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

Question

Hi, 
 The Flow/Capacity design while designing sewer using Darcy-Weisbach method goes upto 102%. 
 Can any one please tell why it happens? When we use manning's for designing sewer network Flow/Capacity design is restricted to 100%. 
 
 Best Regards 
 Jatin Talwar

Jesse Dringoli · Answer

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.

Jesse Dringoli · Answer

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.