Changes of the flood regulation capacity at Lake Dongting since 1950s

Abstract

针对洞庭湖区淤积、围垦和江湖关系变化对湖泊调蓄功能的影响,根据洞庭湖调蓄属于典型复合洪水波的实际情况,提出利用离散小波分解和计算入、出湖径流过程方差的方法揭示1950s以来洞庭湖调蓄特征及其实际调蓄作用的多年变化.结果发现:洞庭湖削减的洪峰主要是32 d以下的中短尺度洪水波,其全年整体削峰系数在0.13~0.56之间;从入、出湖径流方差多年变化体现的调蓄效果看,洞庭湖区近几十年淤积围垦虽然极大地改变了湖区面积和容积,但并未使湖泊调蓄作用发生大的变化.结合洞庭湖削峰系数与城陵矶-螺山段水位落差的对应关系,认为在整个江湖系统关系中洞庭湖的调蓄能力是被动的,其变化主要取决于城陵矶以下河段过水能力对洞庭湖泄流的制约.从整个江湖关系下的洞庭湖调蓄变化特征看,1990s以来湖区小水大灾的原因之一是入湖径流过程的短尺度方差和削峰系数较大,本质上是由荆江裁弯和三峡运行导致的江湖关系变化引起的长江螺山段出流使出湖径流方差减小造成的.;In view of flood regulation capacity at Lake Dongting affected by reclamation, deposition and change of lake-river relationship, the characterization, mechanism and impact of the flood regulation capacity since 1950s were analyzed by using methods of wavelet decomposition and corresponding standard deviation amplitude. The results indicated that the flood regulation of the lake is mainly functioned on the high frequency foods with an average flood peak removing coefficient between 0.13-0.56. Although reclamation and deposition had largely decreased the water storage capacity of Lake Dongting, the flood regulation capacity did not change much revealed by the difference of standard deviation between inflow and outflow in a long term. The area and water storage capacity are only static parameters for the capacity of flood regulation at Lake Dongting. In term of the perspective of lake-river relationship, the dynamic of flood regulation capacity was mainly affected by the stage-discharge relation at the outlet of the lake, which is decided by the river channel morphology. The major reason for the phenomenon of normal discharge but high water level can be explained by the high standard deviation of inflow and the big flood peak removing coefficient, essentially resulting from the decrease of discharge deviation in the Luoshan channel of the Yangtze River which was caused by the channel straightening in the downstream of the Three Gorges.