Custom Conduit Shape Curve in SWMM5 and InfoSWMM
This is a new curve type that is used to construct custom shape for closed conduits that cannot be reasonably defined using any of the existing shape types supported by InfoSWMM. The curve represents depth to full depth ratio on the Y-axis, and width to Full Depth ratio on the X-axis as show below. It specifies how the width of the cross-section varies with height, where both width and height are scaled relative to the section's maximum depth. This allows the same shape curve to be used for conduits of differing sizes.
Excerpt from the EPA manual Storm Water Management Model Reference Manual Volume II – Hydraulics (PDF)