Windhoek flooding not exceptional

DESPITE widespread damage to many homes in Windhoek as the Arebbusch River came down in a flash flood on Friday, the flood was in fact much smaller than the one that caused similar havoc two years ago.

Deputy Director of Hydrology in the Department of Water Affairs Guido van Langenhove told The Namibian yesterday that the flood damage could not be predicted and although the flooding of the Arebbusch River could be considered “major”, it was hardly “exceptional”. The kind of flooding experienced last week, he said, could be expected on average every five to ten years.”But that is only a long-term statistical average.It might happen two years in a row and then not again for another ten years or more,” said Van Langenhove.A flood is termed “exceptional” if it is of the magnitude experienced on average only once every 50 years.Van Langenhove said he was at Acacia on the western outskirts of the city, where the Arebbusch flooding had the greatest impact on human lives and property, shortly after the waters crashed through houses, but said he did not find the flooding to be out of the ordinary.He said the magnitude of a flood could not be determined based on the kind of damage it causes to human infrastructure, if the flooding was as a result of natural conditions.Considering the reasonably small catchment area of the Arebbusch River in the mountains around Windhoek, and the vast amount of rain that fell in the area on Friday, Van Langenhove said it could be expected that the water would arrive downstream at Acacia in full flood within three to four hours of the downpour.The rain that fell earlier in the week had saturated the riverbed and sped up the passage of the water through the city, said Van Langenhove.In his opinion, much of the damage to property could have been minimised, especially at Acacia, if houses had been built further away from the river.”The river cannot be blamed.The flooding was as a result of natural conditions.It depends on people and what they are prepared to accept as a risk,” said Van Langenhove.He added that people could never be fully assured that they were not at risk of flooding if their property was situated in close proximity to a river.”There is no guarantee that it wouldn’t happen soon again.Flood lines are based on how often a flood of a certain magnitude may occur in the long term.The municipality has the flood lines, they can tell the risk you can run [by building close to a river]”, said Van Langenhove.”But flood lines are not 100 per cent accurate.Common sense would tell people that they should be a little back from the river.”In 2004, the flooding of the Arebbusch River was greater than any experienced since 1934, and sparked a series of surveys by the hydrology department to establish its magnitude.According to local authority legislation, development may only take place if the one-to-50-year flood line [the magnitude of a flood expected to occur once in 50 years] has been taken into consideration.Two years ago, the Arebbusch River flooded to above the one-to-100-year flood line.Much of the flooding was put down to obstructions in the river course, such as building rubble and other dumped waste.Since then the municipality has been on regular missions to ensure the watercourses are free from obstruction.Friday’s flooding caused a smaller area to be inundated than two years ago and Van Langenhove said it didn’t warrant further investigation.By the time the Arebbusch River raged through Windhoek on January 16, 2004, the Windhoek area had received 262,2 mm of rain since the beginning of that month.In the last nine days, Windhoek has received 183,8 mm of rain – as much as 160 mm falling in Thursday and Friday alone.On Thursday the largest amount of rain, 80 mm, fell in the Hochland Park area, while on Friday Suiderhof received as much as 85 mm.During the whole of January 2005, only 67,8 mm of rain fell in Windhoek.According to the SADC drought-monitoring centre, most of Namibia is expected to receive normal to below-normal rainfall during the January to March rainy season.In the north-east, where the Zambezi River has the potential to flood, normal to above-normal rainfall is expected.The kind of flooding experienced last week, he said, could be expected on average every five to ten years.”But that is only a long-term statistical average.It might happen two years in a row and then not again for another ten years or more,” said Van Langenhove.A flood is termed “exceptional” if it is of the magnitude experienced on average only once every 50 years.Van Langenhove said he was at Acacia on the western outskirts of the city, where the Arebbusch flooding had the greatest impact on human lives and property, shortly after the waters crashed through houses, but said he did not find the flooding to be out of the ordinary.He said the magnitude of a flood could not be determined based on the kind of damage it causes to human infrastructure, if the flooding was as a result of natural conditions.Considering the reasonably small catchment area of the Arebbusch River in the mountains around Windhoek, and the vast amount of rain that fell in the area on Friday, Van Langenhove said it could be expected that the water would arrive downstream at Acacia in full flood within three to four hours of the downpour.The rain that fell earlier in the week had saturated the riverbed and sped up the passage of the water through the city, said Van Langenhove.In his opinion, much of the damage to property could have been minimised, especially at Acacia, if houses had been built further away from the river.”The river cannot be blamed.The flooding was as a result of natural conditions.It depends on people and what they are prepared to accept as a risk,” said Van Langenhove.He added that people could never be fully assured that they were not at risk of flooding if their property was situated in close proximity to a river.”There is no guarantee that it wouldn’t happen soon again.Flood lines are based on how often a flood of a certain magnitude may occur in the long term.The municipality has the flood lines, they can tell the risk you can run [by building close to a river]”, said Van Langenhove.”But flood lines are not 100 per cent accurate.Common sense would tell people that they should be a little back from the river.”In 2004, the flooding of the Arebbusch River was greater than any experienced since 1934, and sparked a series of surveys by the hydrology department to establish its magnitude.According to local authority legislation, development may only take place if the one-to-50-year flood line [the magnitude of a flood expected to occur once in 50 years] has been taken into consideration.Two years ago, the Arebbusch River flooded to above the one-to-100-year flood line.Much of the flooding was put down to obstructions in the river course, such as building rubble and other dumped waste.Since then the municipality has been on regular missions to ensure the watercourses are free from obstruction.Friday’s flooding caused a smaller area to be inundated than two years ago and Van Langenhove said it didn’t warrant further investigation.By the time the Arebbusch River raged through Windhoek on January 16, 2004, the Windhoek area had received 262,2 mm of rain since the beginning of that month.In the last nine days, Windhoek has received 183,8 mm of rain – as much as 160 mm falling in Thursday and Friday alone.On Thursday the largest amount of rain, 80 mm, fell in the Hochland Park area, while on Friday Suiderhof received as much as 85 mm.During the whole of January 2005, only 67,8 mm of rain fell in Windhoek.According to the SADC drought-monitoring centre, most of Namibia is expected to receive normal to below-normal rainfall during the January to March rainy season.In the north-east, where the Zambezi River has the potential to flood, normal to above-normal rainfall is expected.