I'm not sure if anyone else has had similar problems?

Currently my understanding is that lost manholes simply act as a weir at ground level to 'lose' the flood water. Stored does as it implies and stores the flood water.

I have a situation where there was an observed flood event where a number of manholes popped and flooding occurred. But the manhole covers required x amount of head prior to flooding. Anyone had a get around for situations like this? Only thing I can think of is to set the manhole to lost and artificially raise the ground level?