Oorlope is noodsaaklike hidrouliese strukture wat damme teen oorstroming beskerm deur oortollige water veilig stroomaf te lei. Onder hierdie strukture is die ogee-oorloop een van die mees algemeen toegepaste tipes weens sy hidrouliese doeltreffendheid en stabiliteit. Ondanks hul doeltreffendheid, kan die langtermynprestasie van ogee-oorlope agteruitgaan as gevolg van oppervlakverweer en slytasie oor tyd, wat lei tot verhoogde oppervlak-ruheid. Dit kan die vloeigedrag beduidend beïnvloed, met risiko’s soos kavitasie, luginsluiting en vloeiskeiding – almal faktore wat die strukturele integriteit kan benadeel. Hierdie studie ondersoek hoe verhoogde oppervlak-ruheid, wat veroorsaak word deur slytasie en verwering, die hidrouliese prestasie en veiligheid van ogee-oorlope beïnvloed. Die navorsing is gemotiveer deur die behoefte om die impak van slytasie op die veiligheid en doeltreffendheid van oorloopoppervlaktes beter te verstaan. Soos oppervlak-ruheid toeneem, kan vloeitoestande onvoorspelbaar verander en die risiko van strukturele skade verhoog. ‘n Gekombineerde eksperimentele en numeriese navorsingsontwerp is gebruik. Hierdie studie ondersoek die invloed van eenvormige oppervlak-ruheidshoogtes op die hidrouliese doeltreffenheid van ‘n onbeheerde, beton ogee-oorloop met ‘n standaard Verenigde State Buro vir Reklamasie (USBR) kroonprofiel. Energie-dissiperende strukture is uitgesluit, aangesien die fokus slegs is op die gedrag van die vloei oor die oorloop-oppervlak. Vloei is ontleed met behulp van ‘n fisiese model getoets in ‘n laboratoriumkanaal, en ‘n numeriese model opgestel in FLOW-3D HYDRO. Beide modelle is getoets onder vyf vloeie, naamlik 0.072 m3/s 0.105 m3/s, 0.222 m3/s, 0.447 m3/s en 0.503 m3/s, en vyf ruheidstoestande, glad (0 mm), 0.15 mm, 0.6 mm, 0.9 mm en 1.18 mm. Die numeriese model is met die 0.503 m3/s vloei teenoor die eksperimentele resultate gekalibreer. Die resultate van die gekalibreerde numeriese model is met lineêre regressie ontleed en het getoon dat oppervlakruheid die hidrouliese gedrag oor die ogee-oorloopprofiel beïnvloed. Alhoewel die effekte statisties beduidend was, was die algehele hidrouliese impak gering. Dit dui daarop dui dat ruheid, wat voortspruit uit natuurlike, langdurige agteruitgang, wel in ag geneem moet word in terme van langtermyn instandhouding en materiaalverwering, maar dat die invloed nie groot genoeg is om bo ander bepalende faktore in ontwerp geprioritiseer moet word nie. Verhoogde ruheid het konsekwent die numerise model se vloeisnelhede met ongeveer 17% verlaag oor die oorloop. Beide die fisiese en numerise modelle se druk het naby aan die kruin verlaag en nader aan die toon verhoog soos die vloei toegeenm het. Ruheid het die druk oorwegend verhoog, maar dit was slegs beduidend naby die toon, met ’n gemiddelde persentasieverhoging van 0.02%. Ruheidverhoging het meestal die kavitasie-indeks (𝜎) laat toeneem en dus die risiko van kavitasie verminder, hoewel hierdie beskermende effek net in die middelgedeelte van die oorloop statisties beduidend was. Noemenswaardig is dat die kavitasie-indeks nooit onder die risikodrempel van 0.02 gedaal het nie, en ’n minimum van 8.79 naby die toon bereik het. Die risiko van vloei skeiding is die hoogste naby die kruin onder verhoogde rofheid en lae Reynoldsgetalle, maar afneem stroomaf waar hoër 𝑅𝑒 waardes, sterker turbulensie en versnelling die vloei help om aan die oppervlakte geheg te bly. Die Reynoldsgetal het nie die oorgangs-laminêre vloei drempel vir verhoogde vloeiskeidingsrisiko bereik nie, en slegs ’n minimum waarde van 10 219 getoon. Luginsluiting het verskil tussen die modelle. Die luginsluiting oorsprongpunt het stroomaf beweeg met toenemende vloei in die numeriese model, en hoër ruheid het die luginsluiting versterk, wat ooreenstem met die literatuur. Daarenteen het die fisiese model ‘n afname in luginsluiting met toenemende ruheid getoon, en die hoogste luginsluiting was naby die kruin, wat aandui dat die fisiese reaksie verdere ondersoek benodig. Dit moet beklemtoon word dat die luginsluitingsdata van die fisiese model aan beduidende meetonsekerheid onderhewig was as gevolg van beperkings in die onderdompeling van die sonde, plaaslike oppervlakversteurings en skaaleffekte. Hierdie resultate is dus slegs kwalitatief gebruik en is uitgesluit van die kalibrasie en validering van die numeriese model. Die tendense wat deur die numeriese model gerapporteer is, word gevolglik as meer betroubaar beskou vir die interpretasie van luginasiegedrag. In algeheel beklemtoon die bevindings dat ruheid nie bloot ’n teken van agteruitgang is nie, maar hidrouliese voordele kan inhou wanneer dit strategies toegepas word. Masters 2025-12-19T08:35:39Z 2025-12-19T08:35:39Z 2025-12 Thesis https://scholar.sun.ac.za/handle/10019.1/134626 en Stellenbosch University xxv, 203 pages : illustrations application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Spillways -- Design and construction Computational fluid dynamics Turbulence Surface roughness Open channel flow Groenewald, Janneke Impact of surface roughness on flow dynamics over ogee spillways |
| title | Impact of surface roughness on flow dynamics over ogee spillways |
| title_full | Impact of surface roughness on flow dynamics over ogee spillways |
| title_fullStr | Impact of surface roughness on flow dynamics over ogee spillways |
| title_full_unstemmed | Impact of surface roughness on flow dynamics over ogee spillways |
| title_short | Impact of surface roughness on flow dynamics over ogee spillways |
| title_sort | impact of surface roughness on flow dynamics over ogee spillways |
| topic | Spillways -- Design and construction Computational fluid dynamics Turbulence Surface roughness Open channel flow |
| url | https://scholar.sun.ac.za/handle/10019.1/134626 |
| work_keys_str_mv | AT groenewaldjanneke impactofsurfaceroughnessonflowdynamicsoverogeespillways |