Weirs
Weirs are classified by their flow-diversion purpose as either a side weir or a transverse weir, as described in the following definitions:
- 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. Weirs in an outlet of a detention pond can be treated as one of the control elements in the composite outlet control structure. Another example of a side weir is the emergency overflow weir or spillway at the top of a detention pond. Side weirs are only used to model flow splits in the implicit or explicit solvers. In the GVF based solvers, flow splits should be modeled using diversions or a user notification will be issued when the model is run.
- Transverse weirs or inline weirs are typically placed directly cross 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. Accordingly the computational equations for the weirs are different, the discharge through a rectangular weir is proportional to the 1.5 exponent of the head above the weir crest, and the exponent for the V-notch weir becomes 2.5.
Bentley SewerGEMS users need to specify the weir discharge coefficient. Typically a weir discharge coefficient ranges between 2.65 and 3.10 (English units). Since the weirs in a sewer system are mostly sharp-crested weirs, a value of 3.0 is a common default assumption without knowing the weir specifics and hydraulic conditions.
Weirs can occur in models either as control structures in conduits or in pond outlet elements. The properties of weirs are set in the Components under Conduit Control Structures or Composite Outlet Structures. Once they have been defined, they can be assigned to individual conduit or pond outlet elements.
There are three types of in-line weirs: