PNEUMATIC BREAKWATERS
Ju-Jitsu Methods Overcome Wild Sea Waves
WELDMESH
EROSION
CRATES
Erosion crates or gabions are used for
In October 1955 the Marley Tile Com- that the bubbles serve to create a state pany, whose range of products includes of disorder against which the harmonic Marleythene (polythene) pipes, gave facili- motion in the incoming waves is distorted building dykes, piers, groynes, weirs and ties to Mr. A. H. Laurie, principal and and converted into energy of random
chief scientist of Pneumatic Breakwaters turbulence. Ltd. for the developing of plant and ap paratus to test his invention for reducing seawaves.
In short the necessary power to form a wave barrier is for the most part found from the energy in the waves themselves. Attempts to calm the sea by using com- and the Pneumatic Breakwater as now pressed air have been made intermittently designed can be regarded in the light of since 1916, without much success. All a "ju-jitsu" exercise in which the strength these attempts were made on the assump- of the attacker is used to bring about the tion that the function of air bubbles was attacker's downfall. to create strong vertical and, at the sur- face, horizontal streams of water which would literally push back the waves, and therefore prevent their passage beyond the
line of the undersea installation,
A
that
retaining walls to prevent erosion and landslides, and to assist in the control of storm water and tidal conditions. The crates are made up from welded steel wire mesh of suitable aperture size and gauge into cages of convenient size to suit the particular conditions; they can be supplied in collapsed form, readily erected at site and them filled with stone or any sub- stance obtainable in the locality where used.
The Pneumatic Breakwater does not
The Manchester Corporation recently attempt to produce calm water: would be uneconomic. wave height used collapsible Weldmesh Erosion Crates reduction of 50% is in most situations 72" long x 39" wide x 39" deep, to stop enough to improve conditions to the point erosion at Moston Brook. The wall is where, for instance, at a tanker jetty or approximately 90' long and follows the an offshore drilling rig, the availability of bend of the river, it has a second rein- the site for ship handling can be enormous-
forcing tier at the sharpest point of the ly improved.
bend. The crates were made
run
up from
The Lancashire County Council has also used Weldmesh Crates in various places. At Savick Bridge, undermining was threat- ening adjacent property on a bend in the river; a three-tiered Erosion Crate Wall some 200 ft. long was erected оп the location and has proved completely suc- cessful. At Foundry Bridge, the founda-
While it is true that waves can be dealt with in such a manner, the demands on air are phenomenal and quite outside the range of commercial operation. The method used in these attempts, which have been made in Britain, America, Germany, Whereas, by the old perforated pipe | 3" x 3" x 10 gauge dip galvanized Weld- Russia and Japan, was to blow compress- method, a fifty per cent wave reduction mesh. ed air through a perforated pipe on the was achieved at a cost of 4 horsepower seabed.
per yard
of breakwater, the new patented system achieves the same result Pneumatic Breakwaters Ltd. approached with 0.3 horsepower per yard. According- the problem in a different light. They ly, the running cost of a temporary har considered that the air bubbles could be bour or
an improvement to an existing made to perform a different function, and harbour has now for the first time become that what was wanted was a system of
a commercial proposition. air distribution from the sea-bed which would create a local turbulence, and thus All this work has been greatly facilitated tions on the lower side of the bridge were interrupt the normal wave action. In by the use of Marleythene pipes. Nothing being undermined; Weldmesh crates were pursuing this line of attack they were but a. first-class polythene could have used the greater part of which are deliberately attempting to imitate what stood up to the continual corrosion, submerged and now after four years occurs in nature. It is a matter of com- erosion and battering of the sea. Marley- they are still in perfect condition. At mon observation that waves do not pass thene has undoubtedly done a very good Stang Bridge, silting and erosion were through the turbulent water in the wake job in this respect and given the lead to overcome with a four-tier retaining wall. of a ship, nor do they pass through the all who require economical pipes for under, On each of these jobs the crates were zone of eddies generated in a "tide-rip" over or by the sea use.
made up and assembled in the site from where a local tidal stream conflicts with
Ref 310 and 48 hot dip galvanized still water.
In the assembly and laying stages of Weldmesh. The mesh was found to be these breakwaters, it was the flexibility, very easy to handle and skilled labour They therefore devised a system where- light weight and ease of handling of was not required to assemble the crates by large bubbles were released intermit- Marleythene pipes that made the difference which can be of any size to suit the tently from the seabed. This churned up between a possible and impossible job. | particular conditions. the water along the line of the installation, Metal joints were obviously "out" for use
results which surprised even the in
the corrosive sea, so Vulcathene polythene joints made by J. S. & F. It was found that the eddies resulting thene by the Polyfusion (Folkard) method, Folkard were welded to the Marley from the bubbles were themselves power- less to reduce any but the smallest local and the installation stood "chop." But, provided the oncoming rigours of the moving sea for months waves exceeded a certain minimum height, without maintenance. the thin line of eddies spread gradually Fittings made of polythene have been over a period of about five minutes until available and suitable for joining Marley- thene for some years, thus affording con- tinuous pipe runs entirely of polythene. steel wire mesh made by B.R.C. Engineer- Weldmesh is an electrically cross-welded However until the polyfusion technique ing Co., Ltd. It is available in a very the electrically welded polythene wide range of gauges and mesh sizes and joints came along, the all-polythene pipe is supplied in sheets or rolls. It is special- was rarely seen outside chemical ly suited for Erosion crate construction be- works and laboratories. The sole agents cause it is flexible, efficient and economical. The horsepower required to produce this for the Marley Tile Co., Ltd. in Hongkong defence in depth is considerable and does are Messrs. Dreyer & Co., Ltd. of not come from the bubbles. It appears Alexandra House.
with inventors.
there was established to seaward of the bubble line a zone of broken water, from twenty to forty yards wide. It was this wide zone of turbulence which served as a "defence of depth" through which a wave could not pass without losing a large part of its original energy.
and
run
up
to the
A great construction
advantage in this method of crates which when filled permit the entry is the permeability of the
and drainage of water in them. An im- portant feature has been the use of foundation mattresses (a form of crate) which are laid on the river bed and then filled. The mattresses settle down and conform to the uneven surface thus giving a rigid and level foundation for the walls.
The Agents in Hong Kong are: Davie,
Boag & Co., Ltd.
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