HUMAN CAUSES

PHYSICAL CAUSES

• Deforestation of hillsides. This reduces the strength of hills by removing route systems. It also reduces the amount of interception and transpiration. The reduced interception and transpiration means that the soil becomes saturated more quickly increasing surface run-off and the stress on slopes. This increased stress can cause landslides to happen.
• Building on marginal land. Increasing rates of rural-urban migration have meant that more and more people are building on marginal land. Marginal land includes floodplains and steep slopes which are unsuitable for settlement building.
• No building regulations. Most settlements built on marginal land are informal. This means that residents have built the houses themselves, using any available material. The houses will be weak with no real foundations so are very vulnerable during times of flood. In Brazil and Rio many of these informal settlements are called favelas. Most of these settlements will not have any drainage. This will increase the saturation of soil and increase the likelihood of floods.
• Population density. Most informal settlements have extremely high population densities. These high population densities mean that any floods are going to affect a larger number of people.
• Poor transport and communication. Poor areas were worse affected, which meant many people received no warning of the floods because they did not have access to media sources. In addition they would have had little opportunity to live the area, even if they did know. The poor transport and communication also meant that rescue efforts were made much more difficult.

• Steep drainage basins and valley sides. Much of Rio and the surrounding area is very mountainous. The steep valleys means that any rainfall reaches streams and rivers very quickly causing flash floods.
• High levels of precipitation. It is estimated that areas around Rio received the equivalent of a months rainfall in just two days, 11th January and 12th January.
• Brazil experiences a tropical climate and the south east of Brazil (around Rio de Janeiro) can experience over 4 metres of rain a year. The majority of the rains falls during the summer months (including January). This means that during the summer the ground remains largely saturated, reducing infiltration rates and increasing surface run-off.
• La Nina - During 2010 and early 2011 Brazil has suffered increased rainfall caused by La Lina. La Lina is an oceanic-atmospheric system that can cause increased rainfall by changing temperatures.
• Mudslides. Although many of the causes of mudslides were human (deforestation, building on marginal land) the mudslides themselves were a secondary hazard of the flood water. The flood water saturated the ground, increasing the stress on the slopes and causing mudslides to happen.

MORE HUMAN FACTORS

MORE PHYSICAL FACTORS

• Urbanisation can increase flooding, because the construction of roads and buildings increase the amount of impermeable surfaces.
• Global warming can increase the frequency of tropical storms, cause greater snow melt and increase the magnitude and frequency of storm events.
• Terracing and contour ploughing can actually reduce the speed of surface run-off, but other farming methods like building irrigation canals, deforestation and ploughing against the contours of the land can reduce interception, increase surface run-off and therefore the risk of flooding.
• Protecting one area with embankments or river straightening can cause increased risk of flooding further downstream.

• Impermeable rock tends to increase the risk of flooding because less precipitation can infiltrate.
• Some countries suffer snow melt during late spring and summer, this can greatly increase the discharge of rivers and therefore increase the risk of flooding.
• Tropical storms can increase precipitation and also create storm surges which can increase the height of the water table and increase the risk of flooding.
• Sedimentation of rivers either naturally or because of human actions can cause rivers' cross-sections to become smaller and increase the risk of flooding.