About


Flood

Flooding is a temporary overflow of water onto land that is normally dry. Floods can result from rain, snow, coastal storms, storm surges, overflows of rivers, and dam failure. Floods can be dangerous.

Introduction

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".

con

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.

Definition & Nature




"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)."
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: Types


Minor Flood

Minor Flooding is defined to have minimal or no property damage, but possibly some public threat. A FLOOD ADVISORY product is issued to advise the public of flood events that are expected not to exceed the minor flood category.

Examples of conditions that would be considered minor flooding include:

•water over banks and in yards
• personal property in low lying areas needs to be moved or it will get wet
• water overtopping roads, but not very deep or fast flowing
• water in campgrounds or on bike paths
• inconvenience or nuisance flooding
• small part of the airstrip flooded, and aircraft can still land
• one or two homes in the lowest parts of town may be cut off or get a little water in the crawl spaces or homes themselves if they are not elevated.

Moderate Flood

Moderate Flooding is defined to have some inundation of structures and roads near the stream. Some evacuations of people and/or transfer of property to higher elevations may be necessary. A FLOOD WARNING should be issued if moderate flooding is expected during the event.

Examples of conditions that would be considered moderate flooding include:

•several buildings flooded with minor or moderate damage
•various types of infrastructure rendered temporarily useless (i.e. fuel tanks cannot be reached due to high water, roads flooded that have no alternates, generator station  flooded)
•elders and those living in the lowest parts of the village are evacuated to higher ground
•access to the airstrip is cut off or requires a boat
•water over the road is deep enough to make driving unsafe
•gravel roads likely eroded due to current moving over them
•widespread flooding, but not deep enough to float ice chunks through town
•water deep enough to make life difficult, normal life is disrupted and some hardship is endured
•airstrip closed
•travel is most likely restricted to boats

Major Flood

Major Flooding is defined to have extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations are necessary. A FLOOD WARNING should be issued if major flooding is expected during the event.

Examples of conditions that would be considered major flooding include:

•many buildings flooded, some with substantial damage or destruction
•infrastructure destroyed or rendered useless for an extended period of time
•multiple homes are flooded or moved off foundations
•everyone in threatened area is asked to evacuate
•National Guard units assist in evacuation efforts
•erosion problems are extreme
•the airstrip, fuel tanks, and the generator station are likely flooded
•loss of transportation access, communication, power and/or fuel spills are likely
•fuel tanks may float and spill and possibly float downstream
•ice chunks floating though town that could cause structural damage
•high damage estimates and high degree of danger to residents

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.

Storm Surge

Storm surge is an abnormal rise of water generated by a storm, over and above the predicted astronomical tides. Storm surge should not be confused with storm tide, which is defined as the water level rise due to the combination of storm surge and the astronomical tide. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 feet or more in some cases.

Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The impact on surge of the low pressure associated with intense storms is minimal in comparison to the water being forced toward the shore by the wind.

The maximum potential storm surge for a particular location depends on a number of different factors. Storm surge is a very complex phenomenon because it is sensitive to the slightest changes in storm intensity, forward speed, size (radius of maximum winds-RMW), angle of approach to the coast, central pressure (minimal contribution in comparison to the wind), and the shape and characteristics of coastal features such as bays and estuaries.