re: Need the brains of the OB

What is the goal?

trying to prevent contaminated runoff to surrounding areas?

It sounds like you need to have a basic P trap design with small diameter back drains.

During low rainfall the ground absorbs and filters the water through the sand then gravel and then perforated pipe and exits

During short durations of high rainfall the over flow pipe collects ground water not being absorbed quickly enough to prevent runoff. The pipe itself fills acting as an underground retention vessel. The volume of the shaded portion of the overflow pipe will be back filtered through small diameter drain pipes into the sand.

During longer duration high rainfall the entire system fills and the volume in the overflow pipe/vessel exceeds the shaded area causing the overflow pipe to discharge dirty water. When the volume once again drops below the shaded area the pipe retains water and back filters it as per above.