Imágenes de páginas
PDF
EPUB

'tween the piles. These will serve to prevent the washing away of the filling during construction. The pile driving, and the preparation of the fascines may be done before the closing of the channel with earth is commenced, and if upland clay or gravel, to be mixed with the local mate rial, can be economically brought to the place by boats or wagons, it will be an advantage. Everything being in readiness, a sufficient force of laborers to finish the dam in six hours should commence the work a little before dead low-water, and, (with the aid of wheelbarrows, if necessary,) throw the earth in rapidly behind the row of stakes and fascines, giving the dam sufficient width to resist the pressure of the water from without, and keeping the work always in advance of the rising of the tide, so that, during the whole operation, none of the filling shall be washed away by water flowing over its top.

If the creek has a sloping bottom, the work may be commenced earlier,—18 soon as the tide commences to recede,--and pushed out to the center of the channel by the time the tide is out. When the dam is built, it will be best to heavily sod, or otherwise protect its surface against the action of heavy rains, which would tend to wash it away and weaken it; and the bed of the creek should be filled in back of the dam for a distance of at least fifty yards, to a height greater than that at which water will stand in the interior drains,—say to within three feet of the surface, so that there shall never be a body of water standing within that distance of the dam.

This is a necessary precaution against the attacks of muskrats, which are the principal cause of the insecurity of all salt marsh embankments. It should be a cardinal rule with all who are engaged in the construction of such works, never to allow two bodies of water, one on each side of the bank to be nearer than twenty-five yards of eack other, and fifty yards would be better. Muskrats do not bore through a bank, as is often supposed, to make a pas

ne the tide is one or otherwise propould ten

sage from one body of water to another, (they would find an easier road over the top); but they delight in any ele vated mound in which they can make their homes above the water level and have its entrance beneath the surface, so that their land enemies cannot invade them. When they enter for this purpose, only from one side of the dyke, lhey will do no harm, but if another colony is, at the same time, boring in from the other side, there is great danger that their burrows will connect, and thus form a channel for the admission of water, and destroy the work. A disre. gard of this requirement has caused thousands of acres of salt marsh that had been enclosed by dykes having a ditch on each side, (much the cheapest way to make them,) to be abandoned, and it has induced the invention of various costly devices for the protection of embankments against these attacks.*

When the creek or estuary to be cut off is very wide, the embankment may be carried out, at leisure, from each side, until the channel is only wide enough to allow the passage of the tide without too great a rush of water against the unfinished ends of the work; but, even in these cases, there will be economy in the use of fascines and piles from the first, or of stones if these can be readily procur. ed. In wide streams, partial obstructions of the water

* The latest invention of this sort, is that of a series of cast iron plates, set on edge, riveted together, and driven in to such a depth as to reach from the top of the dyke to a point below low-water mark. The best that can be said of this plan is, that its adoption would do no harm. Un. less the plates are driven deeply into the clay underlying the permeable foil, (and this is sometimes very deep) they would not prevent the slight infiltration of water which could pass under them as well as through any other part of the soil, and unless the iron were very thick, the corrosive action of salt water would soon so honeycomb it that the borers would easily penetrate it; but the great objection to the use of these plates is, that they would be very costly and ineffectual. A dyke, made as described above, of the materia: of the locality, having a ditch oply on the inside, and being well sodded on its outer face, would be by cheaper and better.

course will sometimes induce the deposit of silt in such quantities as will greatly assist the work. No written de scription of a single process will suffice for the direction of those having charge of this most delicate of all drain. age operations. Much must be left to the ingenuity of the director of the work, who will have to avail himself of the assistance of such favorable circumstances as may, in the case in hand, offer themselves.

If the barrier to be built will require a considerable outlay, it should be placed in the hands of a competent engi. neer, and it will generally demand the full measure of his skill and experience.

The work cannot be successful, unless the whole line of the water-front is protected by a continuous bank, suffici. ently high and strong in all of its parts to resist the action of the highest tides and the strongest waves to which it will be subjected. As it is always open to inspection, at each ebb tide, and can always be approached for repair, it will be easy to keep it in good condition; and, if properly attended to, it will become more solid and effective with age.

The removal of the causes of inundation from the upland is often of almost equal importance with the shutting out of the sea, since the amount of water brought down by rivers, brooks, and hill-side wash, is often more than can be removed by any practicable means, by sluice gates, or pumps.

It will be quite enough for the capacity of these means of drainage, to remove the rain-water which falls on the flat land, and that which reaches it by under-ground springs and by infiltration,-its proper drainage-water in short,—without adding that which, coming from a higher level, may be made to flow off by its own fall.

Catch-water drains, near the foot of the upland, may be 80 arranged as to receive the surface water of the hills and carry it off, always on a level above that of the top of the embankment, and these drains may often be, with advan. tage, enlarged to a sufficient capacity to carry the streams as well. If the marsh is divided by an actual river, it may be best to embank it in two separate tracts; losing the margins, that have been recommended, outside o the dykes, and building the necessary additional leugth of these, rather than to contend with a large body of water. But, frequently, a very large marsh is traversed by a tortuous stream which occupies a large area, and which, although the tidal water which it contains gives it the appearance of a river, is only the outlet of an insignificant stream, which might be carried along the edge of the upland in an ordinary mill-race. In such case it is better to divert the stream and reclaim the whole area.

When a stream is enclosed between dykes, its winding course should be made straight in order that its water may be carried off as rapidly as possible, and the land which it occupies by its deviations, made available for cultivation. In the loose, silty soil of a salt marsh, the stream may be made to do most of the work of making its new bed, by constructing temporary “jetties,” or other obstructions to its accustomed flow, which shall cause its current to de posit silt in its old channel, and to cut a new one out of the opposite bank. In some instances it may be well to make an elevated caral, straight across the tract, by constructing banks high enough to confine the stream and deliver it over the top of the dyke; in others it may be more expedient to carry the stream over, or through, the hill which bounds thr marsh, and cause it to discharge through an adjoining valley. Improvements of this magnitude, which often affect the interest of many owners, or of persons in. terested in the navigation of the old channel, or in mill privileges below the point at which the water course is to be diverted, will generally require legislative interferenca

But they not seldom promise immense advantages for a comparatively small cutlay.

The instance cited of the Hackensack Meadows, in New Jersey, is a case in point. Its area is divided among many owners, and, while ninety-nine acres in every hundred are given up to muskrats, mosquitoes, coarse rushes and malaria, the other one acre may belong to the owner of an adjacent farm who values the salt hay which it yields him, and the title to the whole is vested in many individual proprietors, who could never be induced to unite in an im. provement for the common benefit. Then again, thanks to the tide that sets back in the Hackensack River, it is able to float an occasional vessel to the unimportant villages at the northern end of the meadows, and the right of navigation can be interfered with only by govermental action. If the Hackensack River proper, that part of it which only serves as an outlet for the drainage of the high land north of the meadows, could be diverted and carried through the hills to the Passaic; or confined within straight elevated banks and made to discharge at high water mark at the line of the Philadelphia Rail-road ; —the wash of the highlands, east and west of the meadows, being also carried off at this level,—the bridge of the railroad might be replaced by an earth embankment, less than a quarter of a mile in length, effecting a complete exclusion of the tidal flow from the whole tract.

This being done, a steam-pump, far less formidable than wany which are in profitable use in Europe for the same purpose, would empty, and keep empty, the present bed of the river, which would form a capital outlet for the drainage of the whole area. Twenty thousand acres, of the most fertile land, would thus be added to the available area of the State, greatly increasing its wealth, and inducing the settlement of thousands of industrious inhab. itants.

As the circumstances under which upland water reacher

« AnteriorContinuar »