Culverts are commonly required in road construction for carrying under the road the small streams which may be crossed by the road, or sometimes for carry ing the water collected in the gutters or ditches on the upper side of the road to the lower side.
The waterway provided by a culvert must, for safety, be sufficiently large to pass the maximum flow of water that is likely to occur, while for economy it must be made as small as may be without danger.
The maximum flow of a stream depends upon a number of local conditions, most of which are very difficult of accurate determination. These are: the maximum rate of rainfall, the area drained by the stream and its position, the character of the surface drained, and the nature of the channel.
The maximum rate of rainfall varies in different localities, and differs in the same locality from year to year. It is commonly taken at about an inch an hour.
This is sometimes exceeded for a very short time and over a small area, but is usually a safe value for a watershed of any considerable area.
The approximate area of the watershed drained by a stream is readily found, and its form is also impor tant as determining the distance the water must flow in reaching the culvert under consideration, and to some extent regulating the rate at which the water falling upon the area will reach the culvert.
The maximum flow of a stream is also affected by the physical characteristics of the watershed. The permeability of the surface largely determines what portion of the rainfall shall reach the stream; while the slope of the surface, its evenness, and its vegetation have an effect upon the quickness and rate with which the rainfall is received by the stream.
The determination of the maximum flow to be ex pected in any case from an examination of the locality is therefore possible only as a very rough approxima tion. A number of formulae have been proposed for such estimation, the use of which for the case of an ordinary culvert ,simply amounts to estimating the quantity of water which would fall on the watershed in the heaviest probable rain, and judging as well as pos sible from local conditions how much of it may arrive at one time at the culvert. In some cases where a
more accurate determination is desirable it may be advisable to measure the flow of the 'stream at high water, and form an idea from such measurement as to what may be expected at a maximum stage.
The amount of water that will pass a culvert in a given time depends upon the form of the section, the smoothness of its interior surface, its slope, and' the head under which the water is forced through. A well-constructed culvert may be considered in comput ing its capacity as a pipe flowing full. Other culverts or bridges must be treated as open channels.
Prof. Talbot gives (Selected Papers•C. E. Club, Univ. of Illinois, 1887-8) a formula for the rough determina tion of area required for waterway, derived from ex perience: Area waterway in feet = C ? (drainage area in acres) 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 3 or 4 times the width, C = k. When the valley is longer, decrease C. If not affected by snow and with greater lengths, C may be taken at 1, }, or even less. For steep side slopes C should be increased.
For most cases in practice the size of waterway required may be determined from the knowledge which usually exists in the vicinity regarding the character of a stream, from the sizes of other openings upon the same stream, or from comparison with other streams of like character and extent in the, same locality. Where data of this kind do not exist, careful exami nation of water-marks on rocks, the presence of drift, etc., may be made to determine the height to which water has previously risen. The shape of the valley and the slope of the surface is of more importance than the area of country drained. The use of a for mula like Talbot's assists the arrangement of the factors which enter into the determination, and is only intended as an aid to judgment in selecting the size of opening required.