BARCHAN DUNES ON THE KUISEB RIVER DELTA, NAMIBIA

Abstract

ABSTRACT The shape and behaviour of the barchan dunes on the Kuiseb River delta, south of the town of Walvis Bay, and the temporal and spatial variability of those characteristics are the focus of this study. Data from a field survey undertaken in August 1999 are used to investigate a number of salient aspects of dune morphology. The Kuiseb dunes are found to exhibit many of the same morphometric characteristics as those found in other dune fields around the world, such as the strong interrelationship between dune height and horn width (r = 0.853). Abnormalities in the transverse and longitudinal profiles of the dunes are noted however, and the causative factors investigated. Grain size analysis provides an insight into the intra-dune processes that may cause horn asymmetry and the absence of a slip face. Aerial photographs taken in 1961, 1976, 1988 and 1997, in conjunction with the results of the August 1999 field survey, are used to calculate rates and directions of barchan dune movement for 58 dunes between 1961 and 1997, and for 10 dunes from 1961 through to 1999. Between 1961 and 1997 the dunes moved at an average rate of 13.15m a′1, in an approximately northwards direction ([xbar]= 0.67°N). Over this period there has however been a substantial fluctuation in the rate of movement, from a minimum mean value of 4.24 m a″′ to a maximum of 18.9m a″1. The direction of dune movement has also fluctuated considerably between mean values of 12.58°N and 338.2°N. In the past two years rates of movement appear to have been very high, at 56.1m a″1, with the dunes moving in an average direction of 355.1 °N, although the data available for this stage has considerable limitations. Composite meteorological records derived from the National Centre for Environmental Prediction (NCEP) climate model suggest that the probable cause of this fluctuation in rates of dune movement (1961–97) is a variability in the regional wind regime. Comparative plots of mean wind speed during the three subsections of this 36 year period—1961 to 1976, 1976 to 1988 and 1988 to 1997—show significant changes in conditions. The first and last periods are characterised by markedly stronger mean winds, which can explain the faster rates of dune movement. This change in mean wind speed is related to fluctuations in the strength of the South Atlantic anticyclone, also illustrated by the NCEP data.