Cause of flash flooding


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Cause of flash flooding

  1. 1. The cause of flash flooding11.0 IntroductionNatural disaster can be the consequence of natural hazard or even from humanactivities, which may lead to destruction of buildings, environmental problem even lossof life. Malaysia is fortunate to be freed from natural disaster such as typhoon, earthquakeand volcano eruption.However, our country does not escaped from the natural disaster totally as themost occurring natural disaster in Malaysia is flood. In recent years, severe flooding hasoccurred in several parts of Malaysia, both as localized flash floods and as basin-widefloods on major river systemsFloods are the major disaster affecting many countries in the world year afteryear. It is natural phenomenon that cannot be avoided occurring from time to time in allrivers and natural drainage systems. It causes damage to lives, natural resources andenvironment as well as the loss of economy and health. The impact of floods has beenincreased due to a number of factors such as rising sea levels and increased developmenton flood plainTwo major floods that occur in Malaysia are monsoon flood and flash flood.Monsoon floods occur from cyclical monsoons during the local tropical wet seasonroughly around the month of October to March. Flood occur when heavy rainfall thatprolonged for several days or intense rainfall over short period of time causing river orstream increase in the water level that submerges the land. However, flash floodinghappens suddenly and without any warning.Most flash flooding occurs due to slow-moving thunderstorm that repeatedlymoving over the same area or heavy rain from hurricane and tropical storms. It takesabout several minute to hours to develop. It also could occur when man-made structuresfail such as collapse of dam. Flash flood occurs six hours after heavy rainfall, mean whileflood is longer lasted a week or more..
  2. 2. The cause of flash flooding22.0 Flash floodFlash flood was one of the most common and destructive weather-relatedphenomena that Malaysia experienced. In fact, due to the rapid development that thecountry is undergoing, more incidences of flash floods were reported than before andthey were not confined to the monsoon seasons only but rather occurred at any period ofthe year. Flash flood water moves at such a high velocity and usually carries a substantialamount of debris. Several factors contribute to flooding is the rainfall intensity andduration. Flash flooding occurs when a barrier holding back water fails or when waterfalls too quickly on saturated soil or dry soil that has poor absorption ability. Flash floodsmost often occur in normally dry areas that have recently received precipitation.Figure 1. Flash flood in Kuala Lumpur2.1 CauseFlash flooding have many reason for it occurrence, one of them is the due toclogging of drain. In Kuala Lumpur, the clogged drains and poor drainage system are themain factors causing flash floods. The rubbish inside the drain prevents the smooth flowof water. When heavy rainfall occur, the drain unable to support the amount of water thusan overflow occur causing flash flooding.Figure 2. Clogged drain
  3. 3. The cause of flash flooding3Flash flood also could occur when man-made structure such as dam and leveefailed. All types of dams may fail with the sudden release of water into the downstreamdrainage. Examples of dam failures such as in 1928, the Saint Francis Dam in LosAngeles failed. The Johnstown, Pennsylvania dam, built of earthen material collapsedafter a period of heavy rainfall in 1889.The Vaiont Dam in Italy failed when landslides that moved into the reservoirbehind the dam caused water to overtop the dam and in New Orleans, the levee systemsdesigned to prevent flooding failed during hurricane Katrina due to overtopping of leveesand floodwalls by the storm surge.Figure 3. Saint Francis Dam. Figure 4. New Orleans levee.Other than that, the increased of floods occurrences especially in urban area arelargely due to urbanization process that result in increase of impervious surface whichshortened the time of concentration and increase the magnitude of the runoff discharge. Italso decreases the amount of infiltration of the rainfall thus causing the water toaccumulate and flash flooding occurs.Figure 5. The peak in urban area is higher due to impervious area and water flowfaster in concrete drain.
  4. 4. The cause of flash flooding4It also occurs when the ground becomes saturated with water that has fallen tooquickly to be absorbed. The runoff collects in low-lying areas and rapidly flows downhill.Flash floods most often occur in normally dry areas that have recently receivedprecipitation, but may be seen anywhere downstream from the source of the precipitation2.2 EffectFloods make an enormous impact on the environment and society. Floods destroydrainage systems in cities, causing raw sewage to spill out into bodies of water. Also, incases of severe floods, buildings can be significantly damaged and even destroyed. Thiscan lead to catastrophic effects on the environment as many toxic materials such as paint,can be released into the rivers, and ocean, killing marine life.Floods may also cause millions of dollars worth of damage to a city, both evictingpeople from their homes and ruining businesses. Floods cause significant amounts oferosion to coasts, leading to more frequent flooding if not repaired. Other than that, thedestruction of farms has cause the increase of vegetable price. The monsoon season inother country has cause the imported price of greens soar.2.3 PreventionFlash flood cannot be stopped, however there are ways to prevent it fromrepeating itself. Flood plains can be created and the stream also can be widened. It helpsby hindering the accumulation of water by providing a route for the drainage of water.Also, the protection of wetlands helps to maintain a natural drainage system to provide aplace for the excess water to gather. Such devices allow the water to evaporate before itcan accumulate, creating flooding conditions.Other ways of preventing floods include placing sandbags along the riverbank toserve as levees and placing large rocks on banks to prevent erosion, which preventsriverbanks from deteriorating, remaining high enough so that flooding will not occur asfrequently. Levees also impede the collecting of water. With a levee, a huge amount ofrain water can be accumulated before flooding occurs. However, one drawback to theiruse is that if water is able to get past the levees, then the flooding is significantly worseand causes several times more damage.
  5. 5. The cause of flash flooding5Figure 6. LeveeIn Malaysia, there are several project that been done in other to reduce the flashflooding such as in Kuala Lumpur, Stormwater Management and Road Tunnel(SMART)had been build to divert large volumes of flood water from entering the holding pond,bypass tunnel and storage reservoir. This will reduce the flood water level at the JalanTun Perak Bridge, preventing spillover. According to the Star Online, there are severalprojects are implemented at various areas in Kuala Lumpur to reduce the flash floods. Anew retention pond in several areas in Kuala Lumpur and the drainage upgrading.3.0 ConclusionFlooding can cause many bad effect to our activities life, commercial orindustrials activities. However, floods can also bring many benefits, such as rechargingground water, making soil more fertile and providing nutrients in which it is deficient. Atplace that needed water, flash flood is a good way for water resources in particular in aridand semi-arid regions where precipitation events can be very unevenly distributedthroughout the year.There are ways to reduce the effect of flash flood to our life, such asimplementation of a flash flood guidance system. A key benefit of the flash floodguidance system is providing early awareness of impending local flash flood threats.Other than that, Floodplain mapping can be prepared now to delineate flood prone areason the map. Such kind of maps will help the civil authorities for quick assessment ofpotential impact of the flood prone areas and initiation of appropriate measures can beconducted for reducing the impact..
  6. 6. The cause of flash flooding65.0 ReferenceMusa Bashir, Basema. Flash FloodMohd. Safie Mohd., Dr. Buang Alias and Dzurllkanian Daud. (2006) GIS Analysis forflood Hazard Mapping: Case Study; Segamat, Johor, West MalaysiaZulherman M. Sosi and Susilawati Sulaiman. Analysis on Kuala Lumpur flood risk zone (20 December 2009) (20 December 2009) (1 January 2010) December 2009)