Product(s):	PondPack, Civilstorm, SewerGEMS
	Version(s):	08.XX.XX.XX and higher
	Area:	Modeling

Background

Water in a pond can be released downstream via several different standard outlet structure types, one of which is a "Riser". A riser represents a vertical structure inside the pond with an opening on the top, and in some cases orifices on the sides and/or a notched opening. These are also sometimes referred to an inlet box or standpipe.

For low water surface elevations, flow is slowly released through the orifices on the side, into the riser barrel, and typically passes downstream via an underground pipe. If there are no orifices on the side, then the riser top elevation will set the normal water surface elevation of a "wet pond". When the water surface exceeds the top of the riser, water spills down into the top as weir flow, at a higher rate.

In many cases you may also have an emergency overflow weir elsewhere in the pond to handle even more extreme events, to release water at an even faster rate. This can either be incorporated into the same composite outlet structure, or added as a second outlet structure depending on where the flow will go. See: Modeling a separate spillway (weir) and Riser in a pond

Riser Components

When defining the properties of a riser in the Composite Outlet Structure Editor, you will notice there are input fields for both weir and orifice information. The orifice parameters in the riser component refer to the hole at the top of the riser. As the pond water level reaches the crest of the riser, it first spills over and into the riser as weir flow. However if the riser is completely submerged (at higher water surface elevations), the flow into the top of the riser acts as orifice flow, which is why you must enter those parameters.

Weir length - represents the perimeter of the top of the box where water is spilling over as the pond headwater elevation first starts exceeding the crest.

Orifice area - this refers to the hole on top of the riser. Basically when the water surface goes just a little bit above the top of the riser, it's acting as weir flow because it's spilling into the hole at the top. However, there comes a point when it transitions to orifice flow, when the hole at the top is fully submerged and at that point, it's just orifice flow, with the area value being the area of the hole at the top (pointing down into the ground).

Elevation  - this is the elevation of the top of the riser - the elevation above which water starts spilling (as weir flow) into the hole at the top. 

Transition Elevation - the elevation above which a transition from weir to orifice flow occurs (as the hole becomes submerged). Set to the default of zero 

Transition Height - the height of the transition zone, between which the flow transitions from full weir flow to full orifice flow.

Any orifices on the side of the box will have to be entered as separate orifice items in the outlet structure. The separate orifice components should have their "downstream ID" set to the same outlet structure that is also downstream of the riser. For example the Riser's downstream ID may be a culvert 1, and the orifice downstream ID would also be culvert 1 (not the riser). If the downstream pipe/culvert does not present a bottleneck, it is better to use "tailwater" as the downstream ID, to avoid potential convergence problems. For more information on this, see quick start lesson #1, which covers risers with side perforations.

If you have a notch weir on the side of the top of your riser/inlet box, see "A note on weirs on the side of a Riser" in this article: Modeling a multi-stage or step weir