强人类活动驱动下珠江磨刀门河口径潮动力的季节性异变特征

IMPACT OF INTENSIVE HUMAN ACTIVITY ON SEASONAL VARIATION IN RIVER-TIDE DYNAMICS IN THE MODAOMEN ESTUARY OF ZHUJIANG(PEARL)RIVER

人类活动对河口环境影响巨大,揭示在强人类活动驱动下河口径潮动力非线性相互作用的异变特征,有利于了解人类活动影响河口动力地貌的机制,对河口区水利工程建设及环境保护等具有重要指导意义。基于1960—2016年珠江磨刀门河口沿程潮位站(甘竹、竹银、灯笼山、三灶)的逐月高、低潮位数据及马口水文站的月均流量数据,统计分析了磨刀门河口在强人类活动驱动下月均水位、潮波振幅及其空间梯度(即月均水位坡度和潮波振幅衰减率)的季节性异变特征。结果表明,1990年和2000年为磨刀门河口径潮动力的异变年份,1990年前为自然演变阶段,2000年后为恢复调整阶段,1990—2000年为过渡阶段;高强度采砂导致的河床下切使磨刀门河口月均水位及月均水位坡度显著减小,夏季减小幅度最为明显,沿程平均分别减小0.53m和8.93×10^-6;月均水位坡度减小导致潮波衰减效应减弱,进而使沿程潮波振幅增大,多年平均增大0.071m;磨刀门河口径潮动力相互作用具有明显的季节性差异,夏季月均水位坡度随流量增大在上游抬升明显,冬季月均水位坡度在上游显著减小,但在下游略有抬升;随着流量的增大潮波振幅的衰减作用增强,但当流量超过阈值20000m^3/s时,月均水位坡度引起的底床摩擦增大效应不足以抵消横截面积辐散效应,潮波衰减效应略有减弱。

Human activities have a great impact on the estuarine environment.Study of the nonlinear river-tide interaction dynamics in modern time is conducive to understanding the impact of strong human activities on estuarine dynamic geomorphology,and important for the construction of water conservancy projects and environmental protection in the estuarine area.Using monthly high and low tide level data of the Modaomen estuary(Ganzhu,Zhuyin,Denglongshan,and Sanzao gauging stations),Guangdong,South China,and river discharge data of the Makou Hydrological Station from 1960 to 2016,seasonal variations of residual water level,tidal amplitude and their spatial gradients(i.e.,residual water level slope and tidal damping rate)were analyzed.Results show that 1990 and 2000 were the critical points marking the periods of without(pre-1990)and with(post-2000)intensive human activities,respectively,in the variation of tidal dynamics in the Modaomen estuary.Specifically,pre-1990 was the natural evolution period,post-2000 was the recovery and adjustment period,and from 1990 to 2000 was the transitional period.Large-scale sand mining undercut the riverbed considerably,which reduced the residual water level and residual water level slope in the estuary,especially in summer(being 0.53m and 8.93×10^-6,respectively).The decreased residual water level slope weakened subsequently the tidal damping effect,which results in an increase of the tidal amplitude in the whole channel(increased by 0.071m after intensive human activities).Seasonal variations in the river-tide dynamics in the Modaomen estuary indicated that in summer the residual water level slope in the upstream increased obviously with substantial river discharge,while in winter it decreased significantly in the upstream and rose slightly in the downstream.With the increase of discharge,the tidal damping effect of tidal wave amplitude increased,but when the discharge exceeded the threshold value of 20000m^3/s,the increase of riverbed friction due to residual water level slope was insufficient to offset the cross-sectional area divergence effect,and thus the tidal damping effect decreased slightly.