In the illustration, the shingle is shown by the dotted part as having accumulated to the full extent due to the length of the groyne. The thick line at right angles to the coast shows the single large groyne. The dotted lines represent the number and position of low groynes as usually placed to protect the same length of coast.

The effect produced by a single groyne in promoting accumulation of material and providing a beach free from all projections and encumbrances, and the advantages of this system as compared with that where a number of groynes are placed at short intervals, are exemplified on the coast at Cromer, a description of which will be found in Chapter VII.

The shingle beach in front of the wall at Hythe has been accumulated, and is held up along its whole length, by a single concrete and timber groyne at the east end; and the result cannot but be regarded as more satisfactory than the unsightly and

FIG. 21.-Plan showing Accumulation of Shingle against Groynes.

uneven condition of the adjoining beach at Seabrooke, which is protected by a number of high groynes at short intervals.

The construction of the west pier of the harbour at Folkestone, and the groyne adjoining it, led at once to the making up of the beach for a width of 500 yards near the pier, tailing off to a distance of half a mile; all the old groynes which formerly obstructed the use of the shore being now buried.

The same has been the case at Shoreham, where the west pier of the harbour holds up the shingle along a frontage of 13 miles, beyond which the beach has been obstructed by some high timber groynes.

The pier at Littlehampton is boarded on the west side, and has prevented the eastward drift, and raised the beach for a considerable distance, the old groynes being buried.

At Hastings the concrete groyne at the east end of the promenade and road led to a large accumulation of shingle, which is gradually spreading westward, making a far more

effective protection to the sea-wall than the high unsightly groynes which obstruct the remainder of this beach.

At Southwold the wooden jetty which has been projected out from the shore for protecting the river Blyth from the littoral drift has led to the accumulation of a bank of shingle half a mile in length and 110 yards wide.

The fine level stretch of sand at Yarmouth, free from any groynes, is due to the projection of the north pier; and at Lowestoft, after the jetties at the entrance to the harbour were built, the beach rapidly accumulated on the windward side, and in 20 years advanced over 100 yards seaward.

Height.—The less the height of the planking of groynes rises above the beach, the less the effect of the waves in disturbing and eroding the beach. Low groynes are therefore more effective in securing an even surface than those which are carried several feet above the beach. The most effective plan, where the groynes are placed at short intervals, is to commence with the planking raised about 18 inches or 2 feet above the surface, and gradually to raise it as the material accumulates.

High groynes are costly to construct in the first instance, and afterwards to maintain. They require to be made very substantial to resist the impact of the waves and the pressure of the large amount of shingle that accumulates on one side only.

They are less effective in accumulating material than low groynes raised from time to time as the beach accumulates.

The profile of a beach where high groynes placed at short distances apart exist, if traced along a line horizontal to the shore, consists of a series of mounds, the shingle being accumulated against the groyne to a great height on the windward side, and tailing off at a steep slope, leaving the leeward side of the bay bare, the drop from the surface on one side to that on the other sometimes being from 10 to 15 feet. Where high groynes are used for the purpose of collecting beach to support a seawall, they offer a very irregular support and protection. The mounds of shingle oscillate between one side of the bay and the other as the direction of the gales varies.

So, also, a profile taken in a direction transverse to the coast will show the shingle heaped up to a great height, and sometimes flowing over the sea-wall and on to the roadway at the top end, and sloping seawards with a rapid inclination, leaving the beach bare lower down.

Temporary Groynes.-Where groynes are placed at short distances apart, if properly arranged and not carried too high, they will become buried by the accumulating shingle, and there is, therefore, no service in making them of a more substantial and permanent character than is necessary to serve the temporary use they have to fulfil.

After a beach has been raised to nearly high-water mark, or to a little above mean tide-level, the accumulation of shingle will proceed without artificial aid when there is a continuous supply of drift.

Sloping Groynes.-In order to avoid the cutting out of the beach which occurs on the lee side of a groyne, a form has been used in France of the shape of the letter A, a centre pile being driven with walings on each side, from which planks are laid at an angle of 45 degrees to other walings attached to shorter piles at the level of the beach. It is claimed for these groynes that there is less disturbance of the beach by the waves, and that consequently the beach makes up equally on both sides. Groynes of this shape have also been used at Mablethorpe, on the East Coast, and at Bridlington, a description and illustration of which will be found in Chapter VII.

Direction. With reference to the most effective direction to be given to groynes, the general practice is to place them at right angles to the shore-line, experience having shown that this on the whole gives the best results. The worst direction is at an angle sloping to the windward side, or towards the source from which the regular supply of drift comes; and although examples are to be found of groynes placed in this direction, experience cannot be said to justify this system. Groynes sloping to leeward, or away from the source of supply, are advocated by Mr. R. F. Grantham, who has had considerable experience in works of coast protection on the South Coast. The groynes he has constructed at Lancing and Middleton have been made to point in a south-easterly direction, and he considers the result as satisfactory.

Length. As to the length of groynes. Where there is a fair supply of shingle along the coast, it is not necessary to carry groynes as far down as where the beach consists only of coarse sand with a small sprinkling of pebbles, and it will be found sufficient if they extend to low water of neap tides; but as groynes are generally required on beaches which have been more or less denuded of shingle, and their purpose is to prevent any drifting

of material away from the beach to be protected, the groyning ought to extend from the cliffs or sea-wall to low water of spring tides. Although shingle accumulates only at the upper part of the beach, there is always-especially after on-shore gales—a certain quantity spread all over the beach down to or below low water. A groyne extending the whole depth of the beach will collect any coarse sand or shingle drifted against it, and material so stopped from travelling will gradually work along the groyne to the upper part of the beach.

If the groyne to the extreme leeward end of the part to be protected be carried as far seaward as practicable, so that it may stop the drifting material, each successive groyne to windward may be made shorter, and lengthened from time to time as the beach accumulates. This will be better understood by a reference to the diagram given on p. 120.

Cost of Groynes.-This will vary with the condition of the coast to be protected and the price of materials and labour. The following, however, may be taken as representing approximately the relative cost of the various systems.

For the purpose of this comparison, a coast-line has been taken where the cliffs are just reached by the spring tides. The width of the beach to L.W.S.T. as 100 yards; rise of tide 15 feet; slope of beach 1 in 10 at upper part, and 1 in 30 at the lower, or mean of 1 in 20. The groynes to extend from the cliff to low water, or 100 yards; the distance of the groynes apart as 120 yards.

For the system of high groynes, the type is taken as that in use at Hove and Hastings; for the low groynes, those at Worthing and Felixstowe; for the single groyne, those at Cromer, supported by struts and spaced 600 yards apart. These groynes are all described in Chapter VII. In order to reduce all to the same standard, the Case groynes are taken the same length as the others, although the practice is for them to commence only at mean high water; but, on the other hand, they are placed nearer together than 120 yards, and it is doubtful whether to be really effective they ought not to reach higher up the beach.

For the larger groynes, the height is taken as 12 feet at the upper end and 4 feet at the lower, or a mean height of 8 feet; and for the low groynes, when raised to their full height, 5 feet at the upper end and 2 feet at the lower end, or an average throughout of 3 feet 6 inches.

The cost of pile-driving, timber, and ironwork for each different system of groynes is taken at the same prices.

Relative Cost of Groynes.

The groynes are taken at 100 yards long and 120 yards apart (14.66 groynes to a mile). High groynes, mean height 8 feet; low groynes, 3 feet 6 inches.

Time of Accretion.-As the beach grows up from the accumulation of the drift, the low water is gradually pushed more seaward and the depth of the water at high tides decreased, and consequently the eroding and destructive action of the waves diminished. The time that must elapse before improvement takes place depends on the abundance or scarcity of the supply of material, but, speaking generally, a period of from two to three years must be reckoned before any substantial increase in the height of the beach will ensue.

Case System.-The late Mr. E. Case, engineer of the Romney Sea Defences, who paid great attention to the system of groyning, and had considerable experience of its effect both at Dymchurch and other places, was a strong advocate of long low groynes, and advised strongly against the use of high groynes, contending that the building up of the foreshore should be gradual and progressive, and that the object to be sought is the remodelling of the beach to its natural inclination of repose. The inclination varies according to the material of which the beach is composed, being steeper where shingle exists than where sand only prevails. For a beach containing a mixture of shingle and sand, he put it at 1 in 12 for the upper part, increasing to 1 in 40 lower down, and towards low water of spring tides 1 in 70. He contended that groynes should accordingly be placed at this angle, the planking being placed horizontally and stepped to meet the varying slope. He also contended that with high