The Store Elevation-Flow-Tailwater Table property in the Composite Outlet Structures dialog (we’ll refer to this as Option 1) and the Store Elevation-Flow-Tailwater Table? Calculation Option (we’ll refer to this as Option 2) are related, but slightly different.

Option 1 allows you to store the results of an interconnected outlet structure (which is a composite outlet structure with a tailwater setting of "Interconnected Ponds") into an Elevation-Flow-Tailwater curve.

You first set this option to "Yes" and then specify the table to store the data in. If there is no table available yet you can select "<Create New EQTW Series>" which will automatically create one for you with the label of the composite outlet structure.

The storing of the results into the Elevation-Flow-Tailwater curve is generally done only when computing from the composite outlet structure dialog box. However, there is an option in Bentley SewerCAD that allows you to store these results when you run the scenario.

Option 2 allows you to store the Elevation-Flow-Tailwater table when computing the scenario that uses the calculation options you just modified. It tells the engine that if the pond outlet link has the "Has Control Structure?" property set to "Yes" and an assigned composite outlet structure that has a tailwater of "Interconnected Ponds" and the "Store Elevation-Flow-Tailwater Table" set to "Yes" to store the results in the specified table.

These two options are used in conjunction with each other. You would not normally set Option 2 to True if you are not using interconnected pond routing or have an outlet structure that has "Store Elevation-Flow-Tailwater" set to No.

If, however, you have Option 2 set to False and Option 1 set to Yes and for the latter have the "<Create New EQTW Series>" selected, a new blank Elevation-Flow-Tailwater Series table will be created with no data.

If you have Option 1 set to Yes with an actual table specified and Option 2 set to False and you compute the scenario the data in the selected Elevation-Flow-Tailwater table will NOT change.

Option 1 can be used alone but Option 2 should be used with Option 1.

EQT Curves

EQT curves refer to the relationship between the flow (Q) through a structure and the elevation (E) of the water upstream of the structure and the water elevation in the tailwater (T) downstream of the structure. They consist of a family of curves with upstream elevation on the horizontal axis, flow on the vertical axis and individual curves for different tailwater elevations.

EQT curves are an intermediate step in the hydraulic calculations for pond outlets (composite outlet structure), and (box and circular) conduits that are culverts.

To view the EQT curve for a structure (after an implicit or explicit solver run),,

1. Go to Components > Composite Outlet Structures.
2. Right clicking in a pond outlet structure and picking "Outlet Structure EQT" graph.
3. Right clicking a conduit with a culvert and picking "Culvert EQT Graph".

When the GVF solver is used, the tailwater effect is ignored so there is only a single EQ line. It usually corresponds to the lowest of the EQT curves.