Design guidelines for multi-stage outlet structures situated in stormwater attenuation facilities of residential developments in South Africa: physical model investigation

Myburgh, Marisa (2016-12)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

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.

AFRIKAANSE OPSOMMING: Vloedbeheermaatreëls vir stedelike gebiede in Suid-Afrika raak al hoe meer van belang omdat plaaslike owerhede se stormwaterbeleide dit vereis. ‘n Verdere rede vir belangstelling in vloedbeheermaatreëls is die toename in stedelike ontwikkeling, wat die indringbaarheid van stormwater in ‘n afloopgebied beïnvloed en die afloop vermeerder. Bogenoemde aspekte kan ‘n wesenlike invloed uitoefen op die morfologie en erosie van strome en riviere. Literatuur dui aan dat as gevolg van die effek van klimaatsverandering, kan die voor-ontwikkelingsvloei wat ooreenstem met soortgelyke herhaalperiodes in die toekoms, toeneem en stormwater vloedvertragingsdamme sal gevolglik onderontwerp word, tensy ingenieurs se bewustheid van veranderende omstandighede tred hou daarmee. Dié aspek sal aanleiding gee tot die genoodsaakte verhoging van die stoorkapasiteit van vloedvertragingsdamme. As gevolg van ruimtelike beperkings, sal die belangrikheid van akkurate berekening van die uitvloeie vir multi-fase uitlaatstrukture van kritieke belang wees. In die verlede is vloedvertragingsfasiliteite ontwerp om slegs ‘n enkele herhaalperiode (RI) ontwerpstorm, soos die 1:50 jaar storm, te demp. Verskeie metropolitaanse owerhede vereis tans van ontwikkelaars om op-terrein vloedbeheerstrukture vir verskillende ontwerpvloeie te voorsien. Vorige navorsing het getoon dat vloedvertragingsfasiliteite wat ontwerp word met multi-fase uitlaatstrukture, meer effektief is vir die nabootsing van voor-ontwikkeling vloeie. Die ontwerp van hierdie multi-fase uitlaatstrukture kan dus help in die voorkoming van erosie. Die doel van hierdie navorsing was om die hidrouliese werking van ‘n multi-fase uitlaatstruktuur te ondersoek, om te bepaal of die multi-fase uitlaatstruktuur die uitvloei van die vloedbeheerfasiliteit kan beperk vir storms van verskeie herhaalperiodes. Dit is belangrik om die kwantiteit van die stormwater so akkuraat as moontlik te beheer. Gevolglik is ‘n fisiese modelstudie onderneem, op ‘n 1:3 skaal, om te bepaal of die teoretiese vergelykings en ontwerphandleidings soos aanbeveel in die literatuur, gebruik kan word om die vloei deur multi-fase uitlaatstrukture akkuraat te kan bepaal. Ses verskillende konfigurasies van die multi-fase uitlaatstruktuur is getoets in Stellenbosch Universiteit se hidrouliese laboratorium om die beheer van die uitvloei vir ‘n wye reeks scenarios te evalueer. Uit die fisiese model se toetsresultate blyk dit duidelik dat multi-uitlate, bestaande uit uitlaatstrukture wat ontwerp is om storms van vier verskillende herhaalperiodes te beheer, voldoende was om die storms van al ses herhaalperiodes te beheer. Daarom is individuele vloeibeheerstrukture om die intermediêre herhaal periode storms (d.i. 5- en 20-jaar storms) te beheer, onnodig, want as dit korrek ontwerp is, sal die multi-fase uitlaat die uitvloei van die ooreenstemmende 5- en 20-jaar watervlak hoogtes kontroleer. Dit verkort dus die iteratiewe ontwerpproses van die multi-fase uitlaatstruktuur. Die eksperimentele data dui ook aan dat veranderings benodig word ten opsigte van die vloei koëffisiënt vir die gaatjiesplaat wat ontwerp is vir die 2-jaar herhaalperiode storm. Indien die werklike waarde van die vloei koëffisiënt hoër is as die teoretiese vloei koëffisiënt (tipies 0.61 vir reghoekige ontwerpe) bestaan die moontlikheid dat die uitvloei van die multi-fase uitlaat die plaaslike owerhede se vereistes sal oorskry. Die eksperimentele data is gebruik vir die verdere ontwikkeling van ‘n Excel sigblad gebaseerde model, asook ‘n Visual Studio program, wat gebruik kan word vir die bepaling van die uitvloei vanaf multi-fase uitlaatstrukture. Dit kan afgelei word van die eksperimentele resultate dat die multi-fase uitlaat struktuur suksesvol was in die demping van die na-ontwikkelingsvloei vloedpiek tot die voor-ontwikkelingsvloei vloedpiek vir storms van al ses herhaalperiodes wat getoets is (d.i. die 2-, 5-, 10-, 20-, 50- en 100-jaar storms) vir kus en binnelandse gebiede. Die ontwerp van multi-fase uitlaatstrukture in vloedvertragingsfasiliteite kan daarom help om erosie te voorkom in die waterliggame waarin dit uitvloei.

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