Water Pressure, small inlet vs. large outlet

[FriscoGary]

If there is a "sewer box" (2' x 2' x 2') with a 8" drainage inlet pipe into it and it has a 18" drainage outlet pipe to
discharge water entering into the sewer box and the large 2' x 2' grate opening at the top...IF the entire sewer box is inundated with storm water, will the incoming water thru the top 2' x 2' grate allow water into (backflow) the 8" inlet pipe as water is also discharging out the 18" drainage outlet pipe? I would think the storm water entering from the grate opening would mainly discharge out the larger 18" discharge pipe and sort negate or
"push back" any water trying to drain into the box from the small 8" pipe. My question is will the volume of water from the top grate not allow any water from the small 8" inlet to enter into this system if it is inundated from the top. My thought was all the pressure from the top would negate or equalize any from the 8" pipe...any insight would help!

 

[anorlunda]

The diameter of the inlet and outlet are less important than the pressures or elevations at the far ends.

In theory, pressure in the middle could cause flow in both directions, backing up the inlet, but whether this happens or not depends on more details than the sizes.

 

[FriscoGary]

The elevations are equal for the 8" inlet and 18" outlet and their far ends are lower than the top of the "sewer box" grate inlet. The 8" inlet only has
pressure from the water draining into it and it is a level pipe.

 

[anorlunda]

The elevations are equal for the 8" inlet and 18" outlet and their far ends are lower than the top of the "sewer box" grate inlet. The 8" inlet only has
pressure from the water draining into it and it is a level pipe.

In that case, yes you will get some backup into the inlet.