Page images
PDF
EPUB
[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

These three Diagrams show the Enlargement of one small Mud Flat to ten times its original size

in many places has been very considerable. 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 succeeded in flooding such low areas, the possibility of their being reclaimed was practically forever ended. The shrinkage 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. 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, reaching a depth in some cases of 20 feet below 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

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 only are the available materials poor, but suitable foundations upon which to erect dikes or, for that matter, any structures whatever, are totally absent. This is the problem that has been solved by the Dutch engineers through generations and generations of experience.

It is not within the scope of this paper to describe the many different kinds of dikes in use: their forms vary as circumstances require, and a lengthy discussion of them would lead into endless technical details. In brief, the principal features may be described as follows:

Compared with similar structures elsewhere, the Holland dikes are noteworthy for their great width; the river dikes are built with a crown, usually 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 feature 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 imperviousness where the hydrostatic pressure 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 construction are sand and clay, and in the case of the ordinary 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 foundations, 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. Wherever riprap or stone revetments are required, as, for instance, on the sea dikes, where the erosive action 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

of the breakers. Nearly all masonry in engineering constructions is of Dutch brick, which is of a very superior quality. In breakwaters or piers, however, concrete blocks are used exclusively, as neither bricks nor basalt would furnish a bond-strong enough to withstand the impact of the waves.

ENGINEERING PROBLEMS

From a hydrographic point of view, the Netherlands present a very unusual spectacle. While the eastern elevated portion has a natural topography of its own, and consequently natural lines of drainage, the western lowlands are devoid of all drainage whatever, and every drop of rain water that falls, as well as all seepage water, must either evaporate or be pumped up and discharged through artificial means into the ocean, if accumulations and inundations are to be prevented.

Statistics show that in 1896, 2,519 square miles of polder land were being maintained with the aid of 444 steamengines and 247 windmills; 1,234 square miles were being maintained with the aid of 1,706 windmills, or in all 2,397 pumping plants were required to drain 3,753 square miles.

What at one time were natural channels and water-courses have been since inclosed between dikes, and the level of their waters is now higher than that of the adjacent land. The large rivers that flow through these low districts are therefore here no longer rivers in the strict sense of the word, as the features and problems which they present are very distinct from those characteristic of natural streams. The smaller streams have in reality ceased to exist as such. For instance, the northern branch of the Rhine, along which general Drusus caused a levee to be built, is no longer a river; its waters no longer flow; it is

[graphic][merged small]
[graphic]

The Dunes near Domburg, in the Province of Zeeland

nothing but an artificial channel, held between embankments and divided into a series of sections closed by means of locks. No longer does it empty its waters into the sea at Katwijk, where the light-house of Caligula once stood on an island in its estuary; but when the lock-tender at that point has orders to do so, some of its waters are allowed to escape at low tide when it is considered perfectly safe. The same condition is true of the smaller streams of the polder lands. Protected on the sea side by the dunes and dikes and partitioned off in the interior by an endless array of dikes which skirt the water-courses and canals, surround polders, and also serve as embankments to railroads and highways, Holland partakes much of the nature of a huge ship with water-tight compartments.

The immense amount of engineering which is required to keep up this com

plicated system of dikes and waterways has always been a source of interest to technical men in other countries. No haphazard guesses are made as to the amount of water permissible in any particular waterway, nor as to the height or size of dikes required. Matters of this nature are determined with great nicety through the accumulations of past experience. As one waterway is frequently made to relieve another and the number of combinations must be varied as circumstances require, a knowledge of the fluctuations in the levels of all bodies of water becomes paramount. In order to supply this information, no less than 172 gage rods are maintained throughout the kingdom along the coasts, at estuaries, on large rivers, on canals, bosoms, and small streams, and a few even are located in foreign countries, as, for instance, the gage on the Rhine River at Cologne, Germany, which

has been maintained there by the Dutch Government since 1772. In order to derive the greatest possible use from the data so obtained, all the gage rods in the kingdom are referred to the same baselevel, mean high water, generally denoted by the symbol AP, and the heights of water thus indicated by them give directly the elevation of the waterlevels with respect to that of mean high water of the sea.

The present kingdom has an area very nearly equal to the combined areas of the States of Connecticut and New Jersey.

Connecticut..... 4,990 square miles.
New Jersey..
7,815 square miles.
Netherlands.... 12,738 square miles.

About 59 per cent of this area consists of alluvial formation, and is inclosed by dikes and provided with artificial drainage. There are, therefore, about 7,515 square miles of lowlands, very nearly equivalent to the area of the State of New Jersey, while the remaining highlands would cover an area about equal to that of the State of Connecticut. The discharge of the Rhine at the point where it enters the country is similar to that of the Tennessee River, while the flow of the Meuse may be compared with that of the Potomac.

SMALL AVERAGE RAINFALL IN THE

NETHERLANDS

The

As a large part of the Netherlands is drained artificially, a few words concerning the rainfall will be of interest. country enjoys the unenviable reputation of possessing a wet soil and a still wetter atmosphere. Both of these attributes are popular exaggerations. The atmosphere of the Netherlands is frequently moist-that is, it contains at times a high relative humidity-but the rainfall nevertheless is moderate, not to say small. As compared with the United States, it will be found that the amount of precipitation that occurs annually in

the Netherlands is about the same as that of the Great Plains region. The normal precipitation for the Netherlands, as derived from observations extending over more than a century, is about 26 inches per annum, or only 5 inches more than half of the amount of rain that falls annually in Washington, D. C.; and, in spite of the reputed moist atmosphere of the Netherlands, the evaporation during the early summer months exceeds the precipitation.

Table of Evaporation and Precipitation from Observations Made at Zwanenburg, near Amsterdam, During 1743-1843.*

[blocks in formation]

In other words, there is a decided dry season, during which droughts are by no means uncommon. In order to keep the water in the ditches at the proper level, to prevent plant-growth from suffering during such droughts, an efficient remedy is found in allowing the water in surrounding bosoms and canals to run back into the polders, and the usual process of their maintenance is thus actually reversed.

The polder lands known as Rijnland,

* From A. A. Beekman, Nederland als Polderland, p. 100.

« PreviousContinue »