Channel Turns Cape Fear River - savethecape.orgsavethecape.org/STC/images/stories/PDFs/Channelturns3.pdf · (PIANC) and the International Association of Ports and Harbors (IAPH) developed

A Look at the

Channel Turns

in the

Cape Fear River

[Draft–March 18, 2010]

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Risingwater AssociatesSouthport, North CarolinaOld Saybrook, Connecticut

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Contents

The Channel in the Cape Fear River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Corps of Engineers Standards for Channel Turns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

The PIANC Design Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

The CH2M Hill, Inc. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

A Look at the Channel Turns in the Cape Fear River

All you gotta do is keep it between the lines-Russell Smith and Kathy Louvin(Recorded by Ricky van Shelton)

The North Carolina State Ports Authority, a component of the North Carolina stategovernment, has purchased 600 acres of undeveloped land on the Cape Fear River nearSouthport, and plans to develop an international container terminal to facilitate the import ofgoods from Asia. The terminal would have an annual capacity of 3,000,000 twenty-footequivalent units (TEU), larger than any container terminal in the eastern United States exceptthe combined terminals at Port Elizabeth and Port Newark, New Jersey. The new terminalwould be called the North Carolina International Terminal (NCIT).

The proposed terminal would serve anew generation of container ships (called“post-Panamax”) that are too large to transitthe Panama Canal, now or after the capacityincreases under construction for opening in2014: 1260 feet long, 185 feet in beam, anddrawing 50 feet.

The existing channel in the Cape FearRiver to Wilmington is maintained at a depthof 42 feet, with a width of 400 feet, widenedto 500 feet in the lower reaches at the mouthof the river. Vessels of 50-foot draft wouldrequire a channel 52.5 feet deep (plus a two-foot overdredge for maintenance of thatdepth) and 600 feet wide.

The channel has a sharp turn in thelower reaches near Southport, followed by areverse turn through the mouth of the riverbetween Fort Caswell on Oak Island andBald Head Island, describing a large “S.”

Wilmington Star-News

Consultants to the North Carolina State Ports Authority, in preparing preliminary plansfor the proposed container terminal, concluded that the standards of the US Army Corps ofEngineers for channel width and turns cannot be met on the route of the existing channelbecause of the confines creating the existing S-curve. The consultants recommend bypassingthe turn with a new channel on a relatively straight route from the proposed terminal site to themouth of the river.

This report examines that conclusion.

We agree that the existing channel alignment cannot be enlarged for the vesselsproposed for the new container terminal. Moreover, we find that the existing channel turns donot comply with the standards of the Corps of Engineers for the vessels now using the CapeFear River. However, we do not endorse the proposal to create a new channel. It appears thatthe capacity of the Cape Fear River for a navigation channel has already been exhausted.

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The Channel in the Cape Fear River

This is the chart of the lower Cape Fear River:

The Corps of Engineers maintains the channel at a depth of 42 feet within the river, and at 44feet south of the mouth to deep water. The width of the channel from Lower Swash northwardsis 400 feet. The Battery Island, Southport Channel, and Smith Island Range sections arewidened to 500 feet.

The inshore channel at the Military Ocean Terminal at Sunny Point, shown at the top ofthe chart, is maintained at a depth of 32 feet.

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The configuration of this channel follows the natural channel in the Cape Fear River. However, when first measured in Colonial times the depth of the river in the natural channelwas three to six feet; the entrance to the river over the bar was ten feet deep. The presentdepth and dimensions of the channel are the result of this succession of dredging projects:

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All of those projects followed the natural course of the river, which includes these turnsnear the river mouth:

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The Wilmington District of the US Army Corps of Engineers initiated a project calledthe Wilmington Harbor Deepening project in 2000 to increase the depth of the channel from 38to 42 feet. The channel was opened at a depth of 42 feet in 2004. Certain aspects of theproject–environmental mitigation and dredging of a turning basin at the Port of Wilmington–remain in progress, and are planned for completion in 2014. The charts presented abovereflect the dimensions and course of the channel at the present time, resulting from thatproject.

In the 1996 feasibility study leading to that project, the issue of the channel turns arose.The “design vessel” for that study and engineering analysis was a “Panamax” vessel–thelargest vessel able to transit the Panama Canal–with an overall length of 965 feet and a beam of106 feet.

Such a vessel raised issues of the adequacy of the turns in the channel. In Appendix D,Engineering and Design, to the Record of Decision released for that project in 1996, theWilmington District compared the turns to the recently released criteria for channel turns in theCorps of Engineers Engineering Manual 1110-2-1613:

Some of the turns did not fit the new criteria as far as the combinations of turndeflection angle and ratio of turn radius to ship length. It was decided that theauthorized turn designs were adequate at this time. The turn widths will bestudied during preparation of the plans and specifications as part of the shipsimulation model.

The Corps of Engineers arranged for tests to be conducted at a private ship simulator,the Raymond T. McKay Simulation Training and Research Center at Dania, Florida, in 1999. The facility has a full-size mockup of a ship’s bridge, and the ability to simulate response of avessel in a programmed channel configuration under various conditions of current.

Test runs were conducted for a “design vessel” of 950 feet overall length and 106-footbeam, drawing 38 or 42 feet, in both ebb and flood tides.

In all runs, five for each test condition, the vessels left the marked channel. Althoughin a few runs grounding would have occurred, most runs were without incident because thenaturally deep water on the outside of the channel provided the necessary space to execute theturn.

At the suggestion of the pilots conducting the tests, a widener was added to the apex ofthe turn between Bald Head Shoal and Smith Island Range. Otherwise, the project proceededwith the original design of the channel configuration.

This issue was not addressed in a later report, an Environmental Assessment issued bythe Wilmington District in February 2000 for modifications of the alignment of the offshorepart of the channel.

A composite diagram of the runs conducted with an inbound vessel of 42-foot draft inflood tide is shown on the next page.

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Corps of Engineers Standards for Channel Turns

The Corps of Engineers Engineering Manual, Hydraulic Design of Deep DraftNavigation Projects, in chapter 8, has this table of recommendations for channel turns:

This is approximate. The turn is not a circle, but a 65° intersection of straight1

channel sections, with a cutoff of approximately 1000 feet at the inside. 1280 feet is the radiusof a channel arc of the required width that would fit within the part enlarged by the cutoff,connecting the straight sections.

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Looking at the two turns around Battery Island, we find these discrepancies between therecommended configuration for such vessels and the actual channel:

Deflection Configuration Radius Turn Angle Recommended Actual Recommended Actual

Southport Channel-Battery I. Channel 30° Apex Angle

Battery I. ChannelLower Swash 65° Circle Cutoff 9650 feet 1280 feet1

The most troublesome ofthese discrepancies is the radiusof the 65° turn between BatteryIsland Channel and LowerSwash. (Shown on the right) Instead of the recommendedradius of ten times the shiplength, the turn has a radius onlyslightly longer than the length ofthe design vessel, 965 feet.

The Corps of Engineersmanual advises that “Channelturns should not be designed forturn radius-to-ship length ratiosless than 3, because ships cannothydrodynamically maneuveraround a sharper turn.” This isconsistent with the the standardfor maneuverability of theInternational MaritimeOrganization (IMO), whichrequires vessels to have amaximum turning circle with aradius of 2.5 times the shiplength, at 35 degrees rudderangle (or the maximum angle, ifless). However, the IMOstandard relates to open waterturns at speed; there are not anystandards for the turning circleat reduced speed in shallowwater.

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The dilemma for the Wilmington District of the Corps of Engineers was that a channelconforming to the recommendations and standards for width and turns cannot be fit betweenthe banks of the Cape Fear River. This is demonstrated by the line on this chart, whichrepresents the centerline of a channel conforming to the recommendations for turn radius. (Butthis line does not have a transition zone between reverse curves–a straight section of five timesvessel length is recommended by the Corps of Engineers manual.)

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The PIANC Design Guide

A working group of the Permanent International Association of Navigation Congresses(PIANC) and the International Association of Ports and Harbors (IAPH) developed andpublished in 1997 a design guide: Approach Channels, a Guide for Design. This is morecomprehensive than the section of the Corps of Engineers manual, taking into account manyfactors and explaining the rationale in some detail. The Corps is a member of PIANC.

The PIANC guidelines offer this advice:

! Vessel maneuverability diminishes with channel depth. Although a vessel with goodmaneuverability may be able to execute a hard-over turn with a radius of two to threetimes vessel length in deep water, that radius increases to about five times vessel lengthin a shallow channel with a depth to vessel draft ratio of 1.1 (a depth of 44 feet for avessel draft of 40 feet).

! Designing a channel turn for hard-over turns is inadvisable. Designing for no morethan 20% rudder is recommended to provide a margin of safety. For a water depth todraft ratio of 1.1, the recommended radius at 20% rudder angle is 8.5 times vessellength.

! Bank interaction, a phenomenon that causes the stern of the vessel to be drawn to theside of a dredged channel and results in instability, must be taken into account indetermining channel width.

For the design vessel used by the Corps of Engineers for the 1996 study, 965-foot length and 106-foot beam, application of the PIANC guidelines would require a minimum turnradius of 8200 feet, with a width of 500 feet. The channel turn from Battery Island Channel toLower Swash, with an effective radius of 1000 feet, is far sharper than the recommendedradius. Increasing the assumed rudder angle from 20% to 30% (which PIANC does notrecommend) would reduce the necessary radius to 6000 feet, still much greater than the actualturn.

Experience

Notwithstanding the failure of the sharpest turn in the channel in the Cape Fear River tocomply with either the Corps of Engineers manual or the PIANC guidelines for Panamaxvessels of 965-foot overall length and 106-foot beam, the Cape Fear River pilots have beenable to pilot such vessels around that turn. By starting the turn at or outside the outer edge ofthe channel (and perhaps using bank effects to help start the turn), clipping the inside apex ofthe turn and finishing the turn at or outside the outer edge, pilots have been able to fit the arcof a turn with a radius of about 3000 feet, about three times the length of a Panamax vessel,into the space available in the river, if not in the marked channel.

But not always. The turn is challenging, with no room for error, and there have beengroundings. Coast Guard records show these grounding incidents in the Cape Fear River sincethe channel was opened at the new depth in 2004:

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Coast Guard Number Date Vessel

2475174 August 14, 2005 Lijnbaansgracht

2701997 June 24, 2006 Nordon

2836586 December 8, 2006 YM South

Container ship YM South

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The CH2M Hill, Inc., Recommendation

The North Carolina State Ports Authority engaged CH2M Hill, Inc., a respectedengineering firm with experience in port projects, to provide preliminary engineering analysisof the proposed North Carolina International Terminal and related infrastructure.

In establishing design parameters, the firm selected as the “design vessel” the latestgeneration of container ships, such as the Emma Maersk, with a length overall of 1263 feet, abeam of 185 feet, drawing 50 feet of water when fully loaded.

In the firm’s Conceptual Dredging Study, provided to the State Ports Authority in 2008,channel dimensions of 52.5 foot depth and 600-foot width were determined for the designvessel. The matter of the channel turn was addressed:

A recent engineering manual (USACE EM 1110-2-1613, Hydraulic Design ofDeep Draft Navigation Projects, Chapter 8 – Channel Width, 2006 –APPENDIX B) was used as the basis for the channel width and channel curves.One-way ship traffic was the design criterion for determining channel width.The existing channel into the Cape Fear River has a dramatic “S” curve in thefirst 4 miles inside the inlet. Once a ship passes through Baldhead and JaybirdShoals, it makes a 45-degree turn to port, immediately followed by a 105-degreeturn to starboard around Battery Island, before straightening into the channelreach known as the Lower Swash Channel and heading upriver. This area isknown to have a high current state on both the incoming and falling tide.Multiple attempts were made to design a channel that followed the existingcourse; however, it was found that the standards in Chapter 8 of the manual(USACE, 2006) could not be met without causing obvious impacts at the eastend of Caswell Beach or the riverfront at Southport.

The table below shows the minimum channel width and turn radius the Corps ofEngineers manual and the PIANC guidelines would require for the CH2M Hill design vessel,and for the slightly narrower “new Panamax” vessel that would be able to transit the PanamaCanal after the new locks and other improvements are completed, sometime after 2014. Sucha vessel would also be about 1260 feet in overall length, but the beam would be 160 feet.

Design Vessel Channel Width Minimum Turn Radius____________ Corps of Engineers PIANC Corps of Engineers PIANC

CH2M Hill design vessel 800 feet 700 feet 12,600 feet 10,700 feet

New Panamax 680 600 12,600 10,700

The chart on the next page shows the path of the centerline of a channel with turnsconforming to the Corps of Engineers manual. However, this does not have a transition zoneto recover from a turn in one direction before beginning a turn in the opposite direction. TheCorps of Engineers manual specifies a straight section of five times vessel length between suchturns, in this case 6300 feet.

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The observation of CH2M Hill, Inc., that a conforming channel cannot be fit within thebanks of the river at the turn between Southport and Battery Island is quite obviously correct. Even if the existing channel at the intersection of the Battery Island Channel and Lower Swashcould be widened to 600 feet by enlarging it on the outside, a full-width channel with a radiusof only about 1400 feet could be accommodated, only a fraction of the recommended radius. However, just as in the case of the existing channel, by using the entire width of the widenedchannel a path 160 feet wide with a radius of 4000 feet can be followed, starting at the outsideedge of the straight section leading to the turn, clipping the inside at the apex, and completingthe turn at the outside edge of the straight section after the exit. Whether a 1260-foot vessel

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can turn in a radius of 4000 feet without tugboat assistance is another matter. Such a turnwould be much sharper than specified by the Corps manual and the PIANC design guide.

There is also the issue, not mentioned by CH2M Hill, Inc., of three Civil War erashipwrecks lying along the side of the channel in this area. All are regarded as being ofarcheological interest by the North Carolina Department of Cultural Resources. One is theCSS North Carolina, one of two ironclads built in Wilmington. That has already beendamaged by dredging operations.

As for the riverbank on the outside of Lower Swash, where a conforming turn wouldhave to be cut, that is occupied by the historic district of the City of Southport, which districtis on the National Register of Historic Places. That district includes two buildings on theRegister, including Fort Johnston, which dates to the Colonial era.

CH2M Hill, Inc., consequently recommended a new channel on the east side of BatteryIsland, by-passing the S-curve. That is shown below (red):

This was not a conclusion reached without some concern. The proposed route wouldbe through undisturbed areas with depths measured in single digits. The additional dredginginvolved would be much more expensive than simply enlarging the existing channel. Thatportion of the river is part of the John H. Chaffee Coastal Barrier Resource System, a Federalreserve, and the Bald Head Island Natural Area, a State reserve. There would be significantimplications for environmental effects from loss of habitat and the increase in tidal amplitudethat would result from creating this large, direct path up the river.

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Conclusion

The existing channel in the Cape Fear River cannot be widened sufficiently toaccommodate larger vessels than now navigate the river. Indeed, the channel is not adequatefor those vessels. This is not a healthy situation. In a recent paper presented to the New YorkMetropolitan Section of the Society of Naval Architects and Marine Engineers, the authors,staff members of the Naval Academy, the Coast Guard, and the Maritime Administration,pointed out that as vessel size has increased, channels have been dredged deeper, but notwider, and maneuverability issues have not been adequately addressed. This is a particularproblem with container ships. The authors identified these problems with newer generations ofsuch ships:

! Container ships have large windage that can complicate ship controllability in narrowchannels as well as during slow speed maneuvering;

! Increases in service speeds may also increase the bare steerage speed. Direct-drivediesel ships with high installed power to achieve design service speeds can, in somecases, have a minimum bare steerage speed of about 8 knots —quite a high speed inconfined waters;

! Even the largest container ships usually have single screws, a maneuvering handicap.

! There is a trend to smaller rudders. While such rudders may be adequate at servicespeeds in open water, maneuverability in confined waters at low speed is compromised.

! Marine architects are taking advantage of the beam increases that will be possible withthe opening of new locks in the Panama Canal to decrease the ratio of vessel length tobeam, which improves load capacity without increasing draft. However, decreasingthat ratio also decreases directional stability. Although this makes it easier to initiate aturn, it also makes it more difficult to check a turn.

Thus while the inadequacies of the channel in the Cape Fear River and the difficultiesof maneuvering large container ships so far have been overcome by skillful piloting, thisconflict should not be exacerbated by opening this channel to even larger vessels. Groundingsare usually only inconvenient, but as the channel is deepened into the rock that lies at thebottom in many places, the risk of hull damage to the deepest draft vessels and consequentspills must be considered.

To create access to the site of the proposed North Carolina International Terminal forthe post-Panamax container ships for which it is intended, a new channel would be requiredover a different alignment east of the lower reaches of the existing channel.

This does not constitute an endorsement of such a new channel. Such a new channelwould introduce substantial environmental and economic problems, which may overwhelm anypotential benefits from admitting larger ships to the Cape Fear River.

There are clear indications that the succession of channel deepening projects over theyears has exhausted the capacity of the Cape Fear River for more deepening. The most notableeffect is that on beaches of Bald Head Island and Caswell Beach, at the mouth of the river. Those beaches had remained stable and even grown through the centuries, but as the channel inthe Cape Fear River was enlarged, the beaches have lost sand and now require regular“nourishment” by adding sand dredged from the ocean bottom and from the channel.

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Sources CH2M Hill, Inc., Conceptual Dredging Study, North Carolina State Ports Authority (2008).

William O. Gray, Jennifer Waters, Alan Blume, and Alexander C. Landsburg, Channel Designand Vessel Maneuverability - Next Steps (2002)

International Maritime Organization, Standards for Ship Manoeuverability, ResolutionMSC.137(76) (2002).

Permanent International Association of Navigation Congresses (PIANC), Approach Channels,a Guide for Design, Final Report of the Joint Working Group PIANC-IAPH, Supplement toBulletin no 95, (1997).

United States Army Corps of Engineers, Deep-Draft Coastal Navigation Entrance ChannelPractice, Coastal and Hydraulic Engineering Technical Note IX-1 (1999).

United States Army Corps of Engineers, Hydraulic Design of Deep Draft Navigation Projects,Chapter 8 – Channel Width– APPENDIX B (USACE EM 1110-2-1613, 2006 )

United States Army Corps of Engineers, Record of Decision, Cape Fear–Northeast Cape FearRivers Comprehensive Study, Appendix D, Engineering and Design (1996)