Flow/Capacity (Design) while using Darcy-Weisbach method

Hello Jatin,

There might not be a better design available with the constraints defined.

Check the diameter of the conduit that has a "Flow/Capacity (Design)" of 102%.
Check your conduit catalog to see if there is a larger diameter available for design.

If that is not the reason then please provide the model files so we can see all of the constraints in your design.
See: Sharing Hydraulic Model Files on the Haestad Forum

Hi Craig, Had it been occurring only for the biggest diameter pipe in conduit catalog it was ok. But the problem is, it happens for all the pipes .

Best Regards
Jatin Talwar

Hi Craig,

Sorry for replying to this thread after such a long gap. I am still facing the issue while using using Darcy-Weisbach method. The model has been uploaded separately today under the name "For Bentley.zip".
The model contains new proposed network as well as existing network, some of which is undersized and not of concern. I have mentioned "Existing" in a user defined field of "Status" in conduits for clarity.
The concern is the proposed network where the Flow/Capacity design value exceeds 100%. This is happening for pipes ranging in diameter from 200 mm to 1800 mm pipe.
Also, there is a new proposed 1800 mm dia pipe which has a flow/capacity design value of greater than 100%. regarding this particular 1800 mm dia pipe, my concern is that although 1800 mm is the maximum size of conduit defined in conduit catalog, the range of slope and velocity at which it can be designed (defined in constraints) is large 200 to 2000 and 0.7 to 3 m/sec respectively. Still the pipe is getting designed at the flattest possible slope of 2000 with a velocity of 1 m/sec and a flow/capacity design value of greater than 100%.

Request you to please look into it.

Best Regards
Jatin Talwar

Hello Jatin,

In the model you sent, I see 21 conduits out of 1988 that are "existing" (Design Conduit = True) and that have a Flow/Capacity (Design) above 100%. With the exception of the 1800mm conduits, they are all under 103%. This may be due to exceptional cases during the design iterations. In such rare cases, you may want to simply adjust the sizes yourself.

The automated design feature is not meant to replace the engineer, and some judgment and tweaks may be necessary.

As for the 1800mm conduits at 117%; I suspect that this is due to hitting the maximum size available, and the prioritization of minimizing excavation (cover). If a larger size (say, 2000 mm) was available, it would prefer that over an increase in slope, since that would increase excavation. Since 1800 mm is the maximum though, it selects that and "stops", settling on the minimum slope. With that being said though, similar to the above, could you adjust these few pipes manually?

Regarding the Mannings method not seeming to cause the same issue; in my testing it does not appear that the Mannings n (0.01) that you have entered are equivalent to the Darcy-Weisbach e (1.5mm). If you compute the model in analysis mode with just those 1800 mm pipes, set at 1800 mm and the same slope (2000 1/S), you will see they have different capacity. Flow/Capacity (design) will be at about 83% for the same flow, vs 117% with the Darcy coefficients.

Hi Jesse, Thanks for the reply.

I would like you to look at the conduits with following ids. I have also mentioned the flow/capacity range and diameter alongside them.
ID Flow/Cap Dia
1431 100.1 400 mm
1466 100.3 900 mm
1499 102.1 200 mm
1515 102.7 400 mm
1886 101.8 250 mm
2808 101.9 250 mm
4682 100.5 200 mm
6975 101.3 800 mm
6976 101.3 800 mm
7012 100.3 900 mm
7539 100.8 250 mm
8669 100.6 200 mm
9100 101.5 900 mm
9101 101.5 900 mm


You can see the range of diameters for which this is happening. All these pipes have flow/capacity above 100% which ideally should not be the case for since they are being designed.

Also, the logic of minimizing excavation without looking at the diameter availability in conduit catalog and range of slopes and velocities for which that pipe can be designed and letting the conduits getting under designed (at that particular slope) is not right since there can be a limitation of size availability.

I am aware that manning's and darcy are two different methods , however, my concern was irrespective of the coefficient being used. If we use manning's method for designing, flow/capacity never exceeds 100%. However, its not the case with Darcy. You can try designing this model with mannings method with a higher coefficient also (if you want maybe a equivalent to 1.5 mm darcy's ) .

Hope i was able to explain my concern.

Best Regards
Jatin Talwar

Also Jesse, I would like to add that manually tweaking the diameters in a small network is acceptable. But with a big model (I have uploaded only a part of it), tweaking numerous pipes, changing diameters, stop inverts, setting it to do not design seems a huge task. I hope you can appreciate the issue i am facing.

Best Regards
Jatin Talwar

I suspect that the issue may be due to the sensitive nature of this particular configuration. If you were to look at only the very last pipe for example (CO-14057, delete all upstream elements and place fixed flow in upstream manhole), the design solver chooses a downstream invert of 12.27 m, meeting the minimum slope but yielding a flow/capacity (design) of 117%. If you make a tiny change to that invert (outfall and conduit stop) from 12.27 m to 12.26 m and run Analysis, it will be at 88% (same size of 1800 mm) and slightly higher slope.

Also, even though the capacity figures indicate that the design run yields a pipe that is flowing beyond its capacity (117%), that capacity figure is based on a normal depth assumption (normal depth equal to the top of the pipe), whereas SewerCAD runs a gradually varied flow analysis through the pipe network. So, if you look at those 1800 mm pipes in profile view, they are not long enough for the flow to reach a normal depth condition and thus those sizes and slopes might actually be OK in your case. More on this is explained here:

Why does the profile for the system appear to be at odds with the results for the capacity?

Using FlowMaster, I found that a D-W e of 1.5 mm is equivalent to a Mannings n of about 0.014, for the flow in question here. If I set the 1800 mm catalog entry to use 0.014 for the Mannings and set the calculation options to use the Mannings friction method, I encounter the same sensitivity with CO-14057; slightly above capacity with automated design at a downstream invert of 12.27, and above capacity at a downstream invert of 12.26.

Let us know if this is sufficient or needs to be escalated further.

I suspect that the issue may be due to the sensitive nature of this particular configuration. If you were to look at only the very last pipe for example (CO-14057, delete all upstream elements and place fixed flow in upstream manhole), the design solver chooses a downstream invert of 12.27 m, meeting the minimum slope but yielding a flow/capacity (design) of 117%. If you make a tiny change to that invert (outfall and conduit stop) from 12.27 m to 12.26 m and run Analysis, it will be at 88% (same size of 1800 mm) and slightly higher slope.

Also, even though the capacity figures indicate that the design run yields a pipe that is flowing beyond its capacity (117%), that capacity figure is based on a normal depth assumption (normal depth equal to the top of the pipe), whereas SewerCAD runs a gradually varied flow analysis through the pipe network. So, if you look at those 1800 mm pipes in profile view, they are not long enough for the flow to reach a normal depth condition and thus those sizes and slopes might actually be OK in your case. More on this is explained here:

Why does the profile for the system appear to be at odds with the results for the capacity?

Using FlowMaster, I found that a D-W e of 1.5 mm is equivalent to a Mannings n of about 0.014, for the flow in question here. If I set the 1800 mm catalog entry to use 0.014 for the Mannings and set the calculation options to use the Mannings friction method, I encounter the same sensitivity with CO-14057; slightly above capacity with automated design at a downstream invert of 12.27, and above capacity at a downstream invert of 12.26.

Let us know if this is sufficient or needs to be escalated further.

Hi,

The issue of flow/capacity exceeding 100% in case of darcy method is not due to this particular configuration because I have couple of other models where I am facing the same issue.

Also, when i run the model with manning's method (0.014 as u worked out) except for 1800 mm dia pipe (which is the maximum dia defined) all other issues pertaining to flow/capacity get resolved.

So I think this is basically an issue with the way Sewergems handles darcy and needs to be looked into.

What exactly is happening with the 1800 mm pipe is that, sewergems is designing the pipes at a slope of 2000 (constraints allow 200 to 2000), velocity of 1 m/sec (constraints allow upto 3 m/sec). So basically if you will see, sewergems ignore all other things like maximum pipe size available, maximum allowable velocity, and maximum allowable slope just to reduce excavation and ends up undersigning the pipe.

I feel the maximum available size in the catalog should also be a criteria while designing the pipes and needs to be incorporated in the software because in this case a solution with 1800 mm diameter pipe and with defined constraints exist which the software is not able to produce.

Regards
Jatin Talwar

Jatin,

My previous comments still apply here, but we can seek to improve the Design solver for future versions of the product, and appreciate your feedback.

In order to do this, I will open a Service Request on your behalf and link it to an Enhancement request. Unfortunately I cannot locate you in our system based on your Bentley Communities email address, so I will send you a private message shortly requesting your company details.

To close the loop on this thread - our developers will look into this for possible future improvement, with reference numbers 720875 and 720880.