DOST PAGASA: Understanding Flood and Flooding

Water, one of the most common substances known to man has become, through the years, a precious commodity. Water is required in practically all facets of human activities. And the need for water is enormous. It is roughly proportional to the population!

However, the Philippines is blessed with a large number of rivers, lakes and streams. Thus, the lack of water has never been a real problem. Where an apparent scarcity exists, the difficulty is traceable to the uneven distribution of rainfall necessary to replenish water in rivers, lakes and other bodies of water.

Paradoxically, it is sometimes excessive abundance of water that spells trouble. Because of the monsoons, the Philippines has a fairly well-defined wet season. In addition, there are other precipitation-producing weather phenomena: tropical cyclones, thunderstorms, the ITCZ, frontal passages, etc.

Singly by themselves, these can generate large amounts of precipitation. In combination with each other or, in particular, with the monsoon, these phenomena are capable of bringing intense and excessive precipitation. Under certain conditions, a surfeit of rainfall results in a potentially disastrous phenomenon - flood.

A layman's conception is often an over-simplification. The Manual of Operational Procedures on Flood Forecasting and Warning states:

"From a strict hydrological sense, flood is defined as a rise, usually brief, in the water level in a stream to a peak from which the water level recedes at a slower rate (UNESCO-WMO 1974). The episodic behavior of a river that may be considered flood is then termed "flood event" (Linsley, 1942) which is described as a flow of water in a stream constituting a distinct progressive rise, culminating in a crest, together with the recession that follows the crest (Linsley, 1942)."

From the foregoing technical definition, flood simply denotes a progressive abnormal increase in the elevation of the surface level of streamfiow until it reaches a maximum height from which the level slowly drops to what is its normal level. The sequence described all takes place within a certain period of time.

The definition merely describes a characteristic behavior. It does not include the element of "flooding" or inundation as implied by the popular notion of flood. The technical definition is rather inadequate. Thus, considering the intents and purposes of flood forecasting and warning, the definition seems rather restrictive in its connotation for the public.

Hence, for operational purposes, the Flood Forecasting Branch, the hydrological service of PAGASA has adopted a more extensive definition.
Flood is "an abnormal progressive rise in the water level of a stream that may result in the over­flowing by the water of the normal confines of the stream with the subsequent inundation of areas which are not normally submerged".

The popular or layman's idea of flood is, in the strictest sense, the process of inundation or the coverage by water of areas not normally submerged. Inundation is due to water overflowing from streams and other bodies of water as well as by the accumulation of rainwater by drainage.

FLOOD: CAUSES

NATURAL

Meteorological Events

- an intense and prolonged rainfall spells

- unusually high coastal and estuarine waters due to storm surges, seiches, etc.

Seismic Activities

- tsunamis (seismic sea waves)

- sinking of land due to earthquakes reduces the elevation of land areas

- uplifting of lake and river beds from seismic causes sometime results in the overflowing of these bodies of water

Atronomically-Influenced Phenomena

- high tides coinciding with the occurrence of heavy rainfall

ARTIFICIAL

Blasting

- causes landslides in the slopes of hills and mountains which may result in the unintentional damming of rivers and streams

Construction of temporary dams

- produces an impediment to the flow of a river or stream which then results in an overflow

Failure of hydraulic and other control structures

- accidents like the breaking of a dike result in the entry of an enormous quantity of water in a protected area

Mismanagement of hydraulic structures

- may necessitate an untimely and sudden release of large amounts of excess water

Denudation of forest and watershed areas

- alter the ecological system in a river basin will have an impact on the hydrology of the catchment

FLOOD: TYPES

MINOR FLOODING

- Inundation may or may not be due to overbanking

- When there is no bank overflow, flooding is simply due to the accumulation of excessive surface run-off in low lying flat areas

- Floodwaters are usually confined to the flood plain of the river along the channel, on random low-lying areas and depressions in the terrain

- Floodwater is usually shallow and there may not be a perceptible flow

MAJOR FLOODING

- Flooding is caused by the over­flowing of rivers and lakes; by serious breaks in dikes, levees, dams and other protective structures; by uncontrollable releases of impounded water in reservoirs and by the accumulation of excessive runoff

- Floodwaters cover a wide contiguous area and spread rapidly to adjoining areas of relatively lower elevation

- Flooding is relatively deep in most parts of the stricken areas

- There is a highly perceptible current as the flood spreads to other areas

FLASH FLOOD

While floods take some time, usually from 12 to 24 hours or even longer, to develop after the occurrence of intense rainfall, there is a particular type which develops after no more than six hours and, frequently, after an even less time. These are what are known as "flash floods".

Flash floods develop in hilly and mountainous terrains where the slope of the river is rather steep. The rapid development of the flood is due to the extremely short concentration time of the drainage catchment. This means that precipitation falling on a point in the catchment farthest from the river takes only a short time to reach the river channel and become part of streamflow. Thus, the amount of streamflow rapidly increases and, consequently, the rise in water level. When the flow capacity of the stream is exceeded, the channel overflows and the result is a flash flood.

Floods are among the most destructive calamities man has to cope with. Even the most minor flooding poses some inconveniences. A really big flood can result in millions even billions of pesos of damages to roads and bridges, buildings and other economic infrastructure, in the loss of agricultural crops and livestocks, loss of productivity in industry, commerce and trade. To this is added the incalculable loss of human lives directly attributable to floods as well as the hardship and attendant socioeconomic problems of forced human displacement and the emotional impact on those affected by floods.

Aside from the direct damages brought by a flood there are also those cascading effects which follow in the wake of the calamity. Among the immediate problems caused by flood are the lack of basic utilities and essential necessities, particularly, food and potable water. Flood also disrupts the sanitary regime in a community. This almost always results in the contamination of the water supply. Thus, in a flood-stricken area an epidemic of gastro-intestinal diseases frequently breaks out. Respiratory ailments due to exposure are also quite common.

MONITORING and PREDICTIONS

For obviously practical reasons, man has always preferred a riverine environment. Almost always naturally fertile and, therefore, able to provide him with an abundance of his needs, flood plains have always attracted man as a place to settle down. The river or a lake, in addition to being a source of his livelihood, also serves as a convenient means of communications, an avenue for trade and commerce.

However, a river is not always placid, serenely flowing along. It has its moods. And it changes its moods, quite regularly. When it does, man comes face to face with the phenomenon of the flood.

But man is also a resourceful creature. Knowing that he cannot prevent the recurrence of floods and because he has to live with it, man has learned to cope. He tried to understand the nature, behavior and the causes of floods.

In some primitive ways, he had learned to relate the vagaries of the weather to the behavior of the river. In doing so, he established a "forecasting procedure" which has persisted up to our times and which remains practically unchanged.

Modern flood forecasting is now based on the standard procedure of monitoring and analysing the hydrological and meteorological conditions in a river basin. While the tools and methods of monitoring may have been modernized with the use of sensitive, telemeterized gauging instruments to effect better observation and faster transmission of data, it is still basically an attempt to paint a bread picture of what is currently happening, hydrologically and meteorologically, in a river basin.

The simple method of associating the weather to the behaviour of the river has given way to the more modern sophisticated analytical methods aided by the computerized flood forecasting models. With further evaluation of the results of the various analysis, hydrologists are able to come up with a prediction of the future state of the river.

If anything new is added to modern flood forecasting, it is the more extensive reliance of hydrologists on the scientific principles of hydrology rather than on mere intuition.

http://bagong.pagasa.dost.gov.ph/learning-tools/floods