WO2008100144A1

WO2008100144A1 - Method for laying a drainage system under a beach on a coast

Info

Publication number: WO2008100144A1
Application number: PCT/NL2008/050083
Authority: WO
Inventors: Adrianus Josephus Maria Van Iersel
Original assignee: Van Iersel Adrianus Josephus M
Priority date: 2007-02-15
Filing date: 2008-02-14
Publication date: 2008-08-21
Also published as: NL1033402C2

Abstract

In a method for laying a drainage system under a beach (2)on a coast in order to protect the coast against erosion, first pipe sections (7a) are placed in the beach parallel to the coastline and second pipe sections (7b) with water-permeable walls are placed at right angles thereto, in which the open ends (7c) of the pipe sections (7b) extend into rip (currents 4) that are present in the water. Moreover, several partitions (10) are placed behind each other in the rip current (4) in order to prevent the rip current from shifting along the coast so that the pipe sections (7b) will come to lie with their open ends (7c) outside the rip currents (4). Finally, the pipe sections (7b) are fastened flexibly to the partitions (10) so that they do not come to lie outside the rip currents (4) because of the current.

Description

Method for laying a drainage system under a beach on a coast

BACKGROUND OF THE INVENTION:

Field of the invention

This invention relates to a method for laying a drainage system under a beach on a coast in order to protect the coast against erosion, which comprises laying a pipe, with a water-permeable wall and at least one open end, under and roughly parallel to the surface of the beach, where the open end extends into the sea. The term beach must be loosely interpreted as a strip of land along a sea coast for example, which extends from a line inland that corresponds to the high-water mark to a line towards the open sea that corresponds to the low-water mark, i.e. the whole area that could be threatened by soil erosion.

Prior art

A system laid using such a method is known from WO-A-93/16235. It has appeared that the known system has had a favourable effect in preventing coastal erosion, i.e. the extent to which the coastal erosion decreases on using such a system. The water current provides continual suction at the open end of the pipe, so that a partial vacuum is generated at this open end. The partial vacuum at the open end produces the suction. The consequence is that the pipe draws water and/or mud from the sand surrounding the pipe through the water-permeable wall. This causes impacting of the sand in the bed around the pipe and the suction force exerted on it holds the sand together.

Summary of the invention

An objective of the invention is to provide a method of the type described in the preamble for laying a drainage system with which an even better protection of a coast against erosion is obtained. For this purpose the method according to the invention is characterised in that the open end of the pipe is placed in a rip current. A rip current is a current between sandbanks that extend along a beach. Water that is swept towards the beach in a wave by a set-up current flows sideways towards a rip current, and then back to the open sea through the rip current. The rip current is relatively strong as a relatively large amount of water, which comes in over the width of two nearby half sandbanks, is carried through the relatively narrow rip current back to the open sea. This creates a stronger partial vacuum in the pipe and therefore more water is withdrawn from the sand than if the pipe had been placed outside a rip current. A coast or seabed as described in this document can mean the strip that is dry at low tide and under water at high tide, but could also include adjoining strips inland and in the sea.

An embodiment of the method according to the invention is characterised in that the pipe comprises an initial pipe section, which is placed under the beach roughly parallel to the coast, and a second pipe section that is placed roughly at right angles to the first pipe section and which is connected to the first pipe section with a first end, so that the pipe sections are connected together, in which the second end of the second pipe section forms the open end of the pipe. This achieves a virtually uniform effect parallel to the coastline, and the coast or bank is protected against erosion over the distance concerned roughly to the same extent.

As coastal erosion is manifested over a length of the coast at right angles to the water line, several first pipe sections are placed after each other in the bed, preferably extending towards the open sea, which sections are all connected to the second pipe section. The second pipe section here forms a main drain and the first pipe sections here form small drains.

The transverse cross-section of the main drain is preferably equal to the sum of the transverse cross-sections of the small drains, so that there is an equal contribution to the build-up of as high a partial vacuum as possible.

The transverse cross-section of the main drain is preferably equal to the sum of the transverse cross-sections of the small drains, so that there is an equal contribution to the build-up of as high a partial vacuum as possible. A further embodiment of the method according to the invention is characterised in that the open end of the pipe slopes, in which the pipe is placed so that the longer section of the wall is on the upstream side. This protects the opening efficiently against the water that flows along the opening and increases the suction force. The water in a rip current for example flows obliquely downwards along the seabed and the longer section of the pipe is preferably at the top of the pipe wall.

A rip current can shift along the coast as a result of changes in the water current and wind direction. This could mean that the open end of an installed pipe no longer extends into a rip current, so that the suction force in the pipe decreases sharply. To prevent the open end of the pipe from falling outside a rip current, a still further embodiment of the method according to the invention is characterised in that at least one partition is placed in the water, which partition extends roughly transverse to the coast in the lengthwise direction. The water flow against the partition will cause turbulence near the partition, so that a rip current is formed along the partition.

A new rip current does not have to form if the partition is placed in a rip current and it is ensured that the existing rip current will not shift. Preferably a sheet pile will be used as the partition. To prevent the pipe from being pulled out of the rip current by a sideways current, a further embodiment of the method according to the invention is characterised in that the part of the pipe extending into the water is fastened to the partition. Preferably the pipe is fastened flexibly to the partition, for example with an elastic rope.

Moreover, further partitions, spaced apart in a line roughly transverse to the coast, will preferably be placed next to the above partition.

The invention also relates to a drainage system, which is laid using a method according to the invention.

Brief description of the drawings

The invention will be elucidated more fully below on the basis of drawings, in which embodiments of a system laid using the method according to the invention are shown. In these drawings:

Figure 1 shows a first embodiment of a system according to the invention laid on a beach from above;

Figure 2 shows a vertical cross-section of the beach shown in figure 1 ; Figure 3 shows a second embodiment of a system according to the invention laid on a beach from above; and

Figure 4 shows a vertical cross-section of the beach shown in figure 3.

Detailed description of the drawings

Figures 1 and 2 show a first embodiment of a system according to the invention laid on a sand beach 2 that is open to the sea 1 from above and in vertical cross- section respectively. There are several sandbanks 3 in front of the coast, between which there are rip currents 4. There are tideways 5 between the sandbanks 3 and the beach, which at low tide extend between the sandbanks and the low-water mark e and at high tide extend between the sandbanks 3 and the high-water mark v. This is a typical situation as is formed in a natural way in coastal regions, but can also be laid artificially. A set-up current S continually brings in water from the open sea 1 towards the beach 2. Once it has reached the water line (usually between low-water mark e and high-water mark v) the water is repelled by the beach 2. The water then flows, at least in a undercurrent, in a tideway 5 following the tideway current, for example ZSe at ebb or ZSv at flood, between the sandbank 5 concerned and the coastline concerned towards a rip current 4, through which the water then flows via the rip current MS back to the open sea 1. The pattern of the current is shown in figure 1. Figure 2 shows how water flows back to the rip current in the direction of arrow A and from there via a rip current MS over the seabed 8 back to the open sea 1.

The set-up current S brings water containing solid particles to the coast. The deceleration of the current near the beach 2 means that some of the particles drop to the seabed 8, where they can then settle. This ensures a build-up of the seabed 8 near the coastline and the beach 2 up to the high-water mark v. On the other hand, the set-up current S, the tideway current ZS and particularly the relatively strong rip current MS cause erosion of the beach 2 between the low-water mark e and the high-water mark v and of the seabed 8, as the current takes particles from the beach 2 and the bed 8 with it on its way back to the open sea 1. This natural phenomenon produces tideways 5, rip currents 4 and sandbanks 3, which remain in existence in a natural way.

First pipe sections 7a extend over the width of the beach 2 parallel to the coastline and form small drains. These first pipe sections are placed behind each other as seen in the direction of the open sea. These first pipe sections 7a are hollow and have a perforated plastic wall, for example a typical drainpipe as is also used for drainage on land. Second pipe sections 7b have been placed at right angles to the first pipe sections 7a, which are connected to the first pipe sections 7a. These second pipe sections 7b form main drains S and also have a perforated wall. The parts of the second pipe sections 7b that extend into the water are preferably not perforated. The pipe sections 7a and 7b are covered with a fibrous structure 9 (see figure 2) in order to increase their water permeability.

The ends 7c of the second pipe sections 7b mat extend into the rip currents 4 are oblique. The openings 7c are oriented so that the top of the pipe wall protrudes0 beyond the bottom. The water that flows with the rip current MS along the bed 8 towards the open sea 1 generates a partial vacuum at the ends 7c of the pipe sections 7b. This means a partial vacuum will also be created in the pipe sections 7a and 7b themselves, which is at least partly compensated in that water and sand are sucked from the beach 2 towards the pipe sections 7a, 7b, as shown in figure 2 by arrows z. Water from the beach 2 and the S seabed 8 between the low-water mark e and the high-water mark v therefore flows through the perforated pipe walls and is carried off to the sea through the ends 7c, because the pipe sections 7b end obliquely in the direction of the sea. An important effect is that the sand in the bed around the pipe sections 7a; 7b will become more compacted due to the sucking action of the pipe sections. This gives better protection against erosion as a result of the0 current. A coast, as a result of the natural current forces, will be less subjected to erosion than a coast without such pipe sections, or will even build up further. A big advantage is that with a strong current, when the risk of coastal erosion is relatively great, the effect of the measure according to the invention will increase because the sucking action as a result of the relatively strong rip current will also increase. Use is thus made of natural forces in5 order to protect the coast against coastal erosion.

The pipe sections have a diameter between 60 - 120 mm in order to obtain a good relationship between the suction force and the removal capacity. The combination of the suction force and the removal capacity is optimum within this range.

Figures 3 and 4 show a second embodiment of a system according to the0 invention laid on a sand beach 2 that is open to the sea 1 , again from above and in vertical cross-section respectively. All parts equal to those in the first embodiment are given the same reference figures. This second embodiment differs from the above first embodiment in that here partitions 10 have been placed in the rip currents to which the parts of the pipe sections 7b that extend into the water are fastened with flexible connections, for example elastic ropes. The partitions 10 are formed by several sheet piles that are placed in the bed next to each other. Water flowing against the partition will cause turbulence near the partition, so that a rip current is formed along the partition. This ensures that the open ends 7c of the pipe sections 7b will remain in the rip currents 4.

Although in the above the invention is explained on the basis of the drawings, it should be noted that the invention is in no way limited to the embodiments shown in the drawings. The invention also extends to all embodiments deviating from the embodiments shown in the drawings within the context defined by the claims.

Claims

CLAIMS:

1. Method for laying a drainage system under a beach on a coast in order to protect the coast against erosion, which comprises laying a pipe with a water-permeable wall and at least one open end under and roughly parallel to the surface of the beach, so that

S the open end extends into the water, characterised in that the open end of the pipe is placed in a rip current.

2. Method according to claim 1 , characterised in that the pipe comprises a first pipe section placed under the beach roughly parallel to the coast, and a second pipe section that is placed roughly at right angles to the first pipe section and which is connected by a0 first end to the first pipe section so that the pipe sections are connected together, in which the second end of the second pipe section forms the open end of the pipe.

3. Method according to claim 2, characterised in that seen in the direction of the open sea several pipes are placed in the bed behind each other.

4. Method according to claim 1 , 2 or 3, characterised in that the open end ofS the pipe is oblique, in which the pipe is placed so that the longer section of the wall is on the upstream side.

5. Method according to claim 1, 2, 3 or 4, characterised in that at least one partition is placed in the water, which in the lengthwise direction extends roughly transverse to the coast. 0

6. Method according to claim S, characterised in that the partition is placed in a rip current.

7. Method according to claim S or 6, characterised in that a sheet pile partition is used.

8. Method according to claim S, 6 or 7, characterised in that the part of the pipe5 that extends into the water is fastened to the partition.

9. Method according to claim S, 6, 7 or 8, characterised in that in addition to the above partition further partitions are placed apart in a line, roughly transverse to the coast.

10. Drainage system laid using a method according to one of the preceding0 claims.