Environmental Engineering Reference
In-Depth Information
Fig. 16.9 Cement weirs built along the Kromme River, Eastern Cape, South Africa. These weirs
are built to restore the river by stopping the headcut from proceeding backwards up the river. This
traps sediments and allows vegetation, such as palmiet in this photograph, and eventually
wetlands to recover
Since 1931, the first recorded flood, there has been a major flood approximately
every decade, the exceptions being the 1940s and the 1970s. In the 1980s there
were two major flood episodes, the first being a series of three consecutive floods
in 1981 and the second in 1983. The 1996 floods were described as the largest ever
experienced in the catchment. In the past decade three major flood events have
been recorded: 2004, 2006, and 2007 (Figs. 16.7 , 16.8 , 16.9 ).
Erosion Damage in the Kromme River
In the Kromme River, headcuts formed as a result of activities which disturbed the
Kromme River's path. Examples from the Kromme were the building of a provincial road
(the R62), the building of the railway line through the wetlands, river or floodplain. The
damage done was exacerbated by farmers allowing animals to graze in these disturbed
areas, or ploughing these areas up for agriculture. These activities created 'nick-points' or
weaknesses which lead to rapid erosion and the loss of the eroded sand and gravel
downstream. The nick-points migrate upstream and create progressively wider and deeper
head-cuts and dongas over time. This process was rapidly accelerated during the large
floods in the Kromme Catchment. The channels formed by the headcuts are detrimental
because they drain groundwater from the surrounding alluvium, drying it out and reducing
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