Product(s):	PondPack, SewerGEMS, CivilStorm, SewerCAD
Version(s):	08.11.XX.XX+
Area:	Modeling

Problem

How do you model a multi-stage weir or step weir on an outlet structure or control structure? For example a v-notch with a wider, rectangular opening above it, or weir with a "staircase" (or "step") shape on the sides (rectangular width that becomes wider at deeper depths).

Solution

The best option to model a multi-stage or step weir would be to use the Irregular weir type. With this, you can define the stage-depth table to model the shape of your weir. For example:

A note on weirs on the side of a Riser

Note that if this is a notch in the side of a Riser and you're also using the riser outlet component, the riser has a built-in weir to model weir flow from the perimeter of the top of the riser as water spills into the opening. If you have already accounted for the weir of the perimeter of the top of the riser in your separate weir outlet component, the Riser component would not be necessary and would otherwise cause you to double-count the flow at elevations above the riser crest. The benefit of the riser component is that it will model the opening as orifice flow when fully submerged. If this is an important consideration and you have a step/multi-stage weir on the side of the riser, consider using the on and off elevation feature in the irregular weir component to turn off the irregular weir at elevations above the weir crest, then let the riser's weir "take over" (with appropriate weir length and orifice area), and transition to orifice flow when submerged. When in doubt, try multiple composite outlet structures with different approaches and compare the overall computed composite rating curve. If the difference is minimal, the model results / designed pond may not be impacted by the choice you make.

Externally computed rating table

Optionally, if you can compute the appropriate rating table for your step weir in another application, you can enter that in PondPack using the outlet component "user defined rating table".

See Also

Units for V-notch vs rectangular vs irregular weir

Using the "Elevation (On)" and "Elevation (Off)" fields in a Composite Outlet Structure