Hydrology of the Limpopo River Basin: Flooding

Flooding is a major problem in the Limpopo River basin, particularly in the lower Limpopo River, across the coastal floodplain in Mozambique. This low lying region is particularly susceptible as during periods of high flow, it receives a large portion of the waters from upper basin.

The Limpopo River basin has been subject to several significant flooding events over the recent years. All of these events are associated with heavy rainfall resulting from tropical cyclones.

As stated in the Flooding section of Principles of Hydrology, the rapid conversion of heavy rainfall to flood waters is increased when precipitation falls on saturated or near saturated soil. Several severe floods have been recorded in the past 50 years - 1955, 1967, 1972, 1975, 1977, 1981 and 2000. The map below shows the paths of four of the major tropical cyclones that struck Mozambique between 1984 and 2000 (Domoina, Filao, Eline and Hudah).

High flows often result in inundation as flat channel gradients mean that the transition beyond bank-full is rapid (PBA 2007).

Root-Causes of Flooding

Flooding in the Limpopo River basin is generally caused by a series of primary factors:

• Heavy, episodic, localised rainfall, often associated with tropical cyclone activity and saturated soils from preceding events;
• Poor land use management practises, including land-clearing and poor agricultural land management in the upper river basin;
• Associated soil erosion and increased run-off; and
• Lack of integrated management of upstream dams and wetlands.

Sources: Amaral and Sommerhalder 2004; Leira et al. 2002

Loss or damage to wetland can have a significant impact on flooding as wetlands can act as a buffer in times of high flow.  Wetlands in the upper river can absorb and retain water, buffering the effects of heavy rainfall and increased stream-flow.  While wetlands do contribute to flow control, their capacity is finite and once saturated, they have little impact in terms of reducing the impacts of flooding.

The animation below illustrates the monthly average rainfall from March 1999 through to February 2000, as measured by the NASA/JAXA TRMM sensor. The animation shows the increased precipitation associated with Cyclone Eline during the period of February 2000 that resulted in the flooding in Mozambique.

Video: Monthly Average rainfall for the southern African region, leading up to Cyclone Eline in 2000.

Video Source: NASA/Goddard Space Flight Center Scientific Visualization Studio

Impacts of Flooding

The negative impacts of the flooding in 2000, and many of the previous events, are not solely limited to the tragic loss of life and the immediate economic impacts of loss of property and crops. The floods of 2000 were estimated to have had a significant long term impact on the economy of Mozambique, with some experts suggesting as much as a 20% reduction in Gross Domestic Product as a direct result of the floods.

Furthermore, the severity of the floods devastated the top soil of large portions of agricultural land in the lower Limpopo River basin. In some cases, soil was completely removed, exposing the bedrock below.

An additional unforeseen impact in Mozambique was the displacement of unexploded remnants of war (landmines) from the civil war. Flood waters are thought to have moved thousands of landmines. These mines were previously located in areas that were known, mapped and avoided, but once the flood waters receded, many were moved to new locations, posing a significant threat to the local people (Wareham 2000). This is an unfortunately common occurrence in former conflict zones at risk from flooding and severe storms.

The floods of 2000 also had significant impacts in Botswana, South Africa and Zimbabwe, with the Limpopo River rising to its highest levels in over 15 years (Dartmouth Flood Observatory 2000), causing wide spread damage to property and loss of life (human and wildlife).

The satellite image below is from the RADARSAT-1 satellite, showing the extent of the floods on the lower Limpopo River basin on the 23rd of February 2000.