Keep my head straight on this: connecting to a culvert under inlet control
Keep my head straight on this: connecting to a culvert under inlet control
(OP)
Situation:
Existing culvert is fairly over capacity. Watershed is 600 acres of mostly undetained urbanized and suburbanized watershed. FIS says there's 480 cfs in it at the 100 year storm. This causes a pretty big head differential across the culvert - upstream head is about five feet above the downstream head, which lends me to believe the thing is inlet control for the 100 year event.
My development is just upstream of this culvert, and my land plan really lends itself to putting by stormwater management system, which will be an underground concrete vault with an offline cistern, at an elevation well below the 100 year elevation upstream of the culvert. But we're close enough to the culvert that I have the opportunity to not discharge into the creek at all, and carry my discharge into the street, dog-housing the culvert and dropping my water in downstream of the head wall that's pretty clearly forcing the thing into inlet control.
What tailwater depth should I assume in this design, for my stormwater management system? Seems to me, unless I'm thinking about this wrongly, that I should be able to calculate the 100 year HGL in the pipe and use that as a tailwater. Right? Am I missing anything?
The flood profile on the FIS doesn't really show it that way. I'm not sure how they generated the drawing, but it shows the HGL for the culvert being high through the whole culvert, and only dropping off after the downstream headwall. Graphical error in the FIS maybe? (profile attached - Hermance Drive is the culvert in question)
Unfortunately, the FIS was done in XP-SWMM instead of HEC-RAS so I can't easily get at the guts of their model.
Disclaimer: I haven't had my coffee today.
Existing culvert is fairly over capacity. Watershed is 600 acres of mostly undetained urbanized and suburbanized watershed. FIS says there's 480 cfs in it at the 100 year storm. This causes a pretty big head differential across the culvert - upstream head is about five feet above the downstream head, which lends me to believe the thing is inlet control for the 100 year event.
My development is just upstream of this culvert, and my land plan really lends itself to putting by stormwater management system, which will be an underground concrete vault with an offline cistern, at an elevation well below the 100 year elevation upstream of the culvert. But we're close enough to the culvert that I have the opportunity to not discharge into the creek at all, and carry my discharge into the street, dog-housing the culvert and dropping my water in downstream of the head wall that's pretty clearly forcing the thing into inlet control.
What tailwater depth should I assume in this design, for my stormwater management system? Seems to me, unless I'm thinking about this wrongly, that I should be able to calculate the 100 year HGL in the pipe and use that as a tailwater. Right? Am I missing anything?
The flood profile on the FIS doesn't really show it that way. I'm not sure how they generated the drawing, but it shows the HGL for the culvert being high through the whole culvert, and only dropping off after the downstream headwall. Graphical error in the FIS maybe? (profile attached - Hermance Drive is the culvert in question)
Unfortunately, the FIS was done in XP-SWMM instead of HEC-RAS so I can't easily get at the guts of their model.
Disclaimer: I haven't had my coffee today.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com





RE: Keep my head straight on this: connecting to a culvert under inlet control
RE: Keep my head straight on this: connecting to a culvert under inlet control
RE: Keep my head straight on this: connecting to a culvert under inlet control
*if the culvert is as you suspect not flowing full, there is another water surface profile inside the culvert (not shown) that you'll need to find to determine the allowable flow you can inject mid-pipe without making things worse.
Then again, looks like the slope is adverse to grade, very possible to be outlet control.
RE: Keep my head straight on this: connecting to a culvert under inlet control
RE: Keep my head straight on this: connecting to a culvert under inlet control
RE: Keep my head straight on this: connecting to a culvert under inlet control
Yep, that's how we design many typical culverts...pass the 10yr and no more than 1' overtopping for the 100yr.
RE: Keep my head straight on this: connecting to a culvert under inlet control
When you add the connection you are creating a storm sewer analysis scenario because you are introducing junction losses at the connection. Many storm sewer packages now can test for inlet control also using the HDS 5 equations.
RE: Keep my head straight on this: connecting to a culvert under inlet control
I agree 480 cfs seems low, but a lot of the watershed is in undeveloped B soil stream buffers, and a lot more consists of university athletic fields, so that probably helps some. I'm inclined to stick with the FEMA flow rates, especially given that their watershed area and the one I did as a check are nearly identical.
A HY-8 analysis utilizing the FIS flows and the tailwater shown in the FIS profile shows inlet control (as I suspected), overtopping, and a lower HGL in the culvert. I'm using a 'n' of .015 to account for junction boxes in the line. I'll utilize the pipe HGL shown in the HY-8 run as my system tailwater.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Keep my head straight on this: connecting to a culvert under inlet control
No, because your new incoming junction flow you are inserting into the culvert will change that existing culvert HGL.
RE: Keep my head straight on this: connecting to a culvert under inlet control
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Keep my head straight on this: connecting to a culvert under inlet control
There are two problems with that:
1) If you try and add your flow at the inlet of the overtopping existing culvert, most of your flow will get eaten up by the weir flow that is running to the 3/2 power vs the inlet running to the 1/2 power.
2) If this culvert is not running supercritical, your connection may create tailwater control below your connection, then possibly all the way back up to the inlet of the culvert.
This all affects the tailwater hydraulics of your incoming system.
I suggest analyzing the system together as one. Calibrate your model first to the existing condition, then add your condition.
RE: Keep my head straight on this: connecting to a culvert under inlet control
I don't intend to add any new junction boxes to the culvert run, just to drop my flows into one of the existing ones along the reach - there are several downstream of the headwall. I'd be diverting existing flow from coming into the culvert, managing it, and then discharging it into the culvert, for a watershed much much smaller than the culvert's watershed. And because the flow is detained, there should be insignificant change in velocities, and therefore head losses, along the culvert run. Plus, I'm not required by code to check the culvert, because of the ratio of my watershed size (very small) to the culvert's (very large). I'm merely trying to figure out the tailwater on my discharge pipe. For that, the Chow (1959) recommendation of n=0.015 for "Concrete sewer with manholes, inlets, etc, straight" should produce an accurate HGL for the calculation. Better than SWAG, actually.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Keep my head straight on this: connecting to a culvert under inlet control
I think that 1959 Chow recommendation was for the singular culvert itself, not to be used like you are proposing for modeling incoming sewer junction hydraulic systems.
RE: Keep my head straight on this: connecting to a culvert under inlet control
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Keep my head straight on this: connecting to a culvert under inlet control
Still don't think we are communicating.
RE: Keep my head straight on this: connecting to a culvert under inlet control
No one can argue the change in momentum and disturbance caused by a 1cfs lateral flow is different than that caused by a 25cfs lateral flow.
The standard engineering method is to calibrate your model to the FIS (or if the use of new and better data can be justified), for the reach of interest, then add your proposed condition. This simple culvert scenario would not take that long to do in this manner.
Is it a software issue? Take a look at the FEMA approved software for some ideas on which software can help you.