If there’s a typhoon, there is always a flood, which is defined as “an overflow of water that submerges land which is normally dry.” But typhoon is not the only phenomenon that causes flood.
Rosalie C. Pagulayan, weather specialist II of Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), explained: “Floods are due to the complex combination of weather, climatic and human activities. Most floods occur as a result of moderate-to-large-scale rainfall events.”
The Philippines, given the location and the topography of the country, experiences four other weather-causing phenomena which can bring floods. These are: thunderstorms, cold front, monsoons, and intertropical convergence zone (ITCZ). The weather bureau gives the following insights on the four phenomena:
Thunderstorms, called local storms, occur when towering cumulus clouds reach a height where the temperature is well below the freezing point. Among the associated hazards are heavy rain (which may cause flashflood) and lightning (which may cause death, burns or fire).
Cold front is formed when cold air moves over areas of warm air. Since cold air is heavier than warm air, the warm air is pushed aloft by the cold air giving rise to widespread cloudiness. The cold front affects the eastern part of the country from November to late April or early May.
There are two types of monsoon: Southwest and Northeast. In the former, the Asiatic continent becomes warmer than the surrounding seas and a low pressure cell develops over the continent. This causes a flow of moist southwest wind over the Philippine area. At times, when this southwest flow becomes thick in depth, it persists for a long period causing continuous rains which may last for weeks during the months of June to September. The Southwest monsoon is responsible for the great portion of rainfall during the country’s wet season.
Known by sailors as the doldrums, the ITCZ is an area where the northern hemisphere trades meet the southern hemisphere trades. According to the weather bureau, ITCZ is characterized by towering clouds of cumulonimbus clouds accompanied with showers of widespread thunderstorms.
“The axis of convergence, which is usually oriented in an east to west direction, does not remain stationery at the equator but migrates north or south of the equator,” PAGASA’s briefing paper explains. In the Philippines, it oscillates during the months of May to October.
All these weather disturbances bring a lot of water causing flooding in affected areas. The same thing happens when there is a storm surge, “a rise of seawater above the normal level on the coast generated by the action of the wind and atmospheric pressure, associated with the occurrence of a tropical cyclone.”
According to PAGASA briefing paper, a storm surge can inundate low-lying coastal communities as the level of the ocean is raised by several feet. This happened in Metro Manila in 2011, when large waves that hammered the coastline of Manila Bay caused flash floods in areas. The huge waves caused by the storm surge battered the bay’s seawall, causing portions of the wall to collapse.
According to the World Health Organization (WHO), floods can potentially increase the transmission of water-borne diseases (such as typhoid fever, cholera, leptospirosis and hepatitis A) and vector-borne diseases (like malaria, dengue fever, and yellow fever).
“Flooding is associated with an increased risk of infection,” says the United Nations health agency. “However, this risk is low unless there is significant population displacement and/or water sources are compromised.”
The major risk factor for outbreaks associated with flooding is the contamination of drinking-water facilities, and even when this happens, as in Iowa and Missouri in 1993, “the risk of outbreaks can be minimized if the risk is well recognized and disaster-response addresses the provision of clean water as a priority,” WHO says.
It adds that “there is an increased risk of infection of water-borne diseases contracted through direct contact with polluted waters, such as wound infections, dermatitis, conjunctivitis, and ear, nose and throat infections. However, these diseases are not epidemic-prone.”
Perhaps, the only epidemic-prone infection which can be transmitted directly from contaminated water is leptospirosis, a zoonotic bacterial disease. Transmission occurs through contact of the skin and mucous membranes with water, damp soil or vegetation (such as sugarcane) or mud contaminated with rodent urine.
“The occurrence of flooding after heavy rainfall facilitates the spread of the organism due to the proliferation of rodents which shed large amounts of leptospires in their urine,” the WHO points out.
On the other hand, floods may indirectly lead to an increase in vector-borne diseases through the expansion in the number and range of vector habitats. “Standing water caused by heavy rainfall or overflow of rivers can act as breeding sites for mosquitoes, and therefore enhance the potential for exposure of the disaster-affected population and emergency workers to infections,” the WHO notes.
Flooding may initially flush out mosquito breeding, but it comes back when the waters recede. “The lag time is usually around 6-8 weeks before the onset of a malaria epidemic,” the WHO says.
The UN health agency says the risk of outbreaks is greatly increased by complicating factors. Among these are changes in human behavior (increased exposure to mosquitoes while sleeping outside, a temporary pause in disease control activities, overcrowding), or changes in the habitat which promote mosquito breeding (landslide, deforestation, river damming, and rerouting).
Other health risks posed by flooding include drowning and injuries or trauma. Hypothermia may also be a problem, particularly in children, if trapped in floodwaters for lengthy periods. There may also be an increased risk of respiratory tract infections due to exposure (loss of shelter, exposure to flood waters and rain).
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