Drainage advice please BOS/diversion

[paul alan]

Hi we had an agreement with Welsh water to divert some surface water pipes-also a BOS but not relevant.

There is a small amount of water left in the drain invert for a few hours after rain.

Is there a tolerance for water in bottom of pipe?

I have already dug it all out once and re-levelled it, there is probably a ton of hardcore on top of the pipe and I really don’t want to dig it all out again if not necessary.

Any advice greatly appreciated