at Petten; but the exposure here is much greater, the bank facing nearly due north, and being therefore exposed to a fetch of over 2000 miles. The waves beat on this part of the coast with great violence. This bank protects about 70,000 acres of land, the -Brick Paving E.H. W. -Concrete A H.W. OSTEND. FIG. 20. surface of which is 6 feet below high water. The tidal rise above low water is 11-20 feet, extraordinary tides rising 5.86 higher, or 17.06 feet. The bank dates from the ninth century, but has within recent times been increased in size and consolidated. It is 2.36 miles in length, and its top is 27.38 feet above low water, and 16·18 feet above average high tides. It was built and is maintained by the Island of Walcheren, the annual cost of maintenance amounting to about £16,000, equal to a rate on the land of about 8s. an acre. The bank is protected by a series of groynes, the construction of which is described in Chapter VII. The width of the bank at the top is 40 feet. The average slope is 1 in 14, divided as follows: lower part, for a horizontal distance of 72 feet, 1 in 6, covered with stone up to average high water. All this part is protected by a stockade composed of eleven ranges of piles 6 inches in diameter and from 8 to 11 feet in length, the heads projecting from 3 to 5 feet above the face, and spaced 1 foot 6 inches apart. Above this for 30 feet the slope is 1 in 7, which is pitched with basalt. Above this for 20 feet the slope is 1 in 8; then for 200 feet 1 in 17. This upper part is protected with clay and straw matting. Above this is basalt pitching, and the upper part is sodded. In the less exposed part the face is defended with stone pitching and fascines. Petten Sea-bank.-For the preservation of the Petten and Hondsbossche Polders on the coast of Holland, about 12 miles south of the Helder, a sea-dyke 3.41 miles in length was built many years ago. This bank was reconstructed during the period 1870–80. The beach in front is entirely sand, and extends from 300 to 350 feet to low water, the slope being about 1 in 60. The bank crosses a gap in the sand-dunes, to which it is joined at each end. The tide here rises about 5 feet, or 2.23 feet above Amsterdam Peil, or mean level of the sea. During a gale in December, 1863, high water rose 8.20 feet above the level of ordinary high tides, and within 11·48 feet of the top of the bank. During another gale in December, 1883, the waves ran up the bank nearly to the top, or to a height of 18.70 feet above mean high water. The top of the bank varies from 19-70 to 23 feet above mean high water. This bank is repaired by two different authorities, the Hondsbossche bank being in charge of the local Polder Commissioners, and the Petten bank under the charge of the Govern ment. The Government bank is 0.62 mile long, and has a total width of 253 feet at the base, the sea-slope averaging about 1 in 8. The lower part, up to the level of high tides, slopes at the rate of 1 in 4; then there is a stretch of 1 in 40; another of 1 in 24; above this 1 in 20. The material for the body of the bank was obtained from the inside, at some distance from the work. It is pitched at the bottom up to the level of the highest tides, a length of 60 feet, with basalt from Andernach, on the Rhine, laid on a thick bed of clay. The blocks as obtained from the quarries are hexagonal in shape, and are broken off into lengths of from 12 to 18 inches, and placed on a matting of osiers made into fascines 5 inches in diameter, and from 40 to 50 feet in length. A row of piles, 6 inches in diameter, is driven into the clay about 1 foot apart, the rows being 9.84 feet apart, the heads being left projecting about 3 feet. Above this is a long, flat slope about 87 feet in length, which is thatched with wheat straw laid in bundles overlapping each other, placed on a thick bed of clay, the covering being renewed every autumn. The bundles are fastened down to the clay by bands of straw laid parallel with the dyke, and pressed 6 inches into the clay with a special tool. This straw covering is only expected to last for one season. The straw thatching is terminated by a row of piles, and above this for about 30 feet is basalt pitching on a bed of clay, the clay covering being continued to the top of the bank. The Polder, or Hondsbossche dyke, which is 2.79 miles long, has been treated rather differently. The dimensions are approximately the same, the top of the bank being rather higher. Instead of the straw thatching, basalt pitching is placed for a length of 92 feet, at a slope of 1 in 24, up to a level of about 5 feet above the highest known tide; and above this to the top the face is covered with clay to a thickness of about 4 feet. The cost of the Polder dyke was £183,333, and of the Government bank £41,666, or an average for the two of £63049 per mile. The cost of maintenance is about £2000 a year.1 The Helder Dyke. The great dyke at the Helder is described by Marsh 2 as being 5 miles in length and 40 feet wide at the top. It slopes down 200 feet into the sea at an angle of 40 degrees. The base is always washed by the swell of the sea, but the highest waves do not reach the top. The sea-front of the bank is composed of Norwegian granite. Groynes at regular intervals run out from the bank to low water. On the Friesland coast, over a length of 150 miles, where there are no dunes, the earthen banks have been protected at their foot by triple rows of piles, bound together by longitudinal and cross timbers, the space between the piles being filled in with stones. The ground adjacent to the piling is secured by fascines, and at the more exposed parts heavy blocks of stone are heaped up as an additional protection. Cranz is a sea-bathing resort on the shore of the Baltic standing on a cliff consisting of loamy clay from 16 to 20 feet high. This cliff was gradually being undermined and washed away by the action of the sea. About 40 years ago some short groynes were constructed in front of the cliff, but, not being carried high enough up the beach, did more harm than good; the waves breaking above them cutting out the beach, a current was set up between the upper ends of the groynes and the cliff, which caused considerable erosion. Subsequently these groynes were carried up to the foot of the cliff, and extended seaward to a depth of 5 feet. 1 Centralblatt der Bauverwaltung, January, 1898. 2 "Man and Nature," Marsh. 1864. For the protection of the Corso and bathing establishment, a retaining wall of rubble masonry was built, having a width of 5 feet at the top and 10 feet at the base, which was carried 3 feet below the mean water-level. This wall was protected by a row of piles The cost was £4 a foot run. 3 feet 6 inches long. The foot of the cliffs was protected by a breakwater consisting of a double row of piles 3 feet 3 inches apart; the front battering at an angle of 1 in 4, the back row being vertical, the space between the piles being filled with fascines. This breakwater was strutted with timbers driven at an angle of 45 degrees, the heads being secured to the back piles, these being strengthened by longitudinal waling and cross-braces to the front piles. In front of the breakwater, and parallel with it, two rows of piles were driven at distances of 6 feet and 13 feet, and having a plank spiked to them at the bottom for the purpose of diminishing the effect of the backwash. These piles were 10 to 12 feet long, and 8 inches square. The cost of this breakwater was 30s. a foot run. The general effect was good; the groynes became covered with sand, and a foreshore was formed between the breakwater and the front row of piles (Min. Proc. Inst. C.E., vol. li., 1877). Nordenay. The face of the bank here is paved with heavy sandstone blocks, laid on concrete to a little above the level of the highest tides, the lower face being concave, with a radius of 14.76 feet, and the lower part convex, and above this brick paving on concrete. The base is protected by a toe 18 feet long, consisting of rows of piling with transverse timbers, the space between being filled in with large blocks of stone, the total thickness being 4 feet. LIBRARY SCRIPPS INSTITUTION UNIVERSITY OF CALIFORNIA LA JOLLA, CALIFORNIA CHAPTER VI. GROYNES. FOR the protection of a shore where cliffs exist to provide the material, and where the object to be attained is only the preservation of the land, shingle-banks are sufficient for the purpose, and a sea-wall is not required. For the purpose of gathering the shingle and raising the beach, groynes are made use of, placed on the beach transversely to the coast-line. The system of groyning generally adopted may be divided into three classes (1) High substantial structures placed at short intervals apart. (2) Those of a less expensive character, the planking of which rises only a short distance above the beach. (3) Where reliance is placed on a single substantial structure to accumulate the material over a considerable length of coast. Groynes in themselves cannot be regarded as a protection, but are serviceable in the effect they produce on the beach. They do not create fresh material, but simply stay the littoral drifting action, and so prevent denudation and assist in accumulation. Provided they stay the drift of the shingle from the length of the coast to be protected, that is all that can be expected from them. There is, therefore, little or no service to be derived from placing them on a coast where there is no material to collect, and no wasting cliffs to provide fresh supplies, except to prevent the formation of lows on sandy shores or beaches. Effect of Groynes on Wave-action.-Groynes are of no service in moderating the force of the waves, or preventing them breaking on the beach, but, on the contrary, tend to add to their destructive effect. When the momentum of a wave is suddenly |