Yard drain system problem

I have a drain box installed in a low area outside. The drain box top is 8 inches square with a grate. A 3 inch round connector is in the side wall of the box, and the top of this connector is 3 inches below the grate. Drain pipe runs from the connector to a lower area in the backyard -- a drop of a few feet over 100 feet.. I am not sure of the diameter of the drain pipe but think is about 4 inches in diameter.
In a heavy rain, I noticed water was pooling above the grate showing the system was unable to drain the water fast enough. With the water several inches above the grate, I was surprised to see no apparent swirl of water going down the drain. It did complete draing the area about 30 minutes after the rain stopped.
After water finished draining, I noticed water was sitting in the drain box covering the bottom 2 inches of the drain connector, and only the top 1 inch of the connector was out of water. I assume this must be because the bottom of the drain pipe rises somewhere in its run to within 1 inch below the level of the top of the drain connector in the drain box.
Does anyone have an estimate of how much the flow is reduced by this rise in the drain pipe ? I want to look at the case when the drain capacity is exceeded, and water rises to the top of the grate. What would be the approximate difference in outflow with the rise in the drain pipe versus no rise in the drain pipe?
You may be thinking that rather than look at estimates, I should just dig up the drain pipe and lower it. It is not so simple because the pipe goes under concrete for some of its run.
PaulF
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Likely debris try snaking it...
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Or just flush it by pushing a running hose (perhaps with a "blaster nozzle") down the drain pipe.
Bob
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