According to history books, the situaions of flood disasters
in several major rivers, such as the Haihe River,
the Yellow River, the Huaihe River, the Yangtze River and the
Pearl River are:
Haihe River: from 1368 to 1948, about 500 years, 387 floods
occurred in the river, with an average of two years of flooding
every three years.
The Yellow River: in the 2,600 years since 602 BC,
there have been 1,593 dam collapses and 26 major riverway changes.
Huaihe river: according to relatively complete data analysis
since 1470, more than big 350 floods occurred over 500 years,
on average of nearly two years, among which nearly
20 times of basin floods.
The Yangtze River: floods in the middle and lower reaches
occurred 214 times during the 2096 years from 185BC to 1911.
Since 1921, there have been 12 major floods, about once every 6 years.
The Pearl River: a total of 408 floods have occurred
since the Han dynasty. Among them, the floods were more frequent
in flat lower reaches of the coast, with an average of
a major disaster in every 30 ~ 40 years and a minor disaster
in every 2 ~3 years.
Floods mainly take place in the downstream of Yangtze River, Huai River,
Yellow River, Hai River, Liao River, Songhua River and Zhu River.
The average annual damage area is 6.67 million hectares, disaster area
is 4.7 million hectares, the deaths are 5000 and the collapsed houses
are more than 2 million. Nearly a half of the country’s population,
a third of the land and more than 90% of the cities are threatened by floods.
The flood risk in Yellow River
There is an aboveground river which is over 300 kilometers long
in the lower reach of Yellow River. In the normal situation, 30% sediment remains
in the slot and 70% remain on the beach. But recent years because of
the less runoff, 90% remains in the slot, caused a secondary suspended river
as “high groove, low beach and wet embankment”. If there is a flood peak,
the phenomenon of incline driver, cross river and rolling river may appear,
and this may increase the possibility of dyke burst.
In history, the Yellow River basin is alternately dry and rainy, large floods
often occur during the rainy season. Since the 1990s, China has entered
the flood season, the flood affect the major rivers in our country
except the Yellow River. After the relatively dry period in the 1970s,
1980s and early 1990s, Yellow River basin may enter into a pluvial period.