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Bathymetry Development and Flow Analyses Using Two-Dimensional Numerical Modeling Approach for Lake Victoria
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Water and Environmental Engineering.ORCID iD: 0000-0002-7005-4632
KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA.ORCID iD: 0000-0002-4375-051x
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Water and Environmental Engineering.
Makerere University.
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2019 (English)In: Fluids, ISSN 2311-5521, Vol. 4, no 4, p. 1-21Article in journal (Refereed) [Artistic work] Published
Abstract [en]

This study explored two-dimensional (2D) numerical hydrodynamic model simulations of Lake Victoria. Several methods were developed in Matlab to build the lake topography. Old depth soundings taken in smaller parts of the lake were combined with more recent extensive data to produce a smooth topographical model. The lake free surface numerical model in the COMSOL Multiphysics (CM) software was implemented using bathymetry and vertically integrated 2D shallow water equations. Validated by measurements of mean lake water level, the model predicted very low mean flow speeds and was thus close to being linear and time invariant, allowing long-time simulations with low-pass filtered inflow data. An outflow boundary condition allowed an accurate simulation to achieve the lake’s steady state level. The numerical accuracy of the linear measurement of lake water level was excellent.

Place, publisher, year, edition, pages
Basel, Switzerland, 2019. Vol. 4, no 4, p. 1-21
Keywords [en]
methods of lake bathymetry; shallow water equations; lake hydrodynamics; numerical accuracy; steady-state analysis; water-level validation
National Category
Water Engineering
Research subject
Applied and Computational Mathematics, Numerical Analysis; Civil and Architectural Engineering, Hydraulic and Hydrologic Engineering; Land and Water Resources Engineering
Identifiers
URN: urn:nbn:se:kth:diva-263581DOI: 10.3390/fluids4040182OAI: oai:DiVA.org:kth-263581DiVA, id: diva2:1368322
Projects
PhD project
Funder
Lars Erik Lundberg Scholarship Foundation
Note

QC 20191106

Available from: 2019-11-06 Created: 2019-11-06 Last updated: 2019-11-06Bibliographically approved
In thesis
1. Data preparation, hydrodynamic and contaminant transport shallow-water simulations of Lake Victoria
Open this publication in new window or tab >>Data preparation, hydrodynamic and contaminant transport shallow-water simulations of Lake Victoria
2019 (English)Licentiate thesis, comprehensive summary (Other academic) [Artistic work]
Abstract [en]

This study explores shallow lake numerical hydrodynamic processes that support model development and validation, extreme events and effects of water circulation in Lake Victoria. Lake Victoria is the second largest freshwater lake in the world, and the largest in East Africa. It is the major freshwater reservoir and source for domestic, agriculture, industrial, fishery, and transport. The resources support livelihoods and ecosystem services for over 40 million people. The lake is severely affected by water quality degradation by pollution. This thesis aims at improving the understanding by following recommendation of the Lake Victoria Environment Management Project, Lake Victoria Basin Commission climate change adaptation strategy and action plan 2018-2023, Lake Victoria Basin Commission operational plan 2015-2020, and Lake Victoria Basin Commission report. These reports suggested detailed lake bathymetry survey, modelling of lake flow, study of lake hydrometeorological processes by modelling and simulation, to identify extreme weather events, assess water circulation effect, and study lake pollution near the shore. A numerical hydrodynamic model was built in the COMSOL Multiphysics (CM) software for assessing lake flows and water turn-over from river inflows which carry pollution. The work included the development of systematic methods for lake bathymetry that are relevant for lake numerical and hydrodynamic modelling. The hydrometeorological driven simulation model was employed to assess lake water balance, water circulation and soluble transport. Paper 1 creates a bathymetry from several methods and from several data sources, and a vertically integrated free surface flow model was implemented in CM. The model was used to investigate outflow conditions, mean velocities driven by river inflow, outflow, precipitation and evaporation. It is shown to be exactly conservative and give water level variation in reasonable agreement with measurements. The results indicate that the shallow water model is close to linear. An outflow model, linear in water level, predicts water level reasonable agreement with measurements. The findings suggest that the model should consider wind stress driven flow to provide more accurate lake flow behavior. Paper 2 performed an assessment of the hydro-meteorological processes and extreme weather events that are responsible for changing the characteristics of lake water balance, and changing streamflow variations, and lake transportation. We compare historical data over a long time with data from the model including water balance, sources of data uncertainty, correlations, extreme rain and inflow years, and seasonal variations. Solute loading and transportation was illustrated by tracing the water from the river inflows. The results indicate that the lake rainfall has a strong seasonal variation with strong correlations between tributary inflows and precipitation, and between lake outflow and water level. The tracer transport by mean flow is very slow. Flow increases somewhat in wet periods and is faster in the shallow Kenya lake zone than in the deeper Uganda and Tanzanian lake zones, where the major inflow, from the Kagera River, appears to strongly influence transportation.

Abstract [sv]

Denna studie undersöker med numerisk metodik hydrodynamiska processer i den mycket grundaVictoriasjön och hur de påverkas av extrem väderlek, inflöden, och nederbörd. Victoriasjön är denandra största sötvattensjön i världen, och den största i Afrika. Den är färskvattenförråd och källa förhushåll, jordbruk, industri, fiske och transporter. Resurserna ger livsuppehåll och ekosystemtjänsterför mer än 40 miljoner människor. Sjön är utsatt för allvarliga föroreningar som försämrarvattenkvaliteten. Detta arbete avser att förbättra förståelsen genom att följa rekommendationer somgivits ut av Lake Victoria Environment Management Project (LVEMP), och Lake Victoria BasinCommissions (LVBC) rapporter om strategi för anpassning till klimatförändringar, åtgärdsplan2018-2023 och översiktsplan 2015-2020. Rapporterna föreslår detaljerad genomgång avdjupkartor, modellering av strömning i sjön i syfte att identifiera extrema väderhändelser,undersöka vattencirkulationen, och studera föroreningarna nära stränder. En hydrodynamisknumerisk modell har byggts i simuleringspaketet COMSOL Multiphysics (CM) för uppskattning avströmning och vattenutbyte från förorenade inflöden. Arbetet innefattade utveckling av metoder förvattendjups-modeller för hydrodynamiska studier. Simuleringsmodellen drivs avhydrometeorologiska data och används för vattenmängds-balans, cirkulation ochföroreningstransport.Artikel 1 skapar vattendjupskartan från flera data-mängder med olika metoder. En vertikaltintegrerad modell med fri yta implementerades i CM. Modellen ger vertikalt medelvärdesbildadehastigheter drivna av flodinflöden, utflöde, nederbörd och avdunstning. Modellen representerarvattenbalansen exakt och ger variationer i vattennivå i rimlig överensstämmelse med mätningar.Resultaten antyder att modellen är nära linjär och tids-invariant. En utflödesmodell ansatt somlinjär i vatten-nivån kan anpassas noggrant till historiska data. Bättre realism kan uppnås omvindens pådrivande verkan inkluderas.Artikel 2 går igenom de hydro-meteorologiska processer och extrema väder-händelser som ändrarvattenbalans, strömningsmönster och transport. Vi har jämfört data över femtio år med modellens,inkluderande vattennivå, källor för osäkerhet i data, korrelationer, år med extrema regn ochinflöden, och årstidsvariationer. Resultaten tyder på att nederbörden varierar kraftigt medårstiderna, och signifikanta korrelationer ses mellan nederbörd och inflöden, och mellan utflöde ochvattennivå.Transport av lösliga föroreningar illustrerades genom spårning av vatten från de olika inflödena.Spårämnestransport med vertikalt medelvärdesbildade hastigheter är mycket långsam.Strömningen ökar något i våta årstider och är snabbare i den grunda zonen i Kenya än i de djuparedelarna i Uganda och Tanzania. Det största inflödet som kommer från Kagera tycks ha stor inverkanpå transporten.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. p. 22
Series
TRITA-ABE-DLT ; 1943
Keywords
Lake bathymetry model, Steady-state analysis, Numerical model validation, Lake water balance, Correlation and seasonal variations, Solute transport, Vattendjupskarta, Validering av numerisk modell, Vattenbalans, Korrelationer och säsongs-variation, Transport av lösliga ämnen
National Category
Water Engineering
Research subject
Land and Water Resources Engineering
Identifiers
urn:nbn:se:kth:diva-263593 (URN)978-91-7873-374-3 (ISBN)
Presentation
2019-11-25, Sahara, Teknikringen 10B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20191106

Available from: 2019-11-06 Created: 2019-11-06 Last updated: 2019-11-06Bibliographically approved
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