Some physical and chemical properties of sediments exposed in a gully (donga) in northern KwaZulu-Natal, South Africa and their relationship to the erodibility of the colluvial layers

Gullies (locally termed dongas) are a characteristic feature of the landsca pe in central KwaZulu-Natal and are highly associated with widespread, stra tified colluvial sediments and the buried palaeosols that have formed in th is material. Research objectives were to find out which sediment property o r properties contribute most to the vulnerability of the colluvium to gully erosion; to study the erosion susceptibility of the sequence of buried str atigraphic layers and palaeosols; and to improve understanding of the relat ionship between erosion susceptibility of the colluvial layers and palaeoso ls and their disposition in the landscape. Materials from several colluvial layers and palaeosols exposed in the donga sidewall of the Dabekazi dongs, 13 km north of Nqutu in central KwaZulu-Natal, were subjected to chemical and mineralogical analyses, several dispersion and erodibility tests and we re characterized physically. Exchangeable sodium percentages (ESP) ranged f rom 0% to 23% and the dispersivity and erodibility tests revealed a range o f dispersion potential and erodibility across the sequence of colluvial lay ers and palaeosols. However, a poor correlation was found between the resul ts of different dispersion tests, as well as between dispersion tests and d ispersion-related properties. Dispersion tests showed significant correlati on with silt content. An erodibility test using a flume showed very strong correlation with electrical conductivity (EC) and sodium adsorption ratio ( SAR) of the saturated paste extract and ESP values. The plasticity index ra nged from 0% to 0.23 and estimated hydraulic conductivities ranged from 33 to 0.7 m/day. High correlation with ESP and EC suggests that dispersion pla ys an important role in the erodibility of the materials under conditions o f short duration, turbulent water flow. However, colluvium in which deep gu llying occurs is not necessarily strongly sodic. Also, the degree of erodib ility of the materials differs depending on the test used, illustrating the complexity of the interactions that govern dispersion and erosion. The cre dibility of the colluvial layers and palaeosols from the Dabekazi donga, an d properties with relevance to dispersion, differ considerably down the pro file, indicating that geomorphic threshold conditions and position in the l andscape with respect to surface runoff hydrology also codetermine dongs er osion. Hydraulic conductivity estimates of the investigated materials are r elatively high and do not differ sufficiently to enable lateral subsurface flow through, and piping of, these materials to be the main causes of donga formation. (C) 2000 Published by Elsevier Science B.V. All rights reserved .