A geomorphological investigation of the Klip River Wetland, south of Johannesburg
The Klip River Wetland is located south of Johannesburg. The city is a major conurbation which developed as a result of mining activity. The wetland has, and always had, economic importance to the region, firstly as a water source and later as a purifier of polluted water reporting from the urban-mining-industrial complex. A previous investigation determined that this wetland is in an advanced state of collapse. A network of irrigation canals dug in the early part of the 20th century to support agriculture in the area, provided a preferred discharge conduit for the sewage pumped into the Klip River. These irrigation canals have cut back and avulsed forming an almost continuous channel, with little water flowing through the wetland. The wetland’s capability to remove contaminants from the water is therefore compromised. The geomorphological study presented here aims at the quantification and the better understanding of: • The extent of the degradation • The rate of change to the wetland and • To offer potential remedial solutions. Desktop analysis was conducted on the following data: • Aerial photography (1938, 1952, 1961, 1984, 2003, 2006) • Historical maps from 1917 • High resolution topography data (LIDAR), • Water quality • Water discharge In order to quantify the degradation of the wetland and associated rates, the following factors were considered: • Wetland area and shape • Channel formation, propagation and widening • Knick point migration. The major findings of this study are: • The analysis of wetland area did not prove as effective a method as the study of the channels and knick points. • Two major knick points are distinguished. The first is located at the outlet of the Olifantsvlei Sewage Works and the second represents a major rapid near Kromvlei. The latter has incised 5 m into the wetland. • Knick point migration was not detected between 2003 and 2006, except for the rapid near Komvlei where the knick point migrated a significant 118 m in two years. • Between Kibler Park and Kromvlei the wetland experienced the greatest head cut advance of all the sections, most significantly in the periods 1952 to 1961 and 2006 to 2008. • The period of the most significant change for all channel width monitoring points was between 2001 and 2003. Results of this investigation confirm that the current state of the Klip River Wetland is dire. Large tracts of wetland have disappeared over the last 100 years. The wetlands that remain are not functioning optimally. The importance of the wetland to purify the sewage, industrial and mining effluent is paramount. The window of opportunity to save or restore at least parts of the wetland, and therefore maintain the purification potential of the wetland, is closing. Peat burns will lead to erosion of the soils and therefore the reworking of the pollutants into the river system. It is anticipated that if the Klip River Wetland collapses completely, the water quality will worsen along the Vaal River and in the Vaal Barrage, which will impact negatively on the downstream users. The proposed solution to protect the wetland from further erosion and head-cut advance is to force the water out of the channels and into the wetland. By building a number of weirs in the channels, it is possible to dam the water upstream of the weir, thereby forcing the water to flood the wetland again.