Storm Water Catch Basins 6

[wehr6]

Can anyone provide me with a reference for calculating the required length of a curb opening catch basin with gratings? The catch basin will be located at the bottom of a 15% slope access road. The catch basin has to collect 22 cfs, so that it does not flow directly into a main road thru a development.

 

[Maury]

Go to the Federal Highway Administration web site and download two publications:

Hydraulic Design Series No. 4 "Introduction to Highway Hydraulics"

Hydraulic Engineering Circular No. 22 "Urban Drainage Design Manual"

These should give you the help you need.

 

[tincan]

Check with your local DOT or governing agency for their standards. As a rule of thumb 1 "standard" grate = 1.6cfs and 1 "standard" curb inlet = 2.5cfs. Using a double grate CICI(2-0)would give a value of 2(1.6+2.5) = 8.2cfs. For 22cfs==> (22-2(1.6+2.5))/2.5 = 5.5==> use 6. this would call for a standard CICI with 3 inlets on each side.

best, tincan

 

[bltseattle]

Wow, 22 cfs is a helluva surface flow for one catch! It seems that the spread would be quite wide, so not only does the grate need to be sized appropriately, but you have to think of bypass flow and at what point it will "return to the gutter". What is the gutter/shoulder configuration and road cross slope?

 

[wehr6]

The cross slope of the road is currently 2% to one side. The total road section is 20 feet wide, with 6 inch curb and gutter on one side and a 6 inch curb on the other. A two foot bench is adjacent to each side of the road. The 22 cfs is not from runoff, rather from possible water tank rupture.

 

[cvg]

you said at "the bottom of a a 15% slope access road" - does this mean that the basin is in a sump? If so, the capacity could be greatly increased over the rules of thumb given above, and there might not be any bypass or breakout flow. Use HEC-22 as recommended by Maury.

 

[tincan]

If youre in a sump I would use a grated street inlet extending the width of the street. Another "rule of thumb"
A grated street inlet will intercept approximately 2.5cfs/per foot of width on grade and approximately 4 cfs/foot of width in a sump.

 

[LHA]

I agree with blt...you are not likely to capture 22 cfs with an on-grade inlet at 15%, regardless how long the opening. If it is sumped and you can actually get that much water in, the spread prior to and at the inlet would definitly flood a 2% x-slope, with 10 travel lane; this is only a few inches of allowable head depth, and the roadway is impassable (I assume a crown on the centerline?).

If this is an emergency/contingency design, you should cut a sump and drop an endwall and daylight a pipe outside the roadway, prior to the tank's overflow entering the road. The inlets along a curbed roadway should only be used to capture normal roadway runoff, plus a little from adjoining lands. They are not intended to capture huge concentrated flows like storage tank failures.

 

[cvg]

as noted in the original post - this is an access road - I assume private access and not a public road. Spread may not be an issue and conveying the flow within the roadway perfectly acceptable to the private owner. As I read it, the primary goal is to capture the water prior to reaching the main road and discharging onto the adjacent downstream property owner. A grate spanning the entire street may be a good idea, I have seen it done before (on a city street) with good success.

 

[gbam]

if on grade and the transverse grate is an option look at vane grates. At 15% slope the 22 cfs will scream down the roadway. This will create major splash-over. Check out NEENAH

http://www.NFCO.com

for some precast grates. I am sure there are other options as well.

 

[dicksewerrat]

Space the inlets along the road. The water that spreads out needs time to come back to the curb. Full width trench drains may be the best way to go.

 

[n1cq]

I'd use a version of hydraulic orifice flow.

I've done this before myself - written a small excel program to look a various cases.

You can find discussion on such in any undergraduate fluid mechanics text.

n1cq

 

[Cherie]

Hi everyone, I'm working with wehr6 on the contingency drainage design. I have 2 questions for tincan (or anyone else).

1) When you refer to 1 "standard" grate + 1 std curb inlet having an inlet capacity of 1.6+2.5= 4.1 cfs, which standard are you refering to, and what are the dimensions? We're looking at APWA 302-2, which is ~3' long (add ~3'6" for each additional grate) and 2' wide with a curb inlet. So would an APWA 302-2 single grate with a curb opening have an inlet capacity of 4.1 cfs? Or were you refering to a smaller 'standard' inlet?

2) When you refer to a grated street inlet extending the width of the street (essentially a big trench drain), you say it has a capacity of 2.5 cfs per foot of width on grade. What length accompanies the 'foot of width' to give the 2.5 cfs capacity?

Thanks.

 

[cvg]

use FHWA HEC No. 12 - Drainage of Highway Pavements for design.

 

[dicksewerrat]

Call Neenah in Wi. they have all sorts of castings and the info you need. there is one casting manufacturer on the East coast but the name escapes me. Anderson I think. The maunfacturers have more design data than you will ever need. I think the trench drain is one foot wide and one foot long. You will want to use the shaped vanes on whatever castings you get. They allow more water to flow into the drain structure. Remember to make the pipe leaving the structure is able to take all the flow. Two trench drains 8' to 12' wide the width of the street about 10 feet apart should take everything the road can deliver.

 

[dicksewerrat]

8" to 12" wide not feet. sorry

 

[tincan]

Our standard x-street grated drainage is set up for a 26' or 32' wide clear roadway, The grated portion comes in 3'-0" by 3'-3" sections placed over a 3'x3' concrete collector. There are CICI at each end.

The CICI are a nominal 2'-8" in length (2'-6" opening. The grates are2'-8" x 1'-8" (2'-6" x 1'-6" opening).

We use the x-street grated drainage in flat areas where we experience sheet flow or flooded street conditions.

Neenah has excellent cataglogs and data. As suggested above , contact them. Also as noted above, spacing the inlets along the road should work.