MATEC Web of Conferences 25, 001 0 6 (2015) DOI: 10.1051/matecconf/201525001 0 6 C Owned by the authors, published by EDP Sciences, 2015 Analysis of Tidal Prism Evolution and Characteristics of the Lingdingyang Bay at Pearl River Estuary Shenguang Fang, Yufeng Xie & Liqin Cui Key laboratory of the Pearl River Estuarine Dynamics and Associated Process Regulation, Ministry of Water Resources, Guangzhou, Guangdong, China ABSTRACT: Tidal prism is a rather sensitive factor of the estuarine ecological environment. The historical evolution of the Lingdingyang water area and its shoreline were analyzed. By using remote sensing data, the evolution of the water area of the bay was also calculated in the past 30 years. Due to reclamation, the water area was greatly decreased during that period, and the most serious decrease occurred between 1988 and 1995. Through establishing the two-dimensional mathematical model of the Pearl River estuary, the tidal prism of the Lingdingyang bay has been calculated and analyzed. The hybrid finite analytic method of fully implicit scheme was adopted in the mathematical model’s dispersion and calculation. The results were verified though the method of combining the field hydrographic data and empirical formula calculation. The results showed that the main tidal entrance of the bay is the Lingdingyang entrance, which accounts for about 87.7% of the total tidal prism, while Hong Kong’s Anshidun waterway accounts for only 12.3% or so. Combining the numerical simulations and the historical evolution analysis of the water area and tidal prism, and compared with that in 1978, it showed that the tidal prism of the bay was greatly decreased, and the reduced area was mainly the inner Lingdingyang bay, which accounted for 88.4% of the whole shrunken areas. As weakening trends of the tidal powers of the medium and neap tides are obvious, the tidal prism is decreased during the medium and neap is larger than that during the spring. Keywords: Lingdingyang Bay; tidal prism; empirical formula; shoreline; mathematical model 1 INTRODUCTION sediment mining, the wetland wildlife resources were greatly reduced and the wetland environment was The topography and geomorphology as well as hy- destroyed[4]. In 1980s, due to the large area of wetland drodynamic environment were gradually formed in the reclamation in Liaohe delta, the landform of the orig- past thousands of years of evolution. The main three inal wetland has been considerably changed, which estuary types of the automatic adjustment and the substantially leads to the reduction of natural wetland geomorphological characteristics formation mecha- area while artificial wetlands such as rice fields are nism including the river dominant, the tide dominant gradually increased [5]. Human reclamation activities and the wave dominant have been elaborated by Li[1]. have not only directly caused the decrease of wetland It pointed out that, through factors contained in the area but also have caused the degradation of the wet- system such as velocity, slope, water depth, estuary land’s habitat environment and the decline of biodi- width and the composition of the sediment, the func- versity and ecological functions [6]. The fundamental tion of the estuary was constantly adjusted and formed reason is that the reclamation and the construction of some kinds of most optimal morphology to satisfy and estuary engineering and so on have changed the envi- meet the demand of energy input, output and transfer ronmental condition of hydrology, hydrodynamics and from two directions of the river and sea. Deposition, sediment transportation. While sediment scouring and biological and chemical features are also important silting variations that caused by fluctuation of tidal characteristic elements of the estuary because the dif- channels and flow conditions in the estuary area often fusion and self-cleaning of the pollutants, ecological lead to great disasters. Therefore, it is necessary to do species and environmental mutations also have intrin- basic research on the pattern of the water and sediment sic connections with the estuarine system [2]. In recent movement characteristics and its natural adaptability, years, the tidal flat reclamation and the construction of while weight and balance the reclamation and wetland wading engineering such as port terminals have protection [7]. changed the estuary morphology sharply, leading to The shrink of intertidal zone and wetlands in the ecological wetland shrunk rapidly. According to sta- estuary area directly changed the sea topography and tistics [3], the area of land reclamation from the sea hydrodynamic boundary conditions, which also led to reached to 34 thousand hm2 in the Yangtze River es- the change of tidal prism and water exchange in the tuary from 1978 to 2002. In the Yellow River Delta, bay. Tidal prism is an important index for the bay due to beach reclamation, indiscriminate catching and environment evaluation, which reflects the 4 Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20152501006 MATEC Web of Conferences N N Dahu Jiaomen Tide Level Stations Flow Stations Nansha Dock east Baoan Airport )DQVKL west east Inner Lingdingyang Bay Hengmen Wanqingsha beach trough middle west trough ay Chiwan n B Lingding 2# he S z h n e e n h z S h beach n e ctio n Se ross B beach n C a iwa y Ch C r o s s S e Qiao Island tion c Sec ti Cross o gding n Neilin Neilingding Island Jinxingmen Cross Section Jinxingmen T o n g g u Tangjia Bay C r Anshidun Waterway o s s S e c t i o n tion on s Sec Secti Cros Hongkong Airport Cross rway Xiangzhou Bay gzhou Wate Qin West Jiuzhou Cross Section Dahao Island Hongkong Dahao Jiuzhou Port n s Sectio th Cros ay Mou gyang B Lingdin ◣ 䰘 Guishan Island ᵪ ൪ Hengqin Island Open Sea 1# Open Sea 2# Huangmao Island Dawanshan Islands E Figure 1. Sketch of the Lingdingyang bay self-purification capacity of the bay and decides the compared and analyzed the results. Ye et al. [11] sepa- exchange strength between the bay and the open sea, rated intertidal zone and water area, calculated the and also plays a significant role on the environment, tidal prism and water exchange time of Sansha Gulf. ecology, sediment scouring and silting in the bay[8]. Wu et al. [12] used remote sensing image data to obtain The ADCP (Acoustic Doppler Current Profiler) the water area of Haikou Port and then calculated the method was used to cruse and observe the Jiaozhou tidal prism changes. Considering the situation of rec- Bay estuary and obtain the tidal prism by Qiao[9]. lamation in the estuary, Yang et al. [13] improved the Chen et al. [10] used both methods of charts and ADCP traditional calculation formula of tide, recalculated to calculate tidal prisms of the Jiaozhou Bay, and then and analyzed the tidal prism of Jiaozhou Bay. 01006-p.2 EMME 2015 In the recent thirty years, engineering constructions especially since 1990s. Simultaneously, deep trough such as of sea reclamation, together with significant area which is below 5 meters under the surface shows changes of the estuary shoreline as well as hydrologic that the trend of constant was decreased by 19.6 km2 and topographic conditions, have substantially affect- on total from 1974 to 1998, and becoming narrower ed the tidal prism and water exchange of the bay. Most and deeper with the improvement of water and sedi- research results [14][15][16][17][18] are focused on the as- ment transport capacity. pects of estuary hydrodynamics, water environment, Comparing Chen et al[15]’s and Xu et al[14]’s data on water geomorphology and hydrology, which are the evolution of the shoal and trough in the Lingding- closely and indirectly related to the tidal prism chang- yang estuary, their results are basically the same, es although still lack of fruits about it straightfor- namely the area of inner Lingdingyang which is above wardly. Therefore, based on summarizing and analyz- 5 meters under the surface was respectively decreased ing the existing research results, combining with the by 112.0 km2 and 112.6 km2 from 1974 to 1998. Deep hydrodynamic evolution and changes of water envi- trough areas of the elevations of sea bed which were ronment in the waters of the Pearl River estuary, this below 5 meters from the surface have respectively article discusses the process of the tidal prism evolu- decreased by 24.0 km2 and 19.6 km2 with a little dif- tion and the present condition of the estuary, adopting ference between them. the method of combining historical data, empirical formula, field data and mathematical model. 2.2 Historical evolution of Lingdingyang coastline The historical evolution of the Pearl River Delta can 2 OVERVIEW ON WATER AREA AND SHORE- be described as the processes such as the sediment LINE EVOLUTION OF LINGDINGYANG BAY which is constantly piling up outside the estuary, the coastline which is constantly pushing towards the sea, 2.1 Historical evolution of tidal water area the river course which is constantly extending, the estuary which is correspondingly moving outside and As shown in Figure 1, the landform of the Lingding- the branching bay which is gradually silting up. The yang bay generally maintained the pattern of “three artificial land reclaiming as well as the joint enclosing beaches and two troughs” in the recent 100 years. The and floodgate building have accelerated the speed of evolution of scouring and silting in the sea area is that the delta’s deposition and the coastline’s evolution the west beach is extending, the middle beach is ex- over the recent 100 years. The related data analysis [21] panding, the east beach is stable, and the west and east [19][20] shows that most shores of the east coast of Lingding- troughs are narrowing .