Monitoring and evaluation of sand nourishments on an embayed beach exposed to frequent storms in eastern China
Graphical abstract
Introduction
Due to the global climate change with sea level rise and more frequent and severe storms, serious beach erosion is observed all over the world (Castelle et al., 2007; Houston and Dean, 2014; Qi et al., 2010; Scott et al., 2016; Smith et al., 2014). At least 70% of sandy beaches experienced widespread erosion around the world (Bird, 1985), and approximately 50% of sandy coasts were regressed in erosion in China (Third Institute of Oceanography, 2010). Under this circumstance, there are growing numbers of beach protection or restoration projects in China, particularly in the tourism hotspots, to meet increasing public requirements (Kuang et al., 2011).
The common approach adopted in the past to protect or restore beaches is the hard engineering based (Cai et al., 2011; Pan, 2011). Although hard engineering, such as artificial coastal structures, can effectively mitigate shoreline retreat caused by storms, it may have negative impacts on its adjacent beach (Hamm et al., 2002). For decades, beach nourishment has become a preferred method to protect beaches in developed countries (Castelle et al., 2009; Cooke et al., 2012; Hamm et al., 2002; Hanson et al., 2002; Pan, 2011), which has also been frequently applied in China in recent years (Luo et al., 2016).
However, beach nourishments can often cause large-scale nearshore disturbances that affect the balance of alongshore and cross-shore sediment transport (Dean, 1983). Under natural conditions, storm-eroded sandy beaches may recover gradually over seasons to a decade timescale (Harley et al., 2015; Scott et al., 2016), while the beach processes with nourishment disturbance vary significantly (Elko et al., 2005; Seymour et al., 2005). Previous studies focused on nourishments with regular frequency and fixed implement timing in European countries (Hanson et al., 2002), the USA (Leonard et al., 1989) and Australia (Cooke et al., 2012). However, the beach protection approach at Zhoushan Archipelago in China is different due to frequent occurrence of storms. To prevent the severe storm erosion and maintain the recreational beach, small-scale and irregular nourishments have been frequently implemented and the occurrence time of those nourishments is often close to the storm period. Up to the present, beach morphodynamic evolution involved with this beach protection approach remains unclear.
Understanding the self-adjustment of beaches after nourishments is important for management (Elko and Wang, 2007), while there is always no regular monitoring of changes in beaches after nourishments (Chiva et al., 2018; Leonard et al., 1989; Stauble, 1988). Video monitoring systems are proved to be adequate in detecting and quantifying spatial and temporal beach responses (Archetti and Romagnoli, 2011). Especially, Argus system has been widely used in beach morphodynamic research in recent decades (Angnuureng et al., 2017; Balouin et al., 2013; Karunarathna et al., 2014).
Nourishment can be further evaluated based on the continuous imagery data obtained by video monitoring. Beach morphological variations and longevity of borrowed sediments are criteria to evaluate the effectiveness of beach nourishment (Liu et al., 2019; Psuty and Moreira, 1992). Factors affecting the beach nourishment longevity include parameters of nourishment projects and physical characteristics of beaches. The former mainly includes sediment characteristics, such as grain size distribution (Chiva et al., 2018; Pranzini et al., 2018; Stauble, 2007), mineral component (Pagán et al., 2018), volume scale (Basterretxea et al., 2007; Stauble, 2007) and spatial location (Karambas and Samaras, 2014) of borrowed sediments. The latter consists of wave/wind regimes (Karambas and Samaras, 2014) and the native morphological characteristics of beaches (Liu et al., 2019). Different combinations of those factors will result in different nourishment impacts on the beach.
In this study, we selected Dongsha beach, a 1.5 km-long embayed sandy beach in Zhoushan Archipelago of China, to observe its morphodynamics and evaluate the effectiveness of nourishments. An Argus video monitoring system was used to record variability of beach morphology. Beach morphological parameters including: dry beach width, shoreline displacement, dry beach area, beach orientation and rotation, and unit width volumetric change for more than a year were analyzed using Argus imagery data. Beach responses to individual storm events in five different cases were also revealed in detail. Based on morphological analysis, the evaluation of nourishment effectiveness and factors affecting nourishment longevity were discussed. Beach nourishment implications were proposed for further beach management.
Section snippets
Study area
Approximately 50% of sandy coasts were regressed in erosion in China over past several decades (Third Institute of Oceanography, 2010). Thus, a large amount of beach nourishment projects have been accomplished along China's coast since the 1990s (Cai et al., 2011; Kuang et al., 2019; Luo et al., 2016). Zhoushan Archipelago is a popular tourism destination with more than 30 embayed beaches (Xia, 2014), at a location connecting the Hangzhou Bay and the East China Sea. Typhoon is a common coastal
Storm characteristics
A total of 19 storm events (S1–S19) were extracted during the study period from June 1, 2016 to July 1, 2017 (Fig. 3). Hmax and Tp, D, storm direction θp (wave direction at the storm peak with respect to north), E and Ps were summarized for all the storm events. The strongest storm event (S3) occurred in September 2016 with a Ps of 1157.76 m2h, which is identified as significant severity (Ps > 500 m2h) according to Mendoza et al. (2011). Overall, four storm events over the study period were
Evaluation of beach nourishments
The effectiveness of sand nourishments in this study has been evaluated in terms of their abilities to meet the project goals, which were to prevent storm erosion and widen the recreational beach. As for preventing storm erosion, 12 storm events with none erosion and 3 storm events recovered within several days were observed (Table 2). However, seawall constructed on the beach and the intensive storm events resulted in long-term erosion. Guo et al. (2018) found that Dongsha beach didn't recover
Conclusions
Due to the increasing storm erosion of beaches, beach nourishment has become a wide-used measure. Argus video monitoring system is an effective means to monitor continuous storm-induced erosion of beaches and corresponding nourishment effectiveness. In this case study of Dongsha beach, video-derived morphological parameters of the beach were analyzed over a year, and the following main conclusions are obtained.
Seasonal morphological variation related to storm-intensive period existed on this
Declaration of competing interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
This research was funded by the National Key Research and Development Program of China (No. 2017YFC0405503), the National Natural Science Foundation of China (NSFC) (No. U1706214). The authors thank Dr. Giorgio Santinelli for his help in data processing and Google Earth for the open-source remote sensing data. We also thank the editor and reviewers for their insightful and constructive comments.
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