Hydraulic modelling of the Lower Orange River (Republic of South Africa) and evaluation of integrated water resources management in the Orange-Senqu-Fish catchment

Jan Putteman Bert Schepens
Persbericht

Hydraulic modelling of the Lower Orange River (Republic of South Africa) and evaluation of integrated water resources management in the Orange-Senqu-Fish catchment

 Jan Putteman en Bert Schepens

Duurzaam waterbeheer in Zuid-Afrika: de Oranjerivier bekent kleur

“Let there be work, bread, water and salt for all”, deze wijze woorden sprak Nelson Mandela bij zijn inauguratie in 1994. De verwezenlijking van dit droombeeld blijkt echter minder eenvoudig te zijn, althans voor wat de beschikbaarheid van water betreft. In een land waar water een schaars goed is en de behoeften nog steeds toenemen zal enkel een weldoordacht waterbeheer enig soelaas kunnen bieden. Of hoe ingenieurswetenschappen, ecologie en ethiek meer dan ooit complementair zijn.

Begin september 2009 vertrokken Jan Putteman en Bert Schepens, allebei student burgerlijk ingenieur bouwkunde, voor een maand richting Zuid-Afrika. Het doel van deze reis was het vergaren van informatie in verband met de plaatselijke waterproblematiek, waarbij hoofdzakelijk het Oranjerivierbekken werd bestudeerd. Tijdens dit bezoek werd voornamelijk gepraat met de lokale bevolking, officiële instanties en de verschillende stakeholders, om zo een gefundeerd en genuanceerd beeld te krijgen van de belangrijkste pijnpunten. Ook werd voldoende data (geometrie van de rivier, debiet, ...) verzameld, met de bedoeling later een computermodel van de rivierstroming op te bouwen.

De Oranjerivier ontspringt in het hooggebergte van Lesotho en kronkelt vervolgens doorheen de Kalahari halfwoestijn. De rivier tekent zich af als een groene lijn in een onherbergzaam landschap, en mondt uiteindelijk uit in de Atlantische oceaan. Het rivierbekken beslaat bijna 50 % van de totale oppervlakte van Zuid-Afrika. Zonder het water van de Oranjerivier zou de nationale economie slechts een schim zijn van wat ze nu is. De Oranjerivier creëert werkgelegenheid in deze desolate streek. Een rit langs de rivier illustreert al snel het belang van irrigatieprojecten, bijvoorbeeld voor het verbouwen van druiven, die tevens een belangrijk exportproduct vormen.

Maar de Oranjerivier is wispelturig ... Zo verwoestend de zondvloed tijdens de zomermaanden, zo behoedzaam stroomt het water in de winter. Teneinde de watervoorziening op elk moment te verzekeren werden daarom sinds de zeventiger jaren verschillende dammen en stuwen op de rivier gebouwd. Het reservoir van een stuwdam wordt gedurende de zomermaanden opgevuld, terwijl dit gestockeerd water tijdens de wintermaanden gebruikt wordt om het watertekort te compenseren. Een netwerk van irrigatiekanalen verdeelt het water over de vruchtbare gronden langsheen de rivier.

De weelde van de Oranjerivier staat in schril contrast met het gebrek aan water elders in het land.  Daarom wordt de laatste decennia druk gewerkt aan zogenaamde inter-basin transfers. Men transporteert het rivierwater dus naar een ander rivierbekken waar het in natuurlijke omstandigheden onmogelijk terecht zou kunnen komen. Zo versast een ingenieus netwerk van kanalen, tunnels en dammen water van de Oranjerivier naar de zuidkust van Zuid-Afrika, zo’n 350 km verder. Op dezelfde manier wordt ook het industriële hart van Zuid-Afrika, dit is de regio Johannesburg-Pretoria, van bijkomend drinkwater voorzien.

De vele bouwkundige ingrepen die recent plaatsvonden in en langs de Oranjerivier hebben ertoe geleid dat de rivier geëvolueerd is naar een gereguleerd systeem. Door analyse van debietmetingen sinds de jaren ’30 kunnen we exact nagaan wat precies de impact van de mens op het stromingsregime in de rivier is. Het jaarlijks opgemeten watervolume is meer dan gehalveerd in vergelijking met de periode voorafgaand aan de constructie van verschillende stuwdammen. Maar er is meer aan de hand. Ook de seizoenale variatie van de getransporteerde watervolumes is sterk gewijzigd. Door debietregeling met behulp van stuwdammen kan men immers een constant debiet in stand houden. Dit constant debiet is een gevolg van de vrij constante waterbehoeften van de afwaartse watergebruikers.

De impact van stuwdammen en irrigatieschema’s gaat veel verder dan louter hun fysieke aanwezigheid: het resulterende gewijzigde stromingsregime heeft een grote impact op het aquatisch ecosysteem aangezien er nieuwe leefomstandigheden gecreëerd worden. Zo kampt men momenteel met een plaag van ‘black flies’. Dit zijn kleine vliegjes die door de mildere stromingsomstandigheden veel makkelijker kunnen overleven en bijgevolg in onnatuurlijk hoge aantallen voorkomen. Ook verkeert de monding van de Oranjerivier, tevens van internationaal belang voor trekvogels, in zeer slechte staat. De belangrijkste herstelmaatregel bestaat uit het creëren van een meer natuurlijk en seizoensgebonden stromingsregime, waarbij een hoog zomerdebiet wordt afgewisseld door een laag winterdebiet.

Ontwikkeling, gelijkheid en natuurbehoud: drie mooie begrippen die in de geschiedenisboeken niet blijken samen te gaan. Het nieuwe Zuid-Afrika is niettemin ambitieus: de National Water Act wil water én de hiermee gerelateerde ecosystemen teruggeven aan de gehele bevolking. De gelijke toegang tot drinkbaar water vormt een belangrijke stap in de richting van de verhoging van de levensstandaard van de zwarte bevolking, denk maar aan het gebruik van water voor sanitair, irrigatieprojecten en industrie. Anderzijds groeit het besef dat het beschikbare water eindig is, en dat overexploitatie drastische gevolgen zal hebben voor het leefmilieu.

Net bij het maken van een compromis tussen deze verschillende belangen kan de technologie een handje helpen. In het kader van deze studie werd, vertrekkend van een bestaand softwarepakket, een hydraulisch model van de Oranjerivier opgebouwd. Een hydraulisch model wordt gebruikt om de voortplanting van een bepaald debiet doorheen de rivier te simuleren. Een debietgolf die ter hoogte van een stuwdam in de rivier wordt gestuurd heeft immers een zekere tijd nodig om tot bij een bepaalde watergebruiker te stromen. Zo is bijvoorbeeld een maand tijd nodig voor het overbruggen van een afstand van 1400 km. De mogelijkheid tot het correct voorspellen van het debiet in de rivier laat toe om duurzame beheersmaatregelen voor de stuwdammen voor te stellen. Door gebruik van een hydraulisch model kan men er immers over waken dat belangrijke ecosysteemcomponenten, zoals de riviermonding, op elk tijdstip een optimaal debiet ontvangen.

Momenteel is er reeds een prangend watertekort. Niettemin stelt men een economische groei voorop. Een hopeloze zaak lijkt het. Hebben de huidige gezagsdragers voldoende daadkracht om het tij te keren? De technologische knowhow laat toe om elke druppel water zo efficiënt mogelijk te benutten, maar de mate waarin prioriteit wordt gegeven aan ecosysteembehoeften wordt door politieke beslissingen gestuurd. Zal er nog heel wat water naar zee moeten vloeien alvorens men de kaart van de duurzaamheid trekt, of net niet?

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Universiteit of Hogeschool
Burgerlijk ingenieur bouwkunde
Publicatiejaar
2010
Promotor(en)
prof. dr. ir. Ronny Verhoeven, prof. dr. ir. Peter Troch
Kernwoorden
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