Puzzled about basin outlflow results
Puzzled about basin outlflow results
(OP)
I have a basin with surveyed bottom elevation at 298. The as-builts show the basin bottom at 296 (there has been 2' of deposition accumulated on the basin bottom raising the bottom of basin elevation). The outlet structure consists of a riser structure with top elevation at 303.79', a 21" RCP pipe to the riser structure with invert elevation 296.07', and a 66" RCP outlet pipe from the riser structure with invert at 296'.
Therefore my current existing model has a basin with bottom elevation at 298'. Even though the 21" inflow pipe to the riser structure is blocked, I have still included it in the outlet configuration (I assumed that since both the invert and crown elevation are below the bottom of the basin, nothing would be routed through this pipe). So, my model shows the outlets as follows
# Invert Description Routing
1 295.60 66" RCP Primary
2 303.79 108" Concrete Riser Device 1
3 296.07 21" RCP Device 1
However, when I run the model, and review the discharge curve, it seems that water is being discharged immediately. How is that possible? I don't have any infiltration assigned to the outlets.
To double check, I took out device #3, and ran it again, and the curve appears as expected, with a horizontal line at discharge=0, upto elevation 303.79, at which point the discharge begins.
Is there something that I need to do to ensure the model doesn't include the 21" RCP pipe in the calculations. I am wondering if the model is creating an imaginary stage storage down to the elevation of the 21" pipe? I'd like to keep this component of the outlet structure in the model, so I can model other scenarios (ie, the as-built basin, an proposed design).
Any advice would be greatly appreciated. I have attached the hydrocad file
Therefore my current existing model has a basin with bottom elevation at 298'. Even though the 21" inflow pipe to the riser structure is blocked, I have still included it in the outlet configuration (I assumed that since both the invert and crown elevation are below the bottom of the basin, nothing would be routed through this pipe). So, my model shows the outlets as follows
# Invert Description Routing
1 295.60 66" RCP Primary
2 303.79 108" Concrete Riser Device 1
3 296.07 21" RCP Device 1
However, when I run the model, and review the discharge curve, it seems that water is being discharged immediately. How is that possible? I don't have any infiltration assigned to the outlets.
To double check, I took out device #3, and ran it again, and the curve appears as expected, with a horizontal line at discharge=0, upto elevation 303.79, at which point the discharge begins.
Is there something that I need to do to ensure the model doesn't include the 21" RCP pipe in the calculations. I am wondering if the model is creating an imaginary stage storage down to the elevation of the 21" pipe? I'd like to keep this component of the outlet structure in the model, so I can model other scenarios (ie, the as-built basin, an proposed design).
Any advice would be greatly appreciated. I have attached the hydrocad file





RE: Puzzled about basin outlflow results
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Puzzled about basin outlflow results
RE: Puzzled about basin outlflow results
RE: Puzzled about basin outlflow results
RE: Puzzled about basin outlflow results
Of course, the actual system will have some storage down to the lowest outlet, even if it is just a vertical feed pipe or drop inlet. But this volume is often insignificant in comparison to the primary storage volume, and can be omitted from the model, as done in the OP. This approach works just fine. If you need verification you can include the lower volume in the storage definition and compare the results.
Also have a look at the stage-discharge curve, and you will see that the discharge starts at the bottom of the storage, not at the depressed outlet.
Bottom line: Omitting the feed volume of a depressed outlet will not adversely effect the results, as long as the volume is minimal.
Peter Smart
HydroCAD Software
www.hydrocad.net