The flood disaster refers to water level increase in rivers, lakes,
seas, which is higher than the normal water level. Flood always threatens
the safety of rivers, lakes and coastal areas and even causes submerging
disasters. Flood disaster is one of the ten natural disasters
which threaten the survival of mankind.
The formation of flood disaster is affected by natural factors
such as climate, underside surface and human activities.
Floods can be divided into river flood, Lake Flood and storm surge flood, etc.
There are several types of river floods according to their causes:
Rainstorm floods are the most common and most threatening floods.
It is formed by heavy rainfall and can be shortly called as rain flood.
The main areas under the threat of rainstorm floods in our country
are 738,000 square kilometer, which are distributed in the downstream
of seven rivers, that is Yangtze River, Yellow River, Haihe, Pearl River,
Songhuajiang, Liaohe and southeast coastal areas.
In the middle and low latitudes, the floods are mainly caused by rain.
River Basin area is large and river, lake and reservoir regulation.
Different water in different tributaries always overlay and form different
flood peaks with gently fluctuation. The water-holding capacity of water
area of rivers and river web is different and a rain may cause huge waves
with wild fluctuation.
The main characteristics of river flood are high peak, long duration and
widespread disastrous area. Several major floods in modern times,
such as the flood of Yangtze River in 1931 and in 1954, the flood of the
Pearl River in 1915, the flood of Haihe in 1963 and the flood of Huaihe
in 1975 all belong to this type of floods.