Product(s):	SewerGEMS, CivilStorm, StormCAD
Version(s):	08.11.XX.XX and later
Area:	Output and Reporting

Problem

Why is it that no matter how large the inlet on grade is the gutter spread always remains the same?

Solution

HEC-22 inlet computations are concerned with when the gutter spread is maximum, which is immediately upstream of the inlet (see image below).  Just downstream of the inlet, the spread is at its minimum because most, if not all, of the water has been intercepted.  The spread is calculated for the flow in the gutter immediately upstream of the inlet, so the size of the inlet does not impact the width of the  gutter spread.  To remedy this situation, try increasing the size of the inlet(s) upstream of the inlet you have been working on.

If you still see spread results at the catch basin, this is an indication that a large amount of runoff is coming from the catchment, since this is also a component of the surface flow. Considerations may be needed to reduce the runoff, such as splitting up large catchments into smaller subcatchments and including additional catch basins to capture surface flow.

 

See Also

Order of constraint based design priorities for StormCAD (GVF-Rational) and SewerCAD (GVF-Convex)