Interaction Between Coastline Change and Tidal System of the Bohai Sea

Dramatic change of coastline due to economic development in the last two decades has posed significant challenge to the marine environment and ecosystem in China. Little is known regarding the impact of rapid coastline change on the hydrodynamic of semi-enclosed Bohai Sea. In this study, we collected and digitized the nautical charts and satellite images of last decades to construct a coastline and bathymetry database. ADCIRC finite element model was utilized to simulate the tidal system of the Bohai Sea. The 2D model was calibrated and validated using data from coastal tide gauges. Scenario model runs was conducted to quantitatively evaluate the impact of coastline change on tidal system. Our study indeed revealed the significant change of tidal amplitudes and phases due to coastal development. Our results suggested that the change of tidal system might be a risk to the coastal structure and need to be considered in coastal management.

Hua Zhang, Yanfang Li, Tao Zou, Cheng Tang Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences

Findings

Background

Abstract Numerical Model

M2 Coastline 2002

M2 Coastline 2013

K1 Coastline 2002

O1 Coastline 2002

S2 Coastline 2002

K1 Coastline 2013

O1 Coastline 2013

S2 Coastline 2013

Table 2 Location of amphidromic point simulated with year 2002 and 2013 coastline s M2 North M2 south K1 O1 Lat Lon Lat Lon Lat Lon Lat Lon

Coastline 2002 39.88 119.88 38.05 119.04 38.16 120.78 38.03 120.95 Coastline 2013 39.47 121.30 37.83 119.26 39.47 121.30 38.00 121.03

Figure 6. Co-tidal chart of M2, K1, O1, S2 constituents produced from ADCIRC 2DDI model simulation with coastlines derived from Landsat images of year 2002 (left column) and 2013 (right column) . Color patterns and white lines indicate tidal phase lags in degree. Black lines depict tidal amplitudes in centimeter.

Figure 4. Calibration and validation of ADCIRC 2DDI model simulated sea surface elevation at selected coastal tidal monitoring stations. Empty blue circles denotes hourly observed sea surface elevations and blue lines depicts model simulated sea surface elevations. Observation data in May 2012 was used as calibration dataset. The simulation results were further validated using tidal station data from August 2012. Figure 1. Location and bathymetry of the Bohai Sea. Bathymetry data set was

calculated from 1:250K nautical charts.

Simulation Results

The Bohai Sea is a semi-enclosed shallow water body connected east to the Yellow Sea through the narrow Bohai Strait. It has a surface area of 77,109 m2 and an average depth of 18.7m. Coastal zone around the Bohai has been one of the fastest growing economy in China. Driving by drastic industrialization and urbanization, a rapid increase of coastal reclamation activities has been witnessed in last decade. Particularly, mega-projects such as Tianjin new coastal district, Caofeidian Industrial Zone, and the Yellow River Delta High-efficiency Economic Zone has made Bohai into a hotspot of anthropogenic coastline change. With the construction of harbors, seawalls, and urban complex on tidal flats and coastal wetlands, physical environment including the tidal system of the Bohai Sea has been permanently changed. The objective of this study is: 1) to develop a 2D hydrodynamic model for accurate prediction of tidal system of the Bohai Sea; and 2) to reveal the impact of coastline change during last decade on the hydrodynamics based on model simulation.

Table 1 ADCIRC-2DDI model scenario configuration Scenarios Nodes Elements Resolution Duration Time Step Friction 2002 14132 27281

<3km 365days 30s 0.0015 2013 13475 25894

• The ADCIRC 2DDI model successfully simulated tidal system of the Bohai Sea using satellite derived coastline and updated bathymetry. However, the Le Provost tidal database underestimated tidal amplitude for most constituents in studied region and need to be updated.

• Model simulation suggests that dramatic change of coastline from 2002 to 2013 substantially altered the tidal system of the Bohai Sea, which is shown by the southward move of amphidromic point of M2 constituent and increase of amplitude of K1 and O1 constituents.

• Further studies will be required to fully evaluate the potential risks associated with the interaction between tidal system and coastline change. Our model need to be further tested for its capabilities of predicting the tidal dynamics using updated bathymetry and boundary conditions .

Acknowledgements This research was supported by the "Strategic Priority Research Program" (XDA01020304) and “Key Research Program” (Grant NO. KZZD-EW-14) of the Chinese Academy of Sciences. We thank Rick Luettich at the Institute of Marine Sciences, University of North Carolina, for providing the ADCIRC-2DDI code. Landsat images courtesy of the U.S. Geological Survey .

Figure 2. Dramatic change of coastline from year 2002 to 2013 in the Bohai Bay as shown by the Landsat Images. Landsat image derived coastline of May 2002 and May 2013 was used in our model simulation.

Year 2002 Year 2013

Caofeidian Tianjin Huanghua

Figure 3. Finite element mesh used in ADCIRC 2DDI model simulation.

Figure 5. Maximum tidal surface elevation (top, in meters) and tidal current (Bottom, in m/s) from ADCIRC 2DDI model simulation with coastlines derived from Landsat images of year 2002 (left column) and 2013 (right column) .

Maximum Elevation (m) Coastline 2002

Maximum Velocity (m/s) Coastline 2002

Maximum Velocity (m/s) Coastline 2002

Maximum Elevation (m) Coastline 2013

Coastline 2002 Coastline 2013