Natural Hazards: Causes and Effects - Disaster Management Center ...

The most noted floods are associated with the world’s great rivers. However, the lesser floods
on smaller rivers or upstream tributaries may cause cumulatively more damage, even though
receiving less public attention.
Natural Preconditions for Disasters
All of the earth’s water (including the atmosphere) is part of a system referred to as the
hydrological cycle (Fig. 6.1). Beginning with the moisture in the air, water vapor enters the
atmosphere by evaporation from bodies of water and by transpiration (the giving off of water
vapor) from plants and trees. Once aloft, the moisture cools and collects into clouds as it rises
higher into the atmosphere. When temperature and moisture content reach the proper stage,
the vapor in the clouds condenses, and the water in the clouds falls to the earth as rain or snow.
Once returned to the surface, the water may evaporate again rapidly, or it may soak down into
the earth and remain as groundwater for thousands of years until at last it again finds its way to
an outlet. But regardless of where the precipitation falls, or how long it remains, eventually it is
recycled.
At any given moment, only about .005 percent of the earth’s estimated 1,360 million cubic
kilometers of water is actively involved in the hydrological cycle. But because of fluctuations in
the cycle, the actual amount of water available to various regions of the world can vary
dramatically, often bringing searing drought or devastating flood. 5
Disaster Event
Flood Types
Flash floods are local floods of great volume and short duration. A flash flood generally results
from a torrential rain or “cloudburst” on relatively small and widely-dispersed streams. Runoff
from the intense rainfall results in high flood waves. Discharges quickly reach a maximum and
diminish almost as rapidly. Flood flows frequently contain large concentrations of sediment and
debris. Flash floods also result from the failure of a dam or from the sudden breakup of an ice
jam. Flash floods are particularly common in mountainous areas and desert regions but are a
potential threat in any area where the terrain is steep, surface runoff rates are high, streams
flow in narrow canyons, and severe thunderstorms prevail.
Riverine floods are caused by precipitation over large areas or by melting of the winter’s
accumulation of snow, or by both. These floods differ from flash floods in their extent and
duration. Whereas flash floods are of short duration in small streams, riverine floods take place
in river systems whose tributaries may drain large geographic areas and encompass many
independent river basins (see Fig. 6.2). Floods on large river systems may continue for periods
ranging from a few hours to many days. Flood flows in large river systems are the distribution of
precipitation. The condition of the ground (amount of soil moisture, seasonal variations in
vegetation, depth of snow cover, imperviousness due to urbanization, etc.) directly affects
runoff.
In most cases the most devastating flooding from rainfall is that associated with tropical
cyclones. Catastrophic flooding from rainfall is often aggravated by wind-induced surcharge
along the coastline. Rainfall intensities are high and the area of the storm is broad-based; these