10.2.1 Flash floods and their causes
Flash floods are floods that happen as a result of intense rainfall in a localised area and they usually drain away quite quickly (Few, 2006). If a large amount of rain falls onto the land in a short time and it cannot soak into (infiltrate) the soil, the water runs downhill across the land surface. This heavy flow of surface run-off can cause a flash flood.
There are many factors that increase the risk of flash floods in Ethiopia, including changes in land use and poor soil permeability (Abaya et al., 2009). As you read in Study Session 6, when forests and grasslands are removed for the construction of roads and buildings, permeable soil is replacedby hard surfaces such as concrete. This reduces the infiltration of water into the ground and increases run-off,making flooding more likely. The proportion of Ethiopia’s land area that is covered by forests has decreased significantly in recent decades, as Study Session 1 demonstrated (see Figure 1.3).
How do forests help rain to soak into the soil?
Forests play an important role in the water cycle because the roots of plants reach deep into the soil and create space between soil particles; this increases soil permeability, so when it rains the water can infiltrate the soil.
Failures of dams and reservoirs can also lead to flash floods. Many micro-dams have been constructed in Ethiopia for irrigation, water supply and generating hydroelectric power. Dam failure can occur if they are not properly constructed, or heavy rainfall may cause ‘overtopping’ (water flowing over the top of the dam when it is full).
Flash flooding has happened many times, in many parts of Ethiopia. This causes significant harm to human life and property because it happens so unexpectedly and local people are unprepared (Greenough et al., 2001). For example, the flash flood on 5 August 2006 in Dire Dawa caused 256 deaths and displaced 9956 people (Early Warning Department, Federal Disaster Prevention and Preparedness Agency, 2007).
10.2 Floods