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Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study
Thesis (MEng)--Stellenbosch University, 2020.
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| Format: | Thesis |
| Language: | English |
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Stellenbosch : Stellenbosch University
2020
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| _version_ | 1867613875757645824 |
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| access_status_str | Open Access |
| author | Brooks, Johannes Andreas |
| author2 | Bosman, Adèle
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| author_browse | Bosman, Adèle
Brooks, Johannes Andreas |
| author_facet | Bosman, Adèle
Brooks, Johannes Andreas |
| author_sort | Brooks, Johannes Andreas |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (MEng)--Stellenbosch University, 2020. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/108286 |
| institution | Stellenbosch University (South Africa) |
| language | English |
| last_indexed | 2026-06-10T12:43:06.129Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2020 |
| publishDateRange | 2020 |
| publishDateSort | 2020 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/108286 Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study Brooks, Johannes Andreas Bosman, Adèle Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. Culverts -- Design and construction Boulders -- South Africa -- Western Cape Culvert blockage Drainage Streamflow -- Management Hydraulic engineering UCTD Thesis (MEng)--Stellenbosch University, 2020. 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. AFRIKAANSE OPSOMMING: Padduikers dien as belangrike strukture om dreinering te bied vir strome onder deur die pad oppervlak. Die nadeel van kasduikers is dat die strukuur ‘n obstruksie in die vloei van die water is. As gevolg van die obstruksie, neem die vloei snelheid af wanneer die vloei die duiker nader. Die opdamming en stadige vloei veroorsaak dat klippe uitsak en die kasduiker toe spoel. Bergagtigegebiede bied die benodige hidroulliese styl helling om klip beweging in die rivier aan te moedig. Die studie het gefokus rondom die berggebiede van die Wes-kaap, die gevolg hiervan is dat natuurlike geronde klip vir die eksperimentele toetse gebruik is. Die motivering vir die studie was dus gegrond op die toespoel van kasduikers as gevolg van klippe wat stroomaf spoel. Vloeiarea van die kasduiker word verminder deur die klippe wat die inlaat versper, wat tot ‘n hoër risiko van oorstroming kan lei, en in sekere gevalle kan die vloei die pad ook weg spoel. Die doelstelling van die studie was dus om klip blokasies by kasduikers te ondersoek en te bepaal hoe die klip by die kasduiker uitsak. ‘n Metode moes ontwikkel word om die blokkasies deur klippe by kasduikers te verhoed. Om die doelstelling van die studie te bereik was gebruik gemaak van veldwerk om die probleem te ondersoek en ‘n fisiese model in die Hidrouliese Laboratorium van Stellenbosch Universiteit. Tydens die eksperimentele toetse is daar bepaal dat vir ‘n prototipe ontwerpdeurstroming van 28.53 m³/s, ‘n 1 m diameter klip teen ‘n helling van 1:25 weggespoel kan word. Die wydte van die opstelling was 12 m wyd (prototipe). ‘n 5 × 2 m (B × D) kasduiker is gekies vir die experimentele toetse teen ‘n model skaal van 1:16, deur ‘n Froude skaal te gebruik. Tydens die toetse is daar bepaal dat die klippe by die inlaat van die kasduiker uitsak. Daar is dus gemotiveer om ‘n inlaat te ontwikkel wat die vloei van die water belyn met die inlaat van die duiker om sodoende die vloei te versnel en klip uitsakking te verminder. Drie kasduiker inlaat uitlegte is ontwikkel, waarvan twee getoets is in die laboratorium as fisiese modelle, naamlik ʼn vernouing model en saamgestelde vernouing model. ‘n Vernouing model met ‘n vernouing aan die kant mure in die rigting van die inlaat, asook ‘n vloer wat na onder sak om die vloei te versnel is getoets. Die vernouing inlaat het onstabiele vloei stroomop van die kasduiker inlaat veroorsaak in die vorm van ‘n skokgolf. Die tweede model, die saamgestelde vernouing model, het soortegelyke vernoude kant-mure gehad, maar teen ‘n verhouding van 1:4, die vloer het ook ʼn styl helling van 1:9.6 (V × H) om die vloei te versnel. Addisioneel het die tweede model ‘n gedefinieerde kontrole punt sroomop van die inlaat gehad, met die idee dat geen klip stroomaf van die kontrole punt, mag uitsak nie. Die resultate het aangedui dat die vloeidiepte deur die saamgestelde vernouing model verlaag, wat dus die vloei versnel het. Die self-uitskuur vermoë van die vinnige vloei het verhoed dat klippe in die duiker uitsak. Klippe wat wel uitgesak het, het uitgesak stroomop of by die nuwe kontrole punt. Dit was dus bepaal dat die saamgestelde vernouinging inlaat uitleg verhoed dat die uitsak van klippe naby die inlaat en binne die kasduiker plaasvind. ‘n Twee-opening en drie opening saamgestlede vernouingduiker was ook getoets. Die twee-opening model het op die selfde wyse as die enkel duiker model gefunksioneer. Die drie-inlaat kasduider het getoon dat die kontrole punt goed gedefinieer moet word om die uitsak van klippe in die inlaat te verhoed. Daar was dus bevind dat die saamgestelde vernouingmodel goed werk om klip deponeering te verhoed by die inlaat van ʼn kasduiker, mits die kontrole punt deeglik ontwerp word. Ontwerpriglyne was ontwikkel vir die ontwerp van kasduiker inlate om klip deponeering te verhoed. Masters 2020-02-26T07:19:23Z 2020-04-28T12:30:04Z 2020-02-26T07:19:23Z 2020-04-28T12:30:04Z 2020-03 Thesis http://hdl.handle.net/10019.1/108286 en Stellenbosch University xxii, 185 leaves : illustrations (some color) application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Culverts -- Design and construction Boulders -- South Africa -- Western Cape Culvert blockage Drainage Streamflow -- Management Hydraulic engineering UCTD Brooks, Johannes Andreas Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study |
| title | Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study |
| title_full | Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study |
| title_fullStr | Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study |
| title_full_unstemmed | Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study |
| title_short | Culvert blockage caused by boulders in the Western Cape and the development of mitigation measures : physical model study |
| title_sort | culvert blockage caused by boulders in the western cape and the development of mitigation measures physical model study |
| topic | Culverts -- Design and construction Boulders -- South Africa -- Western Cape Culvert blockage Drainage Streamflow -- Management Hydraulic engineering UCTD |
| url | http://hdl.handle.net/10019.1/108286 |
| work_keys_str_mv | AT brooksjohannesandreas culvertblockagecausedbybouldersinthewesterncapeandthedevelopmentofmitigationmeasuresphysicalmodelstudy |