Cross culverts: Hydraulic Circular No. 5 prepared by the United States
Department of Commerce, floodplain information or otherwise approved
by the Director.
Deviations from the above criteria shall require prior approval of
the Director. If, in the opinion of the Director, it becomes necessary,
these criteria may be reduced to cause detention or increased to prevent
flooding.
Storm drain pipes, structural plate pipes, pipe arches and box culverts
shall meet the requirements of the following referenced standards
or specifications:
In the design of roadway cross drains, the culverts shall operate
under either inlet or outlet control conditions. The value of HW/D
may be greater than 1.2 only with the approval of the Engineering
Division Chief.
When outlet control is encountered, applicable charts from the hydraulic
charts for the selection of highway culverts should be used to compute
pipe sizes and highway elevations.
The computed highwater created by either inlet or outlet control
shall not cause damage to existing properties or proposed development
and shall be at least 0.5 foot below the edge of the roadway shoulder
at the low point in the roadway profile.
When a change in direction or slope of pipe is required, an inlet,
manhole, or accessible bend structure shall be placed at the point
of change of direction.
The minimum slope of pipe shall not be less than 0.50% and the velocity
in the pipe shall not be less than two fps unless otherwise approved
by the Engineering Division Chief. The maximum allowable slope for
storm drain pipe shall be 15%, beyond which designed anchors shall
be provided at a maximum fifteen-foot spacing.
The crown of all pipes shall be at least six inches below the subbase
elevation of the pavements or 1.5 feet from finished grade, whichever
is greater.