WATERWAY REQUIRED.
The determination of the amount of waterway required in any given case is a problem that does not admit of an exact mathe matical solution. Although the proportioning of culverts is in a measure indeterminate, it demands an intelligent treatment. If the culvert is too small, it is likely to cause a washout, entailing possibly loss of life, interruptions of traffic, and cost of repairs. On the other hand, if the culvert is made unnecessarily large, the cost of con struction is needlessly increased. Any one can make a culvert large enough; but it is the province of the engineer to design one of sufficient but not extravagant size.
The area of the waterway required depends upon (1) the rate of rain-fall, (2) the kind and condition of the soil, (3) the character and inclination of the surface, (4) the condi tion and inclination of the bed of the stream, (5) the shape of the area to be drained and the position of the branches of the stream, (6) the form of the mouth and the inclination of the bed of the culvert, and (7) whether it is permissible to back the water up above the culvert, thereby causing it to discharge under a head.
1. It is the maximum rate of rain-fall during the severest storms which is required in this connection. This certainly varies greatly in different sections; but there are almost no data to show what it is for any particular locality, since records generally give the amount per day, and rarely per hour, while the duration of the storm is seldom recorded. Further, probably the longer the series of ob servations, the larger will be the maximum rate recorded, since the heavier the storm the less frequent its occurrence; and hence a record for a short period, however complete, is of but little value in this connection. Further, the severest rain-falls are of compara tively limited extent, and hence the smaller the area, the larger the possible maximum precipitation. Finally, the effect of the rain-fall in melting snow would have to be considered in determining the maximum amount of water for a given area.
2. The amount of water to be drained off will depend upon the permeability of the surface of the ground, which will vary greatly with the kind of soil, the degree of saturation, the condition of cultivation, the amount of vegetation, etc.
3. The rapidity with which the water will reach the water courses depends upon whether the surface is rough or smooth, steep or flat, barren or covered with vegetation, etc.
4. The rapidity with which the water will reach the culvert depends upon whether there is a well-defined and unobstructed channel, or whether the water finds its way in a broad thin sheet. If the water course is unobstructed and has a considerable inclina tion, the water may arrive at the culvert nearly as rapidly as it falls; but if the channel is obstructed, the water may be much longer in passing the culvert than in falling.
5. Of course, the waterway depends upon the amount of area to be drained; but in many cases the shape of this area and the position of the branches of the stream are of more importance than the amount of the territory. For example, if the area is long and narrow, the water from the lower portion may pass through the culvert before that from the upper end arrives; or, on the other hand, if the upper end of the area is steeper than the lower, the water from the former may arrive simultaneously with that from the latter. Again, if the lower part of the area is better supplied with branches than the upper portion, the water from the former will be carried past the culvert before the arrival of that from the latter; or, on the other hand, if the upper portion is better supplied with branch water courses than the lower, the water from the whole area may arrive at the culvert at nearly the same time. In large areas the shape of the area and the position of the water courses are very important considerations.
6. The efficiency of a culvert may be materially increased by so arranging the upper end that the water may enter it without being retarded (see 4 1127). The discharging capacity of a culvert can also be increased by increasing the inclination of its bed, provided the channel below will allow the water to flow away freely after having passed the culvert. The last, although very important, is fre quently overlooked.