Gully erosion in the Sandspruit catchment, Western Cape, with a focus on the discontinuous split gully system at Malansdam

Olivier, George (2013-12)

Thesis (MSc)--Stellenbosch University, 2013.

ENGLISH ABSTRACT: Gully erosion is a major environmental problem not only having direct influences on site but also indirect influences felt further down the catchment. Combating gully erosion has proven to be elusive due to the difficulty in finding the causal factors and developing mechanisms involved. Soil is the medium in which gully erosion occurs yet few research have investigated it as a driving factor behind gully development and those that have has done it in a very elementary way. The first aim of this project was to physically and chemically characterise and classify the discontinuous gully system at Malansdam to establish the relationship between landscape hydrology and geomorphologic gully development with a focus on control factors. This was done by field observations, physical measurements and spatial and hydrological analyses with a Geographical Information System (GIS). The Malansdam gully system was the first ever recorded Strahler stream order (SSO) 5 classical gully system with the most active region being in the upper reaches where a steeper slope is experienced. Although piping was observed the V-shape channels and SAR data from traditional wet analysis indicated runoff to be the dominant formation process. A duo of factors, consisting of one anthropogenic and one natural factor respectively, was found that the major control factors behind the gully formation. A unique anthropogenic factor that has never been published beforehand was found to be the anthropogenic driving factor namely the ploughed contour cultivation technique employed by the farmers in the Sandspruit catchment. The ploughed contours act as channels firstly collecting and secondly moving water that would have drained naturally downwards in the valley to one exit point in the gully system. This allows increased erosive energy because of the larger volumes of water entering one single point in the upper reaches of the gully system where a steeper slope is experienced. The driving factor in the natural group was determined to be weak soil structure due to an abundant amount of exchangeable Mg2+ cations occupying the exchange sites on the clay fraction. This would cause soil to disperse in the presence of water even with a low amount of exchangeable Na+. Combat methods would accordingly exist in the form of rectifying the soil structure and finding an alternative to the ploughed contour system currently employed, but also planting vegetation especially grass or wheat in the gully channels. The second aim of this project was to determine the capability of Near Infrared (NIR) spectrometry, with wavenumbers 12 500 – 4 000 cm-1, to predict indicators used in soil science to establish the dispersive nature of a soil. These indices included the Exchangeable Sodium Percentage (ESP), Sodium Absorption Ratio (SAR), Magnesium Saturation Percentage (MS%), Electrical Conductivity (EC), Potential Hydrogen (pH) as well as the four main exchangeable cations namely calcium (Ca2+), potassium (K+), sodium (Na+) and magnesium (Mg2+). Surface and subsurface soil samples were collected from active gully heads. These samples were minimally pre-processed thus only dried, milled and sieved. Thereafter it was subject to NIR analysis making use of the Bruker multi-purpose FT-NIR Analyser (MPA; Bruker Optik GmbH, Germany) with a spectral range of 12 500cm-1 to 4000cm-1 which is. Partial Least Square Regression (PLSR) models were built for each index and the exchangeable cations making use of QUANT 2 utility of OPUS 6.5 (MPA; Bruker Optik GmbH, Germany) software. Five different regrssion statistics namely the coefficient of determination (r2), Root Mean Square Error of Cross Validation (RMSECV), Ratio of Performance to Deviation (RPD), Bias and the Ratio of Performance of Quartiles (RPIQ) were used to assess the legitimacy of each PLSR model. Upon validation all the PLSR models performed in line with previously published work and in certain cases better. The only exception was MS% which would require further investigation. NIR thus possess the capability to predict a soil’s dispersive nature in a fast, reliable, inexpensive and non- destructive way, thus implying whether or not it contributes to gully erosion at a significant level or only minimally.

AFRIKAANSE OPSOMMING: Donga erosie is 'n groot omgewingsprobleem. Dit het nie net ‘n direkte invloed op die area waar dit geleë is nie, maar het ook ‘n indirekte invloed wat elders in die opvangsgebied ervaar word. Bekamping van donga erosie is moeisaam aangesien die faktore wat aanleiding gee tot die vorming en dryf daarvan moeilik is om te bepaal. Grond is die medium waarin erosie plaasvind, maar daar is nogtans steeds min navorsing wat grond ondersoek het as ‘n moontlike faktor aanleiding gee tot donga erosie. Die wat dit al wel ingesluit het, het dit slegs op n baie elemntêre manier ondersoek. Die eerste doel van hierdie projek was om die diskontinue donga stelsel fisies en chemise te karakteriseer en klassifiseer om soedoende die verhouding tussen die landskap hidrologie en geomorfologiese donga ontwikkeling te bepaal met n fokus op die faktore wat dit dryf. Dit was gedoen deur middel van observasies gedoen terwyl veldwerk uitgerig was, fisiese metings asook ruimtelike en hidrologiese analises deur gebruik te maak van n Geografiese Inligting Stelsel (GIS). Die klassieke Malansdam donga stelsel is ‘n Strahler stroomorde (SSO) van 5 toegeken en is die eerste een ooit wat dit behaal het. Die mees aktiefste area was in die bolope waar die steilste helling ervaar was. Alhoewel ondergrondse pyp formasie waargeneem was het die V-vormige donga kanale en SAR data van die tradisionele nat analise aangedui dat afloop die dominante vorming proses was. Daar was gevind dat 'n duo van faktore, wat bestaan uit een menslike en een natuurlike faktor onderskeidelik, die faktore was wat donga ontwikkeling in die area dryf. 'n Unieke menslike faktor wat nog nie vantevore gepubliseer is, was bevind as die menslike faktor wat aanleiding gee tot donga erosie. Hierdie faktor is die bewerkiingsmetode wat in die Sandspruit opvangsgebied gebruik word naamlik geploegde kontoerbewerking. Die geploegde kontoere tree op as kanale om eerstens water te versamel en tweedens om die vloeirigting daarvan te wysig. Water wat onder natuurlike toestande afwaarts sou dreineer tot in die vallei word vasgevang deur die kontoere en gekanaliseer na een invloei punt in die donga. Hierdie proses verhoog die erosiekrag van die water aangesien groter volumes by 'n enkele punt in die steiler bolope van die donga stelsel invloei. Die dryf faktor in die natuurlike groep was swak grond struktuur. Die oorsaak hiervan was die besetting van ‘n grootmaat uitruilbare Mg2+ katione op die uitruil plekke van die kleifraksie. Dit sou veroorsaak dat grond in die teenwoordigheid van water maklik sou dispergeer, selfs in die teenwoordigheid van 'n lae hoeveelheid uitruilbare Na+ katione. Metodes om donga erosie te bekamp sal dienooreenkomstig bestaan uit die herstel van die grondstruktuur en die toepassing van 'n alternatiewe gondbewerkings stelsel. Die aanplanting van plantegroei, veral gras en koring binne die donga kanale sal verder help met die veg tot bekamping Die tweede doel van hierdie projek was om te bepaal indien naby infrarooi (NIR) spektrometrie (met golfnommer van 12 500 – 4 000cm-1) oor die vermoë beskik om aanwysers wat traditioneel in grondkunde gebruik word om die dispergering van grond te meet te voorspel. Hierdie aanwysers sluit vyf indekse in naamlik die Veranderlike Natrium Persentasie (ESP), Natrium Absorpsie Verhouding (SAR), Magnesium Versadiging Persentasie (MS%), Elektriese Geleidingsvermoë (EC) en die Potensiële Waterstof (pH) sowel as die vier hoof uitruilbare katione naamlik kalsium (Ca2+) , kalium (K+), natrium (Na+) en magnesium (Mg2+). Oppervlak en ondergrondse grondmonsters is ingesamel by die punt van oorsprong by aktiewe dongas. Hierdie monsters is minimaal voorberei, dus slegs gedroog, gemaal en gesif. Daarna was dit onderworpe aan die NIR analise. Die Bruker meerdoelige FT-NIR Analiseerder (MPA; Bruker Optik GmbH, Duitsland) met 'n spektrale omvang van 12 500cm 1 4000cm-1 is hiervoor gebruik. Parsiële kleinste kwadraat regressie (PLSR) modelle is gebou vir elke indeks asook die uitruilbare katione deur gebruik te maak van die nutsprogram Quant 2 van die OPUS 6.5 (MPA; Bruker Optik GmbH, Duitsland) sagteware. Vyf verskillende regressie statistieke naamlik die bepalingskoëffisiënt (r2), vierkantswortel fout tydens kruis validasie (RMSECV), verhouding van prestasie teenoor voorspellingsafwyking (RPD), sydigheid en die verhouding van prestasie van kwartiele (RPIQ) was gebruik om die geldighied van elke PLSR model te asseseer. Alle PLSR modelle het goed presteer, behalwe vir MS% wat verdere navorsing vereis. NIR beskik dus oor die vermoë om die aard van dispergering van grond te bepaal op 'n vinnige, betroubare, goedkoop en nie afbrekende manier. Dit kan dus effektief aangewend word as ‘n substitusie vir die traditionele metodes om te bepaal as grond a beduidende faktor is of nie.

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