Yuliang Zhu

Numerical simulation and analysis of saltwater intrusion lengths in the Pearl River Delta, China

ABSTRACT Zhang, W.; Feng, H.C.; Zheng, J.H., Hoitink, A.J.F.; Van Der Vegt, M.; Zhu, Y., and Cai, H.J., 2013. Numerical simulation and analysis of saltwater intrusion lengths in the Pearl River delta, China. In recent years, large-scale saltwater intrusion has been threatening the freshwater supply in the metropolitan cities surrounding the Pearl River delta (PRD). Therefore, a better understanding of the saltwater intrusion process in this region is necessary for local water resource management. In this paper, a one-dimensional flow and salinity model of the Pearl River networks was established to improve our understanding of saltwater intrusion problems in deltas. The model has high spatial resolution, discretized into 328 reaches and 5108 cross-sections, and the time step is 300 seconds for the hydrodynamic model and 30 seconds for the salinity model on the basis of the Courant–Friedrichs–Lewy condition. The model is calibrated and validated against the field measurements of the water surface elevation, discharge, and salinity at around 40 gauges in 2005 and 2001, respectively. The estimated results are in reasonable agreement with the observational data, suggesting that the model is sufficiently robust to simulate the movement of flow and salinity in the Pearl River networks. The simulated 0.5 parts per thousand salinity isohaline in the Pearl River networks displays a shape similar to “S” and slanting to the right, indicating that the maximum saltwater intrusion length occurs at the Humen outlet. In 2005, the saltwater intrusion lengths intruded far upstream at an average length of 32.4 km from the eight outlets, which is nearly two times that in 2001. Four representative upstream flows were also simulated to acquire quantitative knowledge of the response of the saltwater intrusion to discharges. Finally, historical data were collected to compare the situations of saltwater intrusion in the river networks in the 1960s and 2005. The result implies that the abrupt change in topography due to intensive dredging campaigns in the river networks is probably the most crucial factor leading to the saltwater intrusion outbreaks in large areas of the PRD in recent years.

Redistribution of the Suspended Sediment at the Apex Bifurcation in the Pearl River Network, South China

ABSTRACT Zhang, W; Du, J; Zheng, J; Wei, X., and Zhu, Y. 2014. Redistribution of the suspended sediment at the apex bifurcation in the Pearl River network, South China. Bifurcations in tidal networks strongly control the distribution of flow and sediment flux over a river network system. These distributions are important in determining the fluvial sediment dispersal and transport from river networks to shorelines, thereby greatly affecting river channel, estuary, and coastal environments. Long-term observations of flow and suspended sediment division at the apex bifurcation of the Pearl River network indicate that there has obviously been more water and sediment transport into the North River network since the 1990s. Wavelet analyses of the sediment division at the apex bifurcation show that common spectral structures were dominated by annual fluctuations from 1959 to 2005. However, significant, intra-annual and annual fluctuations have been detected since the early 1990s, corresponding well with the abrupt changes in flow and sediment division. By analyzing the controlling factors (water discharge ratio ηL and suspended-sediment concentration ratio KS) of the suspended-sediment flux distribution in a braided river, we concluded that both the riverbed downcutting from intensive sand mining in the lower river and the lack of suspended-sediment supply because of dam construction in the upper river are primarily responsible for the sediment redistribution in the apex bifurcation of the Pearl River delta. Furthermore, the wavelet analysis indicates an energy growth in the signal of the suspended-sediment division ratio. The observed spectral composition was used to estimate the potential suspended-sediment division in the absence of human influences by removing the effect of that energy growth, which helped to explain the extent of the effect of intensive anthropogenic activities on the redistribution of suspended-sediment flux. The estimate indicates that there has been a decrease of 6.27 × 107 tons of suspended-sediment load flowing to the West River network for sediment redistribution at the apex bifurcation of the Pearl River delta between 1992 and 2005.

Understanding Space–Time Patterns of Long-Term Tidal Fluctuation over the Pearl River Delta, South China

ABSTRACT Zheng, J; Zhang, W; Zhang, P, and Zhu, Y., 2014. Understanding space–time patterns of long-term tidal fluctuation over the Pearl River delta, South China. Principal component analyses (PCA) were used to identify the space–time patterns of long-term tidal fluctuation in the Pearl River delta (PRD) region. The tidal levels of the 35 stations over the entire PRD were divided into four regions by the PCA. The results show that different regions display different trends, indicating spatial variability of long-term tidal fluctuations in PRD region. The regional pattern of the first component loadings is mainly located near the outlets. Time series of the factor scores of the the first principal component (PC) shows a significant increasing trend. The regional pattern of the second component loadings is concentrated in the upper part of the PRD region. The factor scores of the the second PC, however, display a significant deceasing trend. The trend in the factor scores of the the third PC of the middle part of the North River is not significant, but it has to face the highest flood risk. As for the upper part of the East River, the factor scores of the the fourth PC also show a significant downward trend that demonstrates that the tidal level shows an obviously decreasing trend in the upper part of the East River. The impacts of the intensive human activities on these changes in the PRD region in the latest three decades are the dominant factor. In particular, the large-scale and long-term sand excavation in the whole PRD region directly results in the riverbed down-cutting, leading to the tidal level in the upper part of the delta decreasing obviously. More tidal prism can enter the delta much more easily also due to the sand excavation, which can fill out the volume not only to keep the tide level, but also even increase the tide level in the middle part of the North River. Whats more, intensive reclamation in the estuary is another main factor for tide increase at the outlets of the PRD region.

Analytical solution for salt intrusion in multiple-freshwater-source estuaries: application to Humen Estuary

Salt intrusion has some negative impact on the estuarine eco-environment as well as the water resource potential. The paper proposes an analytical model to describe salt intrusion in the estuaries with multiple freshwater sources. The impact of river discharge on the salinity distribution changes along the multiple-fresh-source estuaries, which is different from estuaries with single source of freshwater. Our analytical model is derived from the advection–dispersion equation for salinity while taking into account the hydrodynamic variation along the estuary. In this paper, we take the Humen Estuary, a strongly tide-dominated estuary with two major source of freshwater, as an example to illustrate the model. By testing against eight surveys over a complete spring-neap tidal cycle, the analytical model’s capacity to describe salt intrusion in the Humen Estuary is calibrated and validated. The results show that the analytical method can be used to compute the salinity distribution in the multiple-freshwater-source estuaries. In comparison with the field data in the Humen Estuary, the calculated results indicate that the salt intrusion process exhibits remarkable segmentation in the multiple-freshwater-source estuary, although the estuary’s inherent characteristic remains the same throughout the estuary. Moreover, by analyzing the multi-segmental features of the Humen Estuary, an efficient and effective model to predict the salt intrusion length of the Humen Estuary is presented and satisfactory results are obtained to illustrate its practical application.