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2 Coastal Engineering Applications The coastal engineer operates in a dynamic, intricate, and mul- tifaceted environment. Application of coastal engineering knowledge to the solution of problems is complicated by a host of physical and environmental factors. For example, in order to design and build a structure, engineers need a firm understanding of coastal ocean motions, sedimentation rates, stresses of the wave and water motion, and other forces on the shoreline and on the structure. Further com- plications result from the great range of ocean movement over space and time. Extreme events like breaking storm waves, storm surges, tides, and tsunamis add to the already complex nature of the coastal engineering discipline. To perform the job properly requires detailed and accurate information on the conditions under which a structure must perform and survive. Measurements and measurement systems are required to determine the range of influence, strength, and timing of the forces of nature in the coastal zone. For this study, the committee first identified those issues or problem areas recognized as important to the engineering commu- nity; some issues are more urgent and must be given greater priority. Then it was necessary to identify and evaluate the state of knowI- edge of coastal processes related to each engineering issue. This evaluation in turn considered the state of theoretical development and analytical and numerical modeling in each coastal process area. 19
20 The latter consideration was necessary because modeling plays such an important role in coastal engineering. ENGINEERING APPLICATION AREAS The committee identified four major problem areas where coastal engineering skills must be applied. (See Figure ~l, adapted from the Corps of Engineers' Shore Protection Manual, tI984~.) The four engineering problem areas are identified simply as shorelines, backshoresi, entrances (or inlets), and harbors. These broad areas, either exclusively or in combination, encompass most coastal engi- neering problems, as defined by the U.S. Army Corps of Engineers. Coastal Process Categories Taken in the broadest sense, four categories of coastal processes act on the coastal areas identified in Figure 2-~. Two of these cate- gories may be considered primary and the other two, interactive. Primary processes consist of: . kinematics2 and dynamics3 of high-frequency coastal water motions (periods of 0.~-5 minutes) and . kinematics and dynamics of low-frequency coastal water mm tions (periods greater than 5 minutes). Interactive processes consist of: fluid/sediment interaction and . fluid/structure interaction. in general, the short-period, high-frequency phenomena are re- lated to wind-wave generated water motions; long-wave period, low- frequency phenomena are generated, for example, by pressure effect, tidal motions, and such catastrophic events an slides, slumps, or earthquakes. The categories were specified because of the need to 1~Backshore~ is a general term referring to the area above and behind the normally active beach face. The backshore is typically affected only during storms or extreme high tides. 2aKinematics" refers to the motions of a fluid element, in this case the trajectory of a small patch of water. 3 aDynamicsn refers to the eRects of external forces on fluid motion (effects on kinematics).
21 CLASSI Fl CATION OF COASTAL ENGINEERING PROBLEMS SHORE ~ | BACKSHORE l l INLET I STABILIZATION I I PROTECTION | | STABILIZATION | SEAWALL l l SEA!ALL | | DREDGING l . . . . . | BULKHEAD l l PROTECTIVE LEACH l ~ | ( WITH OR WITHOUT ~tSfORAT10. ) | I REYETHENT I . ~ ' ' I SAND DUNE I . . BEACH NOURISHMENT WITH OR WITHOUT REStORaT10N ) | REVET~ENr I | DETACHED I ~ BULKHEAD I BREAKWATERS | ' I | GROINS l SAND BrpAsslN6 AT INLET CONSIDERATIONS: Hydraulics Sedimentation Control Structure Maintenanca Legal Requirements Environment Economics ' - JETTIES | HARBOR PROT ECT I ON 1 r JETTIES | . . r SHORE-CONNECTED BREAKWATER NAVIGATION | | Of FSHORE CONSIDERATIONS: I BREAKWATER . CONSIDERATIONS: Hydraulics Sedimentation Control StrucnJre Maintenance L~l R - u~rcmcnts Environment Economics Hymen - Sedimenmion N - Simon Control Structure bbint~nce Legal R - uir~na Environment Economics BAY CIRCULATION | - CONSIDERATIONS: Hydraulics Sedimenmion Control Structure Legal Requir~nn Environment Economics FIGURE 2-1 General classification of coastal engineering problems. CONSIDERATIONS: Hydraulics Sed i men u t ion Navigation Control Structure Main~n~ Legal Requir~nu Environment Economics recognize spatial and temporal differences in the coastal measure- ment systems and instrumentation required for each category. Coastal Features and Coastal Engineering Applications Each of the four general coastal features previously categorized in Figure 2-! and illustrated in Figure 2-2 (shoreline or shore, back- shore, entrance inlet, and harbor) presents a distinct set of problems for instrumentation and measurement system development. Some measurement problems cross two or more features. The fodow~ng paragraphs provide a brief overview of each feature or area of en- gineering application and emphasize a few perceived measurement goals. Shore Shore stabilization is a primary engineering goal along large sec- tions of the U.S. coastline. Achieving this goal requires understand- ing the behavior of the shore or shoreline and effects adjacent to both hard engineering structures (e.g., seawalIs, revetments, groins) and
22 Shore and 3ackshore Extreme High Water (EHW) Mean High Water (MHW)- Mean Low Water (MLW)- ~ Shore _ Back-_I Coast , shore - - A 1/ Barrier Island `~~~ Harbor with Breakwater Entrances Ocean Swells ; ~ Shoal \ . :j Or '''a :] - Harbor Tiptoe toenail Breakwater _ Jetties I I \: \ \ ~ Channel N\\\~,~ ~ \\ \ \ \ FIGURE 2-2 Illustrative examples of coastal engineering problems. B C D soft structures (e.g., beach nourishment). Basic to the knowledge required is an understanding of shore response to wave action and currents. Because present modeling capability to predict shoreline response is inadequate, measurements of the sediment transport rate, concentration, and distribution are necessary in both longshore and cross-shore directions. It is important to make these measurements under conditions of moderate-to-high-wave energy. Likewise, the ability to rapidly and accurately measure beach and nearshore profile changes under a broad range of wave-energy conditions is essential to verification of prediction models. An essential consideration for all of
23 these measurements is the ability to carry them out successfully un- der high-energy episodic conditions associated with storms, because these energetic conditions result ~ maximum sediment transport. The need for understanding shore processes ~ illustrated by the erosion of the shore at the Cape Hatteras Lighthouse on the Outer Banks of North Carolina. The 110-year-old historic tower is endangered by changes in the shoreline that have brought the high- water line to within 100 feet of the structure. A series of studies has sought to ascertain the reasons for past variations in the position of the shore, ~ order to forecast the future. But the basic understanding of shore processes Is inadequate for reliable prediction of the rate of shoreline change during high-energy wave conditions (NRC, 1987~. Another factor in shoreline processes is relative sea-level change. Rising relative sea level exerts an inexorable pressure on most sections of the worId's shoreline (but not all). For instance, many high latitude shoreline segments ~ Canada and Europe are emergent, contrasting with the U.S. shoreline which is primarily submergent. Whereas past relative sea~leve! rise ~ the United States has averaged about 30 cm per 100 years, this value exhibits considerable spatial variability. Relative sea-level rise presents a future challenge to coastal en- gineers, but it is one that can be anticipated in engineering planning (NRC, 1987~. If projections of increases in relative sea-level rise are correct, engineering projects designed for 2~50 year time scales will have to incorporate rising sea levels more directly into their design phase. Meanwhile, existing facilities and structures will have to be shored up to account for this long-ignored factor in the design equa- tion. Nevertheless, unpredictable changes caused by coastal storms and hurricanes pose a greater concern than does the effect of sea-level rise. Backshores Much like shore stabilization, backshore protection requires an understanding of processes that vary in nature and importance from one location to another. Knowledge of dune, bluff, and beach re- sponse to extreme wind and wave events is essential to this under- standing. Measurements of runup4 and setups under high-energy 4"Runup~ is the travel of waves up the face of the beach above the still water level. 5 "Setups refers to a general local increase in sea level caused by the momentum of breaking waves.
24 wave conditions and a knowledge of storm surges histories are nec- essary. Measurement problems involving immersion, burial, and ex- posure of sensors may be more pronounced and problematic ~ these locations. Unusually high lake levels along many shores of the Great Lakes during the m~-19BOs provided an example of where backshore pro- tection was paramount. When lake levels rise, backshores are likely to suffer damage from wave and current erosion during storms, em pecially when low-pressure systems combined with wind setup ac- centuate the already high water levels. Then, severe bluff and beach damage often result in significant environmental impact and prom erty damage. The ability to measure wave direction and runup would support more reliable predictions of areas of greatest imp act and how to safeguard them. Entrances Entrances include both natural inlets and constructed harbor mouths and channels. Stabilization of entrances is a primary engi- neering goal in certain natural and almost all constructed channeb. The annual cost of maintenance dredging of inlets and harbors by the Corps of Engineers alone is rapidly approaching $400 million. A major measurement problem related to maintenance drecig~g of inlets and channels Is the determination of transport and deposition of sediment during high-energy wind and wave events that frequently close navigation passages. This fact reinforces the need for measure meets of sediment transport and concentration during high-energy tidal flows. A good example of this problem is the entrance of the Columbia River leading to the major ports of Portland, Oregon, and Vancou- ver, Washington. At this entrance, a large curving sandbar often produces serious depth restrictions to the passage of ships as Pacific swells interact with strong river currents. The severity of navigation problems requires a specialized pilot for bar passage, separate from the river navigation pilot. Extensive studies of the pitching motion of ships crossing that bar under varying wave, tide, and current con- ditions (Wang and Noble, 1982) have verified the critical need to predict the movement of the shifting bar in order to avoid grounding or broaching. 6Storm surge is increased sea level over a broad area caused by wind forcing.
25 Systems for diverting sand are being constructed to keep en- trances open and to maintain sand nourishment to downdrift beaches. These systems require prediction of sand transport volumes that are dependent on local wave and current conditions. Presently, the use of sand diversion systems is severely limited by inadequate capability to predict sediment transport, largely owing to the lack of coastal engineering measurement systems. Harbors Design of safe, elective harbors with low operation and mainte- nance costs is another primary coastal engineering goal. Essential to achievement of this goal is an understanding of the stability of break- waters formed from mounds of rock, the failure of concrete elements used to increase this stability, the leakage of wave energy through the breakwater, and scouring away by the waves of the sediments that form the breakwater's foundation. The cost of these struc- tures is very large. Therefore, there is a strong economic pressure to improve prediction capabilities, thereby eliminating over-design. Measurement of wave forces on and within breakwater structures is required, as weD as measurement of wave and current forces adja- cent to and along the breakwaters. This is a particularly complex area of engineering, where theoretical development is sparse and em- pirical determinations are often based on indirect relations between wave forcing and structural response. Few measurements have been made of the forces and structural interactions on actual structures. Only recently have measurements been undertaken on the external structural elements. To the best of our knowledge no measurements intern e] to the structures have been made. This is an engineering area that requires development of specialized measurement systems. A welI-known example of this need is the failure of the harbor at Sines, Portugal. A massive breakwater constructed on the Atiantic coast was designed to provide a vast port and industrial complex. Be- fore the structure was completed, a period of violent Atiantic storms produced waves that severely damaged the breakwater, destroying much of the capwall and roadway and preventing completion of ship berths planned for the lee side. Extensive investigation of the wave conditions that led to the Sines failure did not lead to a consen- sus judgment; rather, it resulted in 13 different opinions as to the principal cause of the breakwater damage.
