Ž . Catena 39 2000 11–31 www.elsevier.comrlocatercatena Some physical and chemical properties of sediments ž / exposed in a gully donga in northern KwaZulu-Natal, South Africa and their relationship to the erodibility of the colluvial layers Suzanne M. Rienks a , Greg A. Botha b, ) , Jeff C. Hughes a a ( ) Soil Science, School of Applied EnÕironmental Sciences, UniÕersity of Natal Pietermaritzburg , PriÕate Bag X01, ScottsÕille 3209, South Africa b Council for Geoscience, PO Box 900, Pietermaritzburg 3200, South Africa Received 16 April 1998; received in revised form 31 May 1999; accepted 27 September 1999 Abstract Ž . Gullies locally termed dongas are a characteristic feature of the landscape in central KwaZulu-Natal and are highly associated with widespread, stratified colluvial sediments and the buried palaeosols that have formed in this material. Research objectives were to find out which sediment property or properties contribute most to the vulnerability of the colluvium to gully erosion; to study the erosion susceptibility of the sequence of buried stratigraphic layers and palaeosols; and to improve understanding of the relationship between erosion susceptibility of the colluvial layers and palaeosols and their disposition in the landscape. Materials from several colluvial layers and palaeosols exposed in the donga sidewall of the Dabekazi donga, 13 km north of Nqutu in central KwaZulu-Natal, were subjected to chemical and mineralogical analyses, several dispersion and erodibility tests and were characterized physically. Exchangeable sodium Ž . percentages ESP ranged from 0% to 23% and the dispersivity and erodibility tests revealed a range of dispersion potential and erodibility across the sequence of colluvial layers and palaeosols. However, a poor correlation was found between the results of different dispersion tests, as well as between dispersion tests and dispersion-related properties. Dispersion tests showed significant correlation with silt content. An erodibility test using a flume showed very strong correlation with Ž . Ž . electrical conductivity EC and sodium adsorption ratio SAR of the saturated paste extract and ESP values. The plasticity index ranged from 0% to 0.23 and estimated hydraulic conductivities ranged from 33 to 0.7 mrday. High correlation with ESP and EC suggests that dispersion plays an important role in the erodibility of the materials under conditions of short duration, turbulent water ) Corresponding author. Tel.: q27-331-456265; fax: q27-331-949342; e-mail: gbotha@pmb.lia.net 0341-8162r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. Ž . PII: S0341-8162 99 00082-X