Click for next page ( 102


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 101
SEDIMENTATION IN HARBORS J. W. Johnson Introduction The discussion to follow is concerned primarily with the type of sedimentation that normally might be expected to create design, operation, and maintenance problems in harbors, ports, and offshore terminals. The most comprehensive and yet concise coverage of this important sediment problem is that presented by Caldwell.i The types of harbors discussed in his treatment are listed as follows, with some actual harbors given as examples: River-Channel Harbors Baton Rouge, Louisiana St. Louis, Missouri Pittsburgh, Pennsylvania Sacramento, California (old harbor) In such harbors, the fresh river waters keep the clays moving; consequently, the principal sedimentation problem becomes one of sand. Dredging, training walls, and diversion of the river, are the usual corrective measures in such harbors. Off-river harbors These harbors have little difficulty with sand and gravel, but do often have problems with silts and clays. The solution to shoaling is dredging, training walls and dikes, and use of locks or floodgates. Fall-line harbors Troy, New York Washington, D.C. Richmond, Virginia 101

OCR for page 101
102 Sedimentation problems generally result from sand and gravel deposits. Solutions usually consist of dredging, training walls, or the creation of an off-channel harbor. Off-channel harbors in tidal estuaries Washington, D.C., Channel Harbor Houston, Texas Sacramento, California (new harbor) Shoaling is usually due to suspended silt and clay. Improvement is the same as for off-channel river harbors--namely, dredging, training dikes, and use of locks. Shoreline harbors Santa Barbara, California Santa Monica, California Camp Pendleton, Californi The problem at such localities is the deposition of sand moved into the harbor by littoral currents (discussed in the Succeeding section). Maintenance of such harbors usually involves a "and-bypassing operation, as also discussed in another section. Sand Transport by Littoral Currents General Considerations The result of waves breaking at an angle to a shoreline is generation of an alongshore or littoral current. It is this current, combined with the agitating action of the breaking waves, that is the primary factor causing the movement of sand along a coastline. This movement takes place in two manners--in suspension, and by rolling in a zigzag motion along the beach face. For a beach with an equilibrium profile formed by waves of relatively large steepness, which is characteristic of storm conditions, the sediment movement is mainly in suspension. 2 In the came of an equilibrium beach profile formed by wave" of low steepness, which is typical of calm summer conditions, the transport appears to be the result of rolling or skipping along the beach face. It is believed that an much as 80 percent of the material moved by wave action Is moved in the area eboreward of the breaking point.

OCR for page 101
103 Rate of Drift As yet, no general relationship between wave and sediment characteristics is available for estimating the rate of littoral transport that occurs along a given shoreline. A few early laboratory experiments have assisted in defining the important variables. 3/4 Since these early studies, a considerable number of field and laboratory investigations have been conducted. The fundamental mechanics of littoral transport have been summarized recently by Komar and Inmans and Komar. 6 Numerous measurements of rates of transport along natural shorelines have been estimated from the amount of material trapped by man-made shoreline structures. A summary of such measured rates along U.S. coasts, as recently compiled by the Coastal Engineering Research Center, is presented in Table 1. The reader is referred to the Shore Protection Manual for the general procedure in estimating rates of drift for a locality where the wave characteristics are known. Predominant Direction of Littoral Transport The direction of littoral transport at a particular time is dictated by the direction of the alongshore component of the wave velocity at the breaking point (Figure 1~. On many coastlines, important reversals in the direction of littoral drift occur because of the seasonal variation of the direction of wave attack. Usually, however, the intensity of wave attack predominates in one direction, with the resulting in a net or predominant direction of drift. For the locations for which rates of transport are given in Table 1, the predominant direction also is given. Undoubtedly, the drift occurs in one direction along the various coastlines at certain times of the year, and in the opposite direction during the remainder of the year; however, a net drift occurs in the direction and at the rate indicated. For example, along the south Atlantic coast of the United States the littoral drift is northward during the summer season when light winds from the south and southeast prevail, but during the fall and winter, strong northeasterly storms, accompanied by relatively high seas, drive the sand southward. These winter storms are more severe than the summer storms, with the result that the predominant drift is southward along the south Atlantic coast. The determination of the predominant direction of littoral transport has long been a study of interest to the geologist. In many instances, it is necessary to know both the direction of littoral transport at any one time and the predominant direction of littoral transport over a normal climatic cycle. The predominant direction is the more difficult to determine, and may involve locating the position of natural and unnatural littoral barriers and those areas called nodal zones in which the net littoral transport changes direction. In these zones, the net littoral drift is zero, or in other words, the downdrift components of littoral drift are equal to the updrift components. An excellent example in this respect is the coast of New Jersey where

OCR for page 101
104 TABLE 1. LON(:SHORE TRANSPORT RATES FROM U.S, COASTS* PREI)(]MINANT LONGSHORE (a) LOCATION DIRECTION OF TRANSPORT DATE OF TRANSPORT Ccu.yd, /yr, ) RECORD Atlantic Coast Suffolk Coun~cy, N.Y, W 200,,000 1946-55 Sandy Rook, N. J. N 493 000 1885-1933 Sandy Hook, N. J. N 436 000 1933-51 Asbury Park, N.~. N 200,000 1922-25 Shark River, N. J. N 300,000 1947-53 Manasquan, N.J. N 360,000 1930-31 Barneget Inlet, N.J? S 250,000 1939-41 Absecon Inlet, N.J. b) S 400,000 1935-46 Ocean City, N.J. (I) S 400,000 1935-46 Cold Spring Inlet, N. J. S 200,000 -------- Ocean City, Md . S 150 ~ 000 1934-36 Atlantic Beach, N. C. E 29,500 1850~1908 Hillsboro Inlet, Fla. S 75,000 1850-1908 Palm Beach, Fla. S 150~000 1925-30 to 225,000 (:ulf of Mexico Pinellas County, Fla' S 50,000 1922-50 Perdido Pass, Ala. W 200,000 1934-53 Pacific Coast Santa Barbara, Calif. E280,000 1932-51 Oxnard Plain Shore, Calif. S1,000,000 1938-48 Port Huene~e, Calif. S500,000 _-_______ Santa Monica, Calif. S270,000 1936-40 E1 Segundo, Calif . S162,000 1936-40 Redondo Beach, Calif . S30,000 -----a Anaheim Bay, Calif . E150 000 1937-48 Camp Pendleton, Calif. S100 000 1950-52 Great Lakes Milwaukee County, Wis . S 8,000 1894-1912 Racine County, Wis. S 40,000 1912-49 Kenosha, Wis. S 15,000 1872-1909 Ill. State Line to Waukegan S 90,000 -------- Waukegan to Evans ton, Ill. S 57,000 -------- South of Evanston, Ill. S 40,000 ----a Hawaii Waikiki Beach (b) 10,000 aTransport rates are esti - ated net transport rates, Qn In some cases, these approximate the gross transport rates, Qg. bMethod of measurement is by accretion except f or Abeecon Inlet, and Ocean City, New Jersey, and Anaheim Bay, California, by erosion and Waikiki Beach, Hawaii, by suspended load samples. *SOURCE: U.S. Army Corps of Engineers, Shore Protection Manual, Vol. I (Washington , D. C .: Government Printing Off ice , 1973), p . 9 (Table 4-6) . D.ect_ ~ ht - - ret - ~ - .~' ._ Figure 1 . Components of wave ve lo city when waves break at angle to .~hc~rml; no

OCR for page 101
105 extensive study of sand movement by the Corps of Engineers and others has established that a nodal point occurs at Manasquan, New Jersey, with the predominant drift being northward north of this point and southward south of this point. Although the methods used in determining the direction of littoral transport may differ from place to place, determination of the instantaneous and predominant directions of littoral transport and the location of littoral barriers and nodal zones ordinarily is accomplished by consideration of such factors as (a) shore patterns in the vicinity of headlands, (b) the configuration of the banks and beds of inlets and streams, (c) accretion of erosion effects of man-made structures, (d) statistical analysis of wave energy, (e) characteristics of beach and bed materials, and (f) current measurements. Another item of~importance with respect to currents is the confusion that often occurs between the strength of the littoral currents and the strength of the large-scale ocean currents. These latter currents, as measured by the drift of bottles and floating debris, usually are relatively weak as a sand transporting agent compared to the wave-induced littoral current. At localities where these two currents are opposed, the wave-induced littoral current usually is the stronger of the two, and therefore determines the predominant direction of littoral drift. Sediment Transportation, Deposition, and Erosion at Man-Made Littoral Barriers There are three basic types of man-made coastal structures that function as littoral barriers: a dredged channel, a jetty or groin, and an offshore or detached breakwater. 7 The littoral processes in the vicinity of such works are summarized briefly here. Dredged channels. Harbors are often connected with deep water offshore by means of a dredged channel through the littoral zone (Figure 2~. Such a channel creates greater than normal depths with the result that littoral material accumulates therein. Sediment of small enough size to be moved in the deeper depths seaward from the end of the dredged channel would not, of course, be affected. Measurements indicate that most of the longshore transport of material occurs in the vicinity of the breakers where the available wave energy is converted suddenly from an oscillatory motion into the form of turbulence. For that portion of the wave that moves over a dredged channel, however, breaking does not occur, because of the increased depth, and the wave energy passes the normal point of breaking to be spread by refraction and dissipated further inshore. The degree of turbulence, therefore, is insufficient to transport material across the channel and the material accumulates approximately as indicated in Figure 2. To maintain the channel in a navigable condition, this accumulation of littoral material must be dredged periodically. If this material is removed and redeposited on the downcoast side of the channel, normal littoral transport will occur in that region, and the shoreline will

OCR for page 101
106 remain in an equilibrium position. If, however, the channel deposits are placed elsewhere, then the supply of material to the dowocoast beach is reduced and erosion and retreat of the shoreline probably will result (Figure 2~. In a harbor such as shown in Figure 2, the action of the waves is to restore the natural littoral transport of material and thus reduce the area of the entrance to a size compatible with the tidal prism. Tbe equilibrium size of entrance to be expected might be estimated by the relationships between entrance area and tidal prism as given by O'Brien. B Harbors created by shore-connected breakwaters. The effect of a structure that extends seaward from the shore and across the littoral zone is to act as a dam and trap the littoral drift. The impounding capacity is dependent on the height of the structure, the bottom slope, and the equilibrium alinement of the shore in that region. Tbe equilibrium alinement is one which is normal to the resultant littoral forces. Thus, in Figure 3, if the original shoreline was stable with respect to the material balance and a breakwater is constructed as shown, accretion will first occur in the form of a fillet on the upcoast side with an alinement tending toward equilibrium. This will create a deficiency in material supplied to the downcoast shoreline, in which erosion probably will occur with the shoreline also tending toward equilibrium. As the upcoast fillet approaches equilibrium, littoral material will move along the outer face of the breakwater and be deposited in the relatively calm water in the lee of the structure. Thus, the turbulent character of the wave action upcoast from the breakwater tip is sufficient to transport littoral material at capacity. As the waves reach the tip, however, and are refracted and diffracted into the lee of the structure, the turbulence is insufficient to transport the material and deposition occurs. The deposit continues to grow toward the downcoast shoreline, and when it reaches the shoreline the material balance will be re-established on each side of the barrier. The alinement of the harbor deposit depends primarily on the predominant wave direction. A typical example of such a harbor deposit is that at Santa Barbara, California. A variation of a harbor formed by a shore-connected breakwater is the case where two breakwaters must be provided to ensure protection from storm waves that may approach the entrance from various directions. Pronounced reversals in the direction of littoral drift usually occur in such instances. Detached breakwater. This type of structure intercepts the waves and creates a protected area of relatively calm water. Tbe original theory of suab a breakwater location was that the littoral material would move along the coast uninterrupted by the presence of the structure, and consequently, no maintenance problems from sediment deposition would be created. This assumption, however, is in error. The result of the refraction and diffraction of the waves behind the structure is to reduce the energy available for littoral transport in the lee of the structure as compared with the energy available on both the upcoast and downcoast shorelines (Figure 4~. The result of this

OCR for page 101
107 ~ TRIPOLI ARE ~ ~ , 77~71 ~ [',''' I Predict ~ -~d~" ~~~onote;ce I`ttom' currmt ~ cret~ I ~1 L.llorol It .~81't1 0? t" I _ .ovc' .' of "copac'?~- ~ 'Rc r - ~I ~col.~q d - ' ~ ocean ~ clown -~"e ' - ~s ~ ~0.'.9 ! "come d - r's zesty tne,.'ore _ Figure 2. Schematic representation of transporta- tion, deposition, and scour of littoral sediments at channel dredged through littoral zone. ~~ ~_~s d ~ret_. Fir, 'a I \~\ \ \ \ \ Re.uct~ d ~ron..or' - owlet, at cow Scow at ,.~roct~ _ *~ c_' ec - Ace d mot Figure 3. Schematic representation of transpor- tation, deposition, and scour of littoral sediments at shore-connected breakwater. ; _= ~ K~x,,..~. ' ~ ~ O' ~.d - - In, ht - . Ltt~~' tren'.~'-~ _ ~ / tram. ~ 01~, ~ Add. t~.e _ Moo of lo Ivy ~ d ~~ / Item sac-~ _ ~c''~'-~_ B,-~- I / ,, _ \ _ _ _ Figure 4. Schematic representation of transporta- tion, deposition, and scour of littoral sediments at detached breakwater.

OCR for page 101
108 reduction in available energy is that littoral material accumulates in the protected area. If this accumulation of material is not removed periodically by dredging, the accretion eventually may extend completely out to the breakwater in the form of tombola. On the upcoast side of a detached breakwater, the accretion advances beyond the region directly affected by the structure itself, and corresponding erosion occurs on the downcoast side (Figure 47. A typical example of a harbor of this type is that at Santa Monica, California. Sand Bypassing A coastal inlet may be considered, for the purpose of this section, as any relatively narrow waterway connecting the sea or large lake with interior waters. Such inlets, either in their natural state or improved to meet navigation requirements, tend to interrupt the normal littoral transport along the shore. In the case of natural inlets that have a well-clef ined bar formation on the seaward side of the inlet by way of the outer bar, but intermittent, rather than regular, supply reaching the downdrift shore, the result is that the shore downdrift from the inlet is normally unstable for a considerable distance. If the strength of tidal flow through the inlet into the interior body of water is appreciable, part of the available littoral drift is permanently stored in the interior body of water in the form of an inner bar, reducing the supply available to nourish downdrift shores. In the case of migrating inlets, the outer bar normally migrates with the inlet, but the inner bar does not; the inner bar increases in length as the inlet migrates, thus increasing the volume of material inside the inlet. When the natural depth of an inlet is increased by dredging, either through the outer or inner bars or the channel, additional storage area is created to trap the available littoral drift, thereby reducing the quantity that would naturally pass the inlet. If the material dredged (either for opening or for channel maintenance) is deposited beyond the limits of the littoral zone, as in the case of disposal in deep water at sea, the supply to the downdrift shore may be virtually eliminated, with consequent erosion at a rate equivalent to the reduction in supply. The normal method of inlet improvement teas been to provide jetties flanking the inlet channel. Jetties may bave any or all of the following functions: to block the entry of littoral drift into the channel; to serve as training walls to increase the velocity of tidal currents and thereby flush sediments from the channel; to serve as breakwaters to reduce wave action in the channel; and to prevent further inlet migration. In cases where there is no predominant direction of littoral transports, jetties also serve to stabilize the adjoining coastal shores. In the more common cases where littoral drift in one direction predominates, jetties cause accretion of the updrift shore and erosion of the downdrift ebore. Stability of the shore downdrift from inlets, with or without jetties, may be improved by artificial nourishment to make up the

OCR for page 101
Figure 5. Area of littoral drift affected by waves from several directions, Bahia Blanca, Argentina. Figure 6. Sedimentation patterns in Figure 7. Bahia Blanca. Harbors in tidal estuary.

OCR for page 101
110 deficiency in supply. When such nourishment is accomplished by using the available littoral supply from updrift sources, the process Is called sand bypassing. A number of mechanical methods of sand bypassing have been employed; however, this is still a relatively recent engineering development, and additional methods will no doubt be developed as experience is gained. Several teaboiques bave been (and are now) employed for mechanically bypassing littoral materials at inlets. Sometimes a combination of techniques has proved to be the most practicable and economical. The basic methods which have been used are: . Land-based dredging plants, Floating dredges, Mobile land-based vehicles. For details on these methods, the reader is referred to the Coastal Engineering Research Center's Shore Protection Manual, which describes the use of these methods at specific localities. Examples The action of these forces, and their interaction with harbor and port design, can be seen in a particularly challenging area on the coast of Argentina (Figure 5~. The large estuary of Bahia Blanca has no streams of any importance feeding into it: the sources of sediments are the Rio Colorado, the Rio Negro, or both. Notice that whatever the direction from which waves come--south, southeast, west, even to some extent, northeast--littoral drift will occur along the coast, moving material into the entrance of Bahia Blanca. The sink where all this material arrives is shown in Figure 6. The material is principally sand from the large rivers, and from minor beach erosion and small streams up coast. There are no structures at the entrance: the only developments are the buoys. Some harbors within the tidal estuary are dredged back into the mud flats pictured in Figure 7. The problem in this area of tidal flats is almost entirely one of cohesive sediments, or wash flow. With each range in tide (about 15 to 20 feet), the sediments are washed back and forth. About 10 feet of sediments are deposited each year, and must be removed by dredging. Dredging is accomplished by the dredge shown in Figure 8, a museum piece, and barged to the middle of the stream where they are dumped, most likely to return with the next tidal range. The pier in the harbor pictured in Figure 9 projects into the tidal stream. Notice the steeply banked channels. The high particle velocity in these channels creates turbulence in flow through the pier: the particles collide and settle faster than they would otherwise. The grain elevators shown in Figure 10 store the principal export, grain from the pampas. The principal import is oil for the surrounding area.

OCR for page 101
111 ~b ~ : ~ 3 - l - - ~ l i ~ - - - ~ - - ''skiddy ~ id ~ 3 : . lo;. ~ Cal ~.. .. ; ~ ~ , _- Figure 10. Pier facilities for grain exports. . . ~- Figure 8. Dredge used to remove sedi- Figure 9. Pier projecting meets in channels and harbors. stream. into tidal wit . _ _ _ 1 ~ . . 9 Figure 11. Pier built parallel to tidal stream. am_ _e Hi_ ~ Figure 12. Mud flats. ~) ,\

OCR for page 101
112 Downstream from this pier is another (Figure 11) that offers a useful contrast. Notice that the pier is parallel to the flow of the tidal stream, and almost sediment-free. The depth is always 23 to 30 feet along the pier face. A small-craft harbor for tugs and boats is also illustrated in Figure 11, to the left. Some sedimentation can be seen at the entrance, a dead-water area. The principal tidal currents keep the main channels open, but the accumulations of fine materials eventually become cohesive sediments at the entrance to the tug harbor, which is somewhat restricted. The flow of sediments from the mud flats (Figure 12), owing to the high tide range, cuts extremely steep, sloped channels, and the fine sediments are continuously entrained and redeposited. References 2. 1. Caldwell, J. M., "Sedimentation in Harbors, n Chap. 16, Applied Sedimentation, P. D. Trask, ed. (New York: John Wiley and Sons, Ince ~ 1950) ~ ppe 290 - 291. Saville, Te ~ Jr., n Model Study of Sand Transport Along an Infinitely Long, Straight Beach,. Transactions of the American Geophysical Union, 31 (Aug. 1950~: 555. 3. Krumbein, W. C., "Currents and Sand Movement in a Model Beach," Beach Erosion Board, Techn . Memo. No. 7, U.S. Army, Washington , D. C., 1944 . Saville, op. c it . Komar, P. D. and D. I. Inman. "Lc~nashore Sand qYransnort on 4. 5. 7. Clt. . and D. I. Inman, "Longshore Sand Transport on Beaches," Journal of Geophysical Research, 75 (Oct. 1970~: 5914. 6. Komar, P. D., "The Mechanics of Sand Transport on Beaches, Journal of Geophysical Research, _ (Jan. 1971~: 713. Johnson, J. W., The Littoral Drift Problem at Shoreline Harbors, n Transactions of the American Society of Civil Engineers, 124 (1959~: 525. O'Brien, M. P., "Equilibrium Flow Areas and Tidal Inlets on Sandy Coasts," Journal of the Waterways and Harbors Div., American Society of Civil Engineers, Feb. 1969, p. 43. DISCUSSION KRAY: My question is in reference to the wharf or pier which is free from sediments in Figure 11. What is the washout rate ahead of that pier? The sediments consist primarily of the cohesive soils, and ~ presume that washout and movement are very considerable along that face. JOHNSON: You mean the scour? As far as I know, there are no problems. The penetration of the pile is such that the lower portion of the pile is never exposed.

OCR for page 101
113 KRAY: It doesn't extend too deep? JOHNSON: No. KRAY: Do you know by any chance what the foundation i" of that particular pier? JOHNSON: It is piles. KRAY: Steel sheet piles? JOHNSON: I am not sure of that. KRAY: So there is a solid wall, at any rate? JOHNSON: Right. The solid wall is preferable. It is a smooth wall and you don't get the high turbulence which Is conducive to coalescence and deposition of the material. RIEDEL: I would like to add a cautionary note to your suggestion that you won't have deposition along the base of the pier which is parallel to the main channel. I use as illustration what I called down in Vicksburg a couple of days ago a parking lot problem, sometimes referred to as a marine transportation problem, and it is. A ship is turning around in an anchorage, experiences some failures and runs into the ship which is tied up at the base of a pier very similar to the pier you spoke of, and the ensuing fire closes the port. I would suggest that we have to be careful about the best solution for ease of maintenance; for example, docking along the base of a stream. Sometimes we must counsel ourselves on the safety problems as well as the advantages for maintenance of various solutions. ~ don't think we want to be moved too far in any one direction without a rationalization of all elements. JOHNSON: Your point is well taken. In the particular case you refer to, there isn't any parking area in that main stream, and if you dredge a parking area from the mud flats, it will eventually fill up; nonetheless, T agree with your point. BERTSCHE: Would knowledge of the whole hydraulic water flow of that area at the design stage help you in solving some of the sedimentation problem" that occur? You pointed out the one flat that was essentially draining into the docking basin at the side. That is pretty obvious, perhaps it could be predicted by look ing at the chart, but in more subtle cases , would a f ull , tbree-dimensional hydraulic model--either mathematical or full scale--aid in looking at the sedimentation, or is tbat part of the problem with the design process? JOHNSON: Frankly, I don't think enough data exist for a remote area like that to build a model. The model can only be as good

OCR for page 101
114 as the prototype data. A new pier and expansion of this port have been proposed: tbe pier would be parallel to the stream along the mud bank. BERTSCHE: Let me pursue this point. Would the sedimentation include a sand drop at the entrance? You were talking about transport down the coast. I would assume that as soon as it hits the breakwaters and jetties, it creates a problem. JOHNSON: There are no breakwaters or jetties at that particular port, but at others, material does accumulate against the jetties. In my opinion, the rate in difficult to estimate unless there is a record of experience at nearby harbors. Santa Barbara, for instance, has a long period of record: in that vicinity, 250,000 to 280,000 cubic yards per year seems a reasonable expectation. I think Bob Dean has worked at the Channel Islands Harbor further down the coast. What was the annual rate you estimated, and that dredging records show for that area? DEAN: About a million cubic yards. between ports. JOHNSON: So, there is guise a difference in a short distance SAVILLE: The case of Channel Islands Harbor is interesting. The original design was based on Santa Barbara, and then upgraded to about 700,000 to a million cubic yards per year. Predicting sedimentation rates from past experience is a good practice, but you need present experience, too. JOHNSON: That i. correct. Between Santa Barbara and the Channel Islands Harbor, for example, is the Santa Clara River, which can get out of hand about every 25 to 30 years, suddenly dumping a huge amount of material just up the coast from the Channel Islands Harbor. HERBICH: You mentioned some equations in the Shore Protection Manual that allow one to make estimates of Sediment transport. Other , equations have become available since that manual was published. What, in your opinion, is the accuracy of any estimate of sediment transport? Is it plus or minus 50 percent? JOHNSON: It can be as much as 200 percent, and that is the basis of my concern.