In December 2011, Dar es Salaam city experienced heavy rains that resulted in unprecedented flash floods that devastated many areas of the city.
At least 23 people were killed in the disaster and 4,909 people were displaced. Businesses were forced to close and thousands were left homeless as the city became inundated with floods.
The impact of the floods is still a threat in the memories of most residents of Dar es Salaam (DSM) hence the need to initiate mitigation measures to reduce or prevent occurrence of floods in the future as well as adaptation measures to speed up saving of people lives if floods will occur.
Many people are asking” Will floods affect Dar es Salaam City Again?This article provides a brief description on floods and tentative answer to the question – which the community should take note and contributes to its improvement. Some people believe that the floods in Dar es s Salaam city were a result of God wishes hence praying for better future. However scientific analysis indicates that the severity of the floods was a combination of human environmental degradation in the Coast region and Dar es Salaam city and not an act of God.
There are two main types of floods namely normal floods and flash floods.Normal floods refers to an overflow of water onto normally dry land area caused by rising water in an existing waterway, such as a river, stream, or drainage ditch. It may last days or weeks involving water rising and overflowing from its normal path but with tolerable destruction.
Flash flood is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds and urban streets sweeping everything before them. Flash floods appear quickly and move swiftly across land with little warning.
The water in a flash flood moves at such a high velocity that it can move boulders, uproot trees, demolish buildings, and destroy bridges. Flash floods may transform a normally calm area into a powerful river of death in only a few minutes or hours as many people are caught unprepared for the power and speed of a flash flood, which results in dangerous situations
Floods make an enormous impact on the environment and society. They destroy drainage systems in cities, causing raw sewage to spill out into bodies of drinking water. Depending on severity, floods damage and destroy building, and may cause millions of dollars’ worth of damage to a city, including evicting people from their homes and ruining businesses. Floodscause significant amounts of erosion to coastal ecosystems, leading to more frequent flooding if effective mitigation measures to prevent the situation are not taken in time.
The Dar es Salaam flood of December 2011 is a good example of flash floods. Records shows that on March 12, 1928, the Saint Francis Dam in Los Angeles broke down causing flash floods that within one hour obliterated houses, ranches, automobiles, animals and more than 500 people were killed and Santa Paula was overrun by water. Also on the 18th April, 1997 the Red River flooded over the dikes into Grand Forks, North Dakota. By the 19th April 1997 the flood waters had spread over a large area of Grand Forks and East Grand Forks. Roughly 60,000 people were forced from their homes and downtown Grand Forks was left in flames. Deforestation was cited as one of the factors that caused the floods.
Prevention methods for floods include: creation of floodplains and construction of winding streams to hinder the accumulation of water by providing a route for the drainage of water; protection of wetlands and forest coverthat helps to maintain a natural drainage system to provide a place for the excess water to gather.
Experience has proven that if a river cannot handle the load of water it's required to carry, it must rise. With enough water, it must rise above its banks and flood. The faster water runs from the watershed into the river, the higher a flood will be. Thus anything that increases runoff speed like deforestation,excessive pavement and destruction of wetlands will contribute to floods.Deforestation plays several roles in the flooding equation because trees prevent sediment runoff and forests hold and use more water than farms or grasslands. Main attributes of forest cover in preventing floods that are destroyed through deforestation include:
lSome rainwater stays on the leaves, and it may evaporate directly to the air. The more water used in the forest watershed, the less remains to run off to the down stream
lTree leaves cushion raindrops, allowing more water to enter the soil. But when uplands are stripped of trees, rainwater instead of being absorbed is immediately released to the streams and rivers. As a result rains that might have caused small problems now cause floods
lTree roots absorb water from the soil, making the soil drier and able to store more rainwater. Also tree roots hold the soil in place, reducing the movement of sediment that can shrink river channels downstream.
Various research findings have concluded that deforestation of upstream catchment areasis the main cause of destructive floods to major cities in the world. For example it has been concluded that, deforestation and destruction of wetlands were the main cause of the huge China Yangtze floods of 1998 and not due to heavy rains, El Nino or global warming. To prevent future floods, the Government of China is reported to use over US$ 2 billion per annum to reforest the Yangtze basin and encouraging active community participation to conserve forest ecosystems.
In February 2000, Mozambique experienced devastating floods. The death toll is unknown, but probably is in the thousands. Scientific analysis on the cause of the floods hasconcluded thatdeforestation which had removed over 99 percent of tree cover in the forest watershed of the Limpopo River over the past 50 years was the underlying cause of the floods in Mozambique. The floods arising from deforestation, destroyed homes and businesses, killing livestock, and apparently setting the Mozambique economy back by several decades.
If effective environmental conservation measures of Dar es Salaam city are not taken by linking upstream and downstream initiatives, the Dar es Salaam city could experience worse unprecedented flash floods than those of December 2011 in the future. Concerted efforts are therefore required to intensify awareness of policy makers and the general community on existing opportunities to mitigation the situation and possible costs and impact of not taking appropriate actions in time.
Dar es Salaam is the largest city in Tanzania and its estimated population by end of 2011 was around 4.5 million people with an annual population rate increase of 4.6%.
Dar es Salaam has tropical climatic conditions, typified by hot and humid weather throughout much of the year. Annual rainfall is approximately 1,100 mm and in a normal year there are two distinct rainy seasons: "the long rains", which fall during April and May, and "the short rains", which fall during October and November.
On landscape coverage, Dar es Salaam is surrounded by Coast region with slight higher altitude.Experience has shown that the main contributor of devastating flash floods in DSM is mainly forest degradation and deforestation in the Coast region. Other contributors includes: poor drainage system, unplanned construction and destruction of wetlands within the city ecosystem.
An analysis of forest and vegetation cover of Coast region conducted in 2011 by the Rural Energy Agency (REA) in collaboration with the Ministry of Natural Resources and Tourism, confirmed that Coast region has lost most of its forest cover and utilisation of wood resources in the region is not renewable hence causing environmental degradation.
The Coast region has a total forest reserve area of 444,440 ha. However, most of the forest reserves have been degraded to grass land. The total biomass harvested from the Coast region in 2011 was 2,862,574 m3. However, available biomass in coast region in 2011 that could be harvested on sustainable basis without causing environmental degradation was around 896,288 m3 of biomass. Matching the supply potential on renewable basis of 896,288 m3 and the total harvested biomass of 2,862,574 m3 in 2011, - the region experienced a biomass deficit of 1,966,286 m3 which was met through over harvesting of the existing forest stock, consequently causing forest degradation and deforestation.
To enhance sustainable development of Dar es Salaam city and to prevent future unprecedented floods, urgent and concerted efforts are required to ensure effective conservation of Coast region forest ecosystems at the upstream. Based on experience from the December 2011 floods in DSM, the costs for not taking effective actions could be destruction of Dar es Salaam decades’ of development efforts. Concerted efforts are therefore required from all relevant stakeholders to ensure future rains in Coast and DSM regions should be a blessing, contributing to water supply and robust agriculture production and not devastating flash floods.
Losses resulting from the December 2011 floods in Dar es Salaam were intensified by the un-preparedness of the community on floods adaptation measures (ways to reduce damage). Few on ways to reduce flood damage that the general community should be aware include:
lIntensify community awareness on the need to listen to Dar es Salaam daily weather forecastthrough Radio and TV and to take into consideration technical advices provided. For example recent weather forecast for the next three months period (September to November 2012) has reported possibilities of heavy rains associated with floods.
lIf trapped inside a house, you should climb to the highest place and wait for rescuers to find you and avoid temptation to swim outside.
lThose living in lowland areas prone to floods they should have at least two ladders in their premises to facilitating climbing to higher places when trapped by floods.
lTurn offall electricity and gas utilities when you notice signs of floods. Always keep a working torch (flashlight) at an accessible location to facilitate your location at night during floods.
lWhile floods are ongoing do not travel or drive in flooded areas, do not go near power lines and electrical wires also beware of wild animals like snakes.
lIf your house was flooded and floods have stopped, please make sure your house is safe before entering.Use a flash-light in case of darkness and do not turn on your electricity power until confirmed to be safe.
lClean and remove contaminated flood water in your house and take note of any other flood preventive measures that will be announced by appropriate authorities.
Weather forecasts predict that Dar es Salaam will continue to experience periodic rains associated with floods. Intensity and impact of floods to the community will be influence by mitigation and adaptation measures initiated by the community. As floods will affect all sectors of the community, concerted participatory efforts are therefore required to raise awareness on strategies of reducing floods impact and preventive actions to be considered by those trapped by floods.
Bariki K. Kaale is an energy and environmental specialist
Degraded RuvuSouth Forest Reserves – in Kisarawe
Degraded Kazimzumbwi Forest Reserves – in Kisarawe