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.
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.
