m 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]

=Length of Culvert.=--The clear length between end walls on a culvert
should be at least equal to the width of the roadway between ditches.
This is a minimum of 20 feet for secondary roads and rang