Click for next page ( 2


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
Executive Summary Coastal scientists and engineers are faced with a wide variety of problems in the coastal zone. In most cases the resolution of these problems involves measurements of water and particulate motion and coastal processescurrents, waves, tides, bathymetry, sediment transport, and so on. These measurements may be needed for a variety of purposes: research Into the fundamental processes, design data, ~ ~ ~ . ~ ~ ~ . input data for developing physical and mathematical models and for validating them, monitoring, and . documentation of extreme events. Some coastal engineering problems will present increasingly dif- ficult challenges in the next few years. Examples of these problems are the rapid erosion of the barrier islands along the Atlantic and Gulf coastlines, the possible Trip act of sea level change on shore stability and structures; construction, protection, or preservation of some expensive coastal structures and developments; and the cost and impact of the extensive harbor entrance dredging. Although no one has evaluated the total cost of coastal engineer- ing works in the United States, much less defined the entire scope of such activity, we do know that the cost of maintenance dredging 1

OCR for page 1
2 alone amounts to over one-half billion dollars In fecleral and local government funds annually, hurricane losses along beach areas ex- ceed hundreds of millions of dollars, and more ordinary winter storms cause property damage in the tens of millions. The capability to un- derstand and anticipate how and when coastal changes occur in and near the high-energy wave zone can significantly reduce the public and private costs and losses; the ability to measure change Is directly related to this capability. Unfortunately, many coastal processes are poorly understood, largely due to the difficulty of making good measurements. The processes represent a large range of space and time scales, from the smallest turbulence scales to sea level changes occurring over decades. Often a great deal of sensitivity and precision is needed in the in- struments, yet they must function over an extended period of time in one of the most energetic and corrosive of ocean environments. Instrumentation in remote sites must operate unattended for weeks or years. Biological fouling can be a major consideration in subarctic regions. In some cases sensors do not exist to make some very impor- tant measurementsnearshore sediment transport near the seabed, for example. Only a few of the available coastal engineering measure- ment systems (e.g., tide gauges, current meters, wave gauges) can be bought off-the-shelf. Most systems are built in small numbers and are often ~customized" for the particular measurement requirement. With this background, this report sets out to answer the follow- ing questions: . What are the coastal measurement needs, and what are the engineering requirements having a major influence on these needs? . What is the relationship between engineering measurement needs and the use of physical and mathematical models for analyzing and forecasting coastal changes? What are the implications of the modeling capability and practices on development of measurement systems? . What new technology can effectively be applied to improve present measurement practice, and what gaps will exist in measure- ment techniques and systems? . What improvements in measurement techniques and ap- proaches will have a significant influence in accuracy and cost of analysis and forecasting of coastal change? The committee attempted to be as realistic as possible in prepar- ing its conclusions and recommendations. Every effort was made to

OCR for page 1
3 relate the measurement needs to realistic coastal engineering prom lems. In most cases, the measurements are made to define processes and their impact on the shore or a structure, existing or planned. The measurements may be used to characterize or model the particular process such as sediment transport- or they may be deterministic, an in an actual measurement-of a load on a structure. Coastal en- gineering problems are divided into four general features, according to where they occur: shore areas, backshores, entrances, and harbors. These problems are in turn treated in regard to the following coastal processes: . kinematics and hydrodynamics of high-frequency flows, in- cluding wave and current motions with periods of five minutes or less, such as ocean waves; ~ kinematics and hydrodynamics of low-frequency flows with periods of more than five minutes, including tides and storm surges; fluid/sediment interactions; and fluid/structure interactions. In each general process category, the requirements, present ca- pabilities, and needs are assessed. The status of existing models and the modeling requirements for measurements are treated as well. To assist the committee in identifying the major coastal en- gineering problems, a questionnaire containing the above questions was sent to over 30 leading coastal engineers. In part, the conclusions reached by the committee represent their input. The committee conducted this study to assess the needs and availability of suitable coastal instrumentation and measurement systems, and to provide recommendations regarding specific devel- opments. The committee was also responsible for providing guidance on development priorities. The comrn~ttee agreed that there is a pressing need for the de- velopment of instruments and measurement systems to promote our understanding of coastal processes. Presently, the user community is small and cannot attract sufficient industrial interest in developing the needed technology. To stimulate this development, we need a commitment of resources at the national level, as well as a forum to provide information, collaboration, interaction, and coordination on measurement system development. The committee recommends 19 specific steps for improving

OCR for page 1
4 coastal instruments and measurement systems, each recommends tion derived from a conclusion reached by a consensus of the com- m~ttee members. Of the 19 recommendations, 4 were ranked as high- priority, 5 were ranked high-to-medium, 9 were ranked as medium, and ~ was ranked as medium-t>Iow-priority. The nine recommends tions ranked as high-t~medium are regarded as essential to coastal engineering advancement in the rest of this century. The committee, being well aware that specific local or regional coastal problems may argue for a different ranking of the recornrnendations, would only re- m~nd the reader that there recommendations are based on a national point of view. High-Priority Recommendations Wave data and measurement techniques: 1. Directional wave energy spectra and their rapid dissern~nation to forecasters and researchers are needed; a national coastal mea- surement system to expand a small-area coverage now in place is recommended. 2. Directional resolution improvements are essential for more ac- curate forecasting of littoral processesinstrumentation techniques to measure wave clirectionality (2 to 5 accuracy) both in situ and remotely should be developed. Sed~rnent transport processes and their measurement: 3. High-resolution measurement of concentrated sediment mo- tions near the mobile sea bed cannot now be made in the surf zone and tidal inlets; this capability should be developed. 4. Models of sediment transport are largely empirical and are poorly verified; improvements in understanding the underlying the- oretical relationships and field tests (interactive) of existing models are needed. High-t - Medium Priority Recommendations 1. The internal dynamics of rubble-mound structures are not well understood by coastal engineers, as the number of divergent opinions about the causes of structural failure show. Both modeling and in situ measurements of such structures should be undertaken to enhance the basis for better and safer design.

OCR for page 1
s 2. The causes of breakwater failure are still not well understood. Core pressures In breakwater cross sections should be measured dur- ing storms, and modern pressure-sensing and signal transmission technology should be adapted to acquire and process such data. 3. Wave set-up process is not well quantified; measurements of wave forcing and sea-level response during storms are needed. 4. Sedunent transport prediction is closely linked to understand- ing and predicting turbulence; instruments for measuring turbulence in the nearshore environment are needecI, and related modeling ca- pability should be developed. j 5. Lim~ted-area hydrodynarn~cal circulation models (2-D and 3- D) need to be improved; this progress is dependent on developing low-cost current meters and associated telemetry systems to be able to verify and develop new models. Development of such meters is urged. Medi,,=-Priority Recommendations I. Improve and verify, through field measurements, a 3-D cur- rent model for complex bathymetry. 2. Develop reliable current meters for measuring low-frequency currents in tidal inlets during storms. 3. Trnprove storm-surge modem and verify them through field measurements development to enhance predictions of overland flood- ~ng. 4. Improve means to measure velocity profiles in energetic, rapidly varying flow fields to allow better estimates of boundary shear stress. 5. Determine ways to measure small-scale (centimeter to decime- ter) changes in seabed topography to understand scour processes around structures. 6. Develop measurement capability to obtain cross-coastline profiles under high-wave conditions. 7. Develop and verify, through field measurements, an opera- tionally useful numerical mode! for refraction/diffraction for general use in complex bathymetry conditions. 8. Improve and design for lower cost production of slope arrays to measure wave momentum in shallow coastal water. 9. Improve models of nonlinear wave and current interaction and wave interaction with the bottom and verify these models through field measurements.

OCR for page 1
6 A medium-to-Iow-priority recommendation identified the exten- sion to deep water, seaward to the continental slope off the West Coast, of seismic pressure sensors, to be better able to quantify tsunami modem and coastal tidal models; this action would unprove tsunami forecasting in several Pacific and Alaskan areas. The committee agrees that measurements are most needed un- der high-energy conditions and close to the interfaces (sea surface, bottom, and structure). Many of the processes of interest are sum ject to numerical modeling, but in many cases the data needed to specify mitial and forcing conditions have been lacking. Many of the processes of interest are nonlinear and interactive and require rela- tively complex physics in simulation models. Advances in computer technology and modeling open the door to modeling some of these processes if the data requirements for the models can be satisfied. III short, recent developments in sensor technology, data teleme- try, recoiling systems and computers offer promise of significant advances ~ the resolution of many serious coastal engineering prom terns in the next decade. To meet the recommendations of this report, a national commitment is called forinvolving government departments and agencies, such as the U.S. Army Corps of Engi- neere and the National Science Foundation, as well as universities aunt industry to develop the necessary instrumentation and mea- surement systems.