Hello! Are you designing a new site or evaluating existing?
a) New site:
Does this mean that after 400', the flow has to be intercepted by a channel or inlet? In this case it falls back on you to design it I suppose, but that would be an unconventional restriction.
b) Evaluating existing:
I...
Where I'm at it's standard to use flowable fill when abandoning structures but lightweight concrete for this type of application. We had to raise the bottom of a manhole 3 ft once and used lightweight concrete. But yes, like the others said with this small of an invert change just leave the sump...
