Flood Protection
Geographical Background

Being located on a floodplain, the Umfolozi flats and the sugarcane lands thereon have been subjected to numerous floods in the past with varying degrees of flood damage. The 20 000 ha Umfolozi floodplain is relatively level, is about 30 km long and has an average width of 7 km. The height above mean sea level varies from 20 m at the upstream end to 1 m near the coast.

The Umfolozi river has a catchment of approximately 10 000 km². The average annual run-off appears to be between 393 and 887 million m³.

Three important features of Umfolozi River are:

  • Very erratic discharges.
  • Large floods are fairly common (24 in 43 years).
  • A flood can occur at any time of the year.

Demoina and the Umfolozi floodplain

The flood on the Umfolozi River, which resulted from tropical cyclone Demoina on 31 January and 1 February 1984, is the largest in living memory. In hydrological terms, the event is classified as an extreme one approaching the maximum flood expected from the catchment. The 1984 flood in the Umfolozi resulted in an extreme sand deposit on the sugarcane lands in the upper reaches of the Umfolozi flats. The depth of the 2500 ha sand deposited is generally over 1 m and is as much as 3 m.

The sand damaged area was withdrawn from commercial sugarcane farming and managed as part of the flood control infrastructure which acts as a sediment filter protecting downstream farms and Lake St Lucia.

Various studies were conducted in the years after Demoina and the recommendations of most were uniform. To summarise some of the important findings:-

  • The area covered with sand to a depth of 0.5 m and more can be considered as irreparably damaged.
  • The sand deposit in its present state constitutes an ecological danger to the Umfolozi / Umzinduzi Rivers as well as to the estuary of Lake St. Lucia. In addition the sand deposit is also a potential threat to the adjacent farms. The stabilisation of the sand by suitable vegetation is considered essential.
  • The sand-damaged area should be managed as a floodplain that would act as a sediment filter protecting downstream farms and Lake St Lucia.
  • An area was needed that could be used to trap flood sediments.
  • this should be located on the upper flats, close to where the rock confined river valley waters start to spread out..
  • The site needs to have an elevation lower than that of the river levees and must be of such a size to accommodate many millions of cubic meters of sediment.
  • The trap site should have a vegetative cover that would encourage sedimentation.
  • Some earthworks to improve the trapping ability to retain sediments may be necessary.
    - The geomorphologic relationships, as determined from the 1937 aerial photographs, revealed that the present sand area used to previously function as a sediment trap during floods.
  • It was suggested that a “least interference” management plan for the Umfolozi Flats in which a “Sediment Feeder” is constructed be implemented. This entailed a weir/spillway constructed at the diversion point (position of current spillways) with an overflow height of 16.5 m above sea level. The normal river flow would be confined within the river and higher than normal flow would spill into the sand flat (trap depression).

This has since been implemented.

Drainage & Flood Infrastructure

The first drainage schemes were proposed in 1915 by the then Circle Engineer for Natal, Mr Fincham. These schemes constituted the construction of Wilson’s Drain (still part of today’s drainage infrastructure) as well as a diversion of the Umfolozi on the Western end of the floodplain into the Umzinduzi River. The latter project was unsuccessful as the first flood blocked the canal completely. In the early 1930’s the Mr. Hudson Spence (Circle Engineer for Natal) proposed drainage projects which saw the first mechanical draglines established on the Umfolozi floodplain. 

As mentioned above, the flood protection and drainage infrastructures on the Umfolozi floodplain have evolved over many decades and reference is made to various projects dating back as far as 1930.

In 1969 it was established that when floods in excess of 850 m³/s occurred, the excess water needed to be discharged with minimum damage. Due to the slope of the Flats from the Umfolozi, on the high side (North), to the Umzinduzi River, which meanders along the foot of the dunes at the low side (South), any excess floodwater spilled out of the Umfolozi  and found its way to the Umzinduzi. This resulted in it scouring a wide, deep gap through  which  water ran at a very high velocity taking with it most of the heavier sediment  which spoiled farmlands when deposited to a depth that can not be ploughed under.

In view of this, it was proposed to allow for discharging floodwaters in excess of 850 m³/s over a spillway on the South embankment. This was the first reference to the spillway concept and an effort to control/manage flooding as exercised up to this day.

Various permanent structures have since then been constructed including the following:

  • Three spillways totalling a crest length of 1185 meter.
  • An increase in the height of the river embankments on both sides of the Umfolozi river as to accommodate flows of 1200 m³/s in the river and approximately 9000 m³/s over the spillways, allowing for a total flood of 10200 m³/s.
  • Protection of the areas from excessive flooding through embankment and sluice gate systems.
  • Various main drainage canals (94.4 km) to reduce the inundation time after floods. 
     
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