The Dikes of Holland 



22 



7 



in many places has been very consider- 

 able. Lands that were at first at a level 

 with tidewater have shrunk in the course 

 of years from four to seven feet, until 

 their level has sunk below that of mean 

 low water. When the sea therefore suc- 

 ceeded in flooding such low areas, the 

 possibility of their being reclaimed was 

 practically^ forever ended. The shrink- 

 age of the soil has manifested itself 

 throughout Holland wherever clay and 

 peat are encountered. It is therefore 

 evident that the level of the land of the 

 Holland of today is many feet lower 

 than it was at the time of the Romans, 

 when the first dike was built. The 

 level of the provinces of Zeeland and 

 Holland ranges between two and six 

 feet below mean high water, while that of 

 the drained areas is much lower, reach- 

 ing a depth in some cases of 20 feet be- 

 low mean high tide. Reclaiming land 

 from flats in shallow waters has also been 

 practiced in the northern provinces of 

 Friesland and Groningen, though not as 

 extensively^ as in Zeeland. 



THE CHARACTERISTICS OF THE DUTCH 

 DIKES 



The problem of building dikes in a 

 country possessing a soil which offers 

 so little choice in the way of building 

 material as does Holland is to any one 

 but a Dutchman very perplexing. The 

 country has not a single quarry, nor is 

 loose rock available; the few woods that 

 exist are being preserved with great 

 care, and no timber can be cut from 

 them for lumbering purposes. All that 

 the soil of Holland offers is in the form of 

 sand, gravel, and clay, for peat is worse 

 than useless in construction works ; 

 and not onl}^ are the available materials 

 poor, but suitable foundations upon 

 which to erect dikes or, for that matter, 

 any structures whatever, are totally ab- 

 sent. This is the problem that has been 

 solved by the Dutch engineers through 

 generations and generations of expe- 

 rience. 



It is not within the scope of this paper 

 to describe the many different kinds of 

 dikes in use: their forms vary as circum- 

 stances require, and a lengthy discus- 

 sion of them would lead into endless 

 technical details. In brief, the princi- 

 pal features may be described as fol- 

 lows : 



Compared with similar structures else- 

 where, the Holland dikes are noteworthy 

 for their great width; the river dikes 

 are built with a crown, usuall}^ of from 

 15 to 20 feet wide, while the common 

 type of the Mississippi levees has only a 

 crown width of 8 feet, the height being 

 about the same. The slopes are gentle, 

 a common grade on the water side being 

 three and a half to one, and on the land 

 side two to one. A characteristic fea- 

 ture of the Dutch river dikes is what is 

 technically known as the "banquette," 

 a sudden widening of the dike near its 

 base, which serves to reinforce the dike, 

 and is specially designed to insure im- 

 perviousness where the hj^drostatic pres- 

 sure is greatest. The banquettes are 

 built on either side of the dike, and vary 

 in width from 10 to 30 feet. The larger 

 river dikes range in height between 10 

 and 16 feet above the adjacent land, 

 while the level of their banquettes is 8 

 feet below the top of the dike. 



The materials used in their construc- 

 tion are sand and clay, and in the case 

 of the ordinar}' dikes the water side is 

 rendered impervious by means of a heavy 

 layer of stiff clay. As a rule, no special 

 preparations are made for the founda- 

 tions, except where the soil is of a very 

 treacherous character, when fascine 

 mattresses laid in tiers are used, in very 

 much the same manner as along the 

 Mississippi. W'herever riprap or stone 

 revetments are required, as, for instance, 

 on the sea dikes, where the erosive ac- 

 tion of the surf is considerable, basalt 

 blocks brought from Germany are laid 

 on heavy layers of brush. In many 

 places piles are driven at the base of the 

 sea dikes in order to break the violence 



