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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. What's more, intensive reclamation in the estuary is another main factor for tide increase at the outlets of the PRD region. [ABSTRACT FROM AUTHOR]

At tidal junctions, flow, sediment, and, indirectly, flooding risk are distributed over the downstream river branches. To characterize the hydrodynamic pattern related to the transport and distribution of suspended sediment, an intra-tidal field measurement was conducted at the first order tidal junction in the Pearl River Network, Makou-Sanshui (MK-SS), which was the first order junction and the apex of the delta. Two 13h boat-mounted ADCP surveys were implemented and 468 water samples were taken synchronously across three cross sections at wet and dry seasons during the spring tide. Observed spatiotemporal velocities were applied to elucidate the intra-tidal flow pattern in the tidal junction, and the acoustical backscatter intensity collected by ADCP was the inverse of the cross-sectional suspended sediment concentration (SSC). Additionally, vertical profiles of velocity and SSC allowed us to fit the lateral distribution of shear velocity and Rouse number, respectively. From these, we obtained the spatiotemporal variations of settling velocity, which then made it possible to interpret the time lag between flow and suspended sediment. Intra-tidal substance exchange, including flow flux produced by tidal wave and net suspended sediment, was inferred by Stokes transport, integrating axial velocities and the SSC. As a consequence of tidal impact, the flow processes of MK and SS were out of phase by 1–1.5 h. As the connecting channel of MK and SS, Ganggen (GG) experienced more frequent substance exchange, displaying a significant difference in flow pattern. A substantial clockwise secondary flow field was detected during the intra-tidal survey at GG, which was affected by the curved channel. This phenomenon was related to the cross-sectional distribution of SSC. Furthermore, the intra-tidal transport of suspended sediment exhibited an inverse direction during the wet and dry seasons. Sediment during the dry season favoured delivery from MK to SS, while it displayed an opposite trend during the wet season. The results reveal the pattern of intra-tidal partitioning of flow and sediment at tidal junctions. [ABSTRACT FROM AUTHOR]

Based on the annual maximum water level record spanning about 60 years at 34 gauging stations, we conduct flood frequency analysis in the Pearl River Delta (PRD) with generalized extreme value (GEV) distribution. The GEV model performs sound estimation with the correlation coefficient at most stations larger than 0.99. To better understand the flood risk especially in ungauged regions, the spatial distribution of flood-stage is displayed for three given return periods, 10-year, 20-year and 50-year. Similar pattern can be identified for different return periods except the difference in magnitude, which generally exhibits a decreasing trend from the upper region to offshore area with a scope of about 2–9 m. Additionally, a comparison between pre-1980 and post-1980 is carried out to quantify the flood-stage alteration. The results show that most stations display increasing flood risk except for some stations in the upper estuaries, which experienced slight water level decline. Particularly, the lower part of the PRD is vulnerable to the most severe flood-stage increment. The estimated extreme water level increases by 0.35 m (14.96%), 0.59 m (21.23%) and 1.06 m (29.96%) on average at stations corresponding to 10-year, 20-year and 50-year return period, respectively. [ABSTRACT FROM AUTHOR]

Abstract: This paper focuses on identifying changes in the tidal range and the duration of the flood tide in the Pearl River Delta region, China. Mann–Kendall test and regression methods are employed to verify the existence of trends in the annual tidal range and duration series (from the 1950s to 2005) at 17 stations in the 3 major tributaries of the delta. The research results indicate that generally there is an increasing trend in the tidal range and flood tide duration at most of the stations in the channel network. Moreover, the more upstream the location of the station, the more obvious the change in trend is, and these increasing trends are all significant at the stations in the upper part of the delta. Specifically, the tidal range at Shilong station in the East River has increased abruptly since the middle of the 1980s such that the mean level of post-1990 data is approximately three times that for pre-1985. Most stations near the mouth of the estuary show significant downward trends in the tidal range and flood tide duration. Results of the intersection point of the Mann–Kendall test curves for these Pearl River Delta stations appear mostly in 1980s–1990s, indicating that a noticeable change point occurs during this period. Human activities in the Pearl River Delta reached their peak during the 1980s. Among these, large-scale and long-term sand excavation modified the hydrodynamic regime dramatically in a way that increased the water depth and lowered the level of the riverbed, leading to an increased tidal prism and upstream movement of the tidal limit. Land reclamation is also likely to have had a relatively large effect on the tidal range and flood tide duration at stations near to the estuary mouth. Changes in other factors, such as freshwater flow and sea-level rise, seem to be of relatively minor importance. It is noteworthy that in a dry season the tidal dynamics are greatly enhanced and has resulted in strong saltwater intrusion into the estuary in recent years. [Copyright &y& Elsevier]