Construction Of Jetties Research Articles

CHANGES DUE TO JETTIES AT TILLAMOOK BAY, OREGON

Bayocean Spit, separating Tillamook Bay from the Pacific Ocean on the north Oregon coast, underwent severe erosion following construction of a north jetty at the bay entrance in 1914-17. This erosion ultimately led to the complete breaching of the spit in 1952. Simultaneous to the spit erosion south of the entrance, the shoreline north of the north jetty advanced seaward by some 600 m (2000 ft). This pattern of erosion and deposition following jetty construction has generally been interpreted as the jetty blocking a large north to south net littoral drift in the area, estimated by a previous study at 620,000 m3/yr (800,000 yd3/yr). Our reexamination of the shoreline changes and patterns of erosion and deposition following jetty construction disagrees with this interpretation, and instead we conclude that all of the changes resulted from local rearrangements of the beach due to the disrupted equilibrium following jetty construction, but at the same time maintaining an overall condition of zero net littoral drift. This interpretation is supported by other evidence that indicates a near-zero net drift on this portion of the Oregon coast. Thus severe coastal erosion can result from jetty construction even in areas of zero net littoral drift. A new south jetty has been recently completed (1974). The result has been further realignments of the shoreline with accretion and shore-: line advance immediately south of the south jetty. This provides further confirmation that a zero net littoral drift exists in the area. This study also demonstrates the effects of building only a single jetty rather than a pair of jetties. Following construction of the north jetty, the outer bar or ebb-tide delta at the Tillamook Bay inlet grew appreciably in size.

DESIGN AND CONSTRUCTION OF HUMBOLDT JETTIES, 1880 to 1975

In the Chief of Engineers annual report of 1877, it was concluded that "The only way...in which a safe entrance could be obtained into this harbor would be by the construction of two parallel jetties, of very heavy stone, about 500 yards apart, from the north and south spits at the entrance." In 1882, a special Board of Engineers concluded that the "occupation of the south breaker spit by a structure...carried to low water and running from the south head (spit) in a north-westerly direction..." be built. Construction of the south jetty began in 1889 and of the north jetty, subsequently authorized, in 1891. Due to the severe wave action, a number of rubble-mound construction techniques including stone, concrete cubes, tetrahedrons, and finally dolos armor units have been used. A description will be given of the construction and associated results. Experience with the reinforced and unreinforced concrete dolosse units will also be discussed.

Grand Isle Barrier Island, Louisiana--Human Activity in Natural Dynamic System: ABSTRACT

Grand Isle, Louisiana, is a recently formed barrier End_Page 1692------------------------------ island developed by the marine reworking of the seaward edge of the Lafourche delta, the last major deltaic deposit laid down by the Mississippi River prior to the development of its present delta. Marine reworking of the deltaic front provides sediment which is moved northeasterly and northwesterly away from the deltaic front by longshore currents to form barrier spits and chains of barrier islands, such as Isles Dernieres on the west and Grand Isle-Grand Terre Islands. Grand Isle became a subaerial deposit about 700 years ago and began to lengthen and grow southeasterly by ridge accretion. Its shape and location are controlled primarily by interaction of the longshore currents in the surf zone and the tidal-pass currents that interrupt them. Changes in the tidal channels, such as position or depth, drastically affect the action of the longshore currents. This has been dramatically shown at Grand Isle during 1970. High winds in the summer of 1970 and Hurricane Camille in 1969 deepened and changed the location of the tidal channels northeast and southwest of the island. As a result, accelerated erosion at the island's southwest end has removed several acres of a state park, moved the Gulf shoreline inland 300 ft, and caused the loss or removal of seve al beachfront homes. The causeway to the island and a large marina are threatened. Government money has been made available for jetty construction and for eventual sand-pumping operations to reduce erosion and restore the beach. Destruction of the Lafourche delta and construction of the ephemeral and migrating barrier islands on its flanks provided sites attractive for development by man. The desire for permanency of the developed sites is in conflict with the natural dynamic shoreline processes. End_of_Article - Last_Page 1693------------

VARIATION OF TOPOGRAPHY OF SEA-BED CAUSED BY THE CONSTRUCTION OF BREAKWATERS

In Japan, many breakwaters or jetties have been constructed in the sandy beach from the past decade for new ports to cope with the development of industry It is needless to say that the construction and prolongation of breakwaters or jetties cause the change of bottom topography in their vicinity, but many points remain indistinct on this change of bottom-topography In this paper, some general properties on the change of the topography of sea-bed caused by the construction of breakwaters are discussed on the basis of the hydraulic sounding maps of several ports and the results of model tests The terminology used in this paper is given in Figure 1.

DESIGN OF A SMALL TIDAL INLET

Estero Punta Banda on the Pacific Coast of Baja California was to be improved and stabilized as a small boat harbor by the construction of twin jetties. The throat area-tidal prism relationship at this entrance agreed with that of other inlets on sandy coasts in equilibrium. This throat area was retained in the jetty spacing. The major question to be resolved was whether to retain the entrance in its present position at the north end of the lagoon or to cut a new entrance at the south end. The former position was chosen.

”Placed-Stone” Jetty, Stone Weight Coefficients

Hydraulic model tests and some field experience show that placing of armor stone in rubble-mound jetties with the long axis of each stone normal to the surface of the jetty will increase the life and effectiveness of the jetty. The model study demonstrated that when two layers of stones are placed with the long axes perpendicular to the length of the jetty and tilted slightly downward toward the center of the rubble mound, stone-weight coefficients of 5.7 and 6.7 could be obtained with breaking waves and non-breaking waves, respectively. These coefficients compare to 2.8 and 3.5 for similarly rough stones dropped by the pell-mell method. Because the size of stone necessary to withstand the design wave varies inversely with the stone-weight coefficient, the placed-stone method of jetty construction requires stone approximately one half the size required for pell-mell construction.

Rompeolas en el estuario del río Adour, Bayona

The Bayonne harbour (France) has not been very important until recent years. The industries of this zone, as well as the exploitation of local natural resources, such as sulphur and gas, have been steadily increasing, and it is estimated that the total shipping activity in Bayonne during 1963 will amount to two million tons. The harbour is located along the mouth of the river Adour, which flows out into the Atlantic, along a section of the coastline which is sandy. Hence it is frequent, during winter time, for sand to heap up near the harbour. This noi only increases the wave formation, but becomes a hazard for the shipping traffic. To keep the harbour open it becomes necessary to dredge the entry to the harbour, which is affected by the heavy seas. This in turn also tends to make dredging operations difficult. These are some of the reasons why, after long and careful studies and work on models, a project has been developed for the improvement of the harbour entrance, to make it suitable for use by large ships during most times of the year. To carry out this project, credits have been approved, amounting to 30 million NF. Briefly, the project involves the construction of a stone jetty, 850 m long, along the northern part of the river mouth, forming a circular arch, of 1000 m radius. As the sea and river balance will be altered by the presence of this dyke, it is also planned to protect the nearby coast, to impede its erosion by the sea.

Studies of a Channel Through Padre Island, Texas

Construction of 125 ft wide 12 ft deep navigation canal through Padre Island and Laguna Madre to Port Mansfield; results of investigations on effects of disturbance of natural equilibrium on Gulf shore by construction of jetties and channel; requirements for design and reconstruction of suitable jettied navigation channel through inlet; problems of tidal hydraulics encountered in design of artificial inlet.

Interim Consideration of the Columbia River Entrance

Providing for entrance channel of suitable alignment at mouth of Columbia River; channel would be obtained by dredging and construction of spur jetty on north shore, in combination with existing jetties, at additional cost of $10,- 340,000; re-evaluation of natural forces controlling regimen of entrance area.

BREAKWATER CONSTRUCTION

Upon receipt of an invitation to participate in this conference and present a paper on the subject of Breakwater Construction, I was happy to accept, feeling that I would benefit to a far greater degree than my contribution to a subject in which I, and the concern which I represent, have had a deep interest for many years. Our experience embraces some of the major jetty and sea wall construction in San Francisco Bay, viz: Treasure Island, Mare Island, Alameda Naval Air Station, Sacramento and San Joaquin River bank protection and other jetties along the Northern California Coast, including Crescent City. This paper is limited to breakwater construction on the basis of our experience previously mentioned. The subject has been divided into two parts, namely, (1) construction of jetties, or breakwaters which can be built by use of floating equipment and (2) construction of jetties which are not practicable to be built by use of floating equipment.

DESIGN AND CONSTRUCTION OF JETTIES

The purpose of this paper is to present a brief outline of the general engineering procedure for the siting and design of jetties and the methods of constructing such structures. After a general presentation of the formulae proposed by various engineers to determine the size and weight of individual pieces of stone or other material which should be used under various wave heights, this paper will be devoted principally to the construction of rubble stone jetties. This is the type principally used on the Pacific Coast of the United States.

DESIGN OF BREAKWATERS

As the name implies, a breakwater is a barrier constructed to break up and disperse heavy seas, to shield the interior waters of a harbor from winds and waves, and to provide shelter and protection for ships, shipping facilities, and other harbor improvements. Breakwaters are structures used to improve a naturally protected (sheltered) harbor or to create a sheltered harbor at locations required for shipping, refuge, recreation, etc. Breakwaters may be roughly divided into two main groups, the vertical-wall type and the rubble-mound type. A possible third group, the composite type, consists of the wall-type placed upon a rubble-mound foundation. Since the experience of the San Francisco District, Corps of Engineers, has been limited to the construction of rubble-mound breakwaters and jetties in as much as practically all breakwaters on the Pacific Coast are of rubble-mound construction, the second half of this paper has been limited to the consideration of this type of structure. The first half of the paper discusses general subjects (choice of location and type of breakwater, etc.) relevant to both types. Until recently, the design and construction of breakwaters was largely an empirical "art" based mainly on the designer's observations of the performance of previously constructed breakwaters. Great latitude was given personal discretion and judgment, since those factors which might influence or standardize design were little understood.

Stream Improvements in Michigan

When war-time curtailments of labor and material forced a retrenchment in Michiganˈs stream-improvement program, greater attention was given to design and testing of new materials and methods applicable to specific improvement problems. In a uniform-bottomed artificial drain, fishing was improved by installation of 2-foot-square concrete blocks, cast at the site at a cost of $2.50 per block. In 5 years, there has been no maintenance cost. In a sand- and rubble-bottomed stream, deflectors were installed made of Wakefield sheet piling jetted in with a portable pump, and so designed as to escape damage from trash, floating ice, or flooding. The cost was $23.05 per unit, and no maintenance has been required after 1 year of operation. High, eroding, sand banks are being protected by planting vegetation and by construction of short wing jetties of Wakefield sheet piling. New fishing areas have been created by restoring flow through old oxbows and bayous by the use of diversion dams, and by creation of small ponds on minor spring- fed streams. For 200 earth-filled log crib deflectors, original cost was $17.45 per structure and maintenance cost over a 4-year period was $1.38 per structure per year. Experiments are under way to determine the feasibility of lowering stream temperatures by diverting dam overflows directly into the ground water table.

Platinum conditions

This paper analyzes the physical factors controlling the recent morphology of major deltas along the Pacific and Caribbean coasts of Colombia. The study considers the fluvial, coastal, and oceanographic contributions to changes in delta morphology and uses different approaches, including (1) remote sensing techniques; (2) time series analysis of river discharge, sea level, wave climate and tidal variability; (3) analysis of the relationship between monthly mean sea level anomalies near the deltas related to the El Niño Southern Oscillation (ENSO); (4) development of a database of key physical variables; (5) series of correlation calculations to examine which environmental factors control delta morphology; (6) analysis of shoreline changes for the 1986–2000 yr-period; and (7) classification of each delta system based on the relationship between water and sediment discharges and wave and tidal energies. Overall, Colombian deltas are built under destructive physical conditions. The Pacific deltas, San Juan, Mira, and Patía, are tide-influenced deltas although they exhibit definite characteristics of wave-dominated systems such as the presence of barriers and beach ridges. Also, these deltas exhibit the highest marine energy conditions of all Colombian deltas (marine power values between 9.1 and 11.6) due to the interplay of (1) moderate wave conditions as a result of the effect of swells from the SW with a significant height varying from 1.7 in the San Juan delta to 3.0 m and 3.1 m in the Mira and Patía deltas, respectively; (2) meso-tidal ranges; (3) steep subaqueous profiles; (4) low attenuation indexes of deep-water waves; and (5) strong oceanographic manifestations associated with the ENSO, causing regional sea level rises of 20–44 cm during El Niño events. The Caribbean deltas, Magdalena, Sinú and to a lesser extent, the Atrato, are wave-influenced deltas. The Magdalena, with deep and nearshore wave power values of 45 × 106 erg s−1 and 35 × 106 erg s−1, respectively, and the lowest attenuation index of deep-water waves, is the most wave-influenced delta of Colombia. Statistical relationships show that the area of Colombian deltas is best predicted from average discharge and bank-full width of river entering the delta. The number of distributary channels is explained by the marine power index and the gradient of the delta plain. The average and total width of distributary channels are largely controlled by the tidal range. Further analysis of shoreline changes indicate that the retreating behavior of the western part of the Magdalena delta has been due to anthropogenic causes, including the construction of jetties in 1936. By contrast, processes of rapid erosion in the San Juan delta have been the result of 2.6 mm yr−1 long-term relative sea level rise due to tectonic induced subsidence coupled with a eustatic rise of sea level. Overall, the other deltas are experiencing prograding phases with rates of accretion of 100 m yr−1 in the artificial delta of the Canal del Dique.

Origin of the Gulf Stream and Circulation of Waters in the Gulf of Mexico, with Special Reference to the Effect on Jetty Construction

The origin of the Gulf Stream and the circulation in the Gulf of Mexico have never been satisfactorily settled. Many theories have been advanced from time to time, but none as yet have been established upon acceptable proof. The commonly received theory of the present day, that these currents flow in from the South Atlantic, pass the north shore of Yucatan, follow the coast line of Mexico and Texas in a north-northeasterly direction, and finally escape through the Straits of Florida, appears to be plainly contradicted by evidence gathered from the various surveys of the past, together with recent discoveries in connection with the several deep-water projects along the coast of Texas.