4 research outputs found
Revised regional sediment yield prediction methodology for ungauged catchments in South Africa
No full textCITATION: Msadala, V. C. & Basson, G. R. 2017. Revised regional sediment yield prediction methodology for ungauged catchments in South Africa. Journal of the South African Institution of Civil Engineering, 59(2):28-36, doi:10.17159/2309-8775/2017/v59n2a4.The original publication is available at http://www.scielo.org.zaENGLISH ABSTRACT: This paper presents the research on the revision of the regional probabilistic method (Rooseboom et al 1992) for the prediction of catchment sediment yields in South Africa. The determination of sediment yields using probabilistic or empirical methods is suitable for ungauged catchments in the absence of observed data. The prediction of sediment loads is a key component in the quest to deal with reservoir and river sedimentation which is a potential threat to the sustainability of water resources in southern Africa. The revision was necessitated by increased sediment data availability and improved data analysis tools. Ten new sediment yield regions were demarcated in South Africa and Lesotho. Two analytical approaches were developed, namely probabilistic and empirical. The probabilistic approach is applicable in sediment yield Regions 3, 6 and 9. The empirical approach is applicable in sediment yield Regions 1, 2, 4, 5, 7 and 8. The estimation of sediment yields in Region 10 (Lesotho Highlands) needs to be based on direct measurements and locally observed data since no adequate analysis of sediment loads was possible due to limited data. GIS and electronic portable document file (pdf) copies of maps were produced for the retrieval of catchment data.http://www.scielo.org.za/scielo.php?script=sci_abstract&pid=S1021-20192017000200004Publisher's versio
Sediment yield prediction based on analytical methods and mathematical modelling
No full textThesis (MScEng (Civil Engineering)--University of Stellenbosch, 2009.ENGLISH ABSTRACT: A study of the state of reservoir sedimentation in South Africa based on reservoir
sediment deposit data, has shown that a considerable number of reservoirs have
serious sedimentation problems. The analysis of the reservoir sediment deposit data
showed that almost 25% of the total number of reservoirs have lost between 10 to
30% of their original storage capacity. The average storage loss due to sedimentation
in South African reservoirs is approximately 0.3% per year while the average annual
storage loss for all the reservoirs in the world is 0.8%.
The aim of this research was to develop sediment yield prediction methods based on
analytical approaches and mathematical modelling. The sediment yield prediction
methods can be used in planning and management of water resources particularly in
reservoir sedimentation control. The catchment erosion and sediment yield modelling
methods can be applied in temporal and spatial analysis of sediment yields which
results are essential for detailed design of water resources, particularly in the
identification of critical erosion areas, sediment sources and formulation of catchment
management strategies.
Current analytical methods for the prediction of sediment yield have been reviewed.
Nine sediment yield regions have been demarcated based on the observed sediment
yields and catchment characteristics. Empirical and probabilistic approaches were
investigated. The probabilistic approach is based on analysis of the observed
sediment yields that were calculated from reservoir sediment deposit, river suspended
sediment sampling data and soil erodibility data. The empirical equations have been
derived from regression analysis of the variables that were envisaged to have a
significant effect on erosion and sediment yields in South Africa. Empirical equations
have been developed and shown to have accurate and reliable predictive capability in
six of the nine regions.
The probabilistic approach has been recommended for the prediction of sediment
yields in the remaining three regions where reliable regression equations could not be
derived. The predictive accuracy of both the probabilistic and empirical approaches was
checked and verified using the discrepancy ratio and graphs of the observed and
calculated data.
While the analytical methods are needed to predict the sediment yield for the whole
catchment, mathematical modelling to predict sediment yields is applied for more
detailed analysis of sediment yield within the catchment. An evaluation of available
catchment sediment yield mathematical modelling systems was carried out. The main
criteria for the choice of a numerical model to be adopted for detailed evaluation was
based on the following considerations: the model’s capabilities, user requirements
and its application. The SHETRAN model (Ewen et al., 2000) was therefore
specifically chosen because of its ability to simulate relatively larger catchment areas
(it can handle catchment scales from less than 1km2 to 2500km2), its ability to
simulate erosion in channels, gullies and landslides, its applicability to a wide range
of land-use types and ability to simulate land use changes. Another model, ACRU
(Smithers et al., 2002) was also reviewed.
The aim of the model evaluation was to provide a conceptual understanding of
catchment sediment yield modelling processes comprising model set up, calibration,
validation and simulation. The detailed evaluation of the SHETRAN model was done
through a case study of Glenmaggie Dam in Australia. The flow was calibrated and
validated using data from 1975 to 1984, and 1996 to 2006 respectively. The results
for both the calibration and validation were reasonable and reliable. The sediment
load was validated against turbidity derived sediment load data from 1996 to 2006.
The model was used to identify sources of sediment and areas of higher sediment
yield. The land use of a selected sub-catchment was altered to analyse the impact of
land use and vegetative cover on the sediment yield. Based on the results, the
SHETRAN model was confirmed to be a reliable model for catchment sediment yield
modelling including simulation of different land uses.AFRIKAANSE OPSOMMING: ‘n Studie van die stand van damtoeslikking in Suid-Afrika toon dat daar ernstige
toeslikkingsprobleme by baie reservoirs bestaan. ’n Ontleding van die
toeslikkingsyfers gegrond op damkomopmetings toon dat omtrent 25% van die totale
getal reservoirs tussen 10 en 30% van hulle oorspronklike opgaarvermoë verloor het.
Die gemiddelde tempo van damtoeslikking in Suid-Afrika is 0.3%/jaar, wat laer is as
die wêreld gemiddeld van 0.8%/jaar.
Die oogmerk met hierdie navorsing was om sedimentlewering voorspellingsmetodes
te ontwikkel deur gebruik te maak van analitiese metodes en wiskundige modellering.
Die sedimentlewering voorspellingsmetodes kan gebruik word vir die beplanning en
bestuur van waterbronne en veral vir damtoeslikking beheer. Die opvangsgebied
erosie en die sedimentlewering modelleringsmetodes kan toegepas word in
tydveranderlike en ruimtelike ontleding van sedimentlewering. Hierdie inligting
word benodig vir die detail ontwerp van waterhulpbronne en veral vir die
identifisering van kritiese erosiegebiede, bronne van sediment en die formulering van
opvangsgebied-bestuur strategië.
‘n Literatuuroorsig oor die huidige metodes vir die voorspelling van erosie en
sedimentlewering is gedoen.
Nege sedimentasie streke is afgebaken in Suid-Afrika, gegrond op waargenome
damtoeslikkingsdata en opvangsgebied-eienskappe. Proefondervindelike en
waarskynlikheidsbenaderinge is ondersoek. Die waarskynlikheidsbenadering is
gegrond op die ontleding van waargenome damtoeslikking wat bereken is uit
reservoir opmeting data en rivier gesuspendeerde sediment data, asook data oor
gronderosie.
Die proefondervindelike metode se vergelykings is afgelei vanuit regressie ontleding
van die veranderlikes wat ‘n belangrike invloed het op die erosie en sedimentlewering
in Suid-Afrika. Daar is bevestig dat die ontwikkelde proefondervindelike (empiriese)
vergelykings ‘n akkurate en betroubare voorspellingsvermoë in ses van die nege
streke het. Die waarskynlikheidsbenadering is aanbeveel vir die voorspelling van sedimentlewering in die ander drie streke, waar betroubare regressie vergelykings nie
afgelei kon word nie. Die voorspellingsakkuraatheid van albei metodes is nagegaan
en bevestig deur gebruik te maak van die teenstrydigheidsverhouding en grafieke van
die waargenome en berekende data.
Analitiese metodes van sedimentleweringsvoorspelling is nodig vir ‘n volle
opvangsgebied, terwyl wiskundige modellering om sedimentlewerings te voorspel
gebruik kan word om ‘n meer in diepte ontleding van die sedimentlewering binne ‘n
opvanggebied te doen. ‘n Evaluasie van beskikbare wiskundige modelle wat
opvangsgebied sedimentlewering kan voorspel, is gedoen. Die hoofkriteria vir die
keuse van ‘n model vir gebruik by gedetailleerde ontleding is gegrond op die
volgende: die vermoëns van die model, wat verbruikers benodig en die aanwending
van die model. Die SHETRAN model (Ewen et al., 2000) is spesifiek gekies weens
sy vermoë om relatief groter opvangsgebiede te simuleer (dit kan opvangsgebiede
van 1km2 tot 2500km2 wees) asook om erosie in kanale, dongas en grondverskuiwing
simuleer. Dit kan toegepas word op ‘n wye reeks grondtipes en kan ook die gevolge
simuleer as die gebruik van die grond verander. ‘n Ander model, ACRU (Smithers
et al., 2002) is ook ondersoek.
Die doel van die modelevaluering was om ‘n konseptuele begrip te kry van
sedimentlewering modelleringsprosesse wat die opstelling, kalibrasie, toetsing en
simulasies insluit. Die volledige evaluasie van SHETRAN is gedoen deur middel van
‘n gevalle-studie van die Glenmaggiedam in Australia. Die riviervloei is gekalibreer
en getoets deur gebruik te maak van data wat strek van 1975 tot 1984, en van 1996 tot
2006 onderskeidelik. Die resultate van beide die kalibrasie en die toetswas redelik en
betroubaar. Die sedimentlading is gekalibreer teen velddata van 1996 tot 2006. Die
model is gebruik om bronne van sediment te identifiseer, asook gebiede met ‘n hoër
sedimentlewering. Die gebruik van die grond op ‘n gekose sub-opvangsgebied is
verander om die impak van grondgebruik en plantbedekking op sedimentlewering te
ontleed. Die resultate bewys dat die SHETRAN model ‘n betroubare model is vir groot
opvangsgebied sedimentlewering modellering, asook vir die simulasie van
verskillende grondgebruike.Master
Sediment transport dynamics in dam-break modelling
No full textThesis (DEng)--Stellenbosch University, 2016.ENGLISH ABSTRACT: The prediction of breach characteristics during earth embankment dam-break modelling due to overtopping is dependent on the accurate modelling of the intrinsic breaching processes such as flow hydraulics, bed level changes, soil mechanics and sediment transport dynamics. Earth dams are characterised by steep embankment slopes (up to 1:2 (V:H), 50%). Commonly used sediment transport equations in dam-break modelling were based on slopes of up to 20%. The application of sediment transport equations that were derived from data on mild or moderately steep slopes is one of the sources of uncertainty in dam-break modelling. The general objective of this study was to develop new empirical sediment transport equations for steep slopes that can be applied in homogeneous earth embankment dam-break modelling and to assess the uncertainty that is associated with the input of sediment transport equations.
Physical experimental studies were conducted at the Hydraulic Laboratory of the University of Stellenbosch, South Africa. A 4.7 m long, 0.25 m wide and 0.3 m deep flume was used. Three preselected bed slope configurations of 25%, 33% and 40% were implemented. A total of 87 tests were carried out with median (d50) sediment sizes of 0.2 mm, 1.0 mm and 2.4 mm.
New empirical sediment transport equations for steep slopes were developed based on the data from the experimental study. One of the two proposed sediment transport equations incorporated six parameters in its formulation, namely the particle sizes of the sample for which 30 % and 90% were finer (d30 and d90); shear velocity, average flow velocity, friction slope, dimensionless shear stress and critical dimensionless shear stress. The validity or statistical significance of the proposed sediment transport equations (Equations 5.2-5 and 5.2-6) was determined using statistical tests (F-Test and r2). The predictive capability of the proposed sediment transport equations (Equations 5.2-5 and 5.2-6) was confirmed by the degree of correlation between measured and predicted sediment transport rates. There was a deviation of less than 22% between the measured and predicted sediment transport rates.
A comparative analysis was done between measured and predicted sediment transport rates using selected sediment transport equations from literature. The analytical comparison showed that the sediment transport predictions by one of the newly calibrated sediment transport equations (Equation 5.2-6) were within the same order of magnitude as those of the Meyer-Peter Müller (1948) and Camenen & Larson (2005) sediment transport equations, even though the Meyer-Peter Müller (1948) equation overestimated sediment transport rates at higher unit discharges.
The Smart and Jäeggi (1983) sediment transport equation predicted higher sediment transport rates than the measured sediment transport rates and those predicted by Equation 5.2-6.
The MIKE 21C two dimensional hydrodynamic model of the DHI Group was successfully applied to evaluate and compare the performance of the newly calibrated sediment transport equations in dam-break modelling using five dam-break case studies. The numerical simulation results using the newly calibrated sediment transport equations (Equations 5.2-5 and 5.2-6) were compared with the output results that were simulated by two selected sediment transport equations from literature, namely those of Camenen & Larson (2005) and Smart & Jäeggi (1983).
Specifically, Case studies 1 and 2 were applied to analyse the effect of sediment transport equations in the simulation of temporal bed level changes and the simulated breach shape respectively. Case studies 3 and 4 investigated the numerical modelling of dam-break outflow hydrographs on very steep slopes. Case Study 5 was used to analyse the effect of sediment transport equations on the simulated peak discharge and outflow volume and showed that the newly calibrated equations are applicable at prototype scale. The newly calibrated sediment transport equations performed better in Case studies 1 and 2 where the embankment slopes were within the equations’ calibration range (25% - 40% slopes). Consistent and realistic predictions of numerical model output parameters were not possible when the sediment transport equations were applied to Case studies 3 and 4 where the embankment slopes were outside the calibration range of all four sediment transport equations (Equations 5.2-5 and 5.2-6, Camenen & Larson (2005) and Smart and Jäeggi (1983)).
The original contribution to knowledge of this dissertation is the development of sediment transport equations that are based on steep embankment slopes and are suitable for application in dam-break modelling. For dam-break studies, sensitivity analysis of model input parameters such as resistance and alternative sediment transport equations is recommended in order to evaluate a range of scenarios. The sensitivity analysis that was accomplished pertaining to Manning’s resistance coefficient (n) showed that a lower Manning’s resistance coefficient (n) in the numerical model resulted in a higher peak outflow discharge.AFRIKAANSE OPSOMMING: Die voorspelling van breekkenmerke in die breekmodellering van gronddamme as gevolg van vloedwater wat oor die nie oorloopkruin van die dam spoel, berus op die akkurate modellering van die intrinsieke breekprosesse, soos vloeihidroulika, veranderinge in die bodemvlak, grondmeganika en sedimentvervoerdinamika. Gronddamme word gekenmerk deur steil wal hellings (tot 1:2 (V:H), 50%). Tog is die vergelykings wat meestal vir die berekening van sedimentvervoer gebruik word, gegrond op hellings van tot slegs 20%. Die toepassing van sedimentvervoervergelykings wat afgelei is van data vir matige hellings is dus een van die bronne van onsekerheid in dambreekmodellering. Daarom was die algemene oogmerk van hierdie studie om nuwe empiriese sedimentvervoervergelykings vir steil hellings te ontwikkel wat in homogene gronddambreekmodellering toegepas kan word, en om die onsekerheid te beoordeel wat met die inset van sedimentvervoervergelykings verband hou.
Fisiese modelstudies is in die Hidrouliese Laboratorium van die Universiteit Stellenbosch onderneem. ’n Kanaal met ’n lengte van 4.7 m, ’n breedte van 0.25 m en ’n diepte van 0.3 m is gebruik. Drie voorafgekose damhellings van 25%, 33% en 40% is toegepas. Altesaam 87 toetse is met gemiddelde (d50) sedimentgroottes van 0.2 mm, 1.0 mm en 2.4 mm uitgevoer.
Nuwe empiriese sedimentvervoervergelykings vir steil hellings is op grond van die data uit die proefstudies ontwikkel. Die formulering van een van die twee voorgestelde sedimentvervoervergelykings het ses parameters ingesluit, naamlik die deeltjiegrootte van die monster, waarvan 30% en 90% fyner was (d30 en d90); sleursnelheid, gemiddelde vloeisnelheid, wrywingshelling, dimensielose skuifspanning en kritieke dimensielose skuifspanning. Die geldigheid of statistiese beduidendheid van die voorgestelde sedimentvervoervergelykings (vergelykings 5.2-5 en 5.2-6) is met behulp van statistiese toetse (F-toets en r2) bepaal. Die voorspellingsvermoë van die vergelykings is bevestig deur die mate van korrelasie tussen die gemete en voorspelde sedimentvervoertempo’s, wat minder as 22% van mekaar verskil het.
’n Vergelykende ontleding van die gemete en voorspelde sedimentvervoertempo’s is met behulp van gekose sedimentvervoervergelykings uit die literatuur uitgevoer. Dít het getoon dat die sedimentvervoervoorspellings van die pas gekalibreerde vergelykings (vergelyking 5.2-6) in dieselfde grootteorde as dié van die vergelykings van Meyer-Peter Müller (1948) en Camenen & Larson (2005) val, hoewel Meyer-Peter Müller se vergelyking sedimentvervoertempo’s met hoër eenheidsdeurstromings oorskat het. Smart & Jäeggi se sedimentvervoervergelyking (1983) het hoër
Stellenbosch University https://scholar.sun.ac.za
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vervoertempo’s vergeleke met die gemete waardes sowel as die voorspelde tempo’s van vergelyking 5.2-6 voorspel.
Hierna is die DHI Groep se tweedimensionele hidrodinamiese model MIKE 21C suksesvol toegepas om die akkuraatheid van die pas gekalibreerde sedimentvervoervergelykings in dambreekmodellering in vyf dambreekgevallestudies te beoordeel en te vergelyk. Die numeriese simulasieresultate van die pas gekalibreerde sedimentvervoervergelykings (vergelykings 5.2-5 en 5.2-6) is met die gesimuleerde uitsetresultate van twee gekose vergelykings uit die literatuur, naamlik dié van Camenen & Larson (2005) en Smart & Jäeggi (1983), vergelyk.
Meer bepaald is gevallestudie 1 en 2 toegepas om die uitwerking van die sedimentvervoervergelykings op gesimuleerde veranderinge in die bodemvlak oor tyd en die gesimuleerde breekvorm onderskeidelik te ontleed. Gevallestudies 3 en 4 het die numeriese modellering van uitvloeihidrograwe by dambreke met baie steil hellings ondersoek. Gevallestudie 5 is gebruik om die uitwerking van die sedimentvervoervergelykings op die gesimuleerde piek uitvloei deurstroming en uitvloeivolume te ontleed, en het getoon dat die pas gekalibreerde vergelykings op prototipeskaal toegepas kan word. Die prestasie van die pas gekalibreerde vergelykings was beter in gevallestudies 1 en 2, waar die damhellings binne die kalibreerbestek van die vergelykings geval het (25% - 40% helling). Konsekwente en realistiese voorspellings van numeriese modeluitsetparameters was onmoontlik toe die vergelykings op gevallestudie 3 en 4 toegepas is, omdat die damhellings buite die kalibreerbestek van ál vier vergelykings (vergelyking 5.2-5 en 5.2-6, Camenen & Larson (2005) en Smart & Jäeggi (1983)) geval het.
Hierdie verhandeling lewer ’n oorspronklike bydrae deur die ontwikkeling van sedimentvervoervergelykings vir steil damhellings wat in dambreekmodellering toegepas kan word. Sensitiwiteitsontleding van modelinsetparameters soos weerstand en alternatiewe sedimentvervoervergelykings word vir dambreekstudies aanbeveel sodat ’n verskeidenheid scenario’s beoordeel kan word. Die sensitwiteitsontleding wat met betrekking tot Manning se ruheid (n) uitgevoer is, het getoon dat ’n laer (n) in die numeriese model tot ’n hoër piekuitvloei gelei het.Doctora
