Equations for Weirs in SWMM 5

Equations for Weirs in SWMM 5

In SWMM 5, weirs are classified into four types: Transverse, Sideflow, V Notch, and Trapezoidal. Each type has its own flow calculation method based on specific hydraulic principles.

• Transverse Weir: This type of weir, also known as a broad-crested weir, has a flow equation that typically accounts for weir length and head over the weir. It uses the general weir equation where the discharge is proportional to the weir length and the head raised to the power of 1.5.
• Sideflow Weir: Similar to the transverse weir but positioned along the side of the channel. When flow is reversed, it behaves like a transverse weir. The flow equation for sideflow weirs adjusts for the angle of approach and length of the weir, affecting the discharge coefficient.
• V Notch Weir: Characterized by its triangular shape, this weir's flow is calculated using an equation that reflects the increasing flow capacity as the head rises due to the widening of the notch. The discharge coefficient can vary, but generally, the flow is proportional to the head raised to the power of 2.5.
• Trapezoidal Weir: This weir combines characteristics of both sideflow and V-notch weirs. It features straight sides and a triangular bottom, allowing for a combination of flow behaviors. The trapezoidal weir's flow equation includes contributions from both the side sections (similar to sideflow) and the V-notch portion.

Key points:

• End Contractions: Weirs in SWMM 5 can have zero, one, or two end contractions which affect the effective weir length and thus the flow calculation. For transverse and sideflow weirs, this might reduce the flow due to end contractions (Figure 2).
• Weir Length: The actual length of the weir can be modified by settings within SWMM. This includes the "Weir Setting" which might adjust the effective length based on operational conditions like gate control or physical changes in the weir structure.
• RTC Setting: For V-notch weirs, if the Real-Time Control (RTC) setting is less than 1.0, it behaves like a trapezoidal weir due to the modification in the flow path or the effective opening of the weir.

 Figure 1.   Weir Equations in SWMM 5

 Figure 2.   Valid Number of End Contractions

Figure 3.  Weir Length Calculations

Figure 4.   Weir Equations in SWMM 5