大规模人类活动与洪水灾害──从历史到现实

自然灾害是由于异常变异所引发,表现出它的自然属性。此外,作为承灾体的人类社会对洪水的影响也是不容忽视的。大规模的人类活动有制约洪水为害的一面,也有促使灾害增长的负面影响。因此,灾害又具有社会属性。本文主要探讨大规模人类活动如何影响洪水灾害,以及相应的完善防洪方针的途径。

LARGE SCALE HUMAN ACTIVITIES AND FLOODCALAMITIES

Generally speaking, for flood disasters, the environmental variation due toextension and development of existence conditions, the practical development of landresources has made the construction of flood control works for the purpose ofregulating flood to cause the flood disasters more serious in various levels. Thevariation of environment would bring about the growth and accretion of flood disasters.During foe process of social development the water and soil erosion loss due todeforestation and reclamation has the most direct impact on flood and droughtcalamities.In the nine large rivers in the world, the annual sediment transports in theHuanghe River and the Changjiang River in China rank the top first and fourthrespectively. In recent decades, the average annual sediment transport of the HuangheRiver has reached 1.6 billion ton, in which 0.4 billion ton deposited in the river bedsof down stream reaches, thus making the downstream river beds' elevation raised up10cm on average annually. The sediment silting not only lowers the capacity of floodrelease in the river channel, it will shorten the life of the reservoirs as well. Accordingto statistics, the total capacity of 353 large-sized reservoirs nationwide is 323 billioncubic meter and more. UP to 1987, 17.7 billion cubic meter of the capacity havebeen deposited, sharing 5.5% of the total reservoir capacity. Sediments deposited inthe river channels, reservoirs and the flood detention area will continuously decreasethe capability of flood regulation and will also cause the increase of the flooddischarge as well as the value of flood peak.Piratical reclamation accentuating flood disasterIn recent decades, the building of polder and reclamation of diked marsh,shooting without aim in the lake area have evidently reduced the capability of floodregulation of lakes whereas the capacity of flood control will continuously increase.Formerly, the dyke of polder might be breached under ordinary flood. Yet now itcould be free from collapse. Hence, it would elevate the flood water level. Forexample, the records of Hongdaoshi Guishan Hydrological Station on Lishui river inthe vicinity of Hunan Province have witnessed the fact that the water level under thesame discharge in 1983 has elevated 1.8m as compared to what happened in 1964.Likewise, in the area located at the West Dongting Lake, the average annual sedimentdeposition on the bottom of rivers and lakes is 7.4cm in height. Since 1949, 1 000kmold dikes in the said region have been raised their elevations up to 2.52m on average.The benefits gained from the construction of flood control works even failed tocompensate the cost for the attenuation of flood control capability due to the influenceof inappropriate human activities.construction of flood control works also existing the latent calamity bearing factorsThe construction of dykes will be able to control the flood effectively within thestandard criterion of protection without flood violation. However, owing to the factthat the restriction of flood within dikes will make the water level elevated. Once thedykes were breached, the calamities will be more serious. Leaving out theaccentuation of disaster during dyke breaching, the dyke protection would back upflood level, the confluence of tributaries would be difficult and the leakage of bankswould also accelerate water-logging calamities.Rapid extension of urban development making flood disasters more seriousThe urban area extends towards the lowland area where the drainage condition isrelatively poor. The morphologic and topographic features have been changed duringthe urban construction, thereby breaking the original channels for flood release. Theimpervious ground area in urban areas has been increased, thus quickening the floodconfluence speed and increasing the flood peak discharge and total amount of flood.