Response of Fresh Water Distributions on Abrupt Changes of Topography in the Pearl River Networks of China

A 2-D numerical model was used to study the response of fresh water transports and distributions on the abrupt changes of topography in the Pearl River Networks (RNPRD). The results indicate that both the tidal forces in Jiaomen and Humen and the runoff power in Modaomen are intensified, which leads to a fresh water movement from the northeast to the southwest side of the West and North River Delta Networks. However, the water distributions in the East River Delta Networks remain almost the same. The residual currents in the RNPRD decreased dramatically in the West and North River Delta Networks due to the increasing volume of the river channels. This decreasing trend was intensified in the North River Main Channel due to the annual water discharge redistribution in the RNPRD.


INTRODUCTION
The Pearl River Network (RNPRD) is the world's most complicated tidal river network system located in the northern continental shelf of the South China Sea (CSC, Figure 1).Anthropogenic impacts such as sand excavation and waterway regulation have greatly changed the morphology of the Pearl River Delta (PRD), resulting in a severe and uneven river-bed down cutting in the entire Pearl River Network (RNPRD) during the past 20 years.Luo et al. (2007) report that more than 7×10 7 m 3 /yr of river sand has been dredged from all of the 324 tributaries of the RNPRD during 1986 to 2003, resulting in average down cutting depths of 0.59-1.73m in West River, 0.34-4.43m in North River and 1.77m-6.48m in East River (three major water networks in the RNPRD).It is seven times more than the average amount of sedimentation before the sand mining began.Uncontrolled sand excavation changed the hydrodynamic significantly, which leads to an increased tidal prism and upstream movement of the tidal limit in the RNPRD (Zhang et al. 2010).Therefore, the fresh water transports and distributions in the RNPRD have also changed.

STUDY AREA
The Pearl River is the second longest river in china in terms of mean annual water discharge, and its delta area consists of a tidal river network (called the RNPRD) and an estuary (called the PRE).The RNPRD is mainly comprised of the West River, East River, North River and other tributaries (Figure 1).These rivers repeatedly bifurcate after entering the Pearl River Delta and form an intricately interlaced network of narrow channels, which empty out though eight outlets (Humen, Jiaomen, Hongqimen, Hengmen, Modaomen, Jitimen, Hutiaomen and Yamen respectively form the eastern to western side of the PRE).The RNPRD's mean annual loads of water and sediments was 286×10 9 m 3 /yr and 75.3×10 9 kg/yr (from 1957 to 2006), which is the second largest contributor of water and suspended sediments to the SCS.
In the mid-1980s, the large-scale and long-term sand excavation was initiated to meet the construction needs raised from the rapid economic growth and urbanization in this region.These unofficial and uncontrolled practices have accelerated the riverbed evolution process, altering the hydrodynamics of the entire river network (Luo et al. 2002).Thus concerns on the hydrodynamics of the Pearl River Delta have increased in recent years (Hu et al.2011;Du et al. 2014).However, little attentions have been paid to the possible changes of the fresh water transports and distributions in the RNPRD, which have severe impacts on the river channel's immediate vicinity, the estuary, and the coastal areas.

Model setup
The RNPRD is characterized by one-dimensional flow while the PRE is a typical two-dimensional and three-dimensional flow (Du et  ABSTRACT: A 2-D numerical model was used to study the response of fresh water transports and distributions on the abrupt changes of topography in the Pearl River Networks (RNPRD).The results indicate that both the tidal forces in Jiaomen and Humen and the runoff power in Modaomen are intensified, which leads to a fresh water movement from the northeast to the southwest side of the West and North River Delta Networks.However, the water distributions in the East River Delta Networks remain almost the same.The residual currents in the RNPRD decreased dramatically in the West and North River Delta Networks due to the increasing volume of the river channels.This decreasing trend was intensified in the North River Main Channel due to the annual water discharge redistribution in the RNPRD.The momentum equation in the X direction: The momentum equation in the Y direction: Where t is the time; u and v are the vertically averaged velocities in x and y directions; h is the total water depth; K is the water level relative to calcu- late level; a P is the air pressure;  Considering the complex geometry of the study area, an unstructured grid covering the entire PRD was The coastal boundary is extended to -30m isobaths, while the upstream boundaries respectively stretch to Gaoyao in West River, Shjiao in North River, Boluo in East River, Layagang in Liuxi River and Shizui in Tanjiang River (Figure 1).The main parameters of model are shown in Table1.As shown in Table 3, the annual discharge of Makou station changes from 1614×10 8 m 3 under the  The minimum residual currents are concentrated at the East River Delta Networks (J), starting from the East River.This implies that the runoff plays an essential role in the West and North River Delta Networks such as B, F, C, G, H and D, while the East River Delta Networks (J) is dominated by tidal forces.Compared with the annual mean residual current in the RNPRD under different terrain conditions (Figure 3. and Figure 4.), it can be noted that the mean residual current significantly decreases in the West and North River Delta Networks.For example, the annual mean residual speed of the West River Main Channel (B), North River Main Channel (F), Shunde Channel (G) and Xiaolan Channel (H) have respectively decreased from 0.38m/s, 0.44m/s, 0.26m/s 0.28m/s to 0.29m/s, 0.26m/s, 0.14m/s, 0.25m/s because the river channel volume of the RNPRD has increased dramatically from 1999 to 2005 due to the large-scale and long-term sand excavation.However, a decrease value of 0.18m/s was discovered only in the North River Main Channel (F), it's much higher than the other studied channels, which could have resulted from both topography and the water discharge.Although the river-bed down cutting occurred in the entire RNPRD, the decreasing trend of the annual mean residual current was intensified in the North River Main Channel (F), while compensated in the West and North River Delta Networks, due to the annual water discharge redistribution of Sixianjiao station.It is interesting to find that the annual mean residual current remained almost the same in the East River Delta Networks (J) in spite of the great changes of the bathymetry.This can be explained as follows: (1) the East River Delta Networks is a relative independent river network, which could not be affected by the water discharge redistribution of the West and North River Delta; (2) the hydrodynamics of the East River Delta Networks are dominated by the tidal forces and the runoff is relatively weak.

Changes of fresh water distribution in the eight outlets
Figure 5 shows the annual fresh water distribution in the eight outlets under the topographical condition of 1999.The annual net discharge volume is up to 623×10 8 m 3 in Modaomen which accounted for 24.8% of the total runoff, ranking as the first in the eight outlets, while Humen is ranked as the second and accounted for 21.4%.Jiaomen is another major fresh water entrance, which occupied 514×10 8 m 3 of annual total runoff and accounted for 20.5%.The annual water discharges of Hengmen, Hongqimen and Yamen are respectively 301×10 8 m 3 , 205×10 8 m 3 and 187×10 8 m 3 , ranking as fourth, fifth and sixth in the eight outlets.Hutiaomen and Jitimen are the minimum amount of discharges, which accounted for only 2.5% and 3.1% of the total fresh water discharge.However, the annual net discharge in Modaomen has increased to 778×10 8 m 3 based on the bathymetry of 2005, while the other western outlets remained almost the same.For the Eastern four outlets, the annual discharge of Jiaomen, Humen are 408×10 8 m 3 and 481×10 8 m 3 , a decrease of 106×10 8 m 3 and 56×10 8 m3, respectively, but the discharge of Hengmen and Hongqimen stayed almost stable.As we know, Humen and Jiaomen are located at the top of the Lingdingyang Bay, and the hydrodynamics of these two entrances are dominated by the tidal forces.During the past 20 years, the tidal channels of Jiaomen and Humen are deepened as a result of waterway regulation and uncontrolled sand excavation, so that the tidal waves can be more convenient for us to sail upstream and transport in the river network (Zhang et al., 2010).The runoffs from the upstream of the RPPRD are pushed southwestward by the more powerful tidal forces, which directly reduced the fresh water distribution of Jiaomen and Humen outlets.On the other hand, the fresh water paths in Modaomen dominated by runoffs are also deepened, which draw the water discharge towards Modaomen entrance.Because of the transverse water gradients changes induced by the bottom topography, the annual water discharge distributions of Modaomen dramatically increased while 01004-p.6EMME 2015 the Jiaomen and Humen correspondingly decreased.

CONCLUSIONS
The fresh water distributions in the Pearl River Network (RNPRD) under different topographies were investigated with a 2-D numerical model, which covered the entire Pearl River Delta.The model results show that although the severe river-bed down cutting occurred in the entire PRD, the water redistribution in the RNPRD is totally different in space and time.The tidal forces from Jiaomen and Humen are strengthened, which pushed the runoffs form the upstream of the RPPRD southwestward, while the deepened water paths of Modaomen draws more fresh water towards the Modaomen outlet.Because of the increased river channel volume of the RNPRD, the annual mean residual speed significantly decreased.However, the decreasing trend was intensified in the North River Main Channel, while compensated in the West River Delta Networks due to the annual water discharge redistribution in the RNPRD.

MATEC
Keywords: water distributions; numerical model; the Pearl River networks DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2015 Web of Conferences and sediment transportation in different bathymetries.The depth-averaged 2-D hydrodynamic model was based on the 2-D shallow-water equations while depending on the Bousinesq eddy viscosity and static pressure hypothesis.The governing equations are expressed as Continuity equation: u and f v are the accelerated velocities caused by earth rotation; xx T , xy T , yx T , yy T are components of viscous stress; S is component of sources; xx s , xy s , yx s and yy s are the stress of radiation.

Figure 1 .
Figure 1.Map of showing the Pearl River Delta, bottom topography, major rivers and survey stations The model was tested by two representative periods, including July 16-24 1999 (wet season) and February 7-16 2001 (dry season).The comparisons of model results and measurements of water level, discharge for part of the survey stations are shown in Figure 2. The domain mean error and relative error of the total 32 stations are shown in Table 2 (the locations of the survey stations are seen in Figure 1.).The results of model validation are in reasonable agreement with the measured data, which indicate that the model is robust enough to review the fresh water transportations in the RNPRD.Special information regarding the model calibration and validation were also described in detail by Du and Mu (2014).The hydrodynamic model calculates the fresh water transportations in 2003 under the bathymetry conditions of 1999 and 2005 respectively.

Figure 2a .Figure 2b .Figure 2c .Figure 2d . 1
Figure 2a.Calibration of water level for dry season (solid line signifies model results and points depict observations)

Figure 3 .
Figure 3. Residual currents for the Pearl River Networks in 2003, under the topography of 1999(a) and 2005(b), (A: West River, B: West River Main Channel, C: Xihai Channel, D: Modaomen Channel, E: North River, F: North River Main Channel, G: Shunde Channel, H: Xiaolan Channel, I: East River, J: East River Delta Networks)

Table 1 .
Main parameters of the model

Table 2 .
Main parameters of the model