珠江磨刀门河口水位分布演变特征及其对人类活动的响应*

河口区水位受径流、潮汐、地形、人类活动等多因素影响, 其空间分布结构及变化复杂。作为河口动力结构的重要参数, 水位分布演变特征及机制的研究对水资源高效开发利用和河口治理具有重要指导意义。文章基于珠江磨刀门河口沿程马口、甘竹、江门、竹银、灯笼山和三灶6个测站1965—2016年月均余水位数据及马口站的月均流量数据, 分析人类活动影响下水位空间分布的异变, 采用双变量线性回归模型定量辨识水位分布异变的主要影响因素, 并初步讨论人类活动、动力结构、地形变化之间的相互耦合关系。结果表明, 基于平均水面形态变化参数(即曲率)可较好地指示河床冲淤变化趋势, 河段平均曲率为正, 指示河床趋于淤积, 曲率为负, 河床趋于冲刷侵蚀; 围垦、大规模挖沙、河道疏浚等强人类活动前, 磨刀门河口上、下段坡降大于中段(江门—甘竹段), 即中段存在一个水位坡降变缓的区域, 枯季低流量时中段水位坡降出现由海向陆方向的倒坡降, 强人类活动后, 余水位空间分布结构发生异变, 中段坡降增大、低流量时的逆坡降消失; 强人类活动引起的河床地形变化是磨刀门河口沿程水位空间形态发生变化的主要原因, 使口门段(三灶—竹银)和河口上段(甘竹—马口)水位曲率分别减小0.41×10^-4m·km^-2和1.04×10^-4m·km^-2, 河口中段(竹银—甘竹)水位曲率增大0.21×10^-4m·km^-2, 沿程曲率由正(下凹)-负(上凸)-正转变为负-正-负, 沿程河床也随之由淤积-冲刷-淤积趋势转变冲刷-淤积-冲刷趋势。

Evolution of water level profile dynamics in the Modaomen estuary of the Pearl River and its responses to human activities*

The water level in the estuary area is affected by many factors, such as runoff, tide and topography, resulting in a complex pattern in spatial morphology. Understanding the evolution characteristics of water level distribution is essential for sustainable water resources managements in estuaries. In this study, we use the residual water level data from six gauge stations (Makou, Ganzhu, Jiangmen, Zhuyin, Denglongshan, and Sanzao stations) along the Modaomen estuary of the Pearl River during 1965 to 2016, together with the monthly averaged river discharge data from the Makou hydrological station during the corresponding period. With a bivariate variable linear regression model, we quantitatively identify the influence of human activities on water surface profile dynamics, and attempt to understand the coupling relationship among human activity, dynamic structure and morphological change. The results show that the bulk parameters displaying the shape of water level profile (i.e., curvature) can well indicate the trends of erosion and deposition of river bed, with the positive curvature indicating sedimentation tendency and the negative curvature indicating erosion tendency. In the central part of the Modaomen estuary, such as the Jiangmen-Ganzhu reach, there exists an area where the water level slope is considerably reduced, especially during the dry season when the water level slope even fluctuates negatively in the landward direction. The river-bed deepening caused by human activities such as land reclamation, sand excavation and river dredging are the main reason for the alteration in spatial dynamics of the water surface profiles in the Modaomen estuary. We show that the river-bed deepening causes the water level curvature of the seaward reach (Sanzao to Zhuyin reach) and the upper reach (Ganzhu to Makou reach) decreasing by 0.41×10^-4 m·km^-2 and 1.04×10^-4 m·km^-2, respectively, while the central reach of the estuary (Zhuyin to Ganzhu reach) increasing by 0.21×10^-4 m·km^-2; the water surface shape changes from concave (C>0) - convex (C<0) - concave to convex - concave - convex, and the trends of erosion and deposition of river bed are also adjusted accordingly.