pump at end of irregular channel

I have the channel model running fine except for the pump. I put in an inline pump and I want to run it at 145 cfs. I cannot get the model to run without getting this wave action moving through the channel. I believe it is caused by the pumps, only because I have got it to run with a variable inline pump but, I had to make the offset between the high medium and low depth a greater distance. What I want is the pump to turn on at 1 ft and pump continuously at 145 cfs until the water goes back down to 1 ft.

I have tried everything and the only thing that I can get to work is the variable speed inline lift pump but I have to set it to only pump full speed when it is very full. This is unrealistic and I would like to know if I can set a pump to pump full speed the entire time.

Thank you for all of your help.
Chad Qualley
cqualley@mooreengineeringinc.com

Dear Chad :

When looking at the discharge in each channel section, it is clear that there is a stability problem. I have changed a few things in order to narrow down the problem.

1. I have changed the length of the simulation to (140040) to match the 38.9 hours of inflow hydrographs given in K3 card.

2. I removed the pump and made junction 5 a temporary outfall.

It is then clear when looking at the discharge in each channel section that model is still unstable, which means something else than the pumps are causing the instability.

The problem is that the some of the channel sections are too short, < 20 ft - the Courant number criterion is not fullfilled. The model is already running at 1 sec timestep, so the timestep can not be decreased any further.

3. I have tested if the model will be stable using the "modify short pipe length" in the SWMM engine but without any improvement.

I can therefore only suggest that you merge some of the channel sections in order to get rid of the short channel sections. I believe it is not recommended to have channel sections of less than approximately 100 ft of length.

Attached is a test model with the 1-3 changes

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com