三峡建库后洞庭湖入汇对下荆江顶托特征的变化分析OA

Clarifying the new characteristics of Dongting Lake's backwater effect on the lower Jingjiang River after the impoundment of the Three Gorges Reservoir is a critical issue at present.Focusing on the river section from Jianli to Chenglingji upstream of the confluence between the river and the lake,Copula functions were employed to investi-gate the characteristics of flow coincidences between the river and the lake in different periods.Based on the flow dynamics of alluvial rivers,a balanced-state flow pairing relationship was proposed,and an index for assessing the backwater intensity of the mainstream was developed.On this basis,the occurrence probability and timing of backwa-ter events as well as the variation of backwater intensity during different periods within a year for the lower Jingjiang River before and after the impoundment of the Three Gorges Reservoir were quantitatively analyzed.The results indi-cate that,under the balanced state,the flow pairing between the river and the lake follows a nonlinear functional rela-tionship.This relationship is close to the long-term mean fitted line of river-lake discharges and can be used to deter-mine whether the mainstream is in a backwater or drawdown state.After the impoundment of the Three Gorges Reser-voir,the duration of Jianli discharges at the medium-low flow level increased,resulting in a 44.6％rise in the coinci-dence probability of both the mainstream and lake inflows being at medium-low flow.The coincidence probability of the mainstream at medium flow and lake inflows at flood level(exceeding 30,000 m³/s)increased by 5.4％.As a result,the erosional base level at the river-lake confluence decreased,and on an annual scale,the mean backwater intensity of the lake on the mainstream weakened.However,the timing of the mainstream being in a backwater state shifted from above-bankfull flow levels to medium flow levels.The occurrence of extreme backwater intensity far exceeding the historical values during April-July has increased,warranting long-term attention.