CULVERTS AND CONDUITS Types of Culverts.—The term culvert is usually applied to structures intended to provide small waterways through earth embankments. Such structures are usually constructed according to certain standard plans, depending upon the size of opening re quired. For the smaller openings, pipe culverts of vitrified clay, plain or reinforced concrete, cast iron or corrugated iron, are fre quently used.
For openings larger than 24 or 30 inches in diameter, box culverts or arch culverts of stone or brick masonry or of concrete, either plain or reinforced are commonly employed. Concrete for this purpose has recently been gradually replacing the older types of construction, on account of its ease of application in most localities, and its low cost as compared with other types of equal strength and durability.
Wooden culverts have been largely used in the past upon highway work, but are now rapidly giving way to more permanent structures, for, while cheaper in first cost than the other types, they are very uneconomical on account of their rapid deterioration and high cost of maintenance.
All culverts require walls of masonry or concrete at the ends to prevent the possible penetration of water around the culvert, and to sustain the bank of earth and hold it from falling into and clogging the waterway. For small culverts, such walls are usually parallel to the roadway; they should be long enough to permit the earth to stand at a slope of about 12 to 1 without reaching the waterway of the culvert and sufficiently high to sustain the earth fill above the culvert.
178. Area of Waterway Required.—The waterway provided for a culvert must, for safety, be sufficiently large to pass the maximum flow of water that is likely to occur, while for economy it should be as small as possible. There are at long intervals, in most localities, records of storms of extraordinary character, to provide for which would need large increase of capacity in the culverts and add greatly to their cost, and while these unusual storms can hardly be taken into account in the design of the structures, effort should be made to pro vide for any flow of water that may reasonably be anticipated. The maximum flow of a stream depends upon a number of local conditions, most of which are very difficult of accurate determination. Among these are the maximum rate of rainfall, the area drained by the stream, the shape and character of the surface drained, and the nature and slope of the culvert channel.
The maximum rate of rainfall varies widely in different locali ties, the heaviest occurring over very limited areas and short periods of time, and are therefore important only for small culverts. For larger areas, the maximum rainfall of sufficient duration to permit water from all parts of the tributary area to reach the culvert gives maximum results.
The amount of water reaching the culvert depends upon the per meability of the soil, its degree of saturation, and the amount of veg etation. The rapidity with which water reaches the culvert from the far portion of the watershed depends upon the slope and smoothness of the surface and whether it is covered with vegetation. The shape of the area to be drained is important in that it determines the distance the water must travel in reaching the culvert.
The quantity of water which will pass through a culvert in a given time depends upon the smoothness of its interior surface, the dis turbance of flow at entrance to the culvert, and the freedom with which the water flows away after passing the culvert. If the culvert is so constructed that water may stand against its upper end, causing it to discharge under pressure, its capacity will be considerably increased.
The determination of the area of waterway required in any in stance is a matter of judgment, and there is no way in which it may he accurately computed. A number of formulas have been pro posed for the purpose of aiding in estimating the probable quantity of water from a given area or the size of opening required for a given area. The formula of Professor Talbot has been used to consider able extent in the Middle West with good results. This formula is: Area of waterway in feet /(drainage area in in which C is a coefficient depending upon local conditions. For rolling agricultural country subject to floods at time of melting snow, and with length of valley three or four times its width, C=1. When the valley is longer, decrease C. If not affected by snow and with greater lengths, C may be I aken at A-, or even less. For steep side slopes, C should be increased. Where the ground is steep and rocky, C may vary from s to 1. Table XXVII gives roughly the sizes of openings required for different areas, computed from time formula of Professor Talbot.