WATERWAY REQUIRED (1995)
Determination of the proper size culvert opening requires knowledge of the drainage area, runoff, data on past
performance, such as highwater marks above, at and below the opening, and pertinent formulas, with experience
and good judgment in interpreting them. Consideration of the stream both above and below the opening is
necessary. The ideal opening is one in which the velocity of the stream above the opening is maintained or
somewhat increased through the opening and below to a point where the flow will have no effect on the railway.
See Part 3, Natural Waterways.
4.1.2 SPAN REQUIRED (1995)
a. The span of the culvert should be such that the property above will not be injuriously affected at time of
maximum runoff, nor a head created which will induce destructive velocities.
b. In shallow fills the span may have to be increased to provide the predetermined area. For pipe culverts,
where practical, the cover should be a minimum of 22 feet below the bottom of tie.
c. For practical reasons, a minimum size pipe culvert should be established, even though runoff computations
may show that a size smaller than the minimum might be used. For main line track a minimum diameter of
24 inches is recommended, while for highway crossings and unimportant track, the minimum diameter may
be reduced to 18 inches. Pipe structures smaller than these are difficult to clean out and may be of
insufficient capacity if freezing occurs.