Prediction of changes in tidal system and deltas at Nakdong estuary due to construction of Busan new port

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1 Prediction of changes in tidal system and deltas at Nakdong estuary due to construction of Busan new port H. Gm1 & G.-Y. park2 l Department of Civil & Environmental Engineering, Kookmin University, Korea 2 Hyundai Construction Company, Korea Abstract The deltas at Nakdong Estuary may experience environmental impact because of the construction of Busan Yew Port. There are several physical properties which can influence the bathymetric change at the area. The effect of the New Port on the future bathymetric changes of the deltas was examined in three main directions: the tidal current, wave and wave-induced current, and river flood. This paper describes the tidal current environment and the consequent bathymetric change before and after the construction of the Busan New Port and the Nakdong Tidal Barrage. The model results show that the new port will reduce the tidal current speed around the construction site, but not significantly influence the flows around the river mouth area. 1 Introduction The Busan New Port is situated on the southern coast of Korea near Busan City and Nakdong River, see Figure 1. The Busan New Port was recently planned by the Korean Government to enlarge its berthing capacity, especially for the large

92 Coastal Engineering V1 container ships. The construction work has been carried out for a few years so far since 1998, and the present coastline at the area looks like Figure 1.

2 92 Coastal Engineering V1 container ships. The construction work has been carried out for a few years so far since 1998, and the present coastline at the area looks like Figure 1. The construction work will go on until the designed structures are all completed for several more years, see Figure 2. Like other development projects on the globe, the large construction work of the Busan New Port will invoke serious environmental impacts. One of the major impacts is the bathymetric change around the development area. The birds habitat and plants are famous around a series of deltas at the river mouth. The bathymetric change around the development area is related to a lot of physical parameters, e.g. tides, waves, storms, river floods, resonance, and winds. However, the major causes for the sediment transport at the area are believed to be the tidal currents, waves, and river flows. The effect of the construction work on the tidal system, and the consequent bathymetric change is discussed in this paper. The effect of the development of the port on the wave and wave-induced current fields, and river flows will be discussed in other papers. Another interest is that the Nakdong Tidal Barrage was constructed at the mouth of Nakdong River at about 6 km upstream from the deltas in Although the barrage was already constructed after detailed investigation including numerical modelling work beforehand, there have been some disputes on the accuracy of the prediction of the future bathymetric changes around the river deltas. The effect of the construction of the barrage on the bathymetric change at the area is cross-checked by assuming a ficticious geography: keeping the present coastline and bathymetry, but removing the tidal barrage. Figure 1 : Present topography of the site.

Coastal Engineering V1 93 2 Tidal system at the site Figure 2: Future topography of the site. The average tidal range of M, constituent at the site is about 1.

3 Coastal Engineering V Tidal system at the site Figure 2: Future topography of the site. The average tidal range of M, constituent at the site is about 1.2 m, and the average spring tidal range is about 1.7 m. The ranges are slightly larger on the west boundary line than those on the east boundary line. The phase lag of the M, starts from the north-east, and propagates to the west. The tidal current speed reaches up to 1.2 m/s at a few narrow main channels between islands in the area for spring tides. The tidal currents around the river deltas have become weaker since the tidal barrage at the entrance of Nakdong River was constructed in The project was another large-scale development around this area before the Busan New Port. The bed material at the site is sandy in the main channels, and is muddy at calm areas. The bed material at the river entrance and around the islands is also coarse sand. Large bed forms have not been reported around the area up to the present except the exposed islands. The construction work of the New Port includes reclamation of some shallow tidal flat areas, and dredging work in the future port and the navigation channels. The boundaries between the land and water are composed of the gravity blocks, rubble-mound structures, and pile structures. The Nakdong River is one of the 4 major rivers in Korea. The river is split into two branches at the mouth. Many deltas or islands have developed at the river mouth for long time. It is presumed that the estuary is subject to long-term siltation due to the sediment discharge from the river. Questions arises on why the islands were formed, why they were formed at the places, why the main channels have the present sections areas, and what will happen to the deltas in

4 94 Coastal Engineering V1 the far future? These questions cannot be simply solved because of the inherent complexity. However, short-term responses caused by artificial modification of the topography can be relatively soundly predicted by using adequate tools. 3 Description of numerical model An existing numerical model, KU-2DHF, was used to reproduce the present tidal flow field, and to predict the tidal flow field after the construction of the Busan New Port., see Kim([l], [2], [3], 141, [5]). The model is a typical depth-average, horizontal, two-dimensional, explicit, finite-difference model. The model is equipped with a nesting technique, the treatment of dryinglwetting grid points, and the option to choose a method from several evaluation methods for the wavecurrent bed friction forces, see Kim et a1.[6]. The driving forces of the flow model can be the gradients of tidal levels, wave-induced radiation stresses or wind stresses. The tidal ranges and phase lags along the open boundaries were assigned after integrated consideration of the available data sets including previous field measurements, and other wider area modelling results of Korea Ocean Research and Development Institute. The bed friction forces were computed from the Chezy formulation with the coefficient of 65. The initial condition of the tidal flow model does not affect the final solution, but it should be well provided to reduce the preparatory execution time before the final solution. The constant water level of the lowest water level of M, was assigned in the whole computational domain, and the phase lags were moved near to 180" along the whole open boundary lines. The model was executed for 5 tidal cycles, and the solution for the last cycle was extracted out as the final solution for analysis. 4 Model results The model was applies to three bathymetries, the present one (Bath l), the future one after the completion of the port (Bath 21, and a ficticious one which is basically the same as the present one with but the upstream zones of the river beyond the present tidal barrage was added, and the barrage was moved (Bath 3). The computed strongest flood flow vectors for Bath1 are shown in Figure 3. Strong currents develop between islands, especially at the three channels of Sts. 1, 2, and 3. The strongest current speed for M, constituent is about 70 cmls at St.3. Secondly, the model was applied to Bath 2. The computed strongest flood flow vectors for Bath 2 are shown in Figure 4. The tidal current through the channel of St. 2 is reduced by about 3 % influenced by the new port. The flow field around the river entrance does not change much. The computed strongest ebb flow vectors for Bath 1 are shown in Figure 5. The results demonstrate that the ebb channels are much different from the flood channels. This difference causes the residual flows, and the net sediment transport.

Coastal Engineering V1 95 The computed strongest ebb flow vectors for Bath 2 are shown in Figure 6. The current speed at St. 2 decreases, but the flow field dose not change at other areas.

5 Coastal Engineering V1 95 The computed strongest ebb flow vectors for Bath 2 are shown in Figure 6. The current speed at St. 2 decreases, but the flow field dose not change at other areas. The computed residual flow vectors for Bath 1 are shown in Figure 7. An interesting thing is that large or small-sized vortices develop around sharp land corners or inlets between islands, especially around Sts. 1 and 3. The seperation zones may keep pollutants or sediment, and cause environmental or siltational problems. The computed residual current field for the M, constituent is shown in Figure 8. The vortex diameter of the residual flow at St. 1 in the west entrance of the new port site is smaller than that at the moment. The strength of the vortex at the site is also weaker than before. The residual flow at the future berths is westward for the present plan, but the residual flow becomes negligible after the construction of the new port. Consequently, the residual flow speed at St. 2 becomes slightly smaller after the construction. The contours of the maximum absolute bed shear stress values during a tidal cycle at each grid point for Bath 1 and Bath 2 are shown in Figure 9. and 10, respectively. The computed bed shear stress at St. 2 for Bath 2 is about 0.4 N/m2 which is smaller than that for Bath 1 by about 3 %. The bed shear stress at the west-north channel after the construction is expected to decrease. The effect of the construction of the new port seems to be negligible around the river mouth, or St. 3. The computed maximum absolute shear stress contour for Bath 3 is shown in Figure 1 l. The effect of the tidal barrage on the bottom shear stress is considered to fasten the siltation around the delta area, since the computed shear stresses are reduced for Bath 1 compared to those for Bath 3 (before construction of the barrage). We can expect two opposite effects of the tidal barrage on the siltation rate at the deltas. The present computation results demonstrate that the erosion rate may not be as strong as before. However, the sediment supply from the upstream river must have been reduced since the barrage may act a role of sediment-trap. The net amount of sediment supply from the river amount may be the same, but regular maintenance dredging is conducted at the upper side of the barrage, which reduces the sediment supply to the deltas. The quantitative analysis on the subject is needed in the future. 5 Conclusions The changes of the tidal system around the Busan New Port, Nakdong Estuary, and Gadeok-Do were examined by using an existing numerical model. The effect of the construction of the Busan New Port will include accelerating of deposition at a few local areas in the computational domain. The tidal barrage may also have contributed to the growth of the deltas at the river entrance since it was built, and the trend may go on in the future judging from the bed shear stress reduction at the main channels. However, the long-term variation of the bathymetry is also related to the sediment supply from the river. Therefore, the prediction of the integral bathymetric change at the area should be dealt with by involving the river sediment supply and dredging amout in detail.

6 96 Coastal Engineering V1 N Figure 3: Computed M2 flood current field for Bath 1. Figure 4: Computed M2 flood current field for Bath 2.

7 Figure 5: Computed M, ebb current field for Bath 1. Figure 6: Computed M2 ebb current field for Bath 2.

8 98 Coastal Engineering V1 N Vector Scales Figure 7: Computed M2 residual current field for Bath 1. Vector Scales... Figure 8: Computed M2 residual current field for Bath 2.

9 Coastal Engineering V1 99 Figure 9: Computed largest absolute bed shear stress field for Bath 1. Figure 10: Computed largest absolute bed shear stress field for Bath 2.

10 100 Coastal Engineering V1 Figure 11 : Computed largest absolute bed shear stress field for Bath 3. 6 Acknowledgements The present work was supported by Hyundai Construction Company. References [l] Kim, H., Three-dimensional sediment transport model, Ph.D. Thesis, The University of Liverpool, UK, [2] Kim, H., O'Connor, B.A. et al., Application of WIBATH-00 to wave-induced currents with tide at Algarve, Portugal. HYDROSOFT 2000, Lisbon, Portugal, Vol. 8, pp ,2000. [3] Kim, H et al.., Numerical model system KU-IWPH-02 for wave-driven current with wave reflection. Proc. of China-Korea Joint Seminar on Coastal Sediment Transport 2002, Quindao, China, [4] Kim. H., Shim. Y. & Park. S., Prediction of siltation at Chungdong Harbour using Sedtran-3D. Ocean Cities '95, Monaco, [5] Kim. H., Kim. T. & O'Connor. B.A., Scour around Sukmo Square Foundation on tidal flat, Korea, IAHR - RCEM, Genova, Italy, [6] Kim, H., O'Connor, B.A., Park, I.-B., Lee, Y.-G., Modeling effect of intersection angle on near-bed flows for waves and currents. Journal of Waterway, Port, Coastal, and Ocean Engineering, ASCE, Vol. 127, No. 6, pp

Appendix G.19 Hatch Report Pacific NorthWest LNG Lelu Island LNG Maintenance Dredging at the Materials Offloading Facility

Appendix G.19 Hatch Report Pacific NorthWest LNG Lelu Island LNG Maintenance Dredging at the Materials Offloading Facility Project Memo H345670 To: Capt. David Kyle From: O. Sayao/L. Absalonsen December