Evolution of sedimentary dynamic environment in the western Jiaozhou Bay, Qingdao, China in the last 30 years

Abstract

Intensive reclamation and cross-sea bridge construction occurred in the Jiaozhou Bay, Qingdao, China in the last 30 years. These changes have not only reduced water volume but also altered sedimentation in the bay. Using the data collected in the 1980s, 2000s and early 2010s, bottom sediment evolution in the last 30 years in the western bay were studied. Furthermore, a numerical model was used to study the hydrodynamic environment change in the surrounding sea areas. The seabed sediment became finer in the northwestern area from the 1980s to 2003, which was mostly influenced by the reduced sediment discharge from the Dagu River. However, from 2003 to July 2014 during which the Jiaozhou Bay Bridge was built, the seabed sediment became coarser on the north side of the bridge, and became finer on the south side; in particular, two coarser sediment belts appeared across the channel bridge. Numerical simulation results indicated increased current velocity on the north side and decreased velocity on the south side of the bridge. The tidal-duration asymmetry was calculated from the tidal harmonic parameters of the model output. In the flood dominated JZB, the larger deceased tidal-duration asymmetry on the north side of the bridge could result in more coarser seabed sediment brought in from the northern bay. These could be the reasons for the coarser seabed sediment on the north side of the bridge and finer sediment on the south side after the bridge construction. Deeper water on the north side and shallower water on the south side of the bridge from 2005 to July 2014 further demonstrated the effect of hydrodynamic change caused by the bridge construction.

Introduction

Intensive land reclamation in a semi-closed water can largely reduce tidal prism, water exchange, entrance flow velocity, and among others (Parker et al., 1972, Yanagi and Onishi, 1999, Takekawa et al., 2006). Moreover, artificial projects, for example, cross-sea bridge, may further exacerbate the environmental degradation.

The Jiaozhou Bay (JZB) is situated off the southern coast of the Shandong Peninsula, China, and is connected to the Yellow Sea to the south (Fig. 1a). The surface area of the JZB was 343.09 km² in 2012 (Ma et al., 2014). Its water depth, with an average value of 7 m (Gao and Wang, 2002), becomes deeper from northwest to southeast (Li et al., 2006). The area and the shoreline of the JZB have changed markedly due to intensive reclamation and cross-sea bridge construction in the last 30 years. Specifically, the area of the JZB was reduced by 6% from 1986 to 2003 and by 6.4% from 2003 to 2012 (Ma et al., 2014). The shoreline length was reduced by 30.4% from 1952 to 2009 (Chen et al., 2009). To reduce urban traffic and promote economic development, the construction of the JZB cross-sea bridge began in 2007, and the bridge opened to traffic in 2011. The bridge was built in the central and northern parts of the JZB (Fig. 1b), connecting the east and west coasts of the bay. The total length of the JZB Bridge is 26.707 km, and the part of the bridge over the water is about 25.881 km, and the number of piles bored in the sea area is 5127. The Dagu River Channel Bridge (Fig. 1c) is the largest waterway bridge of the JZB Bridge (Zhang, 2009), with a bridge tower pier, four auxiliary piers and smaller approach piers (Meng et al., 2008).

Previous studies focused on the bay area, coastline and hydrodynamic changes (Sun and Zhang, 2001, Yan et al., 2001, Zhou et al., 2010, Chen et al., 2012). The tides in the JZB are semi-diurnal, with an average tidal range of 2.80 m (Compilation Committee of Bays in China, 1993). The velocity of flood tidal current is greater than that of ebb tidal current, which is between 32.0 and 86.2 cm/s at the entrance of the JZB. The duration of ebb current is longer than that of flood current, being 6.77 and 5.56 h at the entrance of JZB, respectively (Qiao, 2008). Chen and Chen (2012) studied the hydrodynamic evolution of the JZB from 1935 to 2008 using a numerical simulation. They found that the co-phase lag lines of M₂ tidal constituent changed obviously, while the patterns of velocity field in different decades were similar; however, the tidal period averaged velocity decreased by 17% during the spring tide. Gao et al. (2014) investigated changes in tidal dynamic factors in the JZB from 1935 to 2008. According to their model, the M₂ tide had experienced only minor changes over this period, while the M₄ tidal amplitude rose dramatically, by up to 80% at the northeastern part of the JZB, which caused a significant increase in the M₂−M₄ tidal-duration asymmetry. The sediment discharged from the Dagu River is the major source of the bay sediment (Li et al., 2006), accounting for over 94% of the total sediment input. The sediment is mainly deposited in the northwest part of the bay (Wang, 1983, Wang and Gao, 2001). However, the annual runoff of the Dagu River decreased by 52.38% due to climate change (Jiang and Wang, 2013) and became almost negligible after the 1990s due to upstream dam construction (Shi, 2010). Thus, the sediment mainly moves and redistributes under the action of hydrodynamics (Wang et al., 1982; First Institute of Oceanography et al., 1984, Zheng and Shen, 1986, Wang et al., 2000, Lario et al., 2002). Sediment movement in the JZB is mainly controlled by tidal flows (Chen et al., 2012).

Observations of sediment dynamics showed that net suspended sediment transport was directed toward outside of the bay, with an order of magnitude of 10³ ton during a tidal cycle (Wang and Gao, 2013, Wang et al., 2014). Considering the semi-enclosed shape of the JZB, the waves in the bay are mainly wind-generated. The near-bottom suspended sediment concentration fluctuations over an intertidal flat displayed a U-shaped trend during each tidal cycle, which was attributed to the shoaling effect of frequent wind-generated waves (Yang et al., 2007). However, in the last 50 years from 1961 to 2010, the wind in Qingdao decreased in each season with an annual average rate of −0.359 m/s/10a (Ma et al., 2013). Therefore, the waves in the JZB should not be responsible for the seabed sediment change.

A cross-sea bridge can affect sediment movement, pollutant dispersion and biodiversity of the bay (Kitheka, 1997). Li et al. (2014) simulated the JZB's hydrodynamic environment under the conditions of with and without the cross-bay bridge. Model results indicated that the bridge's influences at the entrance, waterways and north side of the bridge are the largest. The studies of the Hangzhou Bay Bridge (Pang et al., 2008) and the Wenzhou Damen Bridge (Han et al., 2002) showed that a bridge has an impact on hydrodynamic environment but the impact of the bridge is confined to a small area near the bridge. The structure of the flow is complex around the pier piles, with the whirlpool being the main factor of local pier scour (Elder, 1959). If the bridge span is relatively large and is built in the seabed topography that is relatively flat with weak hydrodynamic flow, the impact of the changes of water and sediment environment on the deposition and erosion will not be too obvious after the construction. If there are waterway, stronger current and complex terrain near the bridge, the impact of the flow changes caused by piers will be significantly increased (Wang, 2011).

Intensive reclamation and cross-sea bridge construction have not only reduced water volume of the JZB but also increased the occurrence and areal coverage of sea ice and altered sedimentation in the bay (Zhang et al., 2012). However, few studies were conducted on the seabed sediment evolution and topography change concerning the dynamic sedimentary environment. This paper aims to reveal the topography and seabed sediment changes in the last 30 years, during which the Huangdao Island was connected to the mainland in the 1980s, the intensive exploration and reclamation were carried out from the 1980s to the 2000s and the cross-sea bridge construction started in 2007. Considering the great influence by the Dagu River and the cross-sea bridge, the sea area in the central and western parts of the JZB was taken as the study area (Fig. 1b). Based on the field observation data, historical data and numerical model are described in Section 2. Seabed sediment change with its mechanism is reported in Section 3, and topography change is analyzed to further support the hydrodynamic change due to the bridge construction in the same section. Conclusions are in Section 4.