A flood is the overflowing of water onto land that is normally dry.
Causes of flooding:
Heavy rainfall
Dam failure
Storm surge
Tropical storm Ondoy hit the Philippines on September 26, 2009.
Tropical cyclones are warm-core low-pressure systems associated with a spiral inflow of mass at the bottom level and a spiral outflow at the top level.
Tropical cyclones form over oceans where sea surface temperatures are greater than 26 degrees Celsius.
Typhoons form in the western Pacific Ocean (Eastern Hemisphere).
Hurricanes form in the Atlantic and eastern Pacific (Western Hemisphere).
Cyclones form in the South Pacific and Indian Ocean.
Wind Signal 1 - Tropical Depression (TD) has 61 km/h or less
Wind Signal 2 - Tropical Storm (TS) has 62-88 km/h
Wind Signal 3 - Severe Tropical Storm (STS) has 89-117 km/h
Wind Signal 4 - Typhoon (TY) has 118-184 km/h
Wind Signal 5 - Super Typhoon (STY) has 185 km/h or higher
Cyclones in the Southern Hemisphere is where winds pass from high pressure to low pressure on the left (clockwise).
Cyclones in the Northern Hemisphere is where winds pass from high pressure to low pressure on the right (counter-clockwise).
In August 2009, there was a dam failure in Botolan, Zambales.
Storm surge is the abnormal rise of water generated by a storm, over and above the predicted astronomical tides.
A storm surge happened in Zamboanga City on November 28, 2007 due to Typhoon Mitang.
A storm surge happened in Panay on November 13, 2013 due to Super typhoon Yolanda.
The types of flooding according to location are:
Coastal flooding (Shorelines)
River flooding (River floodplains)
Urban flooding (Paved roads & grounds)
Typhoon Ondoy caused both River and Urban Flooding in Metro Manila.
The types of flooding according to duration are:
Flash flooding (upstream floods)
Sheet flooding (downstream floods)
Flashfloods are floods of short duration with relatively high peak discharge.
The types of Flashfloods are:
natural
induced
Examples of induced flashfloods are:
Artificial damming of a river
breaching of dike
Sheet flood covers a large area and is not concentrated in a well-defined channel.
243,425,000 m^3 of water were contributed by the Typhoon Ondoy in a span of 24 hours (can fill up 97,370 Olympic size swimming pools).
Typhoon Ondoy Flooding in the Metropolis: Contributory Factors
Occupation of areas along riverbanks and major floodways mainly by informal settlers
Indiscriminate disposal of waste/garbage
Inappropriate design of drain systems
Heavy siltation
Accumulation of water hyacinth
2 Ways to Mitigate Flood Hazards
Structural
Non-structural
In Structural mitigation, there is the construction of physical barriers which consists of:
Artificial levees or dikes
Flood walls
Flood-control dams
Channelization
Artificial levees or dikes are usually built by piling soil, sand, or rocks on a cleared, level surface. Where the area beside a river or other body of water is in particular danger, levees may even be reinforced by concrete.
Flood walls are freestanding, permanent, engineered structure designed to prevent encroachment of floodwaters.
Flood-control dams keep floodwaters impounded and either release floodwaters in controlled amounts downstream to the river below or store or divert water for other uses.
Channelization is the straightening, deepening, widening, enlarging, clearing, or lining any existing stream channel.
In Non-structural mitigation, there is:
Land use planning/floodplain zoning
Flood hazard mapping
The symbol for Tropical Depression is D.
The symbol for Tropical Storm is S.
The symbol for Severe Tropical Storm is a hollow cyclone symbol.
The symbol for Typhoon is T.
The symbol for Super Typhoon is an all black cyclone symbol.
A coastal flooding happened in Tacloban City on November 10, 2013 due to Typhoon Yolanda.
An example of river flooding is in Marikina River.
An example of urban flooding is in Cainta, Rizal on September 26, 2009 due to TyphoonOndoy