=Culverts.=--Culverts and bridges are a part of the drainage system and the distinction between the two is merely a matter of size. Generally, structures of spans less than about eight feet are classed as culverts, but the practice is not uniform. In this discussion culverts will be defined as of spans of 8 feet or less. Numerous culverts are required to afford passage for storm water and small streams crosswise of the road, and their aggregate cost is a large item in the cost of road improvement. The size of the waterway of a culvert required in any location will be estimated by an inspection of the stream and existing structure, and by determining the extent and physical characteristics of the drainage area. Sometimes there is sufficient evidence at the site to indicate quite closely the size required, but this should always be checked by run-off computations. The drainage area contributing water to the stream passing through the culvert under consideration is computed from contour maps or from a survey of the ground, and the size of culvert determined by one of the empirical formulas applicable to that purpose. In these formulas, the solution depends upon the proper selection of a factor "C" which varies in accordance with the nature of the drainage area. Two of these that are quite widely used are as follows: _Myers' Formula: a = CA_ Where _a_ = area of cross section of culvert in square feet. _A_ = area in acres of the drainage area above culvert. _C_ a factor varying from 1 for flat country to 4 for mountainous country or rocky soil, the exact value to be selected after an inspection of the drainage area. _Talbot's Formula_: Area of waterway in square feet = _C_ [Square root of] ((Drainage area in acres)^3) Transcriber's Note: The above formula used the mathematical square root symbol in the original. One should read it as "C times the square root of the Drainage area in acres cubed." _C_ being variable according to circumstances thus: "For steep and rocky ground _C_ varies from 2/3 to 1. For rolling agricultural country, subject to floods at times of melting snow, and with length of valley three or four times its width, _C_ is about 1/3, and if stream is longer in proportion to the area, decrease _C_. In districts not affected by accumulated snow, and where the length of valley is several times its width, 1/5 or 1/6 or even less may be used. _C_ should be increased for steep side slopes, especially if the upper part of the valley has a much greater fall than the channel at the culvert. The value of _C_ to be used in any case is determined after an inspection of the drainage area." [Illustration: Fig. 2. Design of Pipe Culvert and Bulkhead]