Embankments.—Embankments for restrain ing water are widely applied and have special names: as levees and dykes for protecting lands, banks forming canals, coffer-dams, reservoir embankments and the embankment with spill way adjunct for impounding water, the latter being called a dam.
The levee is built of the alluvium of river bottoms to restrain flood overflows. It is given sufficient freeboard above the greatest known flood, the top width is usually greater than the height and the side slopes are very flat, some times as low as five or six to one. The site is cleared of all humus and other vegetable mat ter and well broken, so as to bond the embank ment to the natural ground, and all such ma terial is excluded in forming the embankment A smuck-ditchs is made when deemed neces sary, and "buck-shots or other water-tight ma terial is puddled in and carried up into the embankment as a core.
Many hundred miles of levees have been built along the Mississippi River and its tribu taries, some of them of very large proportions. When built according to the best practice, well seasoned and turfed over, failures are very rare, though floods often stand against them for weeks and the material is comparatively mobile when saturated. Failures have been due to in sufficient height and the chopping effect of waves, but especially to lack of maintenance and of care at critical periods. The crawfish burrowed through at the turf line when its re moval was omitted, and the board fences built in the levee to intercept this pest, decayed and weakened the levee. In northern latitudes the muskrat has also been a nuisance, as in the embankments of the reservoir system at the head-waters of the Mississippi River built for the purpose of increasing the low water flow.
Canals frequently skirt the sides of valleys or contour slopes, partially in excavation with part or full bank on the lower side. The ma terial for, forming such banks is generally far superior to that available for levee construc tion. The slopes are usually flat, from two to three on one, and a water-tight core or face is added in permeable ground, or the entire prism may be given a puddle lining when the percola tion is liable to be serious. Sheet piling has been driven in such banks, but this divides the bank and is not now considered good practice.
The unwatering of navigable canals in the winter time is also bad practice, as it subjects the inner face to frost action.
Canals are carried across water courses and valleys on supporting embankments, sometimes of great magnitude. Such embankments are given easy slopes and care exercised in form ing a water-tight prism above, so as to avoid saturation and the resulting instability.
Ditches for irrigation and hydraulic mining sometimes reach the dignity of large canals, requiring care in the formation of banks. The soil of their location is usually of fine tilth with an admixture of adobe, and such channels generally quickly with the fine sediment carried in the flowing water of certain seasons.
The embankment used as a coffer-dam usu ally encloses some large site, as for the con struction of a canal lock or for rock excavation in the dry, and is of a temporary character. Suitable material may be dumped from a trestle, the mass of material formed in water by dumping from a height being usually suf ficient. In a notable case, a recent contract for a rock channel in the Saint Mary's River, a heavy embankment more than a mile long was formed of dredged material to a maximum height of 30 feet, and rock, estimated as wet excavation, was taken out in the dry at a large profit. No great care was taken in this em bankment, except to provide suitable material and sufficient mass.
The reservoir embankment proper is usually for a storage or distributing reservoir, as for a municipal water supply. Such reservoirs are often 30 feet deep and the embankments are formed with great care from selected material placed in thin layers, watered and rolled. The inner face is lined and protected by a face wall, a pavement of brick, stone pitching, or even riprap in some large storage reservoirs. The height of the embankment varies with the supporting ground.
Earth Dams.— The impounding embank ment, or earth dam, is usually carried across some drainage line between steep valley slopes, so as to make a reservoir in the valley expanses above, and generally seeks to store a large pro portion of the tun-off. AS ample spillway on an in dent site is provided to carry the ex tremCier: ro of the valley when the reservoir is full.