Normally I will determine the min. slope which will produce a velocity of 2.5 fps (min) when the pipe is flowing full and the hydraulic gradient is equal to the slope of the pipe.

This is will help carry sediment through the pipe system.  And when not possible to achieve this velocity try to at least avoid a drop in velocity in the pipe system.

Less than 1% on your asphalt and even the most experienced contractor has trouble preventing "bird baths."

In terms of minimum slopes; yes, scour velocity should be maintained. Beyond that, pipe material also makes a difference.  We used 1% minimum for CMP and 0.5% minimum for RCP and HDPE.

Other options to consider are eliptical pipes, which on a 2 acre site probably won't help much, and concrete channels.  Curb  cuts need to be very well maintained to remain effective. A little grass taking root will trap sediment over time and turn your parking lot into a dam.

I work in a flat coastal plain area.  Even an 0.5% slope can at times cause difficulties in design here.  (one may soon end up below the outfall elevation).

For RCP, a 3fps min. velocity for a pipe with an 0.013 manning's N yields minimum slopes of 0.32% for 15" dia., 0.17% for 24"dia., and 0.10%for 36"dia.

Using the steepest slope achievable keeps pipe sizes down.

For asphalt pavements, you might consider 1-1/2% to avoid slippery areas (freezing climates).  Some jurisdictions have zoning bylaws that stipulate a minimum slope.  I typically use a .5% min slope for pipes as indicated by TerryScan posting.

Dik

Actually, I was advocating slopes less than 0.5% depending on one's design environment.  Also, we are talking about pipe slopes here...not paving slopes.

TerryScan... thanks, the original posting indicated that he was using 1% for asphalt (and I assumed paving).  I've always used 1/2% for pipes (Rule of Thumb)... not fully understanding why... I didn't know you could use considerably less... Were the slopes obtained by plugging data into a formulae or from a manufacturer's catalogue?

Dik

In my area our state, county, city or town design guidelines provide us with minimum design criteria.  I would suggest that you comply with your governing entity's design constraints.  Just give them a call if you do not have documentation stating their position.

Minimum slopes are usually set according to a minimum velocity required to clear the pipe of debris.  Unfortunately, the minimum velocity is often only achieved in the design storm (or greater), which can be 10 or 50 year recurrence interval.  If your pipe is only cleared every 10 or 50 years, then it will quickly block up beyond the design storm's ability to clear it.  For instance, driveway pipes in my 50-year old subdivision are buried with just 3"-4" open.  Practical minimum slopes are usually used to ensure that the pipe remains clear under less than design storm flows; it doesn't take much sand to get enough water retained inside a very flat pipe to harbour mosquitoes.

I think as often as not, minimum slopes are also dictated by the actual slope of the land and construction economics which limit the ability to make pipes significantly steeper than the actual ground slope.  In these cases, pipes often flow under surcharge pressure, even if the slope is adverse.  I would love to get 0.5% slopes on all the storm drains in the city, but it just can't get done within the existing terrain without resorting to deep tunnels and lift stations to drain to the river outfall

I really like overland drainage for anything > 10 yr event.  If pipes are sized that large it seems to be a maintenance pain like francesca states.

Lndsn,

I would consider keeping as much of your drainage above ground as possible.  The use of concrete drains for parking areas and if you can discharge your roof drains to the surface often helps.  Sometimes you have to adjust the location of buildings.  However, depending on your area you may be able to better fit it with any required stormwater quality treatment and just add as small swale or basin at a corner of the site or at required vegetated areas.  I like slot drains, but mostly interior.  OUtside the plug, grow grass and can just be a pain.  They are useful for pedestrian crossings though.

Two things work in your favor.

1. The probability of the "ten year storm" occuring being equaled or exceeded in the next 10 years is 65%, not 1 %. The probability of that same storm being equaled or exceeded in the next 50 years is 99.5 %.  This, it is more likely than not that the ten year storm will occur mote often than every ten years as suggested by earlier comments.

2. The velocity of flow in a circular pipe is the same full or half full.  Therefor, you might expect to achieve self cleaning velocity ( 2.5 ft/sec) even if the flow is only half the design flow at least once a year 9depending on the weather patterns where you live, of course). This should provide more frequent "flushing" of the pipe than suggested in some earlier comments.

For very large pipes ( 27" and larger), I've seen slopes as flat as 0.001.  These are not desirable but are theoretically possible.  For smaller pipes the 0.50% slope suggested by others seems more reasonable.

Good luck

There is a lot of good slot and trench drain out there.  Granted I work for a company that makes them (Zurn), but they can do a lot in terms of draining a situation such as yours.  They come with a built in slope so the surface can remain flat.  They can be designed to take any loading, ADA, flow rates, or other qualifications.

Please do a web search for trech drains.  There is a ot of good information on them.