Different values for Depth (in), Depth (Middle) and Depth (Out)

Question

I have modelled a sewage collection network and designed it in SewerGEMS V8i. I am getting different values for Depth (in), Depth (Middle) and Depth (Out) for the conduits. I expect them to be same, as there is no addition/ extraction of flow or, change in diameter, or, change in slope within any particular conduit. 
 
 Sample1.stsw.zip

Scott Kampa · Accepted Answer

Hello Indranil, 
 
This is operating as expected. With gradually varied flow, it is expected that the depth will change along the length of the pipe. This tends to be more accurate than a uniform depth. Note that the *flow* is not changing, only the depth and velocity. 
 
The following link has some more information on how the gradually varied flow analysis works in conjunction with the profiles: communities.bentley.com/.../21211.how-are-the-profile-descriptions-derived-in-sewercad-s-gvf-convex-solver 
 
The profiles in your model have a Profile Description of M2, meaning a mild slope classification with flow in Zone 2 (the descriptions of these classifications can be found link above). From the image in the link, this tends to mean that the depth will decrease at the downstream end of the conduit compared to the upstream end. 
 
I would encourage you to check out the Stormwater Conveyance Modeling and Design book referenced in my earlier post as well. The book goes into greater detail on how gradually varied flow works. 
 
If you only need to see uniform flow for a single conduit, you could use a program like FlowMaster to analyze the conduit. 
 
Regards, 
Scott

Sushma Choure · Answer

Depth of water at upstream, downstream & middle of the pipe is different because velocity in & out changes so the depth. These values are same when there is subcritical flow in the pipes. 
Do you mean you have constant/same slope for all the conduits & no addition flow at each manhole? Because the flow gets added up at each manhole, which increases the velocity & ultimately varies the depth.

Nawnit Kumar · Answer

The reason for different depth of water is gradually varying flow in the pipe. Flow is governed by continuity,momentum and energy principal.

Tom Walski · Answer

Even when flow is constant, the depth and velocity can change when you have some kind of condition that causes non-uniform flow. If the flow at either end is at something other than normal depth, the flow will attempt to reach normal depth along the pipe. Some things that might cause non-uniform flow are a weir downstream, backup from a downstream pipe, transition from subcritical to supercritical flow,...

Scott Kampa · Answer

Hello, 
 
I don't see the model file you are referencing. However, this is likely working as expected. 
 
As Tom noted above, even with the flow being uniform, the flow depth and velocity may be different based on the system. This is how gradually varied flow works. The flow is the same, but the depth may be different. The program will use Gradually Varied Flow (backwater) calculations to solve for the depth in a conduit. You can find more information on Gradually Varied Flow in the Help documenation and in section 7.8 of the Stormwater Conveyance Modeling and Design book: www.bentley.com/.../SCMD.htm 
 
While the model files are not explicitly needed since your description of the issue is good, if you need to upload the model, there are two options for sharing your model files on Communities. If you would like the files to be visible to other members, compress the files into a zip file and upload them as an attachment using the ‘Advanced Reply editor’ before posting. If your data is confidential, you can follow the instructions in the link below to send it to us via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley. 
 
 communities.bentley.com/.../7079.be-communities-secure-file-upload 
 
Please post here with the name of the file so we know it is available. 
 
Regards, 
Scott