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# HGL of Storm Drain vs. Inlet Capacity Depth Result

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## HGL of Storm Drain vs. Inlet Capacity Depth Result

(OP)
Hi Everyone!

I'm new to this site and I thought I'd pick peoples brain by asking a question.

What is the relationship between an inlet capacity and a storm drain capacity calculation? Say my inlet is able to handle the flow, and it's determined that it has a 2in ponding depth, but the pipe can't handle the Q, so my HGL backs up above the grate by 5 feet.

In addition, I have a building next to my inlet, so I would like to know how to asses this situation to determine if my building will have the potential for flooding.

Thank you all, I look forward to you input.

:)

### RE: HGL of Storm Drain vs. Inlet Capacity Depth Result

Culvert flow is controlled by either the inlet condition or outlet conditions.

Review the design guidelines on the FHA site:

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

### RE: HGL of Storm Drain vs. Inlet Capacity Depth Result

if it backs up 5 feet than you have a 5 foot deep problem...

think of it this way. you can have a very large inlet that can easily handle the flow but if it is connected to a drinking straw, than it will be ineffective

### RE: HGL of Storm Drain vs. Inlet Capacity Depth Result

(OP)
Thank you bimr for the reply. I'm going to read into this. Honestly, I've always struggled with the understanding of inlet/outlet control.
Never thought of thinking the situation as a culvert. It may shed some light into my understanding.

cvg, that does make sense. If I wanted to account for the capacity of the pipe and the ponding limits of a sump area where my inlet resides, (i.e. say a parking lot low point), how would you calculate this? I may be confusing a volume vs flow analysis here. I'm thinking Hec-Ras, where you would create a hydrograph and route it through the system and include the pond limits. I know this would probably more work then necessary, but technically speaking, I would jut like to know how it's done and what program would accomplish the job best.

### RE: HGL of Storm Drain vs. Inlet Capacity Depth Result

HEC1, HECHMS, HECRAS, SWMMM, all could do it. but depending on the size of your drainage area, might not be enough storage on the parking lot to make much difference in the peak outflow

### RE: HGL of Storm Drain vs. Inlet Capacity Depth Result

What are you using to compute the 5 ft. above the grate? (specific software or hand calc method)

One may not actually be dealing with 5 ft. of water depth: In many models, if one does not define storage above the inlet, the model will compute a "vertical column" of water above the inlet, while if one provides the actual storage above the inlet...there may be only a few inches of flooding.

In many cases this is totally unacceptable and the pipe needs to be larger or the drainage area reduced. However, there are cases where an amount of surface flooding is tolerated.

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