TW depth on closed storm drain sys
TW depth on closed storm drain sys
(OP)
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?
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?





RE: TW depth on closed storm drain sys
I would say do calcs for both cases and pick the one resulting higher head water depth to be conservative.
RE: TW depth on closed storm drain sys
RE: TW depth on closed storm drain sys
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.
RE: TW depth on closed storm drain sys
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?
RE: TW depth on closed storm drain sys
RE: TW depth on closed storm drain sys
You mean to say that my HGL line at the entrance of the culvert will represent my Headwater elevation?
RE: TW depth on closed storm drain sys
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.
RE: TW depth on closed storm drain sys
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.
RE: TW depth on closed storm drain sys
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.
RE: TW depth on closed storm drain sys
RE: TW depth on closed storm drain sys
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.
RE: TW depth on closed storm drain sys
RE: TW depth on closed storm drain sys
Can I send you my TR-20 input to evaluate see what I am doing right or wrong?