WO1996006230A1

WO1996006230A1 - A system for protecting harbors against sedimentation and for nourishing shores

Info

Publication number: WO1996006230A1
Application number: PCT/GR1995/000014
Authority: WO
Inventors: Constantine L. Goudas
Original assignee: Goudas Constantine L
Priority date: 1994-08-23
Filing date: 1995-08-22
Publication date: 1996-02-29
Also published as: ITRM950575A1; IT1277916B1; ITRM950575A0; EP0760036A1; GR1001915B

Abstract

The system for protecting harbors against sedimentation and for nourishing of shores consists of one or more seabed groins (6-8) or rectilinear, polygonal, or curvilinear trace and about elliptical cross section of 1/3 ratio between minor over major axis, each constructed by one-piece of impermeable by water geotextile material, of suitable width, ranging from 1m to 10m, and length, ranging from 20m to 400m, the said geotextile material being transformed by heat welding treatment into a watertight tube and then placed at fully immersed position at the siste of application, suitably tensioned to receive the desired longitudinal shape and finallly, filled in situ by a special mix of cast concrete that secures automatic advancement within the said watertight tube through gravity and hydrostatic forces, and filling of the entire tube, thus, upon hardening, providing the said with the design cross sections along all the length of the tube. Each application of the system to a port for protection of its basin against sedimentation, or shore for nourishment, is preceded by an oceanographic site investigation and a system design phase. The data to be collected refer to site seabed formation and material constitution, resultant current velocity field, and wave-wind climate. The said design involves analysis of the site investigation data and detail definition of the suitable system for the said site. In particular, the said design defines the number of seabed groins to be used, the shape and direction of each groin, and the total length and cross section geometry at steps of suitable length of each groin.

Description

A SYSTEM FOR PROTECTING HARBORS AGAINST SEDIMENTATION AND FOR NOURISHING SHORES

INTRODUCTION

Harbor sedimentation is the result of the translational effect of water cur¬ rents upon the sand deposits of the seabed of the immediate outside the en¬ trance channel area The same currents are responsible for the erosion of beaches, the incoherent materials of which they displace and deposit in deep- er parts of the seabed, thus causing recession of the shoreline.

Currents appear in all large water concentrations and are the result of known and extensively studied causes, such as the tidal action of the Moon and Sun, the variances in salinity and/or temperature, the action of the winds and the differences in atmospheric pressure.

As regards the frequent phenomenon of harbor basin sand accretion, the impact of currents at small angles upon the harbor breakwaters and their sub¬ sequent deflection by the said breakwater leads to the formation of stream- lines that are directed to the basin through the narrow entrance channel. Upon entering the said channel they pick up speed and part of their increased mo¬ mentum is transferred to the loose materials of the seabed, which thus are transported and deposited in the harbor basin. As a result the basin seabed is elevated, the water clearance is decreased and the need for costly mainte- nance works becomes inevitable.

As regards coastal erosion, the phenomenon is realized by the impact at small angles of currents upon and their deflection by the beach. During their deflected course, the currents transfer to the loose nearshore materials, sand grains and gravel, part of its momentum, thus enforcing displacement of these materials toward deeper parts of the seabed. This displacement process is fa¬ cilitated by the favorable negative seabed slope and the waking effect of the waves. Displacement of larger size rocks, often of many kg^'s or even tons, can also occur due to the gradual undermining of their equilibrium by removal of the supporting substratal loose materials and at some instance the sudden ap¬ plication of overturn torque by combined wave and current action and the synergy of the favorable negative seabed slope.

The methods for facing harbor sedimentation vary but they are always successful. Among them, the periodic, often annual, maintenance by mechani¬ cal removal of the latest deposits of sand, fauna, etc., and the construction of remedial external piers, at appropriate locations for diverting seabed currents toward directions other than that of the harbor entrance channel, are shown to be effective.

However, the frequency and cost of the former method, and the high cost and environmental impacts of the second, constitute serious disadvan¬ tages that make necessary and justify the development of better methods against harbor sedimentation Indeed, the need for periodic excavation of the basin of inefficiently designed harbors, in order to preserve the necessary water clearance, is a constant concern of port authorities in many countries. The construction of additional breakwater walls for the protection of har¬ bors against sand accretion, besides its high cost, constitutes a non-acceptable intervention for the environment. Building, for example, a side breakwater long enough to intercept and cut the current regime intruding into the harbor basin, will create harbor basin conditions, Le, permanently polluted water and seabed, in the sea area between the existing harbor wall and the new breakwater. Good example in the case of Carlovasi harbor, in Samos island, Greece, the sedimentation of which was faced by building an additional lOOm wall normal¬ ly to the shoreline at about 2OOm from the harbor entrance channel Thus, 2OOm of beach was condemned to permanent pollution.

The protection of a shore against the erosive action of sea currents by constructing a number of piers normally to the shoreline, a rather old and successful method, leads to the total diversion of sea currents, for at least the triangular area defined by the shoreline, the pier and the direction of the cur¬ rents. As a result the natural recycling of sea water and seabed pollutants is in part inoperative for this area Thus, besides the high cost of construction, the pier systems create conditions of harbor interior for the protected sea area which eventually will develop into a highly polluted place with muddy seabed.

The System for Protecting Harbor Against Sedimentation and for Nourishing Shores or SPPLAA (Greek acronym) and its novel construction method proposed in this paper, secures monitoring and control of the deposits of seabed loose materials, sand and gravel, by proper utilization of the renewable energy of sea current energy.

More specifically, SPPLAA has the capability to prohibit deposition of sand at places where its presence is undesired and to provoke movement and sub¬ sequently deposition of sand at places where the presence of sand is desirable. In fact, SPPLAA is a high inertia, dead load, system, whose presence at the near shore seabed, at suitable locations, as ad hoc oceanographic investigation and design study based upon the findings of the said investigation will identify, se¬ cures transfer of loose materials, at the expense of the sea energy alone, to desirable areas, where these materials will remain permanently on account of the new dynamic equilibrium established by this system

SPPLAA is a system of seabed groins of straight, polygonal, or curvilinear shapes, in general of elliptical cross section, with ratio minor/major axis equal about to 1/3, or even smaller, and of high total inertia and in cases, as the de- sign may require, high strength. Each of the seabed groins is constructed by one-piece impermeable geotextile, often reaching several hundred meters in length, transformed into a long watertight tube by means of heat welding, and highly resistive to tensile stresses. The said tube, being prefabricated under fac¬ tory conditions, is transported, placed and tensioned at the proper seabed location and filled in situ, from the shore, by special cast concrete mix, to be identified later in this text, the said concrete being of such low viscosity and low friction with the interior surface of the geotextile tube, as to secure con¬ veyance and complete filling of the said tube on the basis of gravity and hy¬ drostatic forces. The system of groins is located at the positions identified on the basis of the site investigation, analysis, and design work that precedes point out for each application, the said investigations being carried out on the sea- bed outside the entrance of the harbor to be protected against sedimentation, or on the seabed adjacent to the shore to be protected and nourished, the said design of the seabed groins aiming at securing deflection of the resultant water current regime operating near the seafloor and transferring momentum to the loose seabed grains toward and the said deflected currents and dragged grains to move toward desired directions, such as away from the harbor basin, or upon the beach.

The said design of SPPLAA which defines the number of seabed groins, the geometry of their trace and cross sections, and their precise positions, is based in each case on data identifying the isodepth contours, the velocity field of the prevailing currents near the seabed, the available sand deposits to be handled by the system, and the wind and wave climate of the site. Objective of the application of SPPLAA, as regards harbor protection, is the long term deflection of seabed currents in a way that precludes sedi¬ mentation of their basin as well as sedimentation of the seabed groins. The im¬ pact of the system will be immediately apparent, since there will be no new sand will be deposited there will be no need for removal of sand from the har- bor basia

As regards the nourishment of beaches, the objective of SPPLAA is to pro¬ voke transport and deposition upon the shore and through natural processes, of loose seabed materials, mainly of sand, the total burial of the system under the transported materials, and the increase of the width of the beach. The new shore and seabed will be exposed to the action of the waves and currents but will not be erodable due to the underlying SPPLAA system

Fundamental characteristic of the proposed method for facing the phe- nomena of harbor sedimentation and beach erosion is that it is based upon the correct identification of their cause. The present study begins with the analysis of this cause. Since the antecedent agent is everywhere the same, namely the combined action of seabed and wave driven currents, whereas the results are related to the topography of the seabed, to the material content and topogra- phy of the eroded nearshore seabed and beach or of the seabed surrounding the breakwaters and entrance channel of the port experiencing sand accretion in its basin, the topography of the shores adjacent to the port, the extremes and time dependence of the resultant current velocity field in the said areas, the effective regime of current streamlines intruding in the port basin on ac- count of the erroneously situated entrance channel, and finally the wind cli¬ mate (statistics of power v. direction) and the wave climate (period, height and length spectrum of the significant wave), the technical description of the pro¬ posed arrangement will be general as regards both the objectives and the means for achieving the desired results.

The design specifications, mainly geometrical, of the arrangement to be constructed, in accordance with the theory of momentum transfer boundary layer hydrodynamics with the purpose of blocking the port entrance to sedi¬ ments and reversing the erosion procedure of shores to a procedure of nourish¬ ment, depend upon the specific port to be protected against sedimentation and the specific shore to be protected and nourished. To be specific, structures compatible with the said theory and capable to procreate the desired reme- dial implications are groups of inte elated and synergetic seabed groins of rectilinear and/or polygonal and/or curvilinear shapes and of a wide range of geometrical characteristics pertaining to longitudinal and transverse section sizes and to detail positioning on the application site. The number of seabed groins required for each application site, the shape of each groin and the values of the said geometrical parameters related with each groin have to be optimally determined defined during the phase of the application design to be developed for the given application site on the basis of effectiveness/cost criteria and on the basis of the said site investigation data

DESCRIPTION OF THE SPPLAA METHOD

The System for Protecting Harbors against Sedimentation and for Nourishing of Shores (SPPLAA) is a modification of the Shore Protection and Nourishment System (SPAA) (Patent No 1001234/1992 issued in the name of same author) and consists of a low cost and small physical dimension structure, yet of large inertia and, if necessary, strength, which is placed on the seabed outside the harbor to be protected, or the shore to be nourished. The SPPLAA arrangement properly designed on the basis of thorough oceanographic site investigation and placed at the application site, achieves the objectives and results signified in its title, namely, the termination of sedimentation of the harbor basins and the nourish-- ment of the eroded beaches.

Harbor sedimentation and beach erosion are the results of sea currents, of all types, acting close to the seabed. In general, the motions of water particles, on the basis of the forces applied upon them and their kinematic result, are distinguished as waves, currents and tides. All three types of motion are respon¬ sible for harbor sedimentation and coastal erosion, but each to a different degree.

It is well-known that real sea wave action is confined within a surface lay¬ er of depth about equal to L/4, where L is the wave length. The water particles in this case, perform general ergodic (quasi-periodic) motions describing de¬ formed circular paths. For this reason, waves have limited ability to cause irre- versible displacements of seabed sand and graveL They, nevertheless, have devastating effect as erosion agents due to the expending much of their dense energy in crushing large gravel and rocks to granule size and, more importantly, in stirring and lifting above seabed sand grains and graveL Thus, waves facili¬ tate sea currents running parallel and close to seabed in effecting one-way displacements of sand and gravel, a job that even irregular waves can hardly perform

The sea currents constitute a basic type of sea water motion present in the entire volume of all water concentrations. Surface, interior and seabed currents, produced by density, temperature, and atmospheric pressure differences between adjacent water masses, are present everywhere without exception Wave driven currents, particularly at the surf zone, are an additional agent of sand displacement and erosion The motion of the water participating in the current activity is translational, at least over coastlines a few tens of kilometers long, and hence they are capable to displace loose granules resting on the seabed, particularly at shallow waters, where wave motions act as a catalyst.

Current speeds vary within very broad limits. In Greek seas speeds of about 2 m/s are usual, whereas currents of tidal origin to be described below, move in many cases with much larger speeds. All types of currents operate within the water layer of the surf zone together with the waves and, hence, the water particles, in this layer, perform a resultant motion, to be described as the vector sum of the current (translational) and the wave (ergodic) component motions. This type of water motion is of course very complex and dominates all sea areas with shallow waters.

The tidal current motion caused mainly by the differential of the gravita¬ tional attraction applied by the Moon and Sun upon the terrestrial land and sea with observable kinetic results on the liquid parts of the Earth, is an oscillatory motion of the sea water masses, of amplitudes reaching hundreds of km^'s and of short periods of 12 hours and long period of about 27 days. Associated with the tidal currents is a periodic change, with same periods, of the sea leveL In narrow sea straits tidal action produces horizontal water motions (currents) with speeds bigger than 4 m/s. In spite of the fact that this type of currents extends over the entire water body, and hence reaches the seabed boundary, and the speeds developed are quite large, their erosive effect is often small on ac¬ count of the periodic character of water mass displacements. Nevertheless, in narrow straits of appropriate direction the presence of loose, small size, granular matter is strikingly absent.

It thus becomes evident that the only agent of littoral material displace¬ ments, directly associated with harbor sedimentation and coastal erosion, is the resultant of all types of currents, wave driven, density and tidal, and, more spe¬ cifically, the part of the said resultant current, that acts near the seabed, where displaceable loose matter is in most cases present. We shall henceforth refer to the said part of the resultant current as "seabed current".

The process of seabed matter transfer and shore erosion associated with the resultant current motion, resembles the riverbank erosion process occurring at cases where deposition of heavy obstacles in the riverbed cause diversion and impacting of the water flow upon its banks. The erosion of the riverbank is self-explanatory, as it is extensive as fast, particularly in the cases of corrobo¬ rating turbulent flows and wavelike motions such as those present in the surf zone.

Sea currents are the result of primary and secondary forces. Primary are the forces responsible for their creation and maintenance. Secondary, are the forces that modify the currents produced by the primary forces. Primary are the forces of internal and external pressure due to density and/or temperature dif- ferences, the wind stresses and the forces that create the tides. Secondary are the forces of hydrodynamic friction (viscosity) and the Coriolis force due to the rotation of the Earth.

The forces appearing because of internal pressure differences are due to water density differences between adjacent fluid masses resting on the same equipotential gravity surface. Density differences, in turn, are caused by differ- ences in water temperature and salinity, as well as by accumulation of water masses over certain sea regions, accompanied by mass deficiency in other adjacent sea regions, produced by the wind. The external forces are produced by air pressure differences in the atmosphere above adjacent water masses. The pressure forces can appear as resulting from the slope of the sea surface or the slope of the surfaces of equal pressure, both slopes measured with refer¬ ence to the local gravity equipotential surface.

These forces are perpendicular to the velocity vector of the current. It must be pointed out that the above slopes are too small to be directly observed or measured. For example, the known strong streams, e.g. the Gulf Stream, the sur¬ face slope produces sea level difference of 10 cm over a span of 10 km It should also be mentioned that very frequently sea layers of different density separating layers of strictly higher density water from layers of strictly lower density water, are observed. These discontinuity water layers are slightly tilted with respect to the normal to the velocity vector of the current motion Looking along the current velocity vector of a current in the north hemisphere, we ob¬ serve that the sea is tilted to the left. The tilt is 1OOO times bigger at the depth of the discontinuity layer than at the sea surface.

We shall not go into discussing in this presentation the effects of the sec¬ ondary forces, such as those produced by the viscosity and the rotation of the Earth, upon the sea currents. There is also no need to mention the surface sea currents at the open seas of large depths on account of the fact that their presence is irrelevant to harbor basin sedimentation and beach erosion activity caused by seabed currents.

Tidal and density currents are active within the entire sea water body at both deep and shallow waters and hence are capable to scrape the seabed. On the contrary, water driven currents, being generated by shear wind forces, are active within a surface layer in deep waters and are incapable to scrape the seabed. However, while approaching the shore and some distance from it, the thickness of this current action layer becomes larger than the water depth of the nearshore surf zone, whereupon the wind driven currents become capa- ble to scrape the seabed.

As regards the magnitude of current velocities that can be produced by the three types of current motion, maximum is that of the wind driven type and minimum that of the density difference type. Tidal driven currents, sparing rare locations implicitly outside concern for the arrangement presented in this study, give intermediate magnitude current velocities.

As regards the capability of three types of currents to inflict permanent and irreversible displacement of seabed materials, such as are witnessed in harbor basin sedimentation and shore erosion, as classified by this author on the basis of numerous cases examined, yet in limited geographic areas, top posi¬ tion is given to density currents and last position to wind driven

Finally, as regards the advantages offered by each of the three types of current in achieving elimination of harbor basin sedimentation, top position is given to density currents and last position to wind driven, whereas for achieving restoration of a beach top position is given to wave driven currents and last position to tidaL

After the above analysis we are now ready to proceed to the description of the SPPLAA arrangement and of the method to be followed for the calcula¬ tion of the structural and alignment specifications of the said arrangement, al¬ ways in relation to the sand displacement agent, the seabed currents, that effect harbor basin sedimentation and beach erosion, as well as in relation to the site, inshore and offshore, characteristics and microclimate.

Coastal erosion takes place in the indicative way presented in Figure I The band 6 of seabed streamlines, which for simplicity is treated as a set of rectilinear parallel current lines, is modified upon, upon entering shallow seas, due to interference with the wave motions from which the current translational velocity acquires a vertical oscillatory component. The said streamline band 6 impacts upon the coastline at small incidence angles with the said coastline and is reflected back, thus forming to the returning streamline band 7. This re¬ turning band 7, drags along, toward greater sea depths, whatever loose seabed materials can be displaced by absorbing momentum from the said band 7 of streamlines. Liable to absorb momentum and thus be displaced are the gran¬ ules of sand that have immediately before absorbed energy from the turbulent breakage of waves upon the shore and as a result are rather in condition of random floatation than resting upon the seabed. The displacement of materials toward deeper seas is facilitated by the no-action of the gravity force on ac- count of the slope of the seabed.

The synergy of gravity in such sand transfer explains the displacement of even heavier gravel (of weight perhaps several kg per unit), or even large rocks, the latter in small steps and by a procedure that commences with the undermining first the stability of the large rocks due to removal of the substratal lighter supporting granules and subsequently by applying impulse torque capa¬ ble to roll the rock to greater depth.

The SPPLAA system for the protection and nourishment of shores is pres- ented in Figure 2 and consists of items 12, 13, and 14 that act as inshore supply hoses for the conveyance of ballast concrete into the main offshore body of the seabed groins, items 6, 7, and 8 which are one-piece seabed groins resting entirely upon the sea floor and their upper surface above sea level only for a limited length, the said groins having lengths that may reach several tens or even hundreds of meters, and having elliptical cross section with minor/major axes ration equal about to 1/3. The bands of parallel current lines, shown as items 9, 10, and H, in Figure 2, moving close to the seabed, upon impacting on the above groins are diverted toward the coastline. The said diverted seabed streamlines, while moving toward the shore, drag along loose and light granules, mainly sand. This matter is pushed and deposited at smaller water depths thus raising the sea floor and eventually implementing its emergence above water level This broad line and simplified description refers to the end results, while, in reality, many transient phenomena of sand and gravel transfer, difficult or even impossible to predict or describe, occur. Among these transient displacements of matter are included movements along the shoreline and give the impression that no permanent restorative process is in action and hence no stable form of the beach is under generation The restorative function of the groins takes place at higher rates in com¬ parison to the erosion rates and this in spite of the adverse gravity forces. This is due to the diversion by the groins of the wave driven currents which can often be very strong and, as already stated, their utilization for restoring shores is more effective in comparison to the contribution of the other types of sea cur¬ rents. It is for this reason that SPPLAA for shore protection and nourishment is performing better in winter time. It is, therefore, recommended that this arrange¬ ment be placed in the application seabed during autumn, and thus take full advantage of the restorative effect of the winter that follows.

However, the utilization of the diverted current lines for transferring mo¬ mentum of suitable direction upon the granular seabed matter toward the beach, and for pushing the said matter toward the shore and against the op¬ posing frictional and gravity forces, can prove ineffectual in cases of low speed currents and/or high slope seabed near the coastline. It is, nevertheless, as¬ sumed that proper design of the SPPLAA arrangement can extend the range of application of the current diversion technique and thus encompass even such difficult to treat shore cases. It is obvious, however, that some extreme cases will be left outside the range of application of the SPPLAA arrangement.

In general, and without exception, essential phase of recommended pro¬ cedure for applying the SPPLAA arrangement to any specific beach, is that of conducting a thorough site investigation that will secure the required oceano- graphic data, upon which the design of the arrangement will be based. Site oceanographic investigations must include seabed survey, spectrum of the sea waves, current velocity field, identification of seabed areas with loose granular material (sand) suitable for transfer to the beach, etc. The findings of the site investigations will allow determination of the essential parameters, such as the total number of seabed groins to be built in order to achieve diversion of seabed currents, the precise position of each on the seabed, as well as the shape of their trace, the total length and alignment of each and their cross section. Seabed groins 6, 7, and 8 of Figure 2, are shown as rectilinear only in- dicatively, whereas their trace shape, to be determined by the design, may be curvilinear or polygonal In cases where no sand deposits are found within the area surveyed the arrangement must be extended to greater depths. Figure 3 shows one seabed groin in perspective.

Harbor sedimentation takes place according to the process indicatively shown in Figure 4. The band 5 of seabed current, which for depths larger than L/4 can be considered as parallel, is modified at the vicinity of the harbor, through interference with the prevailing weak wave motions, to which it adds a translational component. On account of the angle of impact upon the break¬ water the modified current changes direction and moves toward the harbor en¬ trance channel, where its velocity increases and thus on its way pushes toward the harbor interior whatever loose materials are present on the seabed. It is as¬ sumed that band 5 of streamlines, originating from the open sea 1, acquire the orientation shown in Figure 4, and hence the breakwater walls 3 and 4 facilitate the entrance of the sea currents at increased speeds in the harbor basin 2 The SPPLAA arrangement for shore protection and nourishment is, above all, a system in the sense that all seabed groins together and by joint and syn- nergic action accomplish the desired result. Unrelated groins, in the mathemat¬ ical sense, will either fail to score the expected target or deteriorate the condi¬ tion of the shore. The relation between member groins of one SPPLAA system is a function involving the total length / of the seabed groins and the distance m between any two consecutive such groins and the site characteristics. The said function can be set to express the quantity of loose materials displaced and deposited upon the nearshore seabed and the beach. For /»m it receives posi¬ tive values (nourishment), while for / »m it receives negative values (erosion). Actual experiments have shown that fulfillment of the inequality m/l <1 secures nourishment of the shore. We used the value m/l = 0.75 several times and had the desired implications. For larger seabed slopes, eg > 10°, it is recommended that to take m/l =< 0.50. Other factors, such as intensity and direction of currents and availability v. distance from the shoreline of loose materials, are essential for the selection of the suitable value of this ratio. The optimization of the said functional relationship between m and / is difficult and expensive, since it in¬ volves experimentation on models and/or in situ. Future experiments will pro¬ vide more reliable data upon which reduction of the number of seabed groins without loss of effectiveness of SPPLAA will be based. The SPPLAA system for the protection of harbors against sedimentation is shown in Figure 5 and consists of the seabed groins 6, 7, and 8 bent to form equal angles (their entire body rests upon the seabed) and have elliptical cross sections with minor/major axes at about the ratio 1/3, and total length of many tens or even hundreds of meters. The band of seabed currents 5, due to the presence of the seabed groins are deflected in part toward the adjacent beach, to which they transport sand, and in part toward the open sea I The number of angled seabed groins and their geometric characteristics (size of angle, total length of groins, and cross sections) and position of installation, are to be defined during the design phase of the project. As already pointed out the design is to be conducted on the basis of the oceanographic data and their analysis to be gathered from the site of application The seabed groins 6,7, and 8 are given indicative shape in Figure 5. They can receive, for example, parabolic or hyperbolic shapes with asymptotes making the same angle as de¬ sign requires, or general curvilinear shapes to match and achieve the basic re- quirement of deflecting seabed currents away from the entrance port channel

The construction of SPPLAA for achieving both objectives, Le. protection of harbors against sedimentation and for protection and nourishment of shores, is made by use of single, impermeable geotextile tubes of appropriate weight, ranging indicatively between 5OO gr. to 12OO gr. per square meter for seabed slopes less than 10°, and cross section perimeter between three and ten meters. Geotextiles with such specification are produced, and with special properties can be custom made. In general, they are delivered in rolls for the width, length strength, and color required. Their transformation into single length watertight tubes is achieved by heat welding along the longitudinal sides of the material and the two ends (case of harbor protection), or by heat welding along the longitudinal sides and one end of the resulting tube, leaving open one end to be used for filling the tube in situ and from the shore by special concrete (case of shore nourishment). At regular intervals along the welded side of the tube and normally and symmetrically to the seam, metal sheets, of suitable weight are attached in order to effect complete sinking of the empty geotextile tube during installation In order to avoid the dangerous for environment conse¬ quences of probable failure of the welding seams during the filling of the geo¬ textile tubes with cast concrete, attention must be paid so that the said tubes be positioned with the welded seams facing the seabed.

Upon completion of the preparatory procedures, as above, the geotextile tube is rolled again, beginning from its sealed end, to produce a cylinder, keeping the longitudinal welded seal at the width center of the external sur- face of the said cylinder and the metal ballast resting on the same side. The unrolling of this and the rest of the cylinders is performed at the application shore by divers until the entire length of the tube is stretched, in the cases of SPPLAA systems aiming at shore nourishment and until the vertex of the angle to be shaped for angled entirely immersed groins, in the cases of SPPLAA systems aiming at harbor protection against sedimentation In the first case, the sealed offshore end of the tube is firmly anchored and then a tensile force is applied at its inshore end so as to attain rectilinear form The inshore end is then fastened upon a fixed piles to keep the entire tube under tension In the second case, the same as above stretching procedure is followed separately for the each rectilinear segment of the tube, starting from tube end closed to the shore, paying special attention to realize the design angle be¬ tween segments. In both cases, the stretching is achieved by using anchors and/or hand winches.

Upon achieving the placement and tensioning of the tubes, the filling with a special concrete mix starts. In both cases the filling is accomplished with the use of one or more concrete presses, the blast pipe of which is simply intro¬ duced to the open inshore end of a geotextile supply hose welded to each offshore groin tube (first case), or to the open inshore end of groin tube (second case).

The special cast concrete mix, capable to flow freely under gravitation and hydrostatic forces and to fill the offshore tubes of SPPLAA to the extend re- quired by the design, belongs to the common types B225 to B4OO, and is made by a minimal coarse (No 4 sieve) aggregate, between O to 5OO kg of pit gravel, and a large fine aggregate of pit sand, to which the proper quantity of the appropriate superplasticizer condensate is added. The exact proportions of each of the concrete ingredients, including portland cement, dependent as they are on the distance between the press and remote position of the im¬ mersed geotextile tube, are given by the design of the particular SPPLAA sys¬ tem, always safeguarding the concrete design strength.

The filling procedure by the said cast concrete is made by use of concrete presses whose output hose is inserted only about one meters inside the geotex¬ tile tube at opening made at the nearest position to the shore. In case of in¬ ability to secure watertight insertion of the outlet hose a geotextile conveyance hose extending from the shore and welded at the other end upon the said opening of the tube is used. The said concrete mixture secures it^'s own ad- vancement and complete filling of the tube interior to its remotest end, and creates the elliptical design cross section with the required ratio of axes. The filling procedure of angled geotextile groins aiming at harbor protection against sedimentation, involves cast concrete conveyance from the shore through the same as above impermeable geotextile hose kept under suitable tension in order to avoid blocking of small cross section The offshore end of this hose is watertightly attached, again by means of heat welding, to the large container groin tube through a hole at it^'s nearest end to the shore.

The following remarks are pertinent. The geotextile material to be used in all cases must be impermeable by water molecules and the weldings, particu- larly those in the immersed parts of the tube, must be watertight. Thus, not even the finest of grains of the mixture will escape from the tube. Second, the longi¬ tudinal tensile stress to be accepted by the geotextile tube should defined by special dynamic calculation on the assumption that the ballast to be filled with will be concrete in cast form and the tensile resistance of the geotextile tube will be the only force to resist the tensile load force along the inclined seabed.

For seabed inclination less than 10° geotextile with tensile resistance of about 4 tons per meter of cross section is found to be efficient. Third, the filling proce¬ dure in all applications of SPPLAA, as well as other offshore constructions, such as breakwaters, piers, jetties, involving laying and filling one or many, single-piece watertight geotextile tubes, placed adjacent or in contact to one another and in the same or different levels, is accomplished from the shore via concrete presses, either by insertion of the output hose of the said presses into an opening at the nearest position of the said tubes, or by insertion of the said hose in watertight conveyance hoses made of geotextile welded securely with the seabed groin tube. Fourth, the concrete mixture placed inside the water¬ tight geotextile tube hardens within a few hours. Fifth, the filling procedure of a 5Om long groin should be completed within about one hour.

The results of the application of SPPLAA are particularly related to the characteristics of the site. The interruption of harbor sedimentation and shore erosion after placing a well designed SPPLAA system can be considered cer¬ tain The nourishment and increase of the width of an eroded beach, however, depending on the available loose materials in the nearshore seabed, on the current and curve climate and on the slope of the seabed, can proceed at fast or slow rates or not be realized at alL Beaches of active wave and current seas and nearshore seabed of small slopes and large quantities of sand are ex¬ pected to gain more than 10m width every year. On the contrary, beaches of non active seas and of nearshore seabed with large slopes and poor sand de¬ posits will exhibit small or negligible rates of width growth. But in no case further beach erosion is to be observed after placement of such a system

A common concrete press stationed near the shore and using transmission tubes long enough is sufficient for the filling of the geotextile tube with the above mixture of concrete. During the filling the use of a shock shaker immedi- ately after the exit point of the concrete will facilitate further its advancement within the geotextile pipe The filling process is terminated when the vertical height of the groin has reached its desired design value. This height will in¬ crease somewhat with the sea depth on account of its slope and the compo¬ nent of the concrete weight parallel to the seabed. It is estimated that 6Om³ of concrete will fill a lOOm long geotextile pipe with 3.86m cross section perimeter and secure a seabed groin of elliptical cross section with major a (horizontal) axis 160m and minor b (vertical) axis O.6m These figures are only indicative, since the wide range of applications of SPPLAA can lead to a proportionately wide range of lengths /and ratios b/a

After hardening of the concrete place inside the geotextile tube, the con¬ crete conveyance hose is removed by cutting the material at the welding cir¬ cumference and breaking, if there is need, of the concrete left and solidified inside the hose. It is evident that all heat weldings are done inshore and hence under dry conditions.

DESCRIPTION OF FIGURES

Figure 1 presents a typical parallel band of current streamlines impacting upon and deflected by a shore. The deflected streamlines, assisted by gravity on account of the seabed slope, displace loose materials toward deeper parts of the seabed and thus erode the beach. The numbers in Figure 1 indicate:

1 The beach area

2 The shoreline. 3, 4, 5: Pairs of indicative isodepth contours.

6: Typical band of seabed current streamlines impacting upon the beach. 7: Band of streamlines after the deflection by the beach. Figure 2 shows the horizontal layout of a SPPLAA arrangement for shore protection and nourishment composed indicatively of four seabed groins, with one of them tensioned and then anchored on its beach and offshore ends by common gravity anchors and ready to be filled with special mix of cast con¬ crete. A typical cross section and gravity anchor is also shown The current streamline bands after impacting upon the elevated sides of the groins are de¬ flected and follow paths leading to the shore. The numbers in Figure 2 indicate:

1 The beach area

2 The shoreline. 3, 4_# 5; pairs of indicative isodepth contours.

6, 7, 8, 9. Single-piece seabed groins filled by special concrete. 10, Tl, 12, 13. Seabed current streamline bands impacting upon the groins, and deflected toward the shore and pushing loose materials to¬ ward it. 1 i5_# 16, 17. The nearshore ends of seabed groins. Two anchors at suit able angles placed after tensioning groin 9 assisted by a second pair of similar anchors placed at the end 18 of the same groin keep the empty geotextile tube stretched during its filling with special conaete mix. is, 19, 20, 21 Offshore ends of the four seabed groins.

22, 23. Anchor and its magnification

24, 25. Cross section of seabed groin 6 and magnification of cross section Figure 3 presents a perspective drawing of the SPPLAA arrangement con- sisting of one seabed groin in the phase of placement. The numbers in Figure 3 represent.

I The shore area 2 Pair of anchors keeping in place the stretched empty geotextile tube.

3. The seabed groin

4. The shore line (zero isodepth contour).

5. The band of the current lines near the sea floor that is diverted by the groin and is directed toward the shore dragging along small size granules of sand and depositing them upon the shore and the near shore seabed.

6. The sea surface.

7. 8. The end points of the seabed groins kept under tension by two an¬ chor each. 9, 10. Cross section position and magnification

Tl 12 Anchor position and magnification

Figure 4 shows an indicative harbor that is subject to sedimentation by the intrusion of seabed currents. The numbers define:

1 The open sea

2 The harbor basin 3. 4: Breakwaters.

5. Band of seabed current streamlines pushing and depositing sand into the harbor basin

Figure 5 presents an indicative SPPLAA arrangement for the protection of the harbor shown in Figure 4 against sedimentation by means of three angled seabed groins. The numbers indicate.

1 The open sea

2 The harbor basin 3, 4: Breakwaters.

5. The regime of seabed currents deflected by the groins. 6, 7, 8: The angled seabed groins made of impermeable geotextile tubes, filled from the shore with the special mixture of concrete. Seabed groin 7 is shown in the phase of placement. At the end point 7 and at the vertex point 9, two and three anchors, respectively, keep the empty geotextile tube under tension and in place for its filling with special concrete mix.

10, U Position of cross section and magnification

Claims

CLAIMS A System for Protecting Hαbors against Sedimentation and for Nourishing Shores (SPPLAA) consisting of seabed groins, such as the one shown in Figure 3, or the three shown in Figures 2 and 5, or of unlimited number, resting upon the seabed for their entire length, of rectilinea, angled or curvilinea horizontal projections, with approximately elliptical cross sec¬ tions and ratio of the order of 1/3 between the vertical over the hori¬ zontal axis, the total linea length of the said groins reaching hundreds of meters or as the design based on the upon complete oceanographic survey of the application site will point out, the said groins being con¬ structed as one-piece tube each made of impermeable geotextile material shaped into tube by watertight heat welding, the said tubes being filled in situ from the shore and by special cast concrete mix of minimal internal viscosity and friction to the interior surface of the geo- textile tube, as to facilitate advancement of the said concrete within the geotextile groin tubes or conveyance hoses, the said concrete mix¬ ture being of type between B225 to B4OO, composed with minimal pit gravel aggregate and lage amount of fine pit sand aggregate, to which mixture the appropriate quantity of superplasticizer condensates ae added to minimize internal viscosity and external friction and facili¬ tate advancement of the concrete mixture hundreds of meters from the shoreline and into the said impermeable geotextile tubes, capable the said system of groins to deflect and direct seabed currents towad the beach and transfer of current energy to sand grains at rest on the seabed in order to effect gradual displacement and ultimately deposi¬ tion of the said sand grains upon the neashore seabed as well as upon the shore itself, thus gradually cause enlagement of the beach, also capable the said system of seabed groins to effect deflection of the seabed currents, before entering its basin, away from the entrance channel of a habor and thus secure its protection against sedimenta¬ tion of its basin Any offshore structure made by prefabricated or in situ cast elements produced by watertight geotextile tubes, or closed elongated tubular surfaces, filled in situ by special concrete mixture, as in claim 1, and aiming at controlling and influencing loose seabed material displace¬ ments or any offshore structure such as groins, jetties, piers, pipeline supports, breakwaters, etc., by use of the technique of filling in situ wa¬ tertight geotextile tubes, or closed surfaces, by special concrete mix- ture, as in claim 1