WaterCAD un-varying pipe flow

Question

Hi Jesse, 
 I have read several of the forum posts and I cannot find anything to help me with my model. 
 I am working on a very simple model to obtain some rough discharge values. The model consists of an elevated reservoir connected to two other branching reservoirs at a lower elevation. I am evaluating a controlled discharge to one reservoir to observe how the discharge to the second reservoir changes based on controlling flow to the initial reservoir. I have attached a screen shot of the model so you may be able to get a better understanding. 
 The way I am modeling the system is by removing the "Arroyo Grande Reservoir" and replacing it with a junction with a set demand. I expected to see a change in the discharge to the "Terminal Reservoir" but that did not happen. I further troubleshooted and tried placing a FCV in P-1 to control the incoming flow and see if that caused the discharge to the "Terminal Reservoir" to change but that did not work either. I don't know what other way I could model this, or perhaps, if I am analyzing it incorrectly. If you have any suggestions as to how this may be achieved please let me know. If need be I can also send you the file. Thank you, in advance, for your help. 
 Best Regards, 
 Angie LopezLake Model.zip

Wayne Hartell · Accepted Answer

Hi Angie, 
 
The assumption that reducing the flow to Arroyo Grande Creek should increase the flow to Terminal Reservoir sounds intuitive, however, it depends on the hydraulics of the system as Scott mentioned. If say, pipe P-1, has sufficient capacity to supply both reservoirs (i.e., insignificant head loss), then reducing the flow to Arroyo Grande Creek will not necessarily affect the flow to Terminal Reservoir in any way. I notice that in your model, pipe P-1 is negligible loss (really short) so you could effectively turn J-1 into a reservoir at the same elevation as Lopez Lake and then it should become clear why changing the flow to Arroyo Grande Creek has no effect on the flow to Terminal Reservoir. 
 
It seems to me that there would be some upper limit on the flow coming out of Lopez Lake, so perhaps using an FCV (flow control valve) in P-1 to limit the flow is what you need to do. Then so long as the maximum flow is less than the total flow through P-1 without the FCV in place, you will see the results you expect to see (that is, the flow to Terminal Reservoir will change as you change the flow to Arroyo Grande Creek). 
 
Perhaps you can give us a little more information about the real-life situation that the model represents? e.g., Is there really a common pipe to supply both downstream reservoirs or do they have separate pipes leading from the source. If there is a common pipe is it modeled accurately in your model? etc. 
 
Regards, 
Wayne.

Scott Kampa · Answer

Hello Angie, 
 
WaterGEMS and WaterCAD will work to satisfy the demands in the system as well as balance the hydraulic grade. When you have flow between a high point and low point, the flow it takes to balance this will not necessarily change with a change to another part of the model. In this case, what is coming from the Lopez Lake reservoir and flowing toward Arroyo Grande Creek does not impact what is flowing toward Terminal Reservoir. 
 
If you change the elements of Terminal Reservoir, you will see a different results. If you increase the discharge coefficient at TCV-2, you will see different results as well. However, changing something in another part of the model will not impact the flow to Terminal Reservoir. Note that if you replace the Arroyo Grande reservoir with a junction with no demand, the flow in pipe P-1 changes to match the flow going to the Terminal Reservoir. The flow to Terminal Reservoir will remain the same because that is the amount of flow it takes to balance the energy in the system. 
 
What are you expecting to happen in this case? 
 
Regards, 
Scott