Flooding is one of the most disastrous natural hazards. Take Huang He Basin in China as example, originally, intense rainfall in summer is one of the main causes of flood. With rapid urbanization and population growth, the built environment and the socio-economic characteristics is getting more important in contributing to flood hazards. To minimize the impact of flooding, take Huang He Basin in China as example, some preventive measures, such as flood control projects, are carried out.
Flooding refers to the inundation by water of any land area not normally covered with water owing to a relatively rapid change of the water level. It often happens in the lower course of Huang He Basin. Firstly, physical environment is one of the main factors leading to flooding. Meteorologically, snowmelt in the upper course in spring causes sudden increase in discharge and blocking of ice-floes in river channels. In summer, the rainfall is concentrated in July and August, being highly variable. Prolonged and abnormally high rainfall may also increase the discharge of lower course of Huang He, allowing the floodwater to overflow its banks. The climatic condition throughout the year leads to the occurrence of flood.
Topographically, there is a sudden decrease in gradient at Kaifeng, between the Loess Plateau and the North China Plain, which is therefore liable to flooding due to its topographic characteristics. It is a flood plain with gentle gradient and is extensive lowland. The river course is meandering with intense deposition and silting, raising the river bed.
Geologically, the middle course of Huang He is a loess plateau region. Loess is a loose fine silt with weak cohesion, especially if unprotected by vegetation cover. It is readily eroded by streams and susceptible to mass wasting. As a result, the river overflows its bank during the time of high discharge.
Apart from the physical environment, the built environment can also make the flooding hazard catastrophic.
Although the North China Plain has been frequently affected by flood hazards, there are still many people living there. The reason behind is that there is a long history of settlement and development. With rapid population increase, the total built-up area increases, thus, the river channels on the flood plain have frequently changed due to meandering for development. Without lakes to hold flood water in between, the risk of flooding increases greatly.
For socio-economic characteristics, due to rapid increase in population, poor farming practices, including over-cultivation and over-grazing, leads to deforestation in the middle course and had induced widespread soil erosion. Runoff increases as a result of deforestation. This causes rapid concentration of discharge, reaching flood stage and peak discharge. Landslides are common, which dam river channels and block channel flow, causing the river to overflow its banks. Besides, the Huang He has dissected the loess plateau region in the middle course. Therefore it carries heavy sediment load to the lower course and flood plain. Hence there is heavy silting and deposition in the lower course and flood plain in river channels, raising the river channel floor and so water level along river banks during flood, hence building up leeves which may breach under great stress on the surrounding floodplain. Together with the perception of people in the region, who are ignorant of the likelihood of floods and their disastrous consequences and are prepared to take a calculated risk that the protective measures will save them from most floods. Yet the risks of flooding are under-estimated. Harmful effects could be disastrous.
To minimize the impact of this hazard, some measures are carried out. The Chinese government has launched a large-scale multi-purpose water scheme, the Huang He Project, to reduce the risks and damage due to flood and drought hazards. Dams are constructed along the main river and some tributaries so as to regulate channel flow and retain flood water and silt, hence reducing the volume of sediment load carried downstream to the floodplains. Embankments are reinforced. Dykes, leeves and banks are strengthened to prevent breaching. Meandering river courses are straightened to quicken channel flow. Flood diversion projects are developed, including flood distribution projects, spillways constructed to collect overspill. Also, river beds and mouth are dredged.
Secondly, some irrigation projects have launched. Flood retention reservoirs are constructed to store excess water behind dams and to provide regulated and reliable water supply for irrigation. Canals are constructed to lead away water to fields for irrigation. Therefore, farming can be improved.
Apart from this, to control the impact of flood, soil conservation project in catchment area has been launched to achieve flood abatement and to check soil erosion. Exposed slopes are protected by afforestation and grassing. Hillsides are terraced for farming. Gullies are dammed. Windbreaks are planted in the north. Moreover, farming methods are improved by adopting contour ploughing and strip farming. Desilting basins are constructed and the silt so-called is dredged and used for soil improvement.
Regarding the above measures, flood control and prevention is possible. Hence the dense settlements on the flood plains are safeguarded against flood hazards. More regular and reliable water supply is available for irrigation and industrial uses. Furthermore, the irrigation project allows farm yield and productivity to be raised because a greater variety of crops can be grown under irrigation. Area of arable land is increased because cultivation can be extended into marginal lands.
Despite the economic benefits, some ecological problems are always resulted from the projects. To begin with, flood control and silt retention result in loss of annual addition of nutrients to the flood plains, so the nutrient status of soil is lowered. Secondly, sediments trapped behind dams cause silting of reservoirs, the aquatic ecosystems are upset. Thirdly, with increasing productivity, the use of fertilizer is widespread, thus causing stream pollution. Moreover, excess irrigation water raise the ground water table, causing water-logging and salinization of soil, this leads to changes in soil and in habitats. Not only are this, increase of irrigated fields causes increase in evaporation and relative humidity, microclimate is then changed.
All in all, flooding can badly affect both the ecology and the economy of a country. Preventive and flood control measures can by no means be neglected. However, it seems that people are still more economic-conscious and at the same time ignoring the effects imposed on the ecological balance through the projects.