A New Strategy for Harbor Planning and Design

Transcription

1 A New Strategy for Harbor Planning and Design Xiuying Xing, Ph.D Research Associate Sonny Astani Department of Civil and Environmental Engineering University of Southern California Los Angeles, CA Jiin-Jen Lee, Ph.D, P.E. Professor of Civil and Environmental Engineering Sonny Astani Department of Civil and Environmental Engineering University of Southern California Los Angeles, CA September 30, 2008

2 1 Introduction The steady increase in global trade in recent years has necessitated the drastic expansion of the cargo handling capacities in modern harbors. This has been the prime forces driving the continuing modification and expansion of the harbor basin in several harbors in the Pacific coast of the United States. The significance of these harbors to the national economy can not be overemphasized. Nearly 50% of the cargos coming to the United States come through the Los Angeles/Long Beach harbor complex. In order to accommodate the large ships as well as heavier traffic volume, water depth in the harbor must be increased by dredging and significantly more berthing facilities need to be expanded. Such activities will require examination of its environmental impact and mitigation measures must be addressed. For harbor planners and designers, it is important to note that any changes in the harbor layout or changes in water depth will alter the resonance characteristics of the harbor basin to incident waves (Lee and Xing, 2008). These changes in response characteristics may also bring undesirable and excessive ship motions which may hamper the cargo loading and unloading activities, damage mooring lines or berthing facilities. Thus it is imperative that, in the planning and design stage, the response characteristics of the harbor basin (or the proposed modified harbor basin) be carefully studied to avoid any undesirable resonant mode which may come close to the resonant periods of the ship-mooring system thereby avoiding excessive ship motions. It has been shown that once the harbor layout is fixed, it is very difficult (and very costly) to alter the response characteristics without major overhaul of the harbor layout (Lee, et al, 1998). In this paper we will demonstrate a convenient strategy using computer model to investigate the harbor response characteristics for the planning and design of harbor basin in order to minimize the harbor resonance effect. 2 Hualien Harbor We have conducted a study recently for Taiwan s Hualien Harbor for the purpose of reducing the negative impact of harbor resonance problems. 2

3 Hualien Harbor is located in the eastern coast of Taiwan and facing the Pacific Ocean (Shown in Figure 1). It has long history of harbor resonance problem induced by typhoons during the typhoon seasons. In the last twenty years, on several occasions, harbor resonances have resulted in mooring lines being snapped, ships and dock facilities being severely damaged. To avoid damages as typhoons approached, the ships have been ordered to move out of the harbor! 3

4 applied for the beach, for which energy will be somewhat dissipated. Energy loss is considered at the harbor entrance due to the flow separation. #08 φ =0 n φ iα 2φ = iαkφ n 2k s 2 #22 φ1 = φ2 + φ = φ2 + g U fe U iω 2g Figure 2. Air photos of Hualien harbor, Taiwan with simulation domain imposed (left) and boundary conditions indicated (right) 3 Confirmation of the Model Results with Field Data The model for the present condition is simulated and compared with the field data measured by Harbor and Marine Technology Center (HMTC) during typhoon Tim in July 1994 (Su and Tsay, 2002). The model results and field data at station #22 and #8 as noted in Figure 2 (right) are presented in Figure 3. Figure 3. Simulated response curves and the observed field data at stations #22 and #08 Figure 3 shows the amplification factor (ratio of response wave height at certain location 4

5 divided by the incident wave height) as a function of the incident wave period. It is seen that the agreement is surprisingly good with respect to resonant periods, resonant bandwidths and peak amplification factors. The result clearly indicate that a broad band of resonant response for wave periods between 100 sec and 160 sec which are the period range that have been troublesome of some of the ships. 4 Strategies for Modification of Harbor Layout To minimize the harbor resonance problem in Hualien harbor, several modification strategies were studied. Two of the modification strategies and their effects will be discussed herein. 4.1 Modification Strategy 1: A Second Opening The first modification strategy is to make a second opening at the east bank of inner harbor with a width of 150 m, as shown in Figure 4. The purpose of the second opening is to release some energy of the oscillation waves during the typhoon seasons, thus hopefully decrease the ship motions. The reflection coefficients used in the simulation are also indicated in Figure 4. The reflection coefficient is assigned to be 0.82 for the beach and 0.98 for the vertical wall boundaries inside and outside of the harbor. Figure 4. Layout of the second opening modification strategy with reflection coefficients indicated The simulated response curves for the second opening strategy at the two stations are shown in Figure 5. The observed data as well as the simulated results for the present 5

6 condition are also included for the comparison. For station #22, the amplification factors increase significantly for the periods from 60 sec to 80 sec and from 120 to 160 sec. It s seen that the oscillation condition in the outer harbor basin becomes worse instead of getting better. For the inner harbor area at station #8, the oscillation condition gets worse for the wave period range from 120 sec to 140 sec and from 170 sec to 200 sec. The results show that the second opening does not help much in reducing the harbor resonance problem. It appears that for second opening, instead of releasing out the energy, more energy can propagate into the harbor basin from the second opening, and the waves could be trapped at the outer harbor area, making the oscillation condition at station #22 worse than the present condition. Figure 5. Simulated response curves for the second opening modification strategy at stations #22 and # Modification Strategy 2: Combination of Energy Dissipaters The most possible location for energy dissipater to be installed is at the end of the harbor. There is enough space for constructing sloping boundary that permits wave overtopping. At the eastern caisson breakwater, rubble mound energy dissipater can be constructed, and along the beach seawalls were partly constructed and can be remedied more effectively in the future. The new modifications strategy is to try to combine the energy dissipaters at strategic boundaries, as shown in Figure 6. The circled area with a diameter of 500 m is proposed to be dredged to 25 m deep for ship turning purpose. 6

7 D=500m h=25m R=0.8 R=0.5 R=0.98 R=0.5 Figure 6. Layout of the combined energy dissipater modification strategy with reflection coefficients indicated The reflection coefficient for the concrete wall inside the harbor remains 0.98 in the simulation. But the reflection coefficient at the caisson breakwater boundary is assigned to be 0.8 and the one for the beach is 0.5. For the overtopping slope boundary at the end of the harbor, the reflection coefficient was tried to be 0.5 in the simulation. The simulated results are illustrated in Figure 7. It can be seen that the combined energy dissipaters at the boundary significantly lower the amplification factors. Most of the amplification factors are lower than 1.0 except those at station #22 and #08 for wave periods about 120 sec, which are only slightly higher than 1.0. It can be expected that if the dissipaters are well constructed and can have reflection coefficients similar to those specified in the computation, the oscillation problem in Hualien harbor can be significantly reduced. Figure 7. Simulated response curves for the energy dissipater modification strategy at stations #22 and #08 7

8 5 Summary Remark The numerical model was applied in the simulation for Hualien harbor to investigate the effectiveness of various modification strategies. The good agreements, which are shown, between the prototype measurement and the computer simulation results reinforce the validity of the computer modeling technique. To investigate wave oscillation problem in a harbor, using physical models will be very costly and time consuming. However, modifications in a numerical simulation can be done easily especially when the original grid is already generated. The computer model can serve as a very powerful and cost-effective engineering tool for harbor planning and design to provide a sheltered environment for moored ships and vessels. For the modification strategy studied, several conclusions can be drawn: 1. Without drastic changes in the harbor layout, the resonant modes could not be changed significantly. That s why the wave oscillation study is very important in the harbor layout planning and design. Once the harbor is constructed, the resonant modes cannot be easily altered. 2. Wave oscillation can be diminished or shifted to some extent by modifying the harbor configuration although the oscillation modes are hard to be completely eliminated. Different harbors might need different remedy strategies suitable to the local condition. 3. More openings for a harbor may not be a reasonable modification strategy since more energy could propagate inside instead of releasing them away from the harbor region. 4. Energy dissipaters should always be considered in harbor design and planning as long as they can be effectively designed and constructed. The total reflection condition at a vertical concrete wall could result in a worse oscillation condition. 8

9 References Berkhoff, J.C.W. (1972). Computation of combined refraction-diffraction. Proc. 13 th Coast. Eng. Conf., ASCE, New York, N.Y., Lee, J.J., Lai, C.P., and Li, Y. (1998). Application of computer modeling for harbor resonance studies of Long Beach & Los Angeles harbor basins. Proceedings of 26 th International Conference on Coastal Engineering, ASCE, Lee, J.J. and Xing, X.Y. (2008). A chapter on Computer modeling for harbor planning and design in the book Handbook of Coastal and Ocean Engineering. Edited by Young C. Kim, World Scientific Publishing Company. Su, C.H. and Tsay, T.K. (2002). Numerical simulation on harbor oscillations in Haw-Lien harbor. Report NO. MOTC-IOT-IHMT-NB Institute of Harbor & Marine Technology Institute of Transportation, Tai-Chung, Taiwan. 9