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# What does the 'depth' in a Catchment Basin mean in SewerCAD and why it is different from the hydraulic grade ?

I am using SewerCAD to analyze a high sewer flow scenario of an existing system using steady state analysis.

The model include some gravity conduits, connected to a wet well (Lift Station), pumps, then to pressured pipes. The model ends with an outfall.

Total of 1,425 gpm of sewage flow reaches to the lift station. There are three pumps at the lift station and they are pumping at 1,545 gpm.

I attached the properties of the wet well. The base elevation is 795.5 ft and the hydraulic grade is 798 ft, which makes sense since the pumps are pumping the sewage until 797.5 ft level.  What I am not understand fully is the 'depth' of 88.57 inch, which is 7.38 ft.  I thought it was a sewer depth in the wet well, but if that is the case, it should be the same as the hydraulic grade, which is 2.5 ft - that is my thought.  Please help me understand what 'depth' mean and why there is a difference with the hydraulic grade.

Thank you.

Yoko Parents
• Hi Yoko,

(edited)

The properties dialog describes the wet well field "Depth (Node)" as "the calculated depth of flow measured from the node invert elevation".

Here's how the field is calculated.

If the wet well's "Elevation (Initial)" is less than the upstream pipe's "Hydraulic Grade Line (Out)":

The wet well "Depth (Node)" field is calculated by subtracting the wet well's "Elevation (Base)" from the calculated "Hydraulic Grade Line (Out)" value of the upstream pipe.

In your example, the "Hydraulic Grade Line (Out)" field for the pipe upstream of the wet well should be about 802.88ft.

795.50+(88.54/12)=802.88

If the wet well's "Elevation (Initial)" is greater than the upstream pipe's "Hydraulic Grade Line (Out)":

The wet well "Depth (Node)" field is calculated by subtracting the wet well's "Elevation (Base)" from "Elevation (Initial)". Regards,

Craig Calvin

Bentley Technical Support

• • Hi Yoko,

(edited)

The properties dialog describes the wet well field "Depth (Node)" as "the calculated depth of flow measured from the node invert elevation".

Here's how the field is calculated.

If the wet well's "Elevation (Initial)" is less than the upstream pipe's "Hydraulic Grade Line (Out)":

The wet well "Depth (Node)" field is calculated by subtracting the wet well's "Elevation (Base)" from the calculated "Hydraulic Grade Line (Out)" value of the upstream pipe.

In your example, the "Hydraulic Grade Line (Out)" field for the pipe upstream of the wet well should be about 802.88ft.

795.50+(88.54/12)=802.88

If the wet well's "Elevation (Initial)" is greater than the upstream pipe's "Hydraulic Grade Line (Out)":

The wet well "Depth (Node)" field is calculated by subtracting the wet well's "Elevation (Base)" from "Elevation (Initial)". Regards,

Craig Calvin

Bentley Technical Support

• 