Weir Junction X overflows because height of liquid (Y) exceeds height of weir (Z)

A Warning message in Fathom

Full Message Text

There are two variations depending on whether the upper or lower chamber of the Weir is overflowing in the steady state solution:

  • Upstream overflow

    • Weir Junction X overflows because the upstream liquid height (Y) exceeds the height of the weir (Z). Weir calculations do not account for the physics of a weir overflowing.

  • Downstream and upstream overflow

    • Weir Junction X overflows because the upstream and downstream liquid heights (Y) exceed the height of the weir (Z). Weir calculations do not account for the physics of a weir overflowing.

Explanation

Weir flow equations determine the flow rate through the weir based on the upstream liquid height, and assume that the flow passes through the notch of the weir. These correlations are no longer applicable when the upstream height of the liquid Zn plus H exceeds the height of the weir A, at which point the fluid would also be flowing over the top of the weir.

A diagram of a rectangular sharp crested weir.

Figure 1: Rectangular Sharp Crested Weir Model

The change in pressure and flow rate across the weir are no longer accurate in this situation. Depending on the height of the liquid and basin geometry, there may be a risk of overflow at other sides of the upper basin.

Note: For full channel width rectangular weirs (no contractions) the fluid does not flow through a notch. The height of the weir A does not provide information about notch geometry, but is still used for this warning so that it can be used to identify overflowing conditions at other sides of the basin.

Weir overflow can also occur when there is insufficient flow out of the lower basin of the weir. Under these circumstances, the elevation of fluid in the lower basin exceeds the elevation of the weir A plus Zu. If the flow magnitude is positive, the level of fluid in the upper reservoir also exceeds the level of the weir.

Required Action

If only the upper basin is overflowing, flow from upstream exceeds what can be accommodated by the notch geometry. Address the upstream flow if excessive or modify the weir geometry.

If both basins overflow, the design of the system downstream of the weir may need to be modified to increase flow to prevent buildup and overflow at the weir.