Using Start and Stop Control Structures for conduits and channels

Product(s): SewerGEMS, SewerCAD, CivilStorm
Version(s): CONNECT Edition, V8i
Area: Layout and Data Input

Using Control Structures in the storm-sewer products

Flow regulating structures, also known as control structures, are common in storm water drainage systems and in combined sewer systems. The most common control structures are weirs and orifices.

In SewerGEMS, CivilStorm, and SewerCAD, you can attach a control structure on a conduit as either a Start or Stop Control Structure, referring to the end of the pipe at which the control structure is located. The latest version of SewerGEMS and CivilStorm supports both start and stop control structures for both the Implicit dynamic and the Explicit (SWMM) dynamic solvers. Control structures can also be placed on Channels.

Conduit and channel control structures are treated as in-line (in series), so if you need to model a lateral/side control structure which diverts water to another location, you will need to place a start control structure on a lateral conduit as seen in the figure below.

Weir start control structure on a lateral pipe for flow diversion:

A control structure can also have a flap gate which allows flow to travel in only one direction. Hydraulically these controls are treated as internal boundaries where the empirical weir or orifice equations are used. These equations will replace the momentum equations in the Saint Venant equations (for the Implicit solver). The continuity equation is simply the flow is the same between the upstream face and the downstream face of the internal boundary (control structure).

Weir Control Structure

Weirs are classified by their flow-diversion purpose as either a side weir or a transverse weir. Side weirs or overflow weirs are used to divert extra high flows to overflow waterways. Typically a side weir is a weir parallel to the main sewer pipe and with enough high crest elevation to prevent any discharge of dry-weather flow, but it is also low and long enough to discharge required excess of wet weather flow. Transverse weirs or inline weirs are typically placed directly across the sewer pipe, perpendicular to the sewer flow and act like a small dam, to direct the low flow, usually dry weather flow, to diversion pipe such as dry weather flow interceptor sewer pipe. Weirs are also classified by their cross section shapes, such as rectangular, V-notch, trapezoidal, and irregular.

In the properties for the weir control structure, the Crest Elevation denotes the top of the weir. The Structure Top Elevation is the top of the weir. In many cases, the Structure Top Elevation will be the crown of the conduit, though it is possible for the Structure Top Elevation to be below this, such as in the diagram below.

If the conduit is running full or the depth is higher that the Structure Top Elevation, the flow through the weir will be treated as orifice flow.

Orifice Control Structure

Orifices are usually circular or rectangular openings in the wall of a tank or in a plate normal to the axis of the conduit. Orifices can be oriented in a variety of ways, such as side outlet or bottom outlet. Orifices are treated the same as weirs to be internal boundaries except that the flow equation of an orifice is used to calculate the discharge.

Depth-Flow Control Structure

Depth-Flow control structure allows you to enter a table to define what the flow will be at a given depth in the conduit. This can be useful if you want a greater level of control on the output of the control structure rather than relying on the weir or orifice equation. You can also model vortex valves in this way, but copy/pasting the vortex valve table data.

Functional Control Structure

The Functional control structure lets you select a structure type, which defines the equation that is used in the calculation of the flow. You will also define a set of coefficients and exponents to aid in the calculation. 

Numerical Stability

  • Using a start control structure tends to be more stable than a stop control structure. If a stop control structure must be used and you're experiencing problems with the results, try increasing the LPI Coefficient (in the calculation options for the Implicit solver) to a maximum of 12 (note that this may have an adverse impact on stability requiring other calculation option changes) 
  • Use a smaller calculation timestep in cases where a large change in flow can occur with a small change in head, such as when using a weir.
  • Significant backwater/tailwater effect against a conduit control structure can be challenging to solve. A very small calculation timestep may be necessary, the Explicit (SWMM) solver may need to be used (with a very small Routing Step).
  • In especially challenging situations, consider using an approximate pond (representing the ponded area upstream of the control) with pond outlet structure, which might work better in some situations.

Supported Control Structures Per Numerical Solver

For more on the differences between solvers, see this article.

The Implicit (Dynamic Wave) solver can model conduits and channels with start and/or stop control structure.

The Explicit (SWMM) solver can model conduits and channels with start and/or stop control structure. (older versions only supported start control structures)

The GVF-Convex (SewerCAD) solver can model conduits and channels with a start and/or stop control structure.

The GVF-Rational (StormCAD) solver can not model conduit/channel control structures.

See Also

Using SWMM Control Sets in SewerGEMS and CivilStorm

Modeling gate valve opening and closing based on pressure transducer reading

Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)

Modeling a weir within a catch basin or manhole

User notification: "Conduit has a Stop control structure, but this is not supported next to the connected outfall element."