Flooding across parts of south-east Kenya

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  • Figure 1. A MODIS satellite image of south-east Kenya from 2009. Source: NASA.

    Figure 2. A MODIS satellite image of south-east Kenya from 10th May 2012. Flooding around the Gallina River can be seen to the north and west of Malindi. Source: NASA.

    Figure 3. Forecast rainfall across Kenya over the 72hours from 0UTC on the 12th May 2012. The red flag is the location of Malindi. (from ECMWF).

  • Flooding across parts of south-east Kenya
    12.05.2012 12:55

     

    Kenya’s rainy season has been particularly wet this year, isolating rural communities, damaging roads and killing at least 50 people since the rains began back in March. The flooding was caused by the Galana River, which broke its banks in several places close to the city of Malindi, situated on the Indian Ocean coast (see figures 1 and 2).

    The rainy season occurs across Kenya when the Intertropical Convergence Zone (or ITCZ) moves over the region. During April and May the ITCZ moves northwards, bringing the first and longest period of wet weather to the country. Then during October through to December the ITCZ moves back south, giving a secondary rainy spell. The ITCZ is a semi-continuous line of cloud, showers and thunderstorms which encircle the Earth, marking the point at which northerly winds from the Northern Hemisphere and southerly winds from the Southern Hemisphere come together, or converge. As these two air-masses meet, they are forced to rise, creating showers and thunderstorms. Over land, the ITCZ moves north and south of the equator in line with the sun’s zenith point (the point on Earth where the sun is directly overhead at midday). This explains why equatorial regions (such as Kenya) have two wet seasons.

    This year’s rainy season has been one of the wettest for several years. The last few years have seen below average rainfall, and one of the causes of this may be the fact that the Pacific has experienced La Nina conditions. In a La Nina year, sea surface temperatures across the Equatorial Pacific are lower than usual, which can have a significant affect on global circulation, particularly across the Tropics. Usually during a La Nina phase, convection across eastern Africa in relation to the ITCZ is suppressed, leading to below average rainfall. The last few months have seen a return to neutral conditions across the Pacific as sea-surface temperatures return to normal, and a likely reason why the rainy season this year has intensified compared to the last couple of years. As well as the climate variability, deforestation of the region has been blamed for the floods. Significant clearance of land for agriculture and timber has left large areas of the country un-vegetated. Trees intercept rainfall, slowing down the flow of water into rivers. On bare ground, water quickly washes off into rivers, causing them to rise rapidly.

    Forecasts models for the next 72 hours indicate that further rain is expected across Kenya (see figure 3). Between 30 and 50mm of rain is expected across the worst affected areas around Malindi, as well as 30-40mm across the Kenyan Highlands where the Galana River begins. Therefore, with the ground already saturated, further flooding may be a possibility over the coming week.

    By: Chris Burton