Product(s):	SewerGEMS, SewerCAD, StormCAD, CivilStorm
Version(s):	08.11.XX.XX and higher
Area:	Modeling

 

Problem

What is the difference between Capacity (full flow), Capacity (design), Capacity (excess full flow), and Capacity (excess design)?

Solution

Capacity (Full Flow) is the flow through the conduit if normal depth were equal to the top of the conduit. This value is calculated with Manning's equation and is based on the assumption that flow through the conduit is at a normal flow condition and the normal depth equals the rise of the conduit (for circular conduit the rise = diameter). If you do not adjust the "part full design" options in the design alternative, then this figure also represents the capacity of the pipe that you are designing for, with constraint based design. Meaning, constraint based design will try to choose a pipe whose capacity is greater than the flow through the pipe.

For pipes with a negative slope the "Capacity (Full Flow)" value can be considered the largest amount of water a pipe of that size and slope can carry by gravity. This would apply if there is a flow reversal, which could happen in a dynamic wave run.

Capacity (Design) is the amount of flow to be used in designing the conduit. The Capacity (Design) will normally be lower than the Capacity (Full Flow) if the desired Percentage Full (part full) is set to less than 100%. The Percentage Full can be set by going to: Design constraint alternative > Gravity Pipe tab > Part Full Design tab > enable "Is Part Full Design?" > Percentage Full. The Capacity (Design) will be the same as the Capacity (Full Flow) if the Percentage Full is set to 100% or "Is Part Full Design?" is disabled. For more about part full design, see: Understanding Percent Part Full Design in Pipes

Note:

• The design capacity (and also the excess capacity result) can change even for pipes that are not set to be designed (design pipe? = false). It is only used by the design algorithm when a pipe is set to be designed, otherwise it is for reference purposes and will change as the pipe size, slope or part full design constraints are changed. So, for example if the pipe size and slope stays the same but you make a change to the global percent full design constraint (see above), the "Capacity (Excess design)" will update accordingly. Even if you do not see the percent full constraint show up in the conduit properties, if will still look at the global constraint to determine the excess capacity figure. 
• It is possible for the Design Capacity to be greater than the full flow capacity for a circular pipe, for Percentage Full values near 94%. The full flow capacity is the flow through the pipe with normal depth equal to the top of the pipe, but that is not the maximum flow through a circular pipe. Circular pipes have greater capacity at depths slightly below the pipe top (usually around 94% depth), due to increased friction (wetted perimeter) at full depth. You can read more about this phenomenon in reference books, papers and articles, and you can see it by solving the Manning equation.

Capacity (Excess Full Flow) is the difference of the flow calculated and the Capacity (Full Flow).

Capacity (Excess Design) is the difference of the flow calculated and the Capacity (Design). Like with Capacity (Design), this value can change even for conduits that are not being designed (see above)

Flow / Capacity (Design) is the ratio of the flow in the conduit to the Capacity (Calculated Design).

Percent Full during an EPS / dynamic run

If you are using an EPS in SewerCAD (GVF-Convex solver) or the explicit or implicit solver in SewerGEMS and CivilStorm and want to see the percent full at the peak flow, as of May, 2020 you would need to create a formula-derived user data extension that divides the "flow (maximum)" by the "capacity (full flow)" as seen below.

It is important to note however that the capacity-related results fields are generally more appropriate for peak flow simulations. For example when using the Rational Method-based GVF-Rational solver, or a steady state with peaking factors with the GVF-Convex solver. In these two cases there is only one flow through the pipe (the peak flow) so the "Flow / Capacity (design)" can be used to see the percent full. Additionally, the constraint based design feature in the GVF-Rational and GVF-Convex solvers will use the capacity figures as a basis for pipe size selection. Lastly, the capacity-related fields are calculated based on a normal depth / Manning equation assumption, whereas with the dynamic solvers (implicit and explicit) in SewerGEMS, pipes are usually not flowing at normal depth. So, the capacity-related results fields may not always be a good reflection of the system. For example if there is a downstream bottle-neck causing a tailwater condition that surcharges upstream pipes, the flow through those pipes may still be less than the normal depth-based capacity feature.

See Also

The profile is at odds with the capacity (Full Flow) for a conduit