Interrelationship of the fluvial morphology and the salinity of the Great Fish River Estuary

Julyan, Enrique Edward (2015-03)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The investigation of the interrelationship of the fluvial morphology and the salinity of the Great Fish River Estuary was performed by the combination of a two-dimensional morphological model and a one dimensional advection dispersion module. Two scenarios were defined for investigation, namely Scenarios A and B. Model bathymetry and grid/network for each model and scenario was compiled from topographical information obtained from aerial photos, SRTM data, LIDAR and 24 measured river cross sections of the area from the river mouth up to 27km upstream of the river mouth. Model boundary conditions were developed from empirical formulas and measured data from the Department of Water and Sanitation (DWS). Both models were calibrated with results obtained during field measurement conducted from 5 – 7 May 2012. Scenario A consisted of a long term 5 year morphological simulation (1 May 2007 to 30 May 2012) with manual mouth closure events for identified river low flow periods. Water levels upstream of the river mouth were extracted from the two-dimensional morphological module and used as the downstream boundary condition of the one-dimensional advection dispersion (salinity) model. For scenario B floods with return periods between 1:2 and 1:100 years were simulated in the morphological model. The resultant bathymetries were then used to compile the network and bathymetry of the one dimensional advection dispersion (salinity) model. The different flood resultant bathymetries were then used with equal boundary conditions (representative of the average flow in the river) in the one dimensional salinity model. The predicted salinity was compared for each bathymetry used. From model results distinct trends were observed. During low flow conditions the estuary basin fills with sediments and during floods the sediments are flushed out of the estuary. Large magnitude floods greatly erode the estuary especially in the middle reach, during floods the tidal inlet experiences overtopping and subsequent erosion, the constriction at the tidal inlet is completely destroyed during larger floods. The estuary mouth in its closed state experiences slight overtopping and the mouth is breached during periods of high river flows. The magnitude of salt intrusion depends mainly on the size (the constriction) of the river mouth (tidal inlet). During periods of mouth closure the average salinity in the estuary decreases, average salinity increases if the tidal inlet area is increased. The extent of salt intrusion is approximately 10 km upstream of the river mouth when the mouth is open and the intrusion length increases during spring tides.

AFRIKAANSE OPSOMMING: Die verwantskap tussen die Groot Vis Rivier Estuarium morfologie en sout toestand is ondersoek deur die kombinasie van ‘n twee dimensionele morfologiese model en ‘n een dimensionele sout model. Twee toestande is gedefinieer vir ondersoek naamlik Scenario A en Scenario B. Die area vanaf die rivier mond tot 27 km stroomop van die rivier mond is deur die modelle gesimuleer. Die gemodeleerde area stem ooreen met die area waar gemete rivier-snit diepte metings beskikbaar was, onbrekende data is aangevul met behulp van lugfotos, LIDAR- en SRTM- data. Die model grens toestande is bepaal met empiriese vereglykings asook gemete data vanaf die Departement Waterwese. Beide numeriese modelle was gekalibreer met veld data verkry vanaf 5 tot 7 Mei 2012. Scenario A het behels ‘n langtermyn 5 jaar morfologiese modellering (1 Mei 2007 tot 30 Mei 2012) met toemond toestande gedurende gedefinieerde rivier vloei toestande. Die watervlakke van die twee dimensionele morfologiese model stroomop van die mond is gebruik as die stroomaf grenstoestand van die een dimnesionele model om die effek van die morfologiese veranderinge te inkorporeer in die sout model. Scenario B het behels die simulering van rivier vloede met herhaal periodes tussen 1:2 en 1:100 jaar in die morfologiese model. Die rivier-bodem vlakke verkry van laasgenoemde simulasiesis toe gebruik in die een dimensionele sout model met dieselfde grenstoestande wat ooreenstem met die gemiddelde toestande in die Groot Vis Rivier. Aangesien die grenstoestande dieselfde was en net die rivier-bodem vlakke gevarieer is, kon die effek van vloede op die souttoestand in die estuarium bepaal word. Uit die model resultate kon duidelike tendense waargeneem word. Gedurende lae rivier vloei toestande is die estuarium gevul met sediment en tydens vloede het die sediment weer ge-erodeer en gedeponeer in die oseaan. Groot vloede veroorsaak baie erosie in die estuarium veral in die middel bereik en by die riviermonding. Indien die vloed groot genoeg is word die riviermond vernouing totaal uitgespoel. Die riviermonding in die geslote staat ondervind effense oorstroming en word oopgespoel indien die rivier vloei groot genoeg is. Die graad van die sout indringing in die estuarium hang hoofsaaklik af van die grootte (die vernouing) van die rivier mond. Gedurende toe mond toestande is die gemiddelde sout vlakke in die estuarium laer, wanner die rivier monding groter raak, word die gemiddelde sout vlakke in die estuarium meer. Die omvang van die sout indringing strek tot ongeveer 10 km stroomop van die rivier mond wanneer die mond oop is en die indringing afstand neem toe gedurende spring getye.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96952
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