Curb-opening inlet - typical spacing

[juke71]

Given:
Gutter spread is half lane width
2-lane road with normal crown
No offsite (drainage is road only)
flat road (say 1.5%)
Standard 12' curb opening

Question:
1. What is the rule-of-thumb inlet spacing for 80% interception?
2. For a curb-opening inlet in a sag, how many feet of a 12' lane can the inlet handle?

I am new to road design obviously, so any help I can get is enormously appreciated!

 

[Maury]

Go the the Federal Highway Administration web site and download the Urban Drainage Design Manual (Hydraulic Engineering Circular No. 22) There is a chapter on pavement drainage that includes design examples.

 

[cvg]

ditto
this is a bit complicated to use any rules of thumb. depends on the design storm, cross slope, lane width, type of gutter and/or local depression, roadway slope, grates etc.
I would suggest that you place inlets upstream of all major intersections or larger driveways and suggest if this is in the city, that minimum spacing might be about 1/8 mile.

 

[gtcivileng]

in addition to maury and cvg's suggestions, i'd like to add that getting an idea of your design flows at proposed/initial inlet locations would help--i only work with 10ft/15ft/20ft inlet opening widths (austin,tx); for the 25 yr storm, capacity is ~ 6.1 cfs for a 10ft inlet on a relatively flat slope (1-2%); for the 100 yr storm, it's ~ 9.2 cfs. also, 80% interception isn't much to brag about--the Qpass will compound quickly as you head overland toward the low point = undesirable amounts of water on the road.
as far as spread at the sag (which i'm sure you're planning to place an inlet at... = sump inlet), the same restrictions on allowable spread should apply. also remember that you must keep the water below the top of curb for 25yr (usually) and for the 100yr (usually), the rise must stay within the ROW.
i'm not sure if this is a textbook problem, or perhaps a conceptual design/cost estimate, but i strongly suggest doing at least some calcs to generate time of concentration, rainfall intensity, runoff coefficent =>runoff flows per drainage area. then you can enter the grade/gutter/sump inlet calcs with more confidence. it's not hard to improperly design/estimate a drainage system.

good luck!

 

[dicksewerrat]

You can put in the inlet for 80% and 25 feet downslope put in another. don't let the storm event overrun the inlets toward any intersection.