evel of high-water of spring tides. Then there are inland cities and towns, through a part of which, flow rivers liable to excessive land floods, and the waters may be impeded by locks, for the purposes of navigation, and by weirs, for mills. . The site to be drained may be partially a flat plain and partially rising land inland, showing a tolerably straight front, or being convex towards the plain. The land may rise on both sides, as in a creek, or bay-or may consist of several such creeks, with natural water-courses passing through each, bringing down the surface-waters of suburban areas, much larger than the town itself-these, and other innumerable combinations, which need not be specified, are all questions, for the treatment of which no fixed rules can be given; each individual case will demand special study, and must have local knowledge and care. That the question may however be discussed, certain rules relative to town sewers will be assumed, which may, or may not be established. 1. They cannot receive the excessive flood-water, even of the urban portion of the site. 2. They ought not to be combined with the natural water-courses, which drain large areas of suburban land previous to entering the urban portion. 3. They should be adapted, exclusively, for removing all the liquid and soil refuse from the houses, in such a manner as to cause the least possible nuisance to the inhabitants. Where the site of a town is a plain, only little elevated above the tidal water-line, the delivery of the refuse from the sewers, must either be by pumping, to render it constant, or it must be intermittent, and, therefore, leaving the refuse, for a time, stagnant in its channels of conveyance. Sewers and drains are frequently required, in situations where high tides, land floods, or both combined, cover to a considerable depth the surface beneath which such sewers must be laid; or, if the area is embanked, the waters rise above the level of the land and the outlet. These circumstances frequently serve as excuses for not attempting the formation of any sewers, or drains, and most certainly it is much safer to the inhabitants, that there should be no sewers, than that there should be sewers of deposit. That towns so situated may, however, be perfectly, and even economically drained, is proved by the condition of Holland, where the land is almost all below the level of the sea, and yet it is the most densely populated country in Europe. In England, great portions of Lincolnshire, Cambridgeshire and other counties, with large areas of agricultural land only worth from one hundred to two hundred pounds per acre, are drained either by intermittent action, or to a great extent by pumping machines, designed by and erected under the direction of Members of this Institution. That which is done profitably for agricultural land, may assuredly be carried out for town sites, where the intrinsic value of the land and of the property placed upon it (exclusive of the importance of preserving human life) is, in many instances, as one hundred to one, or even more. Inferentially it may therefore be stated, that town sites may be profitably drained by pumping, independently of any commercial value attached to the sewage refuse. Where a town site is partially a plain and partially rising ground, sloping towards the level, or low portion, the formation of intercepting lines of sewer, to, receive the contents of the sewers and drains of the higher portion, and to prevent their falling into the lower level, will save much pumping. The sewers formed in level, or low districts should be of minimum dimensions,-they should be laid as near the surface as is consistent with their safety, and should be constructed of a material capable of bearing external pressure, without crushing, and internal pressure, without leaking, or bursting. If sewers, connected with a pumping establishment, are much larger than corresponds with the lifting power employed, the current through them will be sluggish, and deposit will take place. It will be evident that such sewers should not be unnecessarily deep, in order that the artificial lift should be as little as possible; and they should be capable of resisting hydraulic pressure, as they will be liable to be filled above the crown, either through an extraordinary rise of tide, or through heavy land floods. This latter consideration involves the question of sluices and valves which however need not be entered upon in this Paper. Another reason, why sewers in flat districts, liable to surface flooding, should be of minimum dimensions, is that maximum volumes of water, in such places, cannot be provided for in sewers. The River Eden, at Carlisle, rises about 20 feet, and the River Ouse, at York, rises about 18 feet, above summer level, laying under water, areas of many miles square. Large sewers, in such districts, would give no relief, at such times, and in dry weather they would be a nuisance. The assertion that, "Town sewers should not receive suburban waters, or excessive suburban rain-fall," requires some explanation. The area drained by natural streams may be much larger than the urban area,—and, therefore, any arched channel, or sewer, to convey away, safely, the flood-waters of such a district, must be in proportion to such area. The cost, of course, being in proportion to the sectional dimensions of the tunnel, or sewer formed. Another consideration, which ought, also, to have great weight, is, that the flow of flood-waters is intermittent, whilst the flow of sewer-waters should be constant, that is, the sewers should be as much as possible selfcleansing, by the daily action of the fluids passing through them. This cannot be the case, if a small body of water is spread out over a wide bottom, or invert. There must, in such a case, be a deposit, which will be increased by such obstructions, as stones, sticks, road-sand, gravel, etc., which will be carried from the open water-courses into all sewers, or tunnels, receiving the flood-waters of a suburban district, because the channels leading to the sewer, and the end of the sewer itself must be open. The stagnation of sewage refuse, in such places, will also be greatest, during the driest period of the year, when the evaporation will be most injurious. Several reasons may be given for the assertion, that "Sewers, other than main outlets under particular circumstances, should not be calculated to contain the flood-water of an excessive rain-fall,”—though one only ought to suffice, namely, that the waters of an excessive rain-fall cannot be passed through any ordinary gully-grates, into the sewers. In Birmingham, on the evening of Sunday the 6th July, 1845, there fell 1945 inch of rain, in little more than half an hour. This was equivalent to 9-091 gallons upon each square yard of surface, or 44,000 gallons, to each statute acre. This is, no doubt, 1 Vide Report to the General Board of Health on the Town of Birmingham. By R. Rawlinson, May 1849, Page 9. such a flood as is seldom met with, but equal volumes of rain have fallen in the metropolis, and in other places in England. Those who have any given area to drain, for town purposes, taking these figures as multipliers will ascertain about the maximum, if they premise, that sewers ought to be of sufficient capacity to receive the storm-waters, and to retain, or pass them off below the level of the cellars. In many cases the storm-waters will pass over the surface, after the formation of sewers, just as they flowed away before the construction of any artificial conduits. The dimensions of sewers ought, in some measure, to fix their depth below the surface, as there ought to be a fall of not less than one in sixty, from the deepest house, or cellar-drain, to the highest water-line, to which a sewer can safely be allowed to be filled. In arranging a system of town drainage, an Engineer must consider the following questions: 1. How has the flood-water hitherto passed away, seeing that there never has been such a work as a sewer in the district? 2. What has been done on the surface, in the formation 3. What are the effects experienced during land-floods? 5. What additional length of outlet will be involved, to These are all questions of the utmost importance, involving economy and efficiency; but there are other minor questions, which an Engineer should settle, before finally deciding on a system of town drainage. There are sites upon which houses ought never to have been built, and cellars have been dug, where the natural outlets, upon the surface, have been contracted, or destroyed, instead of being preserved and improved. In such cases an Engineer should constitute himself Nature's journeyman, and by carefully observing the natural features of the locality, and using them as much as possible, attain his end more readily than by attempting any forcible control. There are cases in which it is the province of an Engineer, in some degree to oppose Nature, but this must ever be a costly and dangerous undertaking, and nothing short of actual necessity will justify the attempt. The greatest men in the profession have shown, by their works, that they never undertook an opposition course, when they could accomplish their purpose in an easier manner. In town drainage, Nature must first be consulted, and effective assistance may then be given to her operations at the least cost. Where there has never been a sewer, it may not be considered necessary to construct one 5 feet deep by 3 feet wide, at a depth of some 15 feet, or 20 feet below the surface of the street, to the great danger of all the adjoining property, and, in many instances, to the certain destruction of the natural outlet. If it shall be decided, that town sewers are not to convey away the drainage waters of suburban districts, and if it shall, further, be granted, that they cannot be made of sufficient capacity to contain even the urban waters, during maximum rain-floods, it may naturally be asked, of what dimensions should town sewers be constructed? and this may be partially answered by considering the principal intention in constructing town sewers. If it be for house and yard drainage, then to these they should be adapted, and there will be reliable data from which to calculate their dimensions. The sewers and drains will be of minimum size, and will, consequently, not require either very wide, or very deep excavations to be made, and the cost of the work will also be reduced to a minimum. It will be admitted, that many large sewers were not originally constructed to serve the purpose to which recent practice has adapted them—namely, to receive and convey away the contents of water-closets for an entire population. The asser The Cloaca Maxima was designed and used by the Romans for conveying away both fluid and solid sewage. [1852-53.] D |