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When you say "velocity errors", I assume you are referring to the user notifications that say "Conduit does not meet minimum velocity constraint."
These messages are warnings (note the yellow color circle next to them) that do not prevent the model from computing. They are in reference to the automated design feature's design constraints. In the Design Alternative, you will see that the minimum velocity constraint is set to 0.61 m/s. The pipes in question have a velocity greater than this, so the user notifications in questions appear.
This model is set to Analysis mode (not Design), so it will not try to automatically change pipe sizes and slopes to meet the constraints. For Analysis mode, those notifications will still display for informational purposes.
Furthermore, even when running automated design, the minimum velocity constraints are at the bottom of the constraint priority order and are unlikely to be satisfied without violating another constraint (again though, those notifications will still display for informational purposes.) See:
Unable to meet minimum or maximum velocity constraint
Order of automated design priorities for StormCAD (GVF-Rational) and SewerCAD (GVF-Convex)
For minimum velocity problems, you may also want to look into Tractive Stress.
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.
First I have designed the network its showed more no of pipes not meeting minimum velocity constraints, cover constraints and slope . After changed to analysis some pipe diameter and slopes are changed but still out of 171 pipes nearly 71 pipes not meet the minimum velocity constraints
When you are working with small sewers with only a few customers upstream, you will almost always have many pipes that don't continually meet the minimum velocity constraint. When you get into these low flow situations, you need to remember that for these pipes, flow is not steady but comes in pulses when a customer drains a washing machine or dishwasher, flushes a toilet, runs a shower,... During these times, those pulses usually provide sufficient tractive force to move any solids.
Those minimum velocity constraints really don't apply in these situations. All you can do is meet the minimum slope requirements as given in you local regulations or in publications like the ASCE Gravity Sewer manual. If you meet minimum slope, you are probably safe.
If the area where you are working is almost completely flat, you may have trouble meeting even minimum slope requirements. In that case, you may need to consider pressure sewers or multiple pump stations. These are more expensive and difficult to maintain, so you'd like to use gravity sewers.