FR3136487A1 - Artificial algae, system and method for controlling silting/erosion of a coastal site based on such algae - Google Patents

Abstract

The invention relates to an artificial algae (S) characterized in that it comprises - an anchoring means (A) for anchoring the artificial algae (S) in a seabed (FM); - a flexible rod (T) configured to be folded by wave movements; - flexible side branches (B) forming projections around the rod (T), configured to be folded by wave movements; - a flotation means (B1) having a density lower than that of sea water, configured to maintain the rod (T) in a substantially vertical position. The invention further relates to a system and method for controlling silting/erosion of a coastal site (S1, S2) based on several S algae of this type. Abstract Figure: Fig. 2

Description

Artificial algae, system and method for controlling silting/erosion of a coastal site based on such algae

The invention relates to the field of systems and methods for controlling the hydrodynamics of sediments, in particular for controlling the silting of a channel outlet open to a sea, or a shelter port pass for example.

In this area, when a channel is created with an opening to the sea, in particular to facilitate various activities carried out around the channel (maritime traffic, users, sailing, etc.), the movements of the swell and currents generate a movement of marine sediments , in particular sand and debris of marine fauna and flora, so that these sediments accumulate at the level of a channel pass.

Thus, we observe a problem of obstruction of the entrance to a pass which can lead, from the first heat, to severe eutrophication of the body of water.

It follows that the width available for passage is then very small and the draft is low, making navigation perilous and limiting incoming and outgoing flows and causing confinement.

The problem is also observed on shelter ports designed to facilitate the mooring of recreational boats in particular. Boaters report cases of damage to their boats when maneuvering to leave or enter the port due to the low level of water available.

A solution proposed to restore circulation consists of carrying out occasional dredging operations of the accumulated sediments, restoring access temporarily, until the channel is blocked again.

These dredging operations require environmental authorizations and require significant and recurring equipment and funding.

In addition, the environmental and social context around ports, such as the port of Jaï, is complex:
- a pass protection dike can generate a change in local sedimentary dynamics;
- a rapid sediment dynamic can fill in a few weeks the clearances made by dredging, which makes the pass inoperable for boaters and professional fishermen for a good part of the year;
- in general, localized accumulations of rotting macrophytes occur, causing olfactory and ecological nuisances;
- non-compliance with regulations by professional fishermen who throw waste from their fishing into the basin, accentuates the dynamics of eutrophication of the basin internal to the site.

Thus, a first objective of the present invention is to propose a solution making it possible to mitigate the silting of a coastal site, such as a channel entrance or a shelter port pass, for example a marina, notably by attenuating the energy of swells and currents.

A second objective is to limit the ecological impact of silting, particularly on aquatic fauna and flora.

To achieve these objectives, the invention proposes an artificial algae characterized in that it comprises
- an anchoring means for anchoring the artificial algae in or on a seabed;
- a flexible rod configured to follow the movement of the waves;
- flexible lateral branches forming projections around the stem, having a density greater than that of sea water, and being configured to cause hydrodynamic dissipations, and to bend according to the movement of the waves;
- a flotation means having a density lower than that of sea water, configured to maintain the rod in a substantially vertical position.

In the context of the invention, we can speak of “artificial algae” or “artificial sargassum” interchangeably, because the algae has a form of natural sargassum.

Advantageously, a plurality of such artificial algae makes it possible to significantly reduce silting and/or erosion of a coastal site. In addition, the artificial seaweed is colonized by algae and constitutes a habitat for the development of juvenile fish and crustaceans.

Furthermore, the suppleness and flexibility of artificial algae make it possible to have movements similar to those of natural algae and magnoliophytes.

The structure of the artificial algae being modular with separable elements, it allows isolated elements to be exchanged in the event of damage without having to get rid of all the algae. In addition, the algae can be adapted to the depth of the implantation site.

In addition, the use of artificial algae makes it possible to limit the frequency of dredging involving the use of construction machines and heavy and often polluting equipment.

According to a variant, the anchoring means comprises a plate, preferably in the form of a disc, having a density greater than that of sea water, and being configured to be buried in the seabed, the plate being fixed, of preferably threaded over the stem and attached to it. The disc can be made of metal or concrete.

This allows for effective anchoring with a simplified structure. A steel disc at a depth of one meter can withstand 0.5 tonnes of traction. Thus, another object of the invention relates to an anchoring means described in the present application, for a given aquatic structure which may be an algae, a net or other sedimentary screen, or another structure to be anchored in a course bottom. 'water.

According to a variant, the stem and/or the branches are in the form of ropes. This allows for simplified manufacturing of algae, based on materials available in large quantities. The structure thus formed is very effective in controlling siltation and allows the fixation of algae and the population of juveniles of several species. The shape of the rope also makes it easier to attach to other structures.

According to a variant, the branches extend over a stem length greater than 1 m or even 2.5 m, preferably less than 3.5 m, more preferably approximately 3 m. This allows for sufficient length to prevent the movement of sediment towards the coastal site.

According to a variant, the flotation means comprises a float attached to the rod. This allows for simplified manufacturing based on materials available in large quantities. The float also improves visibility and avoidance of boats and other vessels.

The invention further relates to a system for controlling silting/erosion of a coastal site, in particular to limit the silting of a channel entrance, a part of the coast, or a shelter port pass. , the control system comprising a plurality of artificial algae according to the invention, configured to be arranged in a wave field oriented towards the coastal site. In particular, the algae are arranged judiciously taking into account the hydrodynamic conditions of the site.

According to a variant, the control system may include, depending on the needs of the site, at least ten, possibly less than 500 artificial algae (or more); depending on the case, we can consider more than 16, and possibly less than 300 artificial algae. This significantly improves the impact on strong swells and the corresponding sediment displacement. The number of artificial algae is adjusted according to the coastal site, the swell and the currents to be attenuated/dissipated. For large areas, depending on the hydrodynamics, we consider using more than 500 artificial algae.

According to one variant, the control system comprises at least one barrier of underwater givelles. This makes it possible to improve the effectiveness of silting/erosion control at shallow seabed depths and to reduce sediment transport.

According to one variant, the control system comprises at least one net forming a sediment screen. This helps limit the movement of large sediments such as shells towards the coastal site.

The invention further relates to a method for controlling silting/erosion of a coastal site, in particular to limit the silting of a channel entrance, a part of the coast, or a harbor pass. shelter, the control method comprising at least a first step for installing a plurality of artificial algae in a wave current oriented towards the coastal site, the artificial algae each comprising an anchoring means; a flexible rod that folds under the effect of wave movements; flexible side branches that fold under the effect of wave movements; and a flotation means, the first stage comprising sub-stages for
- anchor the anchoring means in a seabed; And
- arrange the flotation means so as to maintain the rod in a substantially vertical position.

Preferably, the control method further comprises at least one step for setting up at least one barrier of underwater ledges in the wave current; and/or at least one step for setting up at least one net forming a sediment screen in the wave current.

The invention will be further detailed by the description of non-limiting embodiments, and on the basis of the appended figures, among which:
- illustrates a side view of an artificial algae module according to a preferred variant of the invention;
- illustrates the adaptability of artificial algae according to the at different seabed depths;
- illustrates an arrangement of several artificial algae according to the with pressure sensors;
- illustrates an aerial view of a first site for the invention, which is a channel entrance;
- illustrates an aerial view of a second site for the invention, which is a shelter pass of a marina;
- illustrates a side view of a gable barrier of a silting/erosion control system according to a preferred variant; And
- a side view of a net forming a sediment screen for a silting/erosion control system according to a preferred variant.

The invention relates to a silting/erosion control system for a coastal site S1 or S2. This is for example an S1 entrance to a CM channel; or an S2 shelter port pass from a marina; or another site with the disadvantages already discussed.

The system of the invention makes it possible to limit the silting of said site, namely in particular the entrance to the channel S1 or the shelter port pass S2 of a marina.

According to the invention, the control system comprises a plurality of artificial algae S configured to be arranged in a field or current of waves CV. The CV wave current is oriented towards the coastal site S1, S2.

The artificial S seaweeds each include a flexible T stem configured to be folded by wave movements. They further comprise lateral branches B (also called fronds B) fixed along the stem T, so as to form projections around the stem T. The lateral branches B are preferably flexible in the same way as the stem T They have a density greater than that of sea water. In addition, they are configured to be folded by wave movements.

In particular, the stem T and the branches B are in the form of ropes. This concerns in particular ropes used as accessories, particularly in nautical activities.

The central rope forming the T rod can have a diameter of approximately 30 mm. The peripheral rope forming the branches B can have a diameter of approximately 16 mm.

Ropes such as those from Socomap and/or Technical Ship Supplies can be used.

The fronds B are hung along the stem T, for a horizontal span of approximately 80 cm (D1 = 80 cm on the ). The B fronds can be spaced 5 to 30 cm apart for example.

The S algae can have a length excluding the float of 4m (H = 4m on the ). They can be deployed in a depth ranging from 2 to 3m (h = 2.5 m on the ). But also, S algae can be adapted to the depth of the implantation site, as illustrated by .

The anchoring distance can be from 60 cm to 1m.

The artificial algae S also comprise an anchoring means A making it possible to anchor the algae S in a seabed FM (or a watercourse).

Preferably, the anchoring means A comprises a plate which may have a given shape, in particular with a density greater than that of sea water, so as to sink to the bottom of the sea. Preferably, said plate is under disc shape, for example a metal or concrete disc. The plate or disc is configured to be buried in the seabed, and to be attached to the rod T. For example one or more holes are provided to pass the rope forming the rod T or a link which can be made of stainless steel, and fix thus the disk A to the rod T.

The disc is preferably anchored in the sediments via a high pressure water jet (or “jetting” in English).

Of course, in the context of the invention, we can envisage an anchoring means of any shape, such as an anchor, resting on the seabed.

The lowest fronds B can touch the seabed FM, but they preferably rise above the seabed to limit sand removal via the phenomenon of scouring, which would affect the anchorage of the algae S .

Indeed, studies seem to show that when the B fronds do not touch the bottom, the maximum scour calculated under most hydrodynamic conditions would be 4 cm.

The flexibility of algae further improves their fixation in the soil because the scour study model is based on a rigid structure. The flexible and collapsible structure of S algae probably results in less scouring.

The artificial algae S also include a means of flotation B1. The flotation means B1 is configured to float on the surface of the sea SM. Furthermore, it is configured to maintain the rod T in a substantially vertical position. For this purpose, the flotation means B1, which is preferably a buoy, is fixed, in particular attached, to the rod T in the variant illustrated.

The flotation means B1 can be a float, or other buoy, for example of the known type, with sufficient buoyancy to maintain the rod T substantially vertical in the installation site. In the preferred variant, EVA (ethylene vinyl acetate) floats with a buoyancy of 5000 gr, such as those from the company Technorope, can be used.

The flotation means B1 can be attached to the rod T using a link L1, for example made of stainless steel. L1 links such as those marketed by the company Etigo can be used.

The control system can include around ten S algae and up to around 300 artificial S algae, or even more depending on the hydrodynamic conditions of the coastal site considered. The algae S can be aligned in a geometric block, for example polygonal, as illustrated in the ; or star with concentric alignment as shown in the . In Figures 4 and 5, the white part represents the sea. Depending on the case, we can speak in terms of module for an artificial algae, or of forest for several artificial algae, which can have different geometries (square, round , staggered, etc.) depending on demand and local hydrodynamics.

In the variant of the , the system includes 16 S algae aligned in a square. The square is positioned such that it follows the edge of the developed dike BD. Each structure is spaced 1 m apart and was anchored by jetting.

In the variant of the , the system includes around sixty S algae.

In a variant such as that illustrated in , the system further comprises at least one barrier of underwater ledges G. The ganvels are illustrated in detail in .

S algae are preferably not established in too shallow depths. It appears more advantageous to use a system of underwater givelles G to act as a shock absorber/retarder and create settling boxes for the transported sediments. This role will be all the more effective as the epibiosis becomes greater. Tests have shown that the potential is very significant.

The alignments of piles, for example made of wood, are therefore used from the shore up to a bathymetric level of 50 cm for example, but they can be placed deeper if necessary.

In order to avoid a possible barrier effect due to possible clogging by the epibiontic cover on the piles, empty spaces of 50 cm for example are provided. The observations carried out also showed that the test beach was very frequented by juvenile mullets and joëls which are very common species in certain ponds. The G cranks therefore have a supporting role for fixed species and a habitat for young fish.

The implementation is carried out by burying by jetting in a closed enclosure with an anti-suspended matter curtain. This method is quick, inexpensive and reversible. The materials used are neutral and biodegradable.

Thus, in the variant illustrated in , the system further comprises at least one net F which forms a sedimentary screen. The net is illustrated in detail in .

Net F is intended to stop almost all residual sediment transit. In the preferred variant, it is arranged along the drop formed by the pass. Parallel to the protective dike in the variant of the , it plays the role of a counter-dike.

It can have a mesh thickness of 15 mm. For example, it is anchored at the foot of the slope of the pass, covering the drop-off created by dredging and which is again anchored at the head of the slope over the entire length of the pass.

Thus, given the composition rich in mollusk shells and the very heterogeneous grain size of the sediment, the veneer of the net web fully plays its role of stabilizing the slope. The 15 mm mesh is sufficient to allow water to pass through the screen but losing a lot of its residual energy, already largely attenuated by the system of successive partitions.

The F net is intended to protect itself from biofouling (“biofouling” in English), and in particular to support, depending on the site, in addition to macrophyte algae, a significant part of the capture of mussels. Buoyancy is therefore studied accordingly in order to support the weight of the biofouling. A system of floats sufficient to obtain 5 kg of buoyancy per meter is preferably chosen, but this can be adapted according to needs.

The L2 anchor line at the foot of the embankment can include a chain of 10 or 2.25 kg/m and buried in jetting. The anchor line at the head of the embankment can include a series of small screw anchors connected to a rope every 2 m, for a total of 16 anchors for 30 meters in length.

The net F naturally reaches the surface and is therefore perfectly visible to users. The F net can be configured not to get tangled in the propellers, due to the size of the mesh and its robust and stiff constitution. The net tablecloth does not crease, the cut is adapted to the morphology of the premises which avoids inappropriate convolutions.

The F net allows the marking of the pass, the channeling of flows maintaining self-cleaning by the passage of ships, maintaining the embankment, and preventing the filling of the pass.

The F net is preferably light, neutral, non-polluting, modular and perfectly reversible. It can also provide support for settled life capable of increasing habitat diversity.

Its location sheltered from the pier head and its very robust constitution give it good durability if there are no acts of deliberate damage.

The F nets used are preferably special nets, their small mesh size and the stiffness of the web make it impossible to accidentally get caught as conventional fishing nets could do.

It can include aquaculture-type multifilament polyamide sheets with 15 mm mesh (side) in 210/180 wire (2.5 mm diameter).

The invention further relates to a method based on a system as described above.

The control method comprises first steps for placing a plurality of artificial algae S, in particular as described above, in said CV wave current. To do this, the method comprises sub-steps for anchoring the anchoring means A in a seabed FM. The method further comprises sub-steps for arranging the flotation means B1 on the surface of the sea SM so as to maintain the rod T in a substantially vertical position.

The method further comprises second steps for setting up at least one barrier of underwater ledges G in the wave current CV. These are in particular G ganivelles as described previously.

Alternatively or in combination with the second steps, the method further comprises third steps for setting up at least one net F forming a sediment screen in said wave current CV. It is in particular a net F as described previously.

The order of the steps and sub-steps can be reversed, or they can be implemented substantially at the same time.

In the implementation of the invention illustrated in , the results are very encouraging and conclusive in terms of the deployment objectives. Indeed, during the various storms in winter and the Cévennes episodes, the structures resisted extreme hydrodynamic conditions. Only 2 algae are assumed to be lost during the entire testing period. It is possible that hydrodynamic conditions favored scouring around the structures, which were subsequently torn from the bottom with the swell.

Additionally, no issues or damage have been reported from the testing port. This shows that the selected area will not impact maritime traffic and the various economic activities present.

Furthermore, the system of the invention plays an important role in local biodiversity. During each intervention carried out on site, a large number of postlarvae and juveniles of fish and crustaceans, of different species, were present.

In the implementation of the invention illustrated in , the presence of algae is observed, including a limited amount of green algae on the FM background consisting mainly of ulvae (Ulva sp.) as well as filamentous brown algae caught in the structures. This presence of macrophytes, inevitable even with a barrier net, is in no way problematic since it helps to dampen hydrodynamic agitation. The artificial algae S, the ganivelles G and the net F were also colonized by algae.

The elements of the system generated the development of schools of juvenile fish.

In addition, the structures were well preserved for several months. No disappearance or damage to the floats or the anchors was noted.

A study based on stake probes shows that the probes showing progressive siltation are mainly located near the bank. The probes placed offshore and those near the pass (before the sedimentary screen F) do not show any variation in level.

It appears that the PL beach is growing faster than the rest of the area. The system leads to a fairly rapid sedimentation of sediment transport, since the level remains stable at the 'end of the journey', just before the pass.

After three years of monitoring, the observation is as follows: the port entrance pass is passable, it has not filled in. The experimental project worked, the operation was a success.

The analysis of sedimentation, using probe stakes, reveals a specific evolution. The coastal fringe shows signs of silting, the rest of the area is stable, even slightly eroded.

In first analysis, the system acted as a sufficiently effective retarder so that the drift current loaded with sediments only passed in front of the system. It would therefore be a hydrodynamic microcell acting as a “bumper” which deflects problematic sediment supplies.

However, the pass has absolutely not silted up.

Claims (9)

Artificial algae (S) characterized in that it comprises
- anchoring means (A) for anchoring the artificial algae (S) in or on a seabed (FM);
- a flexible rod (T) configured to follow the movement of the waves;
- flexible lateral branches (B) forming projections around the rod (T), having a density greater than that of sea water, and being configured to cause hydrodynamic dissipations, and to bend according to the movement of the waves ;
- a flotation means (B1) having a density lower than that of sea water, configured to maintain the rod (T) in a substantially vertical position. Artificial algae (S) according to the preceding claim, characterized in that the anchoring means (A) comprises a plate, preferably in the form of a disc, having a density greater than that of sea water, and being configured to be buried in the seabed, the tray being fixed, preferably threaded onto the rod (T) and attached thereto. Artificial algae (S) according to one of the preceding claims, characterized in that the stem (T) and/or the branches (B) are in the form of ropes. Artificial algae (S) according to one of the preceding claims, characterized in that the branches (B) extend over a stem length (T) greater than 1 m or even 2.5 m, preferably less than 3.5 m, more preferably around 3 m. Artificial algae (S) according to one of the preceding claims, characterized in that the flotation means (B1) comprises a float attached to the rod (T). System for controlling silting/erosion of a coastal site (S1, S2), in particular to limit the silting of a channel entrance (S1), a part of the coast, or a harbor pass. shelter (S2), the control system comprising a plurality of artificial algae (S) according to one of the preceding claims, configured to be arranged in a wave field (CV) oriented towards the coastal site (S1, S2). Control system according to one of the preceding claims, characterized in that it further comprises
- at least one underwater gable barrier (G); and or
- at least one net forming a sedimentary screen (F). Method for controlling silting/erosion of a coastal site (S1, S2), in particular to limit the silting of a channel entrance (S1), a part of the coast, or a harbor pass. shelter (S2), the control method comprising at least a first step for placing a plurality of artificial algae (S) in a wave current (CV) oriented towards the coastal site (S1, S2), the artificial algae (S) each comprising an anchoring means (A); a flexible rod (T) that folds under the effect of wave movements; lateral branches (B) that are flexible and foldable under the effect of wave movements; and a flotation means (B1), the first stage comprising sub-stages for
- anchor the anchoring means (A) in a seabed; And
- arrange the flotation means (B1) so as to maintain the rod (T) in a substantially vertical position. Control method according to the preceding claim, characterized in that it further comprises at least one step for setting up at least one barrier of underwater ledges (G) in the wave current (CV); and/or at least one step for setting up at least one net forming a sediment screen (F) in the wave current (CV).