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