Catch Basin in Parking Garage 1

[arcoiris74]

I am working on a drainage system for a 3 acre site in Florida. All of the catch basins are interconnected and empty out into a control structure and eventually a canal. However, one the CB furthest upstream is located at the lowest point of a 3 story parking garage. I've run my hydraulic grade line from the outlet upstream to the catch basin, and it appears to be below the grate. However, I need to prove that water is not going to bubble up out of this catch basin. The drains from the 2nd & 3rd floors of the garage tie into this catch basin. Is there way I can prove on paper that These flows will not create a jump in the basin, hence causing minor flooding. It's my first time going through drainage calcs., so I apologize if I have not spelled out the problem clearer.

Thanks

Gee

 

[msquared48]

Usually where I practice, catch basins in covered garages have to be connected to the sanitary sewer system, not the storm system.

Do you have combined sewers?

Mike McCann
MMC Engineering

 

[Trackfiend]

Have you modeled or included these flows from the 2nd and 3rd floors into this catch basin? I would suggest something like SWMM but since its your first time doing drainage calcs, that may or may not be over your head (no offense, but it still seems over my head most of the time). If the inflow from the overhead drains don't raise your hydraulic grade line, then you should be O.K..

In my area, its just the opposite, tying into the sanitary sewer is a big NO-NO.

 

[Maury]

The hydraulic design is only going to function up to the assumed design runoff. There are bound to be storm events that exceed the design criteria or high water at the discharge, so don't try to prove that there will never be any flooding or backflow at the low catch basin, you're bound to be wrong. You should look at what happens when there is flooding. Provide an overflow route that limits the water depth. If that's not possible, then look at separating the low structure from the 2nd and 3rd floor piping network.

 

[cvg]

you say this is the furthest "upstream" catch basin. If the hgl is below the grate as you say, then it will not "bubble back out". However you will have an entrance loss which you need to account for. This is the loss in energy / momentum of the flow coming out of the 2nd and 3rd floor pipes which connect into the catch basin and then flow to the outfall. If this flow has sufficient velocity, than the entrance loss could push the hgl above the grate. You should always allow some freeboard in your basins to account for this. Usually one foot minimum freeboard is recommended.

 

[arcoiris74]

I ran the HGL from the tailwater downstream and sized the pipes such that the HGL happens to be 1.3' below the top of my grate in the garage. As you mentioned, I definitely want to allow for the freeboard. Is there a way of quantifying how much the water will rise in this basin due to the high velocity of water dumping into this same basin from the garage drain pipes? having the water enter the basin with high velocities and from a smaller dia. (likely 10" PVC) pipe will raise the water over my HGL realistically, and my exiting Q will obviosuly be lower, especially since the drain pipe wil be approx. 24" CMP.

Also, are there any refeneces you may know of for using the appropriate size catch basin size/type based on the Q in cfs?

Thanks for everyones help by the way; ogreat suggestions

Gee

 

[cvg]

assume a full velocity head and perhaps more for safety factor. use the velocity of the flow coming from the upper floor pipe into your basin. then just V^2/2g. this could be significantly greater than 1.3'. If your incoming velocity is greater than 9 fps, than you have zero freeboard.

 

[civilperson]

Your local plumber can teach you about "backflow preventers" which are one way only valves. Use on lowest levels of catch basins. If your assumed maximum capability flows are exceeded, then water WILL rise out of the catch basin!

 

[cvg]

note of caution, for storm drainage - back flow preventers are typically not used in this application. They might be considered where you are trying to limit sewage or groundwater from backing into a basement. For storm drainage, you need to design your system to handle the flows and reduce the headloss in the storm drain system. a backflow preventer will increase the headloss. this will only move the flooding issue to another location.

 

[PEDARRIN2]

A backflow preventor is on the water supply system.

I think the proper term is a back water valve. There is a big difference.

Backwater valves are used on the inside plumbing and sometimes on the first catch basin to prevent water from coming back inside the building. It typically more prefered to have the exterior system overflow than the interior system - although in a parking garage the difference might be moot.

 

[gbam]

Depending on the angle(s) between the inflow and outflow pipes, the high velocity from the upstream areas may even produce a siphon and reduce the HGL in your CB. You may also want to check the inlet control headwater to your outflow pipe to confirm it does not create a headwater greater than the grate elevation or your HGL calcs.