Flooding

• Flood hazards


  Too much water in the 'wrong' place! Overflowing of water onto land that is normally dry.
• Qatar and Saudi Arabia are the least prone to natural disasters in the Middle East (no cyclones and earthquakes)
• Malta is relatively close to the sea, so no tropical storms have formed there
• Caribbean islands have no volcanoes and faults, but do experience tropical storms, though not as severe
• Barbados and Grenada are Caribbean islands (Rihanna is from Barbados)
• Flood


  Too much water in the 'wrong' place! Overflowing of water onto land that is normally dry, typically pavement; due to overflow, flooding occurs, usually in Manila

  
• Metro Manila experienced severe flooding during Typhoon Ondoy on September 26, 2009
• Causes of flooding

  • Tropical cyclones/heavy rainfall
  • Dam or dike failure
  • Storm surge (coastal areas)
• Tropical cyclones


  Warm-core low-pressure systems associated with a spiral inflow of mass at the bottom level and a spiral outflow at the top level. Form over oceans where sea surface temperatures are greater than 26°C
• Typhoons occur in the western Pacific Ocean (Eastern Hemisphere), hurricanes in the Atlantic and eastern Pacific (Western Hemisphere), and cyclones in the South Pacific and Indian Ocean
• Cyclones in the Northern hemisphere- winds pass from high pressure to low pressure on the right (counter-clockwise)

  Southern Hemisphere - winds from high pressure to low pressure on the left (clockwise), associated with Coriolis effect
• Typhoon, hurricane, and cyclone names vary due to geographic differences
• Dam or dike failure can occur due to too much rainfall causing the dam to overflow and possibly fail
• Dike sections can be breached, causing flooding, when the dike was previously continuous
• Storm surge


  Abnormal rise of water generated by a storm, over and above the predicted astronomical tides. **Another factor is that the land area may recede, bringing the water closer to the land.
• Types of flooding according to location


  • Coastal flooding (shorelines)
  • River flooding (river floodplains)
  • Urban flooding (paved roads & grounds)
  • **Typhoon Ondoy - BOTH river and urban flooding
• Types of flooding according to duration


  • Flash flooding (upstream floods)
  • Sheet flooding (downstream floods)
• Flash floods


  Floods of short duration with relatively high peak discharge

  
  

  Types:

  1. Natural (land induced - mountainous areas
  2. Induced (e.g. artificial damming, dike breaching)
• Sheet flood


  Covers a large area and is not concentrated in a well-defined channel, floodwater does not easily subside, as water slides into lower elevation areas
• Marikina River Basin (53,000 hectares area) 


  During Typhoon Ondoy, 243,425,000 m3 of water were contributed by the typhoon in a span of 24 hrs (can fill up 97,370 Olympic size swimming pools)

  Areas in Marikina City adjacent to the Marikina River were inundated for over 5 meters
• Factors contributing to flooding in Metro Manila during Typhoon Ondoy


  • Occupation of areas along riverbanks and major floodways mainly by informal settlers
  • Indiscriminate disposal of waste/garbage
  • Inappropriate design of drain systems
  • Heavy siltation (e.g. Laguna de Bay)
  • Accumulation of water hyacinth (esp. at the mouth of the Pasig River; particularly Napindan Channel section)
• Factors affecting the nature and extent of flood hazards


  • Land use on the floodplain
  • Magnitude (depth and velocity of the water and the frequency of flooding)
  • Rate of rise and duration of flooding
  • Season
  • Sediment load deposited
  • Effectiveness of forecasting, warning, and emergency systems
• Ways to mitigate flood hazards


  • Structural (construction of physical barriers - artificial levees/dikes, flood walls, flood-control dams, channelization
  • NOTE: Everything affects everything else
  • Non-structural (land use planning/floodplain zoning, flood hazard mapping, community awareness & preparedness)
• Artificial levees or dikes


  Usually built by piling soil, sand, or rocks on a cleared, level surface, may be reinforced by concrete where the area beside a river or other body of water is in particular danger
• Flood walls


  Freestanding, permanent, engineered structures designed to prevent encroachment of floodwaters
• Flood-control dams


  Keep floodwaters impounded and either release floodwaters in controlled amounts downstream or store/divert water for other uses
• Channelization


  Straightening, deepening, widening, enlarging, clearing, or lining any existing stream channel
• Land use planning/floodplain zoning


  Proper planning to avoid hazards in floodplain areas
• Flood hazard mapping


  Indicating areas vulnerable to flood hazards to increase community awareness and preparedness
• Wind Signals:

  Tropical Depression (Signal 1)- <= 61 km/h

  Tropical Storm (Signal 2)- 62-88 km/hr

  Severe Tropical Storm (Signal 3) - 89-117 km/h

  Typhoon (Signal 4)- 118-184 km/h

  Super typhoon (Signal 5)- >= 185 km/h