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does storm inlet have Q discharge capacity?

does storm inlet have Q discharge capacity?

(OP)
if I calculate the Q of a given area draining to an area inlet like the pic.
Inlet is standard install by county. Does this inlet have Q capacity?


RE: does storm inlet have Q discharge capacity?

The "capacity" will depend on how the grate is installed, and how much head is allowed, so there is no single value. A grate in a sump situation can typically handle more then a flush grate. From your picture, the grate actually seems to be raised above the surrounding ground, which could cause much of the flow to bypass the grate.

I'm guessing that you have an issue with the adequacy of this inlet? If so, more information is needed in order to provide meaningful feedback.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: does storm inlet have Q discharge capacity?

suggest calculating the capacity using the following reference and since this inlet is covered with some sort of filter, the capacity will be greatly reduced. suggest also using a generous clogging factor

https://www.fhwa.dot.gov/engineering/hydraulics/pu...

RE: does storm inlet have Q discharge capacity?

In years past I saw a graph of the inlet capacity for a standard Type 1 CB grate. I don't think it was in the APWA manual though...

Mike McCann, PE, SE (WA)


RE: does storm inlet have Q discharge capacity?

(OP)
@psmart, the area surrounding the inlet will be re-graded see attached.

I just need confirmation that the inlet opening can take the Q I'm getting.


RE: does storm inlet have Q discharge capacity?

(OP)
I have hydraflow inlet calculation software.
The inputs are very simple and basic. I just put the width and length of grate and my Q.
But I'm not understanding the output.
If you will look at the output picture, there is water above the grate inlet.
Why would there be water above grate inlet if the inlet was designed for Q?

attached are input and output pic from hydraflow




RE: does storm inlet have Q discharge capacity?

The inlet capacity will be a function of the depth of water "standing" above the inlet. Once the inlet is submerged the orifice equation is used to calculate the capacity based on the open area of the grate (less any assumptions for blockage). You can create a table of depth versus flow and then determine is the peak Q from the drainage area can be passed by the inlet at an acceptable ponded depth.

RE: does storm inlet have Q discharge capacity?

For field catchpit like that we used 20 l/s for 50mm of head.

RE: does storm inlet have Q discharge capacity?

(OP)
@nkrigPE, thanks for the info.
I run a table of depth/flow. What is the "standard" slope of sag?
I think I can play with this number because I'm re-grading the site but want to use "standard" practice.

RE: does storm inlet have Q discharge capacity?

aprons are usually 2% slope. dont worry about the open area, unless your depth is quite large, use the weir equation with the perimeter length as recommended in HEC-22. reduce the capacity to account for clogging, assume 50% clogged

RE: does storm inlet have Q discharge capacity?

If you've got 100 feet of head over that inlet, the capacity will be pretty large.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: does storm inlet have Q discharge capacity?

(OP)
hydraflow is not asking for inlet head length? how does that affect the inlet opening Q capacity.

RE: does storm inlet have Q discharge capacity?

It's honestly pretty dangerous and somewhat frustrating to watch untrained people try to muddle their way through site hydraulics by trusting in computer software that they don't understand the inputs and outputs on. You're begging to get sued over a design failure with this whole chain of questioning.

Read CVG's link to HEC-22. It's comprehensive, and will explain to you step by step what the software is doing. The Hydraflow overlay in Civil-3D basically just automates that PDF.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: does storm inlet have Q discharge capacity?

(OP)
I'm not designing anything just verifying the county inlet is good. I can take your for word for it (some of posters said don't worry about it) that it's good or I can at least try to check it. I'm not checking a complicated drainage here, just one pipe culvert with an inlet in the middle.

RE: does storm inlet have Q discharge capacity?

(OP)
Don't worry nothing dangerous here at all. The worst that can happen is the "standing" water above the inlet is actually a few inches above or even below. The site is also not in the flood zone. As I said, county installed/designed this inlet. Just checking how much "standing" water above the inlet.

RE: does storm inlet have Q discharge capacity?

We had the Neenah engineer give a lunch and learn a few weeks ago. I'm not a hydraulic guy, but I remember him saying that they don't publish numbers for grates, just for curb inlets. I was lunching more than learning then, so I don't remember why, but it made sense. My impression was that they have the data, but it wasn't that important for this type of orientation. I bet if you call Neenah, they could help you out.

RE: does storm inlet have Q discharge capacity?

just judging by eye, I would say you have about 2 cfs capacity

RE: does storm inlet have Q discharge capacity?

you can fiddle around with the link below... i've never used it before but it seems reasonable... i've always had so many grates to handle low spots or cut down on overland flows, that i've ever been concerned with localized concentration of flows exceeding the grate capacity. of course the temp witch hat in the photo changes everything, but i assume you are modeling operational flows....

http://www.nfco.com/municipal/resources/weir-orifi...

RE: does storm inlet have Q discharge capacity?

(OP)
@cvg,

can you link me to inlet volume capacity (LxWxH) design.
Not the inlet grate opening which i have already calculated using hydraflow.
I can also calculate the pipe culvert diameter.
Unfortunately, civil 3d hydraflow does not calculate the inlet volume/area.
I have not looked into civil 3d storm sewer design.

Do I even need to calculate it. I think the volume/area of inlet is bigger than the volume/area of pipe culvert outfall?

RE: does storm inlet have Q discharge capacity?

I've never calculated the inlet volume. It's the smallest possible to fit the pipes (usually) or the grates for inflow (rarely).

RE: does storm inlet have Q discharge capacity?

Your structure has to be big enough to fit the holes they cut for the pipes. Attenuation volume in simple junction boxes is almost always neglected.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: does storm inlet have Q discharge capacity?

3' wide x 3' long x 4' high
a typical storm drain inlet structure has a volume of maybe 30 or 40 cubic feet (up to the top of the pipe) and at 2 cfs, that would be completely full in say 15 seconds at peak flow. Once it is full, there is no attenuation of flow. Some software such as stormnet and SWMM do allow you to enter volume of inlets and manholes. But frankly, it is a waste of time for analysis of peak storm flow.

If you really want to get crazy, than calculate the storage capacity of your pipes as well and include that in your routing also.

RE: does storm inlet have Q discharge capacity?

Delagina,

You need to correctly fill out the input for hydraflow (not done in your screen capture). You should be able to use Hec-22 that cvg linked previously for a reference.

RE: does storm inlet have Q discharge capacity?

Yes. That inlet has Q discharge.

How much Q discharge do you need? How are you figuring it?

What is the maximum depth of flooding that is acceptable?

Take that depth, and use it to drive the Q through the opening with the orifice equation. Then halve it (blockage). There's how much Q that inlet can take before you start to flood.

For instance, assuming those openings are (WAG) 1"x6" (times 40 openings) with 1' of head... drumroll... the Q discharge that inlet has is 8 cfs. Woops! It's clogged, use 4 cfs.

RE: does storm inlet have Q discharge capacity?

The inlet capacity increases with the Head above it (depth of ponding water), assuming it is located in a sump.
I normally use Hydrualic Toolbox.
Now, when you have a Q and want to see if your Inlet can handle it, just enter the maximum allowable ponding depth (Head) at the inlet and determine the Q(inlet). If the Q(inlet) is greater than your Q from drainage area, you are ok. If not, resize the inlet opening. Just like Spartan5 suggested, I would follow that method.

RE: does storm inlet have Q discharge capacity?

Since there is a trash rack at the inlet, the inlet should be designed to accomodate discharge considering some percentage of choking conditions. In hydropower we consider 50% of the net area choking condition.

KARMA

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