RESER VOI RS.
Reservoirs may be classified broadly into im pounding, settling, storage, a lid distributing ervoirs, all of which, except impounding. may be covered or open, the latter being the general form. hopounding reserroirs are formed by throwing a dam across sonic stream and flood ing the country above. Storage reservoirs are often created in the same way; or they may be formed more or less completely by embankments or excavation, or a combination of the two. Both impounding and storage reservoirs are designed to conserve a supply of water above the normal consumption, for times when either the natural yield is below the average or the consumption is unusually great. Impounding reservoirs are always for storage purposes, whether the capac ity be for a few hours' supply or for a much longer period. The larger and more regular the daily yield of the stream or other source of snpply, as compared with the consumption, the smaller need be the storage capacity. In some cases storage is required for months. or even for a year. The new Croton Dam twill provide a storage of about 32.000,000,000 gallons, or near ly enough to supply the whole of Greater New York (3,438,000 people in 1900) with 190 gal lons each for 100 days. The Wachusett Dam, of the Metropolitan Water-Supply District (Bos ton and vicinity), will retain about 63,000,000, 000 gallons of water. It is located at Clinton. Mass., and ranks next to the Croton Dam in height (see table of darns above for dimen sions), but has almost double the storage capac ity. The Periyar, or Pe•iar, Dam in India (see table) forms a reservoir Larger than either of the last two, having, as it does. a total capacity of about 100,000,000.000 gallons; but the outlet
tunnel is at so high a level that only about one half of this capacity can be utilized. Still great er will he the capacity afforded by the great As suan Dam, now being built (see table) Nile, some 500 miles above Cairo. The stor age here will be about 280,000,000,000 gallons, or sufficient to cover 6,400,000 acres of land to a depth of one foot. The Periyar and Assuan dams are for irrigation purposes.
An important sanitary question involved in the construction of impounding and storage reservoirs for public water-supplies is the strip ping of the sites, or flooded areas, of all heavy accumulations of organic matter which, either through their decay or through serving as a food supply to low forms of life, would impart un pleasant tastes and odors to the stored water. It is a common practice to remove stumps and tim ber and to burn over the site, where practieable, before filling such reservoirs: but outside of New England very little more than this has been done toward stripping the sites of storage reser voirs. The estimate for the Wachusett Reser voir, mentioned above, included out of a total cost of 89.105,000. for stripping the reservoir site. Aside from stripping and clear ing the sites. the construetion of impounding reservoirs pertains mostly to the dam and its accessories. One additional element of impor tance in some eases is the excavation at the border of reservoirs to prevent shallow flow age and the growth and decay of vegetable mat ter which would occur when these slopes are alternately exposed and covered by the varying levels of water in the reservoir.