The subject of maximum velocities has received but little attention, probably because the dangers connected with excessive velocities are not so great as those resulting from a too slow rate. Such dangers do exist, however. The more immediate one is that the consequent shallowness of the current which would in many cases result would occasion the deposit of the larger floating solids, which may result in obstinate obstructions in the sewer. In the mains this can be obviated by reducing the size of the sewer to the point where the necessary depth is obtained. But it is usually not in the mains but in the branches that steep grades are possible. To reduce the sewer to such a size as would give any considerable depth to the daily flow on very steep grades would call for a diameter much below that usually adopted as a minimum. An 8-inch sewer whose grade is 0.1 gives a theoretic velocity of 10.04 feet per second when flowing full. To secure a flow in this pipe having an average depth of 4 inches would require the sewage from a population of 6500. general it may be said that the ordinary depth of flow in any sewer should not be less than 2 inches, nor should it be less than the radius of the invert, since if it is so there is much more danger of deposits forming along the edges and even in the centre of the stream. It will sometimes be impossible to meet this requirement fully, but it should be kept in mind as extremely desirable.

In

Another objection to too great velocity is the danger of attrition of the sewer-invert by the scouring action of sand, stones, etc., swept rapidly over it. In brick sewers this objection is frequently and successfully met by lining the invert with granite blocks. A 5-foot two-ring brick sewer in Baltimore, 25 years old, was recently found with its invert in one place cut completely through for a width of 12 to 15 inches and badly worn for a height of 2 feet, and many other places were only a little less damaged. In Omaha's brick

sewers the wear, which is usually 18 to 24 inches wide, became 2 to 3 inches deep in 12 years. In both cities ordinary brick was used, but was replaced with stone blocks.

The first objection is the serious one, since the time taken to wear out a sewer-invert must be considerable if good material is used, and replacing it is a matter of expense only. But the forming of deposits in the sewer endangers the health of the community.

It is difficult to set a maximum limit to the velocity allowable, but it may generally be taken as from 8 to 12 feet per second. From 3 to 5 feet per second is probably the most desirable velocity.

ART. 23. SIZE OF SEWERS.

If a house-sewer were constructed to exactly meet the theoretical requirements as above outlined it would continually increase in size from the head to the outlet, by a small increment below each house-connection, by a larger one below each tributary branch or lateral; but between the first two connections it should be of sufficient size to carry the sewage 6 X 175

of one house only, which would be about

=

7.48 X 86400

= .0016 cubic feet per second, which at a velocity of 2.5 feet per second would call for a pipe of .00064 square feet area, or inch diameter.

This method is not closely followed for the reasons that the data on which are based the calculations of volume of sewage as well as the formulas of flow cannot be exact enough to warrant it; that the estimate of ultimate population may be exceeded; that the per capita water-consumption may increase beyond the maximum assumed, factories or other large contributors of sewage locate at points where they were not expected, or for some other cause the amount of sewage

reaching any lateral may be largely exceeded. This excess can be allowed for in a general way only, but it is advisable to design the laterals of a capacity double that calculated, particularly since the cost is not thereby largely increased, and the velocity in a sewer flowing half full is as great as that in one flowing full.

The house-sewer mains need not have so great an excess of size, since they carry the sewage from many laterals, and it is not probable that all these will receive double the calculated amounts of sewage. It will probably be sufficient to increase these by 50% of the estimated capacity. The volume of sewage reaching the trunk or outlet sewer can be still more closely calculated, and an increase of 25% may be made as giving it sufficient capacity, although it would probably be better to add 50% here also, the additional cost being slight in

most cases.

With this increase the head of each lateral would still be less than inch in diameter. This would be too small to adopt in practice for several reasons: because an individual house will contribute sewage at occasional maximum rates far exceeding 175% of their daily average; because a very small sewer would be too frequently stopped by pieces of paper, or by other legitimate sewage matter; and because it would be too difficult of access for inspection and cleaning. The last two objections could, it is true, be met theoretically by making the house-connection of a size so much smaller that nothing could pass it which would obstruct the sewer. But such construction would be utterly impracticable.

There is no particularly good reason, however, why a house-connection might not be made of 2-inch pipe and the sewer of 3-inch or 4-inch; and systems are in existence and reported working satisfactorily where such sizes are in use. But such construction would generally compel a change in the stock dimensions of all house-plumbing and connected appli

ances, and give rise to inconveniences more than balancina the saving in cost. A 4-inch house-connection is, however, ample for any building containing less than 50 persons and which contributes only ordinary house-sewage (see Art. 82).

The scwer might, then, where the grade is quite steep, be constructed as a 4-inch pipe from the head to such point as the calculations fix for an increase in size; but it is better to make the minimum diameter 6 or 8 inches, for then there would be less probability that anything passing the house-connection, in which the velocity may be considerable, would obstruct the sewer. It is thought that the weight of evidence tends to show that with 4-inch house-connections 8-inch sewers are obstructed much less frequently than are 6-inch. Among other reasons for this is the fact that a 6-inch stick, chicken-bone, etc., will pass a 4-inch trap, but an 8-inch one will not; and that a 6-inch stick is more apt to become wedged across a 6-inch pipe than across an 8-inch one. Some engineers set the 6-inch, more, probably, the 8-inch pipe, as the minimum to be employed for sewers. In England 9 inches is generally the minimum size.

In the case of storm-sewers the only change of conditions. affecting the volume of sewage which is likely to occur is in the imperviousness of the contributing area. If this is taken at the maximum, as for a business district, no allowance need be made. In any case the allowance for change can best be made in the selection of the factor of imperviousness and the sewer built of corresponding capacity. It is probable that no condition of size or character of tributary area will in actual practice call for a storm-sewer of a diameter less than 10 or 12 inches. It should, if possible, be of a diameter at least as great as that of the largest opening in the storm-water inlets, to prevent sticks lodging across it.

A circular or egg-shaped sewer is sometimes limited in size by the amount of covering necessary and the distance below

the street-surface of its invert, where this is fixed by the elevation of the outlet and the necessary grade from that to the point in question. If the whole sewer at this point be lowered the grade and velocity become less and the size of the sewer must be increased, thus raising the crown. The size can be reduced only by increasing the grade, which means raising the sewer. Under these conditions the sewer can be built as an "inverted siphon" to flow under a head (Art. 38), two or more parallel sewers can be substituted for the one, or the shape can be modified. In adopting the last alternative engineers have devised many forms which can be generally classified as those flattened on the bottom and those flattened at the top.

ART. 24. SHAPE OF SEWERS,

Of all possible shapes of sewers of equal area of cross-section the circular gives the greatest velocity when flowing full or half full and, having the shortest perimeter, contains the least material. Also, being devoid of angles, it offers little opportunity for deposits. For sewers intended to always flow at least half full it is therefore the most desirable shape. This is not true, however, of a combined sewer-that is, one which carries both house-sewage and storm-waterfor, as we have seen (Art. 22), the house-sewage may occupy only of the capacity of the sewer and have a velocity only about as great if a circular sewer be used. If the sewer, considered as a storm-sewer, be given a grade adapted to a velocity of 4 feet per second when flowing full or half full the velocity of the house-sewage would be about of a foot per second. If on the other hand the grade be so increased (which is seldom possible) as to give the minimum housesewage flow a velocity of 1 feet per second the depth of this flow would be only about .02 of the sewer diameter. Neither of these conditions is permissible in a good sewerage system.