Hydraflow ponds outflow pipe size

[dukercook]

I have entered three different ponds, but the flows from the popes seem to be half of what would be correct from comparing to manning's or the tables with HW/d.  It states I need 2-36" pipes for 60 cfs of flow to exit the pond.   Everyone is questioning my numbers. 

[wfberry]

Perhaps attaching your .gpw files would be beneficial.

Bill

[dukercook]

The original post would not let me. I think I figured it out. I was trying to make one set of pipes work for the 10, 25 and 100 year. Which was not letting enough headwater happen to make the pipes work.


Thanks,

Christina Cook


Sent from Outlook

[dukercook]

I changed the end to .txt just change to .gpw

[wfberry]

I don't like to repeat myself, but I am not a good person to critique other person's hydraulic work.  However, not knowing anything about your project.  The CN numbers seem a little high for normal projects.

Are the stand-a-lone areas representing the Pre-Development areas?

Are you comparing pre-development to post-development?

If your areas are correct with pre-development comparing post-development it appears that your basins are too small.

Bill

[dukercook]

The preexsiting CN numbers are due to crop land. in D soil. And it is a weighted CN number. This CN has been discussed with the city and others in our office. I usually do not out a preexisting CN number that high.

That being said. EA-1 is the preexsiting drainage area. These are the flows I am to meet with the combined outlets of DA-1A and DA-1B. There is a new road that is dissecting the flow.

Ea-2 corresponds to the preexisting to the DA-2.

So the outflow of the pipes can not be more that existing.

The goal was DA-1A 10, 25, 100 outflow - 58,72,99 and DA-1B 32,39,53. But can be balanced.


Ask any question you want to. Thanks





Thanks,

Christina Cook


Sent from Outlook

[fcernst · Accepted solution]

What stage are you trying to push the 60 cfs through at?

Are you trying to stay below the weir?

[dukercook]

I accidently hit accept.  So there shold not be a weir.  I originally was designing for the 10, 25, and 100 to go through the pipes only.  So DA-1A would have DA-1A flows go through the pipes, DA-1B goes through DA-1B pond and pipes

[fcernst]

Ok.... Hydraflow looks like it's working correctly.

The problem is your pond is about three times too small to attenuate the 100yr incoming volume for a 60 cfs release rate...You're kicking out 103 cfs now...

[dukercook]

Sorry I want clear. The 60 is for the 10 year. 93-105 can come out for the 100 year. What my problem is.... it is showing two 36 inch pipes. For a 60 cfs flow out for the ten year. When using a chart a 36 inch should be almost 50 ish cfs on its own. The ponds do work.

[fcernst]

Interesting, it seems to have a problem with the 36" pipes..I created a SWMM for this pond:

If you go down to twin 30" pipes both software give 62 cfs.

When I go to twin 36" pipes..... SWMM gives 70 cfs and Hydraflow goes down to 61 cfs..

[dukercook]

That is what I am trying to figure out.  Why with the 36" is the flow completely off.  The only thing I figured is it has to do with the headwater on the pipes.  I did end up going down on pipe size and added a weir.  Everyone kept asking me why the pipe sizes were so large.  I did read some info on how the q is calculated when the headwater is not over the top of pipe.  I had even tried to utilize SSA to figure it out.   The ponds are shallow and long, which I think causes some issues.  Kansas is just flat.

[fcernst]

My guess is sensitivity due to the headwater depth is essentially hovering right at the 36" crown elevation and Hydraflow can't iterate back and forth under and over the crown accurately through this particular scenario... Hydraflow will only go down to a 1 minute routing interval, while in SWMM... I went with 1 second.

Autodesk really needs to allow for the User to set a separate pond routing time step going down to one second or less to model pond outlet structures.