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TW depth on closed storm drain sys

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Ekenny_G

Civil/Environmental
Apr 16, 2012
84
I am trying to figure out the Headwater depth of a 24" Culvert that ties into a manhole. The Q = 18.50cfs (100-yr) and dia=24" with Headwall.
What is the tailwater depth? Can I use the 100-yr HGL at the pipe end as the tailwater depth or just use the normal depth of flow?
 
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Seems to me the 100-yr HGL will be more conservative (assume it is under outlet control; otherwise for inlet control tailwater has no effect).

I would say do calcs for both cases and pick the one resulting higher head water depth to be conservative.
 
you need to assume the most critical level. that may involve analysis beyond one manhole if the storm drain is flowing full or surcharged.
 
Do you know what the design criteria is/was for the downstream system?

For example, it would not be unusual if the system was only designed for the 10 or 25 year event and would be surcharged already in a 100 year event.

I'm with cvg - you need to see if you can acquire design calculations on the downstream system, or evaluate it yourself.

If the data is not available AND the system is too complicated to make modeling practical for you needs, I would at least try to find the design criteria for the downstream system and make some practical assumptions from there.

For example: if the design of the system was for 10yr, estimate a downstream flow assuming the downstream pipe is FULL for THAT event, then scale that flow up to the event you are modeling to establish the surcharged tailwater in the downstream pipe.
 
I have been out in the field to investigate this more. The proposed culvert pipe ties into an ex.manhole. This manhole connects to an existing downstream drainage pipe system that eventually outfall to a stream and the 100-YR WSE = 278.15 at the outfall.
The design storm is 100-YR for the culvert itself. My question is: What is the HeadWater depth for the Q(100-yr) flow? I need to find what is the TailWater depth to get started on this. Can I assume the TW elevation is at stream 100-YR Water Surface Elevation even if its thousands of feet away from the culvert entrance location?
or Can I assume if the downstream pipe if flowing full at the 100-YR flow, then the TW depth must be equal to diam of pipe?


 
you need to analyze the thousand foot long storm drain. your "culvert" will become part of the storm drain. you need to do a complete hydraulic analysis of the entire system to determine the hydraulic grade line at the entrance to your "culvert"
 
@ cvg
You mean to say that my HGL line at the entrance of the culvert will represent my Headwater elevation?

 
it might.
just depends on the flow, pipe slope and diameter, friction and minor losses as well as entrance and manhole losses and of course tailwater in your stormdrain. without a lot more information, just can't be determined.
 
Thanks guys.
After an exhaustive process working on maping out the existing storm drain system, I was able to generate the HGL (100-YR) using Inroads S&S.
Fortunately, the headwater is not overtoping the road, however the required freeboard is not being met. Lowering the inverts of the culvert is not an option due to slope/capacity requirement.
 
So is the culvert under inlet or outlet control? There are some things you can do at the inlet to improve the hydraulics (1:1) miter sloped inlet etc... What Ke did you use for your culvert? More improtant is how close to meeting the freeboard requirement are you, feet or inches? You may need to berm up to meet freeboard or enlarge your culvert.

Also if you have a ponded inlet condition you need to lose your velocity head or at least a portion of it. I recall, from my limited knowledge/use of Storm & Sanitary, that the upper end of the hydraulic run does not account for ponding condition or approach velocity in a channel scenario.

Dont forget to run the hydraulics upstream past your new connection in the stormdrain; the additional flow may have adverse impact on the rest of the storm drain.
 
agree that some programs do not consider either inlet loss, trash rack loss, clogging factor or approach velocity nor any impact to an upstream ditch or channel. I have seen drains fail because the designer did not address these critical issues of actually getting the water into the pipe.
 
@ gbam. The culvert is under outlet control and I used a Ke = 0.50 for square edge w/headwall. There is a ponded inlet condition and I am being asked to develop a stage-storage depth at the inlet.

Another issue is that I used Rational method to size the pipes in the system, however the reviewer wants me to use TR-55 method and to run TR-20 for the stage/storage depth behind the culvert. The Qpeak from the TR-55 is twice the Rational method Qp and as a result, my pipe capacity can't handle even the 25-YR storm.

 
if hydraulic control is downstream, than very little you can do upstream to improved things other than reduce your inlet and friction losses in the upstream pipe. but if you have significant storage at the pond, than maybe you dont have to pass the peak Q, if t
 
Thanks CVg. There is not much I can do downstream. Like you mentioned earlier when I ran TR-20 thru culvert, It says that the Qp is 28.2 cfs, however downstream is only 11.7 cfs. I am new using TR-20 and still trying to understand the output and format.
Can I send you my TR-20 input to evaluate see what I am doing right or wrong?
 
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