Abstract
分汊河段冲淤调整会引起汊道分流比的复杂变化.建立准确的分流比计算公式是研究分流比在不同水沙、地形边界条件下变化的基础.以长江上荆江分汊河段——沙市段为例，利用三峡水库蓄水后的实测资料对现有分流比计算式的适用性进行比较，通过引入综合反映两汊糙率、比降差异的因子，建立了较高精度的分流比计算式，在此基础上，分析了沙市分汊段分流比的变化特点.结果表明：（1）当无法准确反映两汊糙率、比降的差异时，枯水分流比计算误差较大，最大计算偏差达15%；（2）两汊平均水深之比与糙率、比降综合影响系数比高度相关，引入两汊平均水深之比，建立了最大计算偏差小于5%的分流比计算式；（3）三峡水库蓄水后沙市段汊道冲淤变化对枯水分流比的影响大于洪水分流比，对微弯分汊段分流比的影响大于顺直分汊段.;Complex changes in flow division ratio can be aroused by channel morphological adjustments in anabranching reaches. Establishing a precise calculation formula of flow division ratio lays a foundation for investigating the changes in flow division ratio under different flow-sediment and channel boundary conditions. Taking the typical anabranching reach, namely the Shashi anabranching reach in the upper Jingjiang River as the case study, this paper compares the applicability in natural reaches of different flow division calculation formulas based on data observed after impoundment of the Three Gorges Reservoir (TGR) in 2003, and proposes an improved flow division formula with higher accuracy by introducing a comprehensive factor which can reflect the differences in roughness and water surface gradient between the primary and secondary branching channel. Based on this formula, the variation characteristics of flow division ratio at the study reach are further analyzed. The results reveal the following points:(1) Without accurately reflecting the differences in roughness and water surface gradient between the primary and secondary branching channels, the calculation error in low flow division ratio result is large and the maximum absolute difference (MAD) can reach 15%. (2) The average water depth ratio of the primary to secondary branching channel is highly related to their roughness-gradient comprehensive influence coefficient ratio and by introducing the average water depth ratio, a flow division calculation formula with a significantly improved accuracy of less than 5% MAD is established. (3) After impoundment of the TGR, the channel scouring and silting adjustments exert greater impacts on the low flow than high flow division ratio and led to a more significant variation of flow division ratio in the slightly curved branching reach than that in the straight branching reach.
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