Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"
NAVIGATION CHANNEL DEPTH FOR IBAKA DEEP SEAPORT IN NIGERIA Ephraim U. Paul1, David A. Brooks2, James M. Kaihatu3, and Uwemedimo R. Ebong4 ABSTRACT A natural harbor is discovered in Ibaka Bay, Nigeria. Ibaka Bay has an average area of 50 km2 and centers on Latitude 4.65oN and Longitude 8.34oE. The harbor has an average non-dredged draft of 13.5 meters. Ibaka Bay is part of Cross River Estuary, which empties into the northern Gulf of Guinea. In 2003, Akwa Ibom State Government proposed siting a deep seaport in Ibaka. Preliminary investigations showed that the proposed deep seaport has several competitive and comparable advantages over existing seaports in the region. This deep harbor has been locally observed and monitored for over six decades. Sediment processes seemed to have reached equilibrium state in the Bay. Tom Shot Island at the Southwest boundary of the Bay shields the harbor from South-south-west Atlantic swells and other extreme events. The present study investigated the viability of deepening the approach navigation channel to the Bay as part of a larger study on stability of morphodynamics in Ibaka Bay. Field studies were conducted, using locally made drifters, to collect current and wave data as well as information on the dominant sediment characteristics in Ibaka Bay. Sampled data were analyzed and used to calibrate and verify a coupled wave, circulation, and sediment numerical model. Numerical results indicate that the flushing characteristics of the Bay will not be hindered when the navigation channel and turning basin for the deep seaport are dredged to accommodate larger vessels. Morphological characteristics of the site also attenuate higher wave energy such that increased offshore wave action has little or no impacts on the inner harbor where vessels will berth. Finally, the harbor is estimated to have minor maintenance dredging requirements with a five-year dredging window. Keywords: Sediment transport, model validation, depth of closure, seasonal variability, experimental design. INTRODUCTION Ibaka is one of Nigerian south-south coastal communities. It is centered on Latitude 4.65 oN and Longitude 8.32oE with an approximate land area of 50 km2 and estimated population of 21,600 people (NPC 2014). It is situated along the Gulf of Guinea (GoG) in the Southern end of Nigeria and has maritime boundaries with Cameroon to the east, Cross River State, Nigeria to the north, and Equatorial Guinea to the south. The average annual land and surface water temperature is 28oC. The water in the Bay is well mixed throughout the year with an average salinity of 28. Maximum spring tidal range observed in the Bay was 3.5 meters (AKSG Official Site 2014). The yearly average tidal range at the offshore boundary of the Bay is 1.7 meters. Ibaka Bay (the project site) is adjacent to and situated Northeast of Ibaka community. Ibaka Bay has a non-dredged draft of 13.5 meters in most part of the Bay (AKSG Official Site 2014). Water level at the Bay is approximately five meters (5 m) above mean lowest low water datum (MLLW) in the region (Antia 2012: Personal Communication). Figure 1 is a physical map of Africa indicating Ibaka Bay (letter “A” in the map). Figure 1 shows the proximity of Ibaka Bay to other countries in Africa. Six other countries (Cameroon, Equatorial Guinea, Garbon, Congo, Benin Republic, and Togo) have direct access to the proposed Ibaka deep seaport. None of these countries has its own deep seaport or any other operational deep seaport closer to it than Ibaka.
1
Corresponding Author, Lecturer I, Marine Engineering Department, Akwa Ibom State University, Ikot Akpaden, 532021, Nigeria, Email:
[email protected] 2 Professor of Ocean Engineering, Texas A&M University, MS 3146, College Station, Texas 77843, USA, T: 979845-5527, Email:
[email protected] 3 Associate Professor, Department of Civil Engineering, Texas A&M University, MS 3136, College Station, Texas, 77843, USA, T: 979-862-3511, Fax: 979-862-8162, Email:
[email protected] 4 Department Head, Marine Engineering Department, Akwa Ibom State University, Ikot Akpaden, 532021, Nigeria, Email:
[email protected]
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"
Figure 1. A section of the physical map of Africa showing Ibaka bay (“A in red background” in the map). (Courtesy: www.mapsofworld.com/africa)
Southeastern coastal region of Nigeria has a wider (≈ 120 km) continental shelf when compared to the southwestern Nigerian continental shelf (≈ 30 km) (Awosika et al. 1993; Awosika and Ibe 1994; and Awosika et al. 2000). Long waves (swells) from the open ocean (northern Gulf of Guinea) shoal and break at a further distance before they get to Ibaka Bay. This is one of the reasons boat mishaps and wave breaking related hazards are minimized in Ibaka Bay and the entire Cross River Estuary. The topography of the estuary also gives this project site a comparative advantage. The northern Gulf of Guinea seafloor drops off quickly, reaching a natural depth of more than 15 meters in less than 2 km from the mouth of the Bay. Large vessels, such as C9 containership, can navigate freely through open waters until they are within about 2 km to the harbor. Using an average vessel speed of 2.5 m/s, piloting and other major navigation guides will be required in the dredged channel for about 15 minutes travel time into and out of the harbor. Other major ports in Nigeria can only be accessed through travelling in dredged navigation channels for some hours. Ibaka community hosts one of the Nigerian Naval bases (Forward Operation Base, Ibaka). The Navy helps to secure Ibaka and its coastal waters against external invasion and sea pirates. It also collaborates with civilian researchers to conduct some Oceanographic and environmental engineering research in the Bay and surrounding waters in Cross River Estuary. Most data collected during these studies are not available in public archives. They are reserved for internal operations of the Navy. The project site also has an adjacent beach, The Ibaka Beach. The sediment on the beach is mostly coarse (d 50 ≈ 0.32 mm) sand. This shiny coarse sand gives the beach an aesthetic look. The beach is not yet developed for tourism and other recreational activities. The beach nourishment is an integral part of the proposed deep seaport project. When
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" the deep seaport is operational and the beach fully developed, the strategic location of Ibaka Beach will make it the preferred vacation resort for both local and international tourists. The project site is endowed with several natural gas and oil fields as shown in Figure 2. It is hoped that the Oil and Gas industries, operating in the area, will collaborate with the state government and its development partners to ensure speedy completion of this project. The port will accommodate larger oil tankers (design vessel is C9 containership with length: 262 m, beam: 32.2 m, and draft: 10.7 m). Oil and Gas operators will benefit from this project in the following areas: no lightering of tankers offshore (as it is currently done); reduced spilling of oil into territorial waters; economy of scale; and enhanced safety.
Figure 2. A section of the Gulf of Guinea showing national boundaries, Oil and Gas fields, and proposed new maritime border between Nigeria and Cameroon. This study focused on designing the most cost-effective and operationally efficient port navigation channel in Ibaka, and then makes recommendations to harbor and port developers, stakeholders, regulators, and policy makers. It is a decision support study for the navigation channel component of the proposed port project. FIELD AND NUMERICAL EXPERIMENTS Surface drifters were designed and built for this study. The total cost of constructing each unit of the surface drifter shown in figure 3 was about $200.00. The drifter design closely followed the work of Brooks (2004). The drifters were constructed with simple materials available in most Marine Engineering laboratories. Students in Akwa Ibom State University (AKSU), Ikot Akpaden, Nigeria were very excited to realize that simple apparatus in their laboratories can be assembled into equipment that can sample oceanographic data in bays and estuaries. This
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" discovery triggered students’ active involvement at all stages of the field campaign. The role of drifters in this study was to gather reliable surface current and circulation data for calibration and verification of the NearCoM-TVD (Shi et al. 2012) model in Ibaka Bay. The validated numerical model was then applied to investigate circulation and sediment transport processes in the Bay using different forcing mechanisms and input scenarios.
Figure 3. Surface drifter design (left) and prototype (right) (Courtesy: Brooks (2004)) The numerical experiments aided in better understanding of circulation patterns and morphological characteristics of the Bay in response to different forcing (both environmental and meteorological) scenarios. The model forcing were wind, wave, current, river input, and bathymetry. For details about the numerical modeling, the interested reader is referred to “Development and Implementation of a Composite model for Wave, Circulation and Sediment processes in Ibaka Deep Seaport, Nigeria”, (Paul 2014).
RESULTS AND DISCUSSION Some results of the experiments are discussed in this section. This study revealed that seasonal changes played a significant role in sediment transport in the Bay. Only rainy season (April to October) meteorological forces exceeded the threshold bottom shear stresses (1.1 Pa based on the dominant sediment characteristics in the Bay, Gardner 1989b) to initiate sediment transport in the Bay. Model results indicated that only wave heights greater than 1.0 meter and currents in excess of 0.24 m/s could mobilize bed sediment in the Bay. Figure 4 shows analyses of quarterly wave (a) and current magnitude (b) in the Bay. Error bars (red is data while blue is model output) indicate two standard deviations. Current and wave data for these analyses were sampled by TOTAL between 2004 and 2008 using wave staff and current meters in Ekundu. The data were retrieved from the System of Industry Metocean data for the Offshore and Research Communities (SIMORC 2012) database.
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"
(a)
(b) Figure 4. (a) Quarterly significant wave heights and (b) quarterly currents in Ibaka Bay The results in Figure 4 confirm that dry seasonal (November to March) meteorological conditions could not cause bed load sediment transport in the Bay. However, we cannot draw a definitive conclusion that there will be no bed level changes in the Bay during dry season because other mechanisms (such as river input) also contribute to morphological changes in bays and estuaries. Nevertheless, we can make an educated guess (based on historical site conditions) that the rate and distribution of sediment deposition during dry season will be minimal. Most tributaries that flow into rivers linked to the Bay dry up during dry season. Also Aeolian transport carries mostly silt into the Bay. Tidal current in the Bay exceeds the
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" threshold shear stress for deposition of silt (0.67 Pa from Tate et al. 2008). Most of the wind-transported sediment will bypass the Bay and flushed into the open ocean. Different locations within the estuary were investigated to determine an optimum navigation channel for the port. Such investigations have been demonstrated to save cost and enhance safety in Barbers Point Harbor, Oahu, Hawaii (Briggs et al. 2003). Figure 5 shows the contoured bathymetry of the study site, the locations where model outputs were taken from, and the dredged channel. This navigation channel was adjudged the optimum channel in terms of cost effectiveness, operational efficiency, and legal considerations. Legal considerations became a major factor in selecting the approach channel location following the International Court of Justice Rulings in 2002, which ceded part of Nigerian territory (Bakassi Peninsula) to Cameroon. Instead of appealing the ruling at The Hague, Nigeria opted for a peaceful and sustainable resolution of the case. Both countries (Nigeria and Cameroon) set up a joint maritime boundary demarcation committee. The parties agreed to implement the recommendations of the committee. Major stakeholders in both countries have applauded the committee’s work. The proposed new national boundary was shown in figure 2 above. The channel in Figure 5 is within Nigerian waters.
Figure 5. The contoured bathymetry of the study site showing the dredged channel, Ibaka Bay, and locations where model outputs were taken (indicated by S followed by the station number). For cost considerations, this channel location has a non-dredged draft of 13.5 meters in most part of the navigation channel. Thus, it requires minor capital (new) dredging to deepen it to 15 meters to accommodate vessels of about 200,000 Dead Weight Tonnage (DWT). In addition to savings in capital channel dredging cost, the analysis of current and wave patterns within this channel indicate a net ebb flow as presented in figure 6. This implies that lowdensity materials such as suspended sediments and contaminants will be flushed out of the channel. With strong near bed currents (and corresponding shear stress), sediment deposition from offshore sources would be minimal in the channel since there is no circulation loop in the channel (circulation pattern results will be presented later in the
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" discussion). This will yield longer window (elapsed time between maintenance dredging) of periodic maintenance dredging of the channel and harbor. Every dredging project requires mobilization and demobilization of the dredge to and from the site. Estimated cost of mobilizing an average hopper dredge to site is about $750.00 per kilometer (USEPA 1994). This translates to $3,000,000.00 for a project site that is 4000 km away. There are three major elements of dredging process: excavation, transportation of removed materials, and disposal or utilization of the materials. Each of these elements has associated cost (monetary and environmental). In a navigation channel dredging, there is also an additional cost of disruption in port operations. If the channel is in coastal waters, then water quality issues are also raised. The findings of this study indicate that most of these dredging costs will be saved in Ibaka deep seaport. Consumers of port products and services will share in these savings. Thus, this port and harbor has comparative and competitive advantages over its competitors. Aside from cost, operational efficiency, safety, and reliability of the harbor will be enhanced. Dredging equipment is a potential source of accident in waterways. Port operations are interrupted during dredging. One of the objectives for a careful evaluation of environmental and meteorological conditions during site selection for port and harbor construction is reliability of operations. Regular dredging within short time intervals will hinder realization of this objective. Data analyses and model results indicate that the Bay exhibits a net ebb flow around the proposed navigation channel as illustrated in Figure 6.
Figure 6. Water current around the deep channel at stations 1 and 2. Long-term trend indicates net ebb flow.
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" Of particular interest to this work were circulation and morphological changes in the navigation channel when the channel was dredged to accommodate larger vessels (200,000 DWT). Figures 7 a and 7b show wave attenuation in the Bay with and without the dredged deep channel. The figures indicate that variations in offshore wave heights might not contribute to significant changes in Ibaka Bay morphological processes. The bathymetric characteristics of the Bay seem to constrain waves (through breaking in the wider continental shelf) that reach the Bay.
(a)
(b) Figure 7. Impacts of wider continental shelf on wave transformation in the Bay: (a) 3-m incident wave and (b) 2.2-m incident wave. Effect of navigation channel was also assessed. As seen in the figures, both a 3-m and 2.2-m waves at the model domain boundary were reduced below the threshold (with respect to bed sediment mobilization) 1.0 m wave height before they reached Ibaka Bay (Bay extent indicated
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" by two vertical red lines). Another important result from this plot is that the dredged navigation channel reduces wave height propagating through the channel into the Bay. This observation agrees with earlier work by Li and colleagues (Li et al. 2000). Morphological changes of adjacent Ibaka Beach were also assessed. Station 4 rate of seabed changes and cumulative bed level change for a 20-year simulation is shown in Figure 8. The figure also shows sediment response to variations in wave heights at that location in the Bay. The limiting significant wave height for mobilization of bed sediment is 1.0 m. Waves below this height do not have enough energy to initiate sediment transport. The cumulative change in seabed height indicates about 1.0 m deposition of sediment over the simulation time period. Similar results (not shown here) were found in other locations closer to the channel. Wave height and seabed changes at station 4.
Figure 8. Wave height and seabed level changes in Ibaka Bay. A morphological factor of eight was applied to the bed level module. Although ship motions and other port infrastructures contribute to channel erosion and accretion in operational seaports (Tate et al. 2008), we do not believe that they will alter the results presented here significantly. Thus, we have estimated a five-year minor maintenance-dredging window for this channel and harbor based on the model results on sediment deposition in the Bay. This estimate was also guided by the Permanent International Association of Navigation Congresses (PIANC) recommendations for ship underkeel clearance for efficient and safe operations in harbors (PIANC, 1997). Current loop and magnitude were assessed in the study site. Convergence and divergence of current are major factors in near shore sediment deposition and erosion. Figure 9 is a snapshot of model output plot of seabed level change and current. It is seen in this figure that the selected navigation channel location is not an area of current convergence. Thus, probability of sediment accumulation in the channel is relatively low compared to other locations within the estuary.
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"
Figure 9. Simulated bed level change with current overlaid on it. Current converging regions indicate likely areas of sediment deposition. The strategic location of Tom Shot Island at the Southwestern limit of Ibaka Bay gives this proposed deep seaport another comparative advantage. Construction of breakwaters and jetties are not necessary. Topographic features at this site shields the harbor against damaging waves from offshore. Wave related hazards will not impede normal operations of this natural harbor. CONCLUSIONS AND RECOMMENDATIONS Ibaka Bay is a natural harbor for a safe and reliable deep seaport. The site of this seaport has several advantages, such as direct access by nationals from neighboring six countries, blockage of offshore waves, favorable weather conditions, no breakwater and jetty requirements, few minutes of piloting services requirements to and from the harbor, presence of shiny coarse sand beach, and natural deep harbor depth. Seasonal influence on sediment processes in the Bay would facilitate efficient long-term simulations of morphological changes in the Bay. Detailed modeling of rainy season processes will yield reliable predictions of yearly seabed level changes in Ibaka Bay. Navigation channel in the Bay will not amplify long wave energy reaching the shore. It will not alter the flushing characteristics and long-term sediment equilibrium of the Bay. The presence of the channel will not refocus incoming offshore waves to erode the adjacent beach; rather model results indicate that it will promote net sediment accretion in Ibaka Beach. The results of this study should be used as a decision making support to get approval for commencement of the project. Detailed feasibility study is required before the final design of port facilities.
Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" It is recommended that effects ship motions, port infrastructures, and berth area activities be represented in the model. The effects of these variables on erosion/accretion rates and distribution within the channel and harbor should also be quantified. Government and development partners in this project should provide more funds for comprehensive field experiments in the Bay. Morphological changes in the Bay should be monitored over time. Data sampled during this monitoring program can be used for sediment model calibrations and validation for the Bay. ACKNOWLEDGEMENT We are grateful to Akwa Ibom State University, Ikot Akpaden, Mkpat Enin, Nigeria for funding the major part of this study and allowing us to publish the findings. REFERENCES Akwa Ibom State Government (2014). “State Government Official Website: Ibaka Deep Seaport Project”. www.aksg.gov.ng/ibaka-project, Accessed December 12, 2014 Antia E. E. (2012). “Personal Communication”. Institute Of Oceanography, University Of Calabar, Nigeria. Awosika, L. F., Dublin-Green C.O., Folorunsho R., Adekoya, E.A., Adekanmbi, M.A., and Jim-Saiki, J. (2000). “Study Of Main Drainage Channels Of Victoria Islands In Lagos, Nigeria And Their Response To Tidal And Sea Level Changes”. CSI-UNESCO Special Report, pp. 1–108. Awosika, L. and Ibe, A. (1994). “Geomorphic Features Of The Gulf Of Guinea Shelf And Littoral Drift Dynamics”. In Proceedings, International Symposium On The Results Of The First IOCEA Cruise In The Gulf Of Guinea, May 17-20, 1994. Center For Environment And Development In Africa, pp. 14-18 Awosika, L. F., Ibe, A.C., and Ibe, C.E. (1993). “Anthropogenic Activities Affecting Sediment Load Balance Along The West African Coastline”. In Proceedings: Coastlines Of Western Africa, Coastal Zone 93. American Association of Civil Engineers, New York, NY, pp. 81-93. Briggs, M.J., Borgman, L.E., and Bratteland, E. (2003). “Probability Assessment for deep-draft Navigation Channel Design”. Coastal Engineering, Vol. 48, pp. 29-50. Brooks, D.A. (2004). “Modeling tidal circulation and exchange in Cobscook Bay, Maine”. Northeastern Naturalist, Vol. 11 (Special Issue), pp. 23-50 Gardner, W. (1989b). “Periodic Resuspension In Baltimore Canyon By Focusing Of Internal Waves”. J. Geophysical Research, Vol. 94, No. C12, pp. 18185–18194. Li, Y.S., Liu, S.-X., Wai, O.W.H., and Yu, Y.-X. (2000). “Wave Concentration by a Navigation Channel”. Applied Ocean Research, Vol. 22, pp. 199-213. Paul, E.U. (2014). “Development and Implementation of a Composite model for Wave, Circulation, and Sediment Processes in Ibaka Deep Seaport, Nigeria.” Ph.D. Dissertation, Texas A&M University, College Station, TX Permanent International Association of Navigation Congresses (1997). “Approach Channels: A Guide for Design”. Final Report for the joint working group PIANC and IAPH, in cooperation with IMPA and IALA, Supplement to Bulletin No. 95. Shi, F., Kirby, J.T. Hsu, T., and Chen, J. (2012). “NearCoM-TVD: A Hybrid TVD Solver For Nearshore Community Model”. Center For Applied Coastal Research, University Of Delaware, Newark, DE, pp. 143. System of Industry Metocean data for the Offshore and Research Community (2012). “Data Access”, www.simorc.org Accessed December 12, 2012. Tate, J.N., Berger, R.C., and Ross, C.G. (2008). “Houston-Galveston Navigation Channels: Texas Project Navigation Channel Sedimentation Study, Phase 2.” ERDC/CHL TR-08-8, July 2008. US Environmental Protection Agency (1994). ARCS Remediation Guidance Document, EPA 905-B94-003-004. Chicago, IL CITATION Paul, E.U., Brooks, D.A., Kaihatu, J.M., and Ebong, U.R. “Navigation Channel Depth for Ibaka Deep Seaport in Nigeria”. Proceedings of the Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015", Houston, Texas, USA, June 22-25, 2015.
