1949-2005兰州城区段黄河演变特征与影响因素分析

Characteristics of evolution and its impact factors of the Yellow River along the Lanzhou urban reach from 1949 to 2005

通过兰州城区段1949、1975、1980、2000、2005年5个时期的黄河河道规模、河心滩规模、主河道河岸线长、平均河宽、河心滩密度以及城区人口规模、建设用地规模、河堤建设与滨河交通等信息资料,分析了1949年以来兰州城区段黄河的演变特征及其影响因素。结果表明:(1)解放后,兰州城区段黄河表现出快速而强烈的萎缩性演变特征,河流变窄,河心滩密度大幅下降,其萎缩过程呈现高速-缓慢-快速的波动性与游荡型-分汊型-弯曲型的空间形态变化。(2)兰州城区段黄河演变是城区人口规模与建设用地规模增长、带状防洪堤与滨河交通建设、城市规划与体制政策以及"蓄枯同期"时的黄河上游径流减小等因素综合作用的结果。

with the help of GIS technology, the channel information of the Yellow River in Lanzhou urban reach were taken from the actual land map in 1949, 1975, 1980, 2000 and 2005. Fistly,the river evolution characteristics were analyzed according to the scale contraction intensity index, such as the channel scale, river islands scale, the main riverbank length, average river width, river islands density and river form index, and then the impact fac- tors of river evolution were discussed in the light of urban population and urban area, urban flood control embankment and strand river road in deferent years. The research results indicated as follows ： （ 1 ） The Yellow River was in a negative contraction process for the river channel exploitting from 1949 to 2005, whose percentage in Lanzhou urban basin area was decreased from 22.3 % in 1949 to 4.6 % in 2005 of the basin space ih Lanzhou urban because large river surface and many river islands were exploited, and because many anastomosing branches were cut and infilled and died away. At the same time, the main channel was shifted toward north and the river bank turned winding. The process of river channel contraction was fluctuant just like ＂ U＂ -shape, and its constraction from 1949 to 1975 was the strongest and from1975 to 1980 was the slowest. All these led up to the evolution of the river channel patterns changing from braided channal to bifurcated one, and to meandered one. （2） The urban expan- sion was an interior driving factor that 1% of the urban area expanding would contract 0.4% of channel area. Urban construction of flood-controlling embankment and riverside road were the direct forces, which could satisfy the urban flood control demands and trafic demands. However, much land by the river channel was surrounded to satisfy urban development. City plannings and system-policies were the regulation factors that could change the exploiting mode, its time sequence and intensity. Store water and low water started from 1969 were the externally inducing factors because the runoff was decreased greatly and distributed balancely among the yeas, which led to many river islands being out of the river surface and connecting each other. In all, the river evolution was driven under the joint effects of the muhi-factors.