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

Applies To
Product(s): StormCAD, SewerGEMS, SewerCAD, CivilStorm
Version(s): CONNECT Edition, V8i
Area: Calculations
Original Author: Scott Kampa, Bentley Technical Support Group

Problem Description

Why does the profile for the system appear to be at odds with the results for the capacity? For example, the profile does not show a conduit as being full, but the capacity results indicate that it is. For example, "Capacity (Full Flow)" = 50 ft^3/s and flow through a conduit is 60 ft^3/s, but the hydraulic grade is below the top of the conduit in profile view. The opposite symptom can also occur, where the profile shows the conduit as being full/surcharged yet the flow through it is less than the reported Full Flow Capacity.


In short, it may be that the water in the conduit has not had a chance to reach a normal depth condition yet, in which case the result you see in the profile view should be considered, rather than the capacity result.

The full capacity is the flow through the conduit if normal depth were equal to the top of the conduit. Using Manning's equation as shown below, the program calculates the "Capacity (Full Flow)" results of a conduit. The "Capacity (Full Full)" results are based on the assumption that flow through the conduit is absolutely at a normal flow condition and the normal depth equals the rise of the conduit (for circular conduit the rise = diameter).

Q = k/n*A*R^(2/3)*S^0.5 [Manning's Equation]

At normal flow:

     •     The water depth, flow area, flow, and velocity distribution at all cross-section and throughout the entire length of conduit remains constant.
     •     The Energy Grade Line (EGL), Hydraulic Grade Line (HGL) and the Channel Slope are parallel.
     •     The velocity/depth ratio is constant.
     •     There is no acceleration or deceleration of the moving mass.

You can verify this by using a simple Manning's equation normal depth calculator, such as Bentley FlowMaster.

However, in SewerGEMS, CivilStorm, StormCAD, and SewerCAD, water does not always flow at normal flow; it computes a varied flow profile. As a result, the HGL slope can change over the length of the conduit. Because of this, the actual depth in the conduit can be different than normal depth and the "full flow capacity" results may not be a good comparison, especially with short conduits that may have otherwise achieved a normal depth condition if extended further.

In the case of a conduit that does not appear full in profile, but has a flow that is greater than the capacity, then the gradually varying flow results that the model computes are telling you that the conduit is not flowing full, but the normal-depth based capacity results indicate that the conduit is full. The gradually varying flow results as seen in the profile should be considered in this case since they are a truer representation of what would happen in the real system, rather than the capacity result, which makes the assumption of normal depth. In this example case, to help visualize this in the actual model, try increasing the length of the conduit in question by 10 times the original length and then look at the profile of that conduit. You will notice that the upstream end of the conduit will extend above the top of the conduit as it tries to find a normal depth condition. If not, increase the length of conduit again. What this means is the initial conduit length is not long enough for the water surface to settle on the surcharge condition even though the flow exceeds the maximum capacity and therefore the HGL in the profile is not above the conduit crest elevation.

If you see a discrepancy between the profile and the capacity results, that likely means that the profiles are likely not flowing at normal depth. In such a case, you may want to look at the "Depth (Average End)/Rise" (called "Depth/Rise" in the latest version) result field instead. This is the middle depth of the conduit divided by the conduit diameter or rise, which provides a better gauge of how full the conduit is at the midpoint when normal depth conditions are not met. In other words, it shows the actual calculated depth in the conduit in the gradually varying flow condition - the truer result that the model predicts and should be considered over the capacity result.

A note on constraint based design

Constraint based design is also based on these same capacity concepts. By default, it will attempt to pick a pipe size whose full capacity is greater than the flow through the pipe. It seems logical to ask that the pipe size instead be based on the actual calculated depth in the pipe, but there are a few reasons why this would not work well. The depth of flow in the pipe based on the gradually varied flow calculations, can be influenced by adjacent pipe hydraulics. For example consider an outfall pipe that discharges below the surface of a body of water. The connected conduit will be at least partially surcharged even when the flow through it is less than the normal-depth based capacity. If the design were instead based on calculated depth, it may choose a very large pipe size that may not be reasonable.

Example #1 - Flowing above full capacity yet not surcharged in profile

Observe the following example conduit, whose "Capacity (Full Flow)" is 50 ft^3/s. This means that if normal depth were equal to the top of this conduit, the flow through it would be 50 ft^3/s. In this case, the flow through the conduit is 60 ft^3/s, yet the conduit is not surcharged.

Now observe what happens if we extend this conduit upstream. Notice that the hydraulic grade eventually reaches the top of the conduit, indicating the surcharge condition. This surcharging is not seen over the entire length of the conduit because there is a transition to a steeper downstream conduit.

Example #2 - Flowing below full capacity yet surcharged in profile

In this example we have a conduit with capacity of 29 CFS and a calculated flow through the conduit of 21 CFS, however, the profile shows that conduit is surcharged. The reason is because there is a tailwater / backwater effect from other flow entering downstream and causing a backup. Since the network is solved as a whole and the downstream HGL is essentially communicated upstream during the backwater analysis (SewerCAD and StormCAD solvers) or dynamic wave calculations (SewerGEMS and CivilStorm Implicit or Explicit solvers), the conduit in question becomes surcharged. The full capacity figure of 29 CFS is based on a normal depth assumption, but a normal depth condition is not experienced in this case, so the capacity figure is at odds with the profile/HGL.