What if semi final is washed out?

For knockouts like the semi-final, there's always a reserve day. The match will simply be played the next day. If some play happened on the first day, it just continues from where it stopped.

But here is the main point. If the reserve day is also completely washed out, then things get interesting. The team that finished higher on the points table during the league stage automatically qualifies for the final. It’s a bit of an anti-climax, I know. But it rewards the team that was more consistent throughout the tournament. Tough luck for the other side