A. 
Generally, storm drain pipes shall be sized in accordance with the following:
(1) 
Closed pipe systems: Manning's Formula and the criteria for hydraulic gradient included within this article.
(2) 
Cross culverts: Hydraulic Circular No. 5 prepared by the United States Department of Commerce, floodplain information or otherwise approved by the Director.
B. 
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.
C. 
The minimum diameter of storm drain pipe shall be 15 inches, or the equivalent elliptical or arch pipe.
A. 
Storm drain pipes, structural plate pipes, pipe arches and box culverts shall meet the requirements of the following referenced standards or specifications:
(1) 
Corrugated polyethylene drainage tubing, Type PS 20: AASHTO M 252.
(2) 
Corrugated polyethylene pipe, 12 inches to 36 inches diameter: AASHTO M 294.
(3) 
Class PS 50 polyvinyl chloride (PVC) pipe: AASHTO M 278.
(4) 
Precast reinforced concrete box sections for culverts, and storm drains, with less than two feet of cover subject to highway loadings: AASHTO M 273.
(5) 
Precast reinforced concrete box sections for culverts, and storm drains: AASHTO M 273.
(6) 
Reinforced concrete culvert and storm drain pipe: AASHTO M 170.
(7) 
Reinforced concrete arch culvert and storm drain pipe: AASHTO M 206.
(8) 
Reinforced concrete elliptical culvert and storm drain pipe: AASHTO M 207.
(9) 
Metallic (zinc or aluminum-zinc alloy) coated corrugated steel culverts and underdrains: AASHTO M 36.
(10) 
Precoated galvanized corrugated metal driveway culverts and underdrains: AASHTO M 245. The minimum allowable gauge shall be 14-gauge.
(11) 
Corrugated aluminum alloy culverts and underdrains: AASHTO M 196. The minimum allowable gauge shall be 14-gauge.
(12) 
Structural plate for pipe arches, and arches: AASHTO M 167.
(13) 
Aluminum alloy structural plate for field-bolted conduits: AASHTO M 219.
B. 
Galvanized corrugated metal pipe shall not be accepted under proposed roadway fill areas or for stormwater management devices.
C. 
Prefabricated end sections shall be used in lieu of headwalls wherever possible.
D. 
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.
E. 
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.
F. 
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.
G. 
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.
H. 
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.
I. 
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.
J. 
A minimum of one foot of clearance shall be provided between storm drain pipe and water and sewer lines.
K. 
Generally, pipe sizes shall not be reduced in the direction of the flow.
L. 
At inlets, manholes, etc., the invert of the pipes upstream shall be a minimum of 0.1 foot above the invert of the pipes downstream.