Proceedings of the symposium on coastal oceanography and littoral warfare

The Harbors and Approaches Working Group discussed the information needs, research directions, andpotential technological developments related to littoral warfare in estuarine areas.. Th

August 2-5, 1993

Navy Committee Ocean Studies Board National Research Council

NATIONAL ACADEMY PRESS 2101 Constitution Avenue, N.W Washington, DC 20418

NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose bers are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine The members of the panel responsible for the report were chosen for their special competencies and with regard for appropriate balance This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee con- sisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

mem-The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters Dr Bruce Alberts is president of the National Academy of Sciences.

The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government The National Academy of Engineering also sponsors engineer- ing programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers.

Dr Robert M White is president of the National Academy of Engineering.

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The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of ence and technology with the Academy's purposes of furthering knowledge and advising the federal government Functioning in accordance with the general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy

sci-of Sciences and the National Academy sci-of Engineering in providing services to the government, the public, and the scientific and engineering communities The Council is administered jointly by both Academies and the Institute of Medicine Dr Bruce Alberts and Dr Robert M White are chairman and vice-chairman, respectively, of the National Research Council.

The work was sponsored by the U.S Department of Defense under Grant No N00014-92-J-1636 Such support does not constitute an endorsement of the views in this report by the sponsors.

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NAVY COMMITTEE

Kenneth Brink, Woods Hole Oceanographic Institution, Woods Hole, MA, Chairman

Robert Cannon, Jr., Stanford University, Palo Alto, CA

Robert Detrick, Woods Hole Oceanographic Institution, Woods Hole, MA

Eileen Hofmann, Old Dominion University, Norfolk, VA

William Kuperman, Scripps Institution of Oceanography, La Jolla, CA

Arthur R M Nowell, University of Washington, Seattle, WA

Project Staff

Mary Hope Katsouros, Study Director

Stewart B Nelson, Consultant

Mary Pechacek, Project Assistant

OCEAN STUDIES BOARD

William Merrell, Texas A&M University, Galveston, TX, Chairman

Robert Berner, Yale University, New Haven, CT

Donald Boesch, University of Maryland, Cambridge, MD

Kenneth Brink, Woods Hole Oceanographic Institution, Woods Hole, MA

Gerald A Cann, Independent Consultant, Rockville, MD

Robert Cannon, Stanford University, Stanford, CA

Biliana Cicin-Sain, University of Delaware, Newark, DE

William Curry, Woods Hole Oceanographic Institution, Woods Hole, MA

Rana Fine, University of Miami, Miami, FL

John E Flipse, Texas A&M University (ret.), Georgetown, SC

Michael Freilich, Oregon State University, Corvallis, OR

Gordon Greve, Amoco Production Company, Houston, TX

Robert Knox, Scripps Institution of Oceanography, La Jolla, CA

Arthur Nowell, University of Washington, Seattle, WA

Peter Rhines, University of Washington, Seattle, WA

Frank Richter, University of Chicago, Chicago, IL

Brian Rothschild, University of Maryland, Solomons, MD

Thomas C Royer, University of Alaska, Fairbanks, AK

Lynda Shapiro, University of Oregon, Charleston, OR

Sharon Smith, University of Miami, Miami, FL

Paul Stoffa, University of Texas, Austin, TX

Staff

Mary Hope Katsouros, Director

Edward R Urban, Jr., Staff Officer

Robin Peuser, Research Associate

David Wilmot, Research Associate

Mary Pechacek, Administrative Associate

LaVoncyé Mallory, Senior Secretary

Curtis Taylor, Office Assistant

Stewart B Nelson, Consultant

COMMISSION ON GEOSCIENCES, ENVIRONMENT, AND RESOURCES

M Gordon Wolman, The Johns Hopkins University, Baltimore, MD, Chairman

Patrick R Atkins, Aluminum Company of America, Pittsburgh, PA

Edith Brown Weiss, Georgetown University Law Center, Washington, DC

Peter S Eagleson, Massachusetts Institute of Technology, Cambridge, MA

Edward A Frieman, Scripps Institution of Oceanography, La Jolla, CA

W Barclay Kamb, California Institute of Technology, Pasadena, CA

Jack E Oliver, Cornell University, Ithaca, NY

Frank L Parker, Vanderbilt/Clemson University, Nashville, TN

Raymond A Price, Queen's University at Kingston, Canada

Thomas C Schelling, University of Maryland, College Park, MD

Larry L Smarr, University of Illinois, Urbana-Champaign, IL

Steven M Stanley, The Johns Hopkins University, Baltimore, MD

Victoria J Tschinkel, Landers and Parsons, Tallahassee, FL

Warren Washington, National Center for Atmospheric Research, Boulder, CO

Staff

Stephen Rattien, Executive Director

Stephen D Parker, Associate Executive Director

Morgan Gopnik, Assistant Executive Director

Jeanette Spoon, Administrative Officer

Sandi Fitzpatrick, Administrative Associate

Robin L Allen, Senior Project Assistant

CONTENTS

EXECUTIVE SUMMARY

Littoral warfare is the use of combined forces, designed for operations in the sea-land-air environment, toinfluence, deter, or contain and defeat a regional threat through the projection of maritime power It is anextremely complex and dynamic part of naval warfare To be waged successfully, it demands long-termcommitment to research and development, acquisition, threat assessments, tactical and operational analysis,training, education, and realistic fleet exercises

On August 2-5, 1993, the third in a series of classified symposia on tactical oceanography was held, with afocus on coastal oceanography and littoral warfare The symposium was organized by the Navy Committee ofthe National Research Council's Ocean Studies Board, and was jointly supported by the Oceanographer of theNavy and the Chief of Naval Research The Navy's new focus is on the littoral regime, examined herein as foursubdivisions: harbors and approaches, straits and archipelagoes, the surf zone, and the continental shelf.Symposium participants discussed the meteorological and oceanographic forcing factors that have an impact onmilitary operations

The symposium brought together knowledgeable individuals from the operational Navy (specialists inoceanographic research and development, and data acquisition) and from academia with the following goals:

• Addressing timely operational problems, fleet mission needs, and other requirements for which researchand development assistance and inputs are sought by naval leaders and program managers

• Enhancing communication and understanding among basic and applied scientists, and between thesescientists and U.S naval personnel

• Enabling an extended group of researchers to become familiar with challenging naval issues applicable

to the littoral regime

The symposium was preceded by a littoral war game designed to provide insight into the critical role thatoceanography plays in achieving and maintaining battle space dominance The war game emphasized the impact

of decisions forced upon warfare commanders and gave insights about the timing involved in developingenvironmental assessments and predictions for influencing combat decisions

The symposium began with several presentations by Navy personnel to set the context for subsequentdiscussions Participants were divided into working groups focused on the four emphases of the symposium:harbors and approaches, straits and archipelagoes, the surf zone, and the continental shelf The working groupssummarized their findings in plenary session, emphasizing the present status and future directions of research ineach area A recommendation that emerged from several of the working groups was that physical regions (e.g.,estuaries and straits) should be categorized in standard classification systems so that the research results fromaccessible regions can be extrapolated during combat situations to relatively unstudied environments in theinaccessible territorial waters of hostile nations

An intriguing proposal that emerged from the symposium concerned the conduct of one or more fieldexperiments in the littoral zone, much like the open ocean measurements conducted in the mid-Atlantic Oceanduring the 1970s These experiments would include observations and modeling of coastal marine, atmospheric,and land environments Such exercises could be used to transfer academic capabilities and expertise to theapplied and operational communities supporting littoral warfare

The Harbors and Approaches Working Group discussed the information needs, research directions, andpotential technological developments related to littoral warfare in estuarine areas The discussions of the workinggroup focused on three scientific topics: tides and currents, acoustic and electrical properties of the water and thesediment, and the transport of materials and other properties having scalar distribution behavior The workinggroup recommended the development of a classification system for estuaries, based on hydrodynamic properties,that will allow simplified prediction of warfare-relevant environmental characteristics from a few measuredparameters

The Straits and Archipelagoes Working Group recommended that a number of issues be addressed as theNavy prepares for future littoral warfare First, like the previous working group, they recommended that aclassification system be developed for the straits of the world Existing information about straits should becompiled and published for scientific review Process-oriented studies of straits must be designed to understandthe key processes that control flow, temperature,

and salinity in straits A variety of strait types, identified according to the new classification system, should bestudied to permit extrapolation of the information to other straits that have not been studied in detail.Archipelagoes are, in essence, series of straits, so that studies of straits are fundamental to understanding themore complex situation in archipelagoes.

The Surf Zone Working Group made recommendations about research and development needs related toimproving our abilities to measure and understand the processes controlling nearshore bathymetry, waves andcurrents, shelfwide propagation of surface gravity waves, acoustical properties, sea level variations, and

“trafficability” through the surf zone The working group noted that several instruments now used in academicresearch could be used by the Navy to improve littoral zone environmental prediction, and thus Navy operations.Examples include bottom-mounted pressure sensors and remote sensing techniques that allow characterization ofwave features and variability The working group also recommended that the Navy pursue an empirical approach

to understanding the evolution of nearshore systems by studying a set of archetypal beaches

The Continental Shelf Working Group identified a number of cross-cutting issues that are important foroperations in the littoral environment These issues included the need for sufficient characterization of coastalregions to permit advanced planning, a better understanding by naval personnel about how to use nonacousticenvironmental information for making tactical decisions, information about the accuracy of sensors under anycombination of environmental conditions, limiting risk to Navy personnel by using more remote methods andpredictions, the need for real-time sensors of environmental properties, and new approaches for data handling,archiving, and dissemination The working group also noted relevant problems in mine countermeasures,antisubmarine warfare, special warfare, and amphibious operations that could benefit from increased researcheffort

1 INTRODUCTION AND OBJECTIVES

Littoral warfare is the use of combined forces, shaped for forward operations in the sea-land-airenvironment, to influence, deter, or contain and defeat a regional threat through the projection of maritimepower It is an extremely complex and dynamic part of naval warfare To be waged successfully, it demandslong-term commitment to research and development, acquisition, threat assessments, tactical and operationalanalysis, training, education, and realistic fleet exercises

A primary need for amphibious operations, made obvious by recent actions, is to detect, locate, and eitheravoid or clear mines and obstacles from shallow water approaches to and through the craft landing zones Theprimary current shortfall is the time it takes for high-confidence Mine Countermeasures (MCM) to be completed.MCM capabilities are required in three categories: (1) rapid reconnaissance and assessment of the mine threat;(2) organic detection and avoidance and/or other means of protecting Carrier Battle Group and Amphibious TaskForce assets; (3) clearance of sea mines, including rapid breakthrough at choke points Operational maneuverfrom the sea is the desired tactic for present and future maritime power projection ashore Whenever possible, it

is initiated from a position at sea that threatens a large part of the enemy's littoral area Current counter-mine/obstacle technology limits our capability to conduct operational maneuvers from the sea across beaches defended

by both mine and obstacle barriers

On August 2-5, 1993, the third in a series of classified symposia on tactical oceanography was held, with afocus on coastal oceanography and littoral warfare The symposium was organized by the Navy Committee ofthe National Research Council's Ocean Studies Board, and was jointly supported by the Oceanographer of

the Navy and the Chief of Naval Research The Navy's new focus is on the littoral regime, examined herein asfour subdivisions: harbors and approaches, straits and archipelagoes, the surf zone, and the continental shelf.Symposium participants examined the meteorological and oceanographic forcing factors that have an impact onmilitary operations The symposium had several objectives.

• To address timely operational problems, fleet mission needs, and other requirements for which researchand development assistance and inputs are sought by naval leaders and program managers

• To enhance communication and understanding among the basic and applied research communities, andbetween these communities and our naval forces

• To enable an extended group of researchers to become familiar with challenging naval issues applicable

to the littoral regime

The symposium was preceded by a simulation of a warfare situation in the littoral zone (war game) Fiftyacademic and military personnel participated in the war game Staff from the Tactical Department of theTraining Center interacted with war game participants for one day, with the goal of emphasizing the decisionsthat are forced upon commanders by rapidly changing littoral environmental conditions The war game simulatedconditions in the Persian Gulf The war game focused on the following issues: Navy platforms (i.e., aircraft,surface ships, and submarines), tactical use of meteorology and oceanography data, tactics, communications,procedures, equipment capabilities and limitations, weapons, and acoustic and nonacoustic systems The wargame was also useful for motivating discussions during the remaining sessions of the symposium

The symposium began with a number of introductory presentations to prepare symposium participants forsubsequent discussions

RADM John Chubb (Commander, Naval Oceanography Command) provided an overview of the challenge

of collecting and analyzing data in the littoral environment (from the shelf break to the surf zone and beyond) toprovide useful products to warfare commanders He challenged symposium participants to identify ways tocollect environmental data in a real-time tactical environment, to improve and apply satellite capabilities, to findways to develop prototype models rapidly, and to correct deficiencies in our present high-resolution modelingefforts

COL Michael Patrow (U.S Marine Corps, Office of Chief of Naval Operations, Expeditionary WarfareDivision) provided an overview of the shift in emphasis (i.e., resources) by the Navy toward the littoralenvironment based on

changes in the international arena The discussion included a summary of the Navy reorganization to sustain thenew focus on littoral warfare, a definition of expeditionary warfare, a review of the threats that U.S forces face,

a review of some systems and initiatives for upgrading them now under way, and several real-world through” examples

“walk-CAPT R.C Mabry (Commander, Naval Special Warfare, Group One) provided an overview of NavalSpecial Warfare from World War II through the current operations in Somalia, focusing on the mission,organization, and capabilities of special forces in the littoral environment The discussion included planningconsiderations and data needs prior to entering the theater of operations, as well as a summary of meteorologyand oceanography data requirements (i.e, essential elements of information in oceanic, land, riverine, and otheroperating environments)

RADM Geoffrey L Chesbrough (Oceanographer of the Navy) provided an overview of operationaloceanography from multiple perspectives, including preparation for executing the strategy set forth in the

internal Navy document entitled From the Sea He also discussed the global changes from World War II to

today, use of the Naval Expeditionary Forces, multiple littoral threats, joint operations required for success,command and control in a vast battlespace, and the need for tailored meteorology and oceanography dataproducts

INTRODUCTION AND OBJECTIVES 8

2 WORKING GROUP SUMMARIES

Symposium working groups were set up to cover four geographic subdivisions of the littoral region:

• Harbors and Approaches

• Straits and Archipelagoes

• Surf Zone

• Continental ShelvesEach working group was asked to focus on the following questions, to stimulate and focus discussion

1 What environmental information is needed to support special operations mine warfare,antisubmarine warfare, and amphibious operations? What are the important technical issues?

2 Why is the environmental information needed and how is it currently applied operationally? Is thereadditional environmental knowledge that can be applied, immediately or over the next three years?

3 How is this environmental information currently acquired?

4 What are the central environmental research issues?

a Database issues

b Collection methods and priority issues

c Modeling and simulation issues

5 What research and development is needed that will have direct impact on these problems? What arethe environmental research issues and warfare issues?

6 List novel, high-risk, “far-out” ideas that could be applied Be creative, not critical

REPORT OF THE HARBORS AND APPROACHES WORKING GROUP

Dr David Jay, University of Washington, Chair CAPT R.C Mabry, SWG1/NSWG, Cochair LCDR Anthony Negron, SWG1/NSWG, Assistant CAPT Charles Mauck, USNR/FNOC, Assistant

The operational aspects considered were counter-mine warfare (CMW), special forces (SF), amphibiousoperations (AO), and antisubmarine warfare (ASW), the first three being of greatest importance in the context ofharbors and approaches

The Environment

Harbors and approaches are almost always identifiable as estuaries An estuary can be defined as “a enclosed coastal body of water which has a free connection with the open sea and within which sea water ismeasurably diluted with fresh water derived from land drainage” (Cameron and Pritchard, 1963) Familiar NorthAmerican examples include New York Harbor, a coastal plain estuary with several adjacent tidal straits; SanFrancisco Bay, a river-estuary attached to a fault-formed embayment; and Puget Sound/Strait of Juan de Fuca/Straits of Georgia, a complex of river deltas, straits, and fjords

semi-We may usefully extend the definition quoted above to include (1) tidal rivers landward of salinity intrusion

to the head of the tide (e.g., the Columbia River to Portland); (2) hypersaline negative estuaries, whereevaporation exceeds the sum of precipitation and river inflow (e.g., Houston's harbor and Galveston Bay); and(3) the estuarine plume outflow area, usually into an open coastal environment, where buoyancy-driven flowsand stratification have a major influence on circulation and sedimentation processes (e.g., the mouth of theAmazon) These additions reflect both operational needs and the research methodology of the estuarineoceanography community

There are undoubtedly some harbors (e.g., on arid islands) that meet the quoted definition only to the extent

of being “semi-enclosed.” As a practical matter, however, the conduct of littoral warfare in these exceptionalharbors would require the same information concerning tides and currents as its conduct in more typicalestuarine harbors and approaches Moreover, the presence of even small sanitary and/or industrial sewer outfallswill raise the same issues concerning

pollution and acoustic and electrical properties of the water column and sediments that are pertinent in estuarineharbors and approaches.

The estuaries that make up the numerous harbors and approaches of the world are characterized by a greatdiversity of physical conditions and processes This is true not only because there are many types of estuaries(ranging from shallow, tropical lagoons to deep, ice-bound fjords), but also because conditions within individualestuaries are extremely variable in both time and space Salinity in a fjord may vary, for example, from zero tooceanic values across the main halocline over a few meters in the vertical direction Salinity variability of thesame magnitude may occur over a tidal cycle at a single point in space as a result of tidal advection in a salt-

How should the oceanographic diversity of the strategically important harbors of the world be handled? It isclearly impossible to study all of them; they are too numerous, and most are within the coastal waters of nationsthat restrict research access It is imperative, therefore, to make good use of available information concerningdominant time and space scales of forcing Process-oriented studies must be carried out in representative andaccessible systems that are analyzed in detail These results must then be extrapolated in a conceptually soundmanner to the many important, poorly known, and inaccessible systems of the world Coordination with otherprograms that seek systems-level understanding of estuarine and coastal environments would also likely beproductive These include the Land Margin Ecosystem Research program funded by the National ScienceFoundation and the upcoming Land-Ocean Interactions in the Coastal Zone (LOICZ) initiative of theInternational Geosphere-Biosphere Program

The variability of estuarine physical environments is reflected in biological processes Supplies of nutrientsand/or organic matter in many estuarine systems are very large relative to open-ocean values, allowing highbiological productivity, a factor that strongly influences acoustic properties of water and sediment Furthermore,estuarine biological communities are commonly, though not universally, structured by physical forcing andgeochemical constraints rather than by biological processes such as competition or predation Thus, theecosystems that have evolved in river-estuaries, for example, are made up largely of organisms that can survive

in an advection-dominated environment where the residence time of an average parcel of water (typically a fewdays) is much less than the generation time of most invertebrates and salinity is variable For example, theChesapeake Bay's ecosystem has been forced to adjust within the past several hundred years to summer anoxiaimposed by high nutrient loadings Although certain opportunistic species may appear in infrequent andunpredictable blooms, most estuarine biological populations respond to the patterns of physical variabilitydiscussed in the previous paragraph This again points to the importance of understanding certain representativesystems in some detail

We should not, as a consequence of a pressing need to deal with the short time scales directly pertinent tonaval operations, lose track of the place of estuaries in recent geological history This context is necessary tounderstanding the structure and evolution of estuarine ecosystems, their sedimentology, and (to the extent thatchannel morphology controls circulation processes) even their physics Most estuarine ecosystems are young andrapidly evolving, because they have existed in their present form only since stabilization of global sea level about5,000-6,000 years ago Sedimentation rates are typically much greater than those in open-ocean environments.Estuaries on tectonically active coasts may change morphology rapidly enough to render bathymetric and tidalinformation obsolete within a few decades Large expanses of bare tidal flats may become vegetated within a fewyears A food chain based on large amounts of detritus exported from marshes may disappear and be supplanted

by one based on river-borne detritus, with important consequences for water clarity and acoustic properties.Finally, because much of the world's population lives adjacent to estuaries, anthropogenic change isextremely important in determining the features of interest to naval operations Estuaries are the part of themarine environment that is most accessible and susceptible to human manipulation The direct effect of harbordredging and construction in the form of jetties, channels, and breakwaters on tidal properties is the mostobvious type of alteration Entering the Columbia River (once the “graveyard of the Pacific”) through themaintained 48-ft navigation channel is a very different problem, for example, from that of navigating the formershifting, natural channel that had a controlling depth of 20 ft in a good year Less direct but equally potentalterations must also be considered Construction of the

Aswan Dam, for example, utterly changed circulation, acoustic properties, and the sedimentation and ecosystemstructure in the Nile delta Industrial and sewage pollution in an estuary may cause a system to have completelydifferent acoustic properties from those of nearby, unimpacted systems that would, in the absence of humanpopulation, be quite similar Moreover, these anthropogenic changes are rapid relative to most natural changes except perhaps those induced by tectonics.

The possible use of flow regulation as a defensive weapon cannot be ignored altogether Nationalist forces

in China used dike breaches and subsequent flooding to immobilize and drown the invading Japanese duringWorld War II, though at great cost to the native population Artificial floods could also be used to increasestratification, decrease water clarity, and bury mines Numerous possibilities for offensive use exist

Working Group Discussion

The charge to the working group consisted of two primary tasks The first concerned environmentalinformation: What information is needed for littoral warfare operations, how is it presently acquired, and howcan this process be improved? For the Harbors and Approaches Working Group, this issue served as a means tofocus further discussion on the second topic, identification of research and development (R&D) priorities forlittoral operations

Review of this table suggested that consideration of research and technological priorities should be organizedinto three unifying topic areas, as follows:

1 Tides and currents

2 Acoustic and electrical properties of the water and sediment

3 Pollutant and other scalar transport

Bathymetric data serve as a good example of how a specific type of data fits into the overall topics listedabove Bathymetric data are needed for modeling of tides and currents in support of all four types of operations(ASW, AO, CMW, and SF) Such data would also play an important role in evaluating acoustic, electric, andpollutant properties of the water and sediment for CMW, ASW, and SF operations, whether this evaluation wascarried out through numerical modeling or more qualitative methods The most systematic and intensive need forthese bathymetric data would, however, come from numerical circulation modeling programs Thus, weassociated bathymetric data acquisition and database storage with the first research area, tides and currents Each

of the other information

types can similarly be assigned to one or more of the three topics listed above The remainder of this report

is organized, therefore, in subsections focused on these areas, followed by a section presenting our Summary andConclusions

It is important to note the considerable overlap of the above three topics with the traditional research agenda

of the estuarine oceanography community: there is both good and bad news here The good news is that thepresent state of knowledge in estuarine oceanography will allow rapid improvement at low cost insome areas,particularly the prediction of barotropic tides and currents The bad news is that certain problems in estuarineoceanography have not been resolved by several decades of research and are of considerable fundamentaldifficulty Transport of pollutants, sediments, and other materials is in this category Consider as an example thetransport of suspended matter that determines the optical properties of the water column and, in manyenvironments, the character of the seabed itself Existing sediment transport models uniformly fail to considerthe effects of horizontal gradients in suspended sediment concentration The rheology of liquid mud (a non-Newtonian fluid), the erosion and deposition properties of cohesive sediments, and the kinetics of particleaggregation as a function of shear, biological activity, and stratification are all poorly understood Clearly, asustained research effort will be required if functional models of estuarine suspended sediment transport are to bedeveloped

Tides and Currents

Improvement of knowledge in the area of tides and currents was the subject most thoroughly treated by theHarbors and Approaches Working Group Ideas for improved prediction of tides and currents were separated intofour categories: planning, operations, research problems, and desirable technology An important conclusion forboth planning and operations was that substantial improvements could be made in the near future with arelatively low investment of funds, on the basis of the existing understanding of estuarine circulation and ofpresent models of barotropic tides and currents

There are two primary needs on the planning level First, an estuarine classification system based onhydrodynamics should be developed that would allow division of most of the harbors and approaches of theworld into about a dozen categories (e.g., lagoons, river-estuaries, weakly stratified and partially mixed bays,tidally forced fjords, and weakly forced fjords) A small suite of parameters (e.g., ratio of semidiurnal to diurnalforcing, tidal range to depth, river flow to tidal velocity, and estuary length to tidal wavelength) would then beused to place estuaries in these categories and a slightly larger number of subcategories Representative estuariesfrom around the world would be classified as to category and subtype Additional systems could be added asthey became operationally important for new or potential operations This categorization could be used focus

research into types of estuaries deemed to be of strategic importance and would be useful for operationalpurposes.

This classification system would be very useful in identifying the primary sources of variance in thevelocity and surface elevation, and would (as discussed below) provide additional information concerningdensity structure and scalar transport processes Following is an example of the practical utility of such a system.Suppose that the barotropic tidal models discussed in the following paragraphs have been run for aparticular harbor of interest, but that the river-flow and bathymetric data used in the model were of poor quality,and therefore the predicted tides and currents were of dubious validity How much effort should be put intoimproving these predictions? Improvement options might include processing of remote-sensing data, covertinstallation of pressure gauges (not possible now, but certainly feasible in the near future), and on-siteintelligence operations If, on the one hand, we know that the system is a macrotidal river-estuary, then furtherefforts might well be merited, because 60 to 90 percent of the total velocity variance in such systems is normallyassociated with tides and river currents If, on the other hand, the system is categorized as a hypersaline,microtidal tropical lagoon, where typically 60 to 90 percent of the current variance is related to atmosphericforcing and density currents, then further refinement of a bathymetric tidal model would likely be pointless, butmodeling of wind and density-driven circulation might be imperative

A considerable improvement in the tide and current predictions presently available for many estuaries could

be made by systematic application of existing tidal models on various scales The methodology has been wellproven in systematic tests in the North Sea [see Waiters and Wemer (1991) for a review] In practice, a large-scale, regional, barotropic tidal model would be run for an entire area of possible operations Such a regionalmodel would usually cover at a low resolution an area the size of the North Sea, or the coastal ocean fromNorthern California to the north end of Vancouver Island This regional model would then be used to formulatethe open boundary conditions for models of individual estuaries or related groups of them Examples of river-estuary systems suitable for regional models would be the Rotterdam Waterway and the Puget Sound/Strait ofJuan de Fuca/Straits of Georgia complex

The smaller-scale models may be either two- or three-dimensional, and adapted to the character of theparticular estuary; such models might include, for example, wetting and drying of tidal flats, fluvial forcing, andsophisticated turbulence predictions, as appropriate Given reasonable bathymetric data and proper boundaryforcing, existing barotropic tidal models can predict tidal

elevations within a few percent and barotropic tidal currents within plus or minus 10 percent For many harborsand approaches, barotropic currents account for the bulk of the total velocity variance In other cases forexample, river-estuaries-nonlinearities related to time variations in the density field create internally forcedmodes that greatly modify the barotropic tide These differences again emphasize the importance of aclassification system: it would allow identification of systems for which existing barotropic models are, or arenot, adequate for current prediction of currents.

A rather different modeling approach is needed within an actual theater of operations The analogy to spill modeling is appropriate here Model predictions must not only be accurate and timely, but the modelingsystem must be relatively easy to use and needs to be capable of being run with assimilation of observations innear real time Experience with oil-spill cleanups suggests that if operational models do not have thesecharacteristics, they will be ignored in favor of tide tables and seat-of-the-pants reasoning Theater operationalmodels should also be relatively small scale, and should run on microcomputers or work stations They would beconnected to large, regional planning models through a network As data are acquired in the course of anoperation, they should be incorporated into a geographic information system (or other similar) database and used

oil-to improve model predictions Relevant data might include bathymetry, velocity profiles from shipboardAcoustic Doppler Current Profilers (ADCP), tidal elevation from moored pressure gauges, and informationconcerning sediment type and bottom roughness These data should also be sent by network back to the regionalplanning center so that the larger regional model could also be modified as data were acquired

Prediction of tides and currents is straightforward for harbors and approaches only in the absence of stronginteractions with other flow processes such as river flow, wind waves, and atmospherically-forced currents Ifboth tides and one or more of these factors are important in a system, the resulting interactions raisefundamental, unresolved questions in estuarine oceanography Estuaries with both strong tides and strong riverflow, for example, have time-varying stratification that drives substantial, nonlinear circulations that are poorlyunderstood at present Interactions of tidal currents with swell can substantially modify the tidal current andgreatly increase wave amplitude Prediction of these wave-current interactions is still an imprecise art Betterturbulence models that account correctly for mixing due to free-shear layer instabilities and internal waveinteractions are essential to the construction of improved models of stratified tidal flows that could properlyrepresent the nonlinear processes mentioned above

Analyses of time series of current and pressure observations need, furthermore, to be improved in light ofrecent advances related to nonstationary

processes Abrupt nonlinear transitions in stratification lead to major reorganizations of the tidal and residualcirculations in many estuaries The advection of a plume by storms and other coastal ocean processes leads tohighly intermittent internal tides These and other examples of forcing of tidal currents by a time-varying densityfield indicate that tidal currents cannot be treated as a stationary process in many systems Conventionalharmonic analysis programs can be modified to add any arbitrary number of frequencies (literally hundreds ofthem), but this does not lead to either conceptual understanding or predictive capability It would be far moreproductive instead to use rapidly emerging methodologies for nonstationary time series; these go under thenames “discrete wavelet transforms” and “short-term Fourier analysis.” Optimum use should also be made oflarge ADCP data sets in which the observations are distributed in space and time in such a way that they do notform proper time series Here partial analogies to tidal analysis via satellite altimetry and other geophysicalinverse problems should be kept in mind Some methods of satellite data analysis may be directly pertinent toADCP data.

Existing harmonic analysis methods all minimize least-squared errors of predicted relative to observedcurrents Because ADCP data are inherently noisy, this optimizes the fit to data plus noise It would be better tofollow the example of other areas of geophysics (Constable et al., 1987) and optimize some combination of least-squared errors and smoothness Finally, scalar data from harbors such as data on salinity, temperature, soundspeed, and turbidity can be subjected to tidal analysis, although the available data will rarely be available astraditional time series

The art of modeling also needs improvement in several areas Most presently available assimilative models,for example, make compromises in representing nonlinear processes that are unacceptable in estuaries withmultiple forcing processes (e.g., tides plus river flow or wind stress) Better methods need to be developed to usenumerical models to identify optimal locations for data collection The cost of data acquisition in a theater of war

is likely to be very high; thus, sensitivity analyses of models should be used to determine which data are mostcrucial

The working group also had several suggestions for improved technology The single item most frequentlymentioned was a pressure gauge that could be covertly deployed (e.g., from the air) and that would periodicallytransmit data to a satellite A low-cost, expendable conductivity temperature-pressure-optical backscatter profilerwould also be highly useful It is worthwhile remembering that temperature and salinity gradients are a factor of

develop because they would need to discriminate among small changes in

temperature and salinity Such sensors would also be highly useful for underwater vehicles and drifters.

Acoustic and Electrical Properties of the Water and Sediment

Determination of the acoustic properties of seawater and various substrates is of vital importance to ASWand CMW because of their reliance on sonar SF operations and AO depend on sonar to a lesser degree Themagnetic properties of the water column and seabed are a dominant consideration for CMW and are of someimportance to ASW Acoustic and magnetic detection methods are important for detecting mines as well as fordetecting other bottom obstacles that may be “false targets” and, in some cases, can distinguish between the two.Acoustic properties of “blue water” environments relevant to the cold war ASW in past decades are entirelydetermined by the thermal structure of the water column This is not the case in littoral environments in general,and particularly not in harbors and approaches

Salinity stratification and the presence of suspended sediment exhibit variability on a variety of time scalesand may have a significant effect on water-column transmission and scattering of sound Sediments in estuarineenvironments are also much more diverse and variable in time and space than they are in open-oceanenvironments Just as salinity stratification may change radically over a tidal cycle as a result of advection on saltwedge, deposits of liquid mud may appear and disappear tidally, and their character may change over the tidalmonth and seasonally This variability is reflected in sedimentary records from numerous coastal and estuarineenvironments that exhibit “varves” (sediment layers) indicative of tidal daily, tidal monthly, or seasonalvariations Sand waves, moreover, may form and disappear, or bury a mine in a single tidal cycle At the otherextreme, more quiescent estuarine environments (fjord basins, for example) may exhibit little or no variabilityover years or decades

Thus, it is vitally important to determine the likely time and space scales of variability for acoustic andelectrical properties, exactly as was the case for tides and currents Any sensible estuarine classification systembased on hydrodynamics must deal directly or indirectly with the time scales of variability of the scalar fieldsthat determine water-column acoustic and electrical properties Existing estuarine classification systems do not,however, consider the relationship of flow properties to the character of the substrate

Sedimentological properties are more difficult to classify than are those of the water, because of the strongroles of regional geology and biology in determining the former For example, we can expect a strong degree ofcommonality in circulation processes in river-estuaries around the world, but the same cannot be said for theirsubstrates Some large river-estuaries with strong

tides are sand-bedded, because bed stresses are too high to allow muds to be permanently deposited This is notthe only possible sedimentary regime, however, for a strongly forced river-estuary If the system is deep enough

or the fine sediment load large enough, then very large concentrations may accumulate in the estuary Densitystratification induced by the suspended field reduces bottom stresses, and the seabed will consist primarily ofsilts and clays rather than sands

The realization that understanding acoustic and electrical properties of the water column and seabed inharbors and approaches overlaps substantially with traditional estuarine research objectives led to identification

of a number of research issues The importance of the estuarine density field in the tidal circulation wasdiscussed in the previous subsection Our understanding of estuarine sedimentology and sediment transport isless mature, however, than is that of tidal processes While the present state of knowledge provides a basis forfuture research, it also emphasizes that sustained funding will be required to improve the present state of the artsubstantially

Cohesive sediment transport is particularly important, and is discussed below in the subsection on scalartransport Here it suffices to note that sedimentary information acquired during the course of operations (e.g.,from helicopter-towed sidescan sonar used in ASW) should be, along with hydrodynamic data, recorded in adatabase and assimilated rapidly into hydrodynamic models The mine warfare tactical environmental displaysystem (MTEDS) database was mentioned by the working group in this regard Basic research is also needed onthe low-angle acoustic scattering by sediments as a function of composition, porosity, degree of bioturbation,bedform character, and presence or absence of active sediment transport The impact of sediment and watercolumn geochemistry on aggregate seabed physical properties is as yet poorly understood, as are water columnaggregation processes that play a vital role in the settling of organic material to the seabed This material may bethe result of in situ productivity, aquatic productivity elsewhere in the river-estuary system, agriculture in thetributary watershed, or industrial pollution Moreover, recent research has emphasized the high numbers,diversity, and productivity of microbial populations in estuarine turbidity maxima These bacteria play a majorrole in aggregation, and thus settling, of particles in many estuaries

Several potential R&D projects were also identified by the working group These included development of aDoppler penetrometer and variable-frequency (“chirp”) sonars for seabed characterization The latter wouldemulate dolphin sonar and would assist in the detection of buried mines that cannot presently be detectedacoustically, except by dolphins Tomographic methods analogous to those now used in medical imaging mightalso be adapted to mine detection

Pollutant and Other Scalar Transport

Scalar transport is important to littoral warfare in several respects Detection of pollutants (e.g., oil, fecalbacteria, and dissolved toxics) and predicting their likely transport are vital issues for SF operations, as wasdemonstrated in the Persian Gulf war They are also an important considerations in the use of dolphins for minedetection As discussed above, suspended sediment transport plays an important role in determining watercolumn and seabed properties of interest to CMW and ASW Several areas of recent estuarine research arerelevant here Studies of estuarine and shelf transport of both cohesive and noncohesive sediments haveconcentrated heavily on the deposition and erosion properties of sediment grains and aggregates Aside from theinfluence of sediment stratification on turbulent mixing, little attention has been paid to the effects of circulationnonlinearities on the fate of sediments Oil spill models are highly specific to petroleum products Other pollutanttransport investigations have tended to focus on the initial dispersion of pollutants from a source rather than onthe long-term disposition of the materials in question

Investigations of scalar transport have focused on the relationship between wave transport of scalar wavevariations (e.g., at the dominant tidal frequency) and the Stokes drift transport of the scalar mean field Thesestudies have shown that the frequencies at which transport occurs are strongly dependent on the scalar inquestion and on the type of estuary In narrow tidal channels where vertical rather than horizontal shear is thedominant agent of dispersion, salt is transported landward both by the tides and by the mean, two-layer flow.Scalar transport is in general not chaotic for this case, although, of course, turbulent mixing adds a chaoticelement to scalar dispersion The direction and magnitude of scalar transport (e.g., of salt or sediment) isextremely sensitive, however, to shear in the velocity field and stratification of the scalar being transported Incontrast, scalar transport in broadly influenced, shallow embayments with complex topography exhibits, likeother chaotic processes, an acute sensitivity to initial conditions Even though it is deterministic, scalar transportcannot be predicted very far into the future in such systems, because the initial conditions cannot be specifiedwith sufficient accuracy Studies of these chaotic processes have considered (to date) only vertically uniformtransport of conservative scalars Theoretical understanding of scalar transport in more complex environments(e.g., fjords) that are both stratified and topographically complex is inadequate

Thus, several decades of research by scientists and engineers have brought about only a preliminaryunderstanding and, in most cases, a rather rudimentary ability to predict scalar transport The large number anddiverse character of the world's harbors and approaches means that research efforts must focus on process studies

in carefully chosen, representative systems Again, the use of a hydrodynamically-based classification system isnecessary for choosing

representative systems for intensive, process-oriented studies Pollutant transport is also an issue of importance

in the Navy's Environmental Program

Sediment transport merits particular mention because of its importance in all aspects of littoral warfare.Recent advances in fluvial transport of mixed-grain, noncohesive sediments provide a solid base of knowledgefor construction of sediment transport models for sand-bedded systems Suspended-sediment transport is subject

to the aforementioned flow nonlinearities, plus the complexities of deposition, erosion, aggregation, compaction,and (for liquid mud) a non-Newtonian response to imposed surface stresses There are at present no adequatemodels of sediment transport, either of sand or cohesive sediments, in the estuarine environment Shelf sedimenttransport models have elaborated in considerable detail the influence of wave-current interactions and sedimentself-stratification on bed stress They emphasize the exquisite level of detail required for successful treatment ofdeposition and erosion about 50 model levels per decade in the vertical direction down to the roughness scalelength for a total of 300 to 400 levels in all This detailed representation has been accomplished, however,through the total neglect of lateral transport and its complexities In contrast, estuarine sediment transport modelshave focused on horizontal transport processes and lack both sufficient vertical resolution and adequateturbulence models Development of a capability to predict the transport, erosion, and deposition of finesediments is vital, and will require sustained effort over perhaps a decade Models of noncohesive transportcould be achieved on a shorter time scale, because of the better level of fundamental understanding available atthis time

Two R&D issues were also mentioned in working group discussions Field detection kits for use in SFoperations should be developed for selected pollutants Pollutant detection capabilities should also be added toremotely-operated underwater vehicles

Summary and Conclusions

Most of the harbors, approaches, and straits of the world are estuaries As such, they are complex, extremelydiverse environments subject to abrupt natural and anthropogenic alteration Prediction of the properties ofinterest for naval operations will require an ecosystems approach that considers the couplings betweenbiological, geochemical, and physical processes, and the ways in which alterations of these processes change theoperational environment Because of the diversity and large number of estuarine systems, a key element of asuccessful strategy for littoral warfare over the coming decades is to define an estuarine classification systembased on hydrodynamics that will allow determination of

basic properties of these diverse systems from a few external parameters This classification system wouldprovide a basis both for operational decisionmaking relative to environmental factors and for choosingproductive research directions.

Discussion of information needs, research directions, and potential technological developments to assist inthe Navy's littoral warfare mission in harbors and approaches led to the definition of three primary topical areasthat unified the previously identified information needs and served as a basis for further discussion The threeareas were tides and currents, acoustic and electrical properties of the water and sediment, and pollutant andother scalar transport Research needs in these three areas overlap considerably with the traditional researchprogram of estuarine oceanography Substantial, short-term improvements can be made at relatively low cost inthe prediction of barotropic tides and currents through systematic use of existing, robust numerical models Insome types of estuaries, barotropic tidal currents are either not important or are strongly modified by otherprocesses, e.g., atmospheric forcing, time-varying vertical mixing processes, and surface waves Practicalprediction of currents in these types of estuaries requires substantial work This is also true in the areas ofacoustic and electrical properties and scalar transport; several decades of research have defined an agenda buthave not resolved the major technical issues A sustained, long-term research effort is required

References

Cameron, W M., and D W Pritchard 1963 Estuaries In: M.N Hill (ed.), The Sea, Vol II John Wiley and Sons, New York, pp 306-324.

Constable, S C., R L Porker, and G C Constable 1987 Occam's inversion: a practical algorithm for generating smooth models from

electromagnetic sounding data Geophysics 52:289-300.

Waiters, R A., and F E Wemer 1991 Nonlinear generation of overrides, compound tides and residuals In: B.B Porker (ed.), Tidal Hydrodynamics John Wiley and Sons, New York, pp 297-320.

REPORT OF THE STRAITS AND ARCHIPELAGOES WORKING GROUP

Dr Michael Gregg, University of Washington, Chair LCDR Timothy Sheridan, Office of Naval Research, Cochair CAPT George Heburn, USNR/NRL(S), Assistant

The constricted topography of straits often produces fast and variable currents and sharp changes in watermasses Consequently, flow through straits is usually characterized by (1) hydraulic controls in constrictions andover sills, (2) strong steering in channels changing direction, (3) density currents throughout the water column,and (4) focusing of surface waves both by the bathymetry and by winds steered by local topography Intensehuman activity further complicates naval operations, particularly with shipping, fishing, and installation of cablesand pipelines Archipelagoes are yet more complicated, with many intersecting channels

Major Research Issues Synthesis of the Literature About Straits

The existing literature on straits should be synthesized in a thorough review article published in a refereedscientific journal This effort should include an attempt to classify straits using known parameters, including thefollowing:

• Geometry, straightness or crookedness, depth or shallowness, and relative importance of sills andconstrictions

• The importance of Earth's rotation, that is the internal Rossby number

• Flow modulation, seasonally and tidally

• The importance of hydraulic controls, given by Froude numbers, in constrictions and over sills

• Hydrography, sources, and nature of water masses and types

• Atmospheric forcing, importance of wind stress, and the surface buoyancy flux

In addition, reviews should be done on particular straits where extensive work has been carried out forexample, Gibraltar

Process-Oriented Studies of Straits

The goal is to understand the major processes controlling flow, temperature, and salinity in straits So manystraits are potentially important for naval operations that only a subset can be studied, and access for research islikely to be denied to some Consequently, once a particular strait is identified during a crisis, predictions fornaval operations must rely on available data and models incorporating processes known or likely to dominatethat strait Straits and processes chosen for further study should be chosen on the basis of the reviewsrecommended above, but several issues are apparent now

a Internal bores and solitary waves are often generated when hydraulic control is lost as flowdecreases for example, every 12 hours in the Strait of Gibraltar when tidal outflow changes toinflow Similar features are often found on continental shelves, propagating in from the sea.Although it is shown to be strongly turbulent, the rate at which they decay has not been determined

b Bottom stress is likely to be a major factor in flow dynamics but is difficult to measure and wasomitted completely from the Gibraltar Experiment Later estimates obtained with ExpendableCurrent Profilers (XCPs) and expendable dissipation profilers outside Gibraltar in the outflow plumedisagreed by a factor of 3

c How surface waves are generated and focused in straits has not been studied This is importantduring minesweeping, as estimates of bottom pressure are used determine the likelihood that otherfactors can trigger mines

d Secondary circulations Most straits have strong secondary flows, across or counter to the main flow.Sometimes accurately described in local pilots, these features are likely to be important to thedynamics of the strait, but are poorly known and have not been studied systematically

e Bottom morphology and sediment dynamics strongly affect bottom stress, acoustic propagation, andmine warfare It needs to be known whether there are general scientific issues other than those aboutbottom features in general

Modeling Issues

Modeling flow through straits has not received the attention given to modeling ocean basins and continentalshelves Several general issues should be addressed: (1) How should flow be modeled in a channel connectingtwo water bodies without simultaneously modeling those bodies? (2) What scale of bathymetry should beincluded to describe the dominant flow, and what numerical schemes are needed to model abrupt changes indepth? (3) How can subgrid-

scales processes be included, as they differ greatly from those in the open ocean? (4) How can models be adapted

to predict stochastic variability instead of dealing only with deterministic flows?

Instrumentation Issues

Available instruments are not adequate for either academic or naval measurements; the same type ofinstrument needs are common to other coastal regimes Short space and time scales require makingmeasurements at many places and times Existing capabilities need to be available in packages that are smaller,cheaper, easier to operate, and in some cases capable of covert deployment

Nearshore salinity often dominates both density and sound speed profiles It is essential, therefore, thatnaval forces measure salinity when conducting antisubmarine operations or sweeping mines Adding a dippingconductivity-temperature-depth (CTD) package to minesweeping helicopters is one step that appears feasiblewith minimal development

Valuable data are being lost when minesweeping helicopters fail to record output from their sidescan sonar.When coupled with positions from the Global Position System (GPS) receiver, these data can be used tocharacterize the detailed bathymetry and bottom type

Process studies are limited by the difficulty of measuring bottom stress ADCPs with standard beamsoriented 45 degrees from vertical usually do not return useful data from within the lower 15 percent of the watercolumn ADCPs with beams oriented at 20 degrees from vertical can measure within 6 percent of the bottom,and sophisticated processing can further improve resolution Nevertheless, full resolution of the bottom boundarylayer requires mounting sensors on the bottom or dropping XCPs, which are presently too expensive for thespatial and temporal resolution required

Archipelagoes

All issues about straits apply to archipelagoes No less important to the Navy, archipelagoes are much morecomplex Consequently, the working group believes that further progress in research on straits should contribute

to the understanding of flow behavior in archipelagoes

Summary and Conclusions

The Straits and Archipelagoes Working Group recommended that a number of issues be addressed as theNavy prepares for future littoral warfare First, they