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Suggested Citation:"D--Sea State." National Research Council. 2014. Force Multiplying Technologies for Logistics Support to Military Operations. Washington, DC: The National Academies Press. doi: 10.17226/18832.
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D

Sea State

Sea state is a factor that affects moving logistics over the shore because it places limits on when and how different systems can operate. Describing the sea is not an easy task, as any oceanographer will admit. Rather than over-simplify the sea state by describing it as a simple train of sinusoidal waves, which it is not, it is more effective to describe the sea condition as an energy spectrum. Sea state also encompasses factors such as confused seas, swells, combined sea and swell dynamics, and other complications. None of these factors are addressed in any description of the capabilities of causeways, mobile landing platforms, landing craft air cushions, or other at-sea transfer methods mentioned in this report.

There are different classification systems for sea states. Researchers such as Pierson, Moskowitz, and Bretschneider, have developed different energy spectra to characterize an open ocean and shallow water conditions. For example, Pierson-Moskowitz describes North Atlantic open ocean sea states generated from steady wind blowing over long distances (known as the wind’s fetch). Joint North Sea Wave Project (JONSWAP) spectra are based on are based on North Sea data, and are more descriptive of fetch-limited coastal waters.1 Three sea-state classification systems are summarized in Table D-1. While oceanographers are able to quantify the energy, commonly used terms such as sea state do not capture the impact of the energy on ships and floating causeways. Consequently, the Beaufort scale is still used, although it is centuries old. The World Meteorological Organization scale is in more common usage.

SEA STATE AND SURF

Surf zone conditions are equally important as sea state when considering littoral logistics. Surf conditions cannot simply be defined by sea state, but rather are affected by the slope of the bottoms and abruptness or gradualness of shoaling. Non-monochromatic waves are a further complication. These surf conditions are greatly affected by the state of the tide, swells, coastal currents, wind strength, and other factors. Surf conditions before, during, and after storms can build up or decay rapidly, and the ability to forecast these changes is limited. Thus, the use of sea state alone as a metric is insufficient. Where possible, it would be desirable to directly analyze the surf conditions in areas where over-the-shore logistics operations are anticipated. One possibility is the use of unmanned watercraft to investigate and update surf conditions. Such watercraft could directly sample and map underwater and surf conditions in a potential area of operation. As an added benefit, they could also identify mines, obstacles and other navigational hazards, bottom conditions (rocks, coral, sand, etc.), and the effects of tide on surf.

There are ways to determine the impact of sea conditions on causeways and watercraft. One is full-scale testing. This, however, has limitations, the chief one being the cost and time that would be required to explore all possible variations in sea conditions. Another is to perform scale model testing in test basins, allowing for more comprehensive data collection in controlled settings. Experimental model basins include those at the Navy’s David Taylor Model Basin and at various universities.

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1 See “Section 2.8.3-JONSWAP Spectrum,” in S. Gran, 1992, “A Course in Ocean Engineering,” Developments in Marine Technology, Vol. 8., Amsterdam: Elsevier Science Publishers, http://research.dnv.com/hci/ocean/bk/c/a28/s3.htm.

Suggested Citation:"D--Sea State." National Research Council. 2014. Force Multiplying Technologies for Logistics Support to Military Operations. Washington, DC: The National Academies Press. doi: 10.17226/18832.
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TABLE D-1 Various Sea State Classification Systems

System Sea State Wave Height (feet) Description
World Meteorological Organization 0 0 Calm (glassy)
1 0.3 Calm( rippled)
2 0.3-1.6 Smooth (wavelets)
3 1.6-4.1 Slight
Beaufort 0 0 Flat
1 0-1.0 Ripples without crests
2 1.0-2.0 Small wavelets
3 2.0-3.5 Large wavelets
4 3.5-6.0 Small waves
Pierson-Moskowitza 0 <0.5
1 0.5-1.0
2 1.5-3.0
3 3.5-5.0
4 6.0-7.5

a Heights are “significant wave heights” or the average of the highest 1/3 of waves. The discontinuity in wave height ranges is not an error.
SOURCE: Bowditch (1984).

OBSERVATIONS

While sea state, which is based on wave height, is a commonly used and understood term, it is too simplistic to encompass all the variables acting on causeways and watercraft. Vessel motions are subject to not only sea state, but also wavelength, celerity, steepness, combinations of different wave trains, and also swell height, length, and direction. Surf conditions, too, limit the capability of causeways and watercraft to operate. Surf conditions are governed by many variables. For the general characterization of the effects of sea and surf conditions on causeways and watercraft, full-scale testing, while effective, cannot capture all the variables acting on causeways and watercraft within a reasonable time and cost. The testing of scale models in basins allows for more comprehensive, timely, and efficient data collection. While the Army has only limited influence in these matters, identifying a new metric to either replace or complement sea state might be useful in understanding under what sea conditions logistics systems can operate. This effort might be assisted by model basin testing to obtain a broad data set to support establishing a new metric.

REFERENCE

Bowditch, N. 1984. American Practical Navigator: Epitome of Navigation, Volume 1. Washington, D.C.: Defense Mapping Agency Hydrographic/Topographic Center.

Suggested Citation:"D--Sea State." National Research Council. 2014. Force Multiplying Technologies for Logistics Support to Military Operations. Washington, DC: The National Academies Press. doi: 10.17226/18832.
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Page 195
Suggested Citation:"D--Sea State." National Research Council. 2014. Force Multiplying Technologies for Logistics Support to Military Operations. Washington, DC: The National Academies Press. doi: 10.17226/18832.
×
Page 196
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The mission of the United States Army is to fight and win our nation's wars by providing prompt, sustained land dominance across the full range of military operations and spectrum of conflict in support of combatant commanders. Accomplishing this mission rests on the ability of the Army to equip and move its forces to the battle and sustain them while they are engaged. Logistics provides the backbone for Army combat operations. Without fuel, ammunition, rations, and other supplies, the Army would grind to a halt. The U.S. military must be prepared to fight anywhere on the globe and, in an era of coalition warfare, to logistically support its allies. While aircraft can move large amounts of supplies, the vast majority must be carried on ocean going vessels and unloaded at ports that may be at a great distance from the battlefield. As the wars in Afghanistan and Iraq have shown, the costs of convoying vast quantities of supplies is tallied not only in economic terms but also in terms of lives lost in the movement of the materiel. As the ability of potential enemies to interdict movement to the battlefield and interdict movements in the battlespace increases, the challenge of logistics grows even larger. No matter how the nature of battle develops, logistics will remain a key factor.

Force Multiplying Technologies for Logistics Support to Military Operations explores Army logistics in a global, complex environment that includes the increasing use of antiaccess and area-denial tactics and technologies by potential adversaries. This report describes new technologies and systems that would reduce the demand for logistics and meet the demand at the point of need, make maintenance more efficient, improve inter- and intratheater mobility, and improve near-real-time, in-transit visibility. Force Multiplying Technologies also explores options for the Army to operate with the other services and improve its support of Special Operations Forces. This report provides a logistics-centric research and development investment strategy and illustrative examples of how improved logistics could look in the future.

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