Cross Slope across bottom of storm box?

[cveg05]

Hi,
Does anyone have suggestions for the minimum cross slope across the bottom of a storm/junction/inlet box?
I guess I've just never considered it before until my surveyor, who's about to do the construction staking, brought it up. I've always performed the pipe design assuming the slope to the center of the box but this would only give <1% cross slope in many instances(or .04'). My surveyor is suggesting a 0.10' drop(as for manholes) is this reasonable? required? suggested? A 0.10' at every box adds up fast on a flat sight.
Thanks

 

[francesca]

We always did a tenth of a foot drop across a storm or sewer MH, regardless of the size of the pipes. For a larger pipe diameter storm junction box, you might consider a greater drop.

 

[cveg05]

Thanks for the response. I just graduated a year ago and the PE who i'm interning under doesn't think the .1' drop is necessary so I hadn't ever heard of it though it is logical. I really wanted to see how common of a practice it is and it sounds like it is quite common. Thanks

 

[jgailla]

We do not typically provide any drop across a storm box. We do provide 0.1' across sanitary boxes as a regulatory requirement.
We are, however, in a very flat area in coastal SE US.

 

[cvg]

The primary purpose of the slope is to provide sufficient velocity to carry sediments out of the box. If you are in an urban area with very little sediment carried by the stormwater, than a slope within the box is probably not necessary. However, in practice, most standard details do show a slope which is formed in the bottom by mortar and hand troweling which directs flow at 1% or 2% towards the outlet pipe. I am looking at a detail now that requires 4H:1V slope towards the pipe.

 

[zabrab]

Many jursdictions in my area have a prescriptive requirement of 0.1 ft to 0.17 ft (2 in) when entering and exiting pipes are the same diameter and matching pipe crowns when the pipe diameter increases at a drainage structure.