Thomas -
Unfortunately, we have extensive experience with this problem where I play in Chicago.
A branch of the Chicago river meanders through our back 9 and it has overflowed its banks onto several fairways every year since the club's inception over 120 years ago. In any given season it happens 3-4 times.
In the past this would render the course (at least the back 9) unusable for 2-4 days.
I believe it happened right before we hosted the US Am in 1902 and it was contested on our fron 9 only. I will skip the reasons for this flooding and focus solely on what we have done to mitigate the damage and ensure the course is fully operational in a shorter period of time. In August 2008, the flood was so bad that water lingered on course for days when a 95+ degree heat wave ensued and the back 9 had to close FOR THE YEAR. Ugh. So...
...we know what we CANNOT DO: 1) re-route the course and 2) create large banks to contain the river. (The latter being basically illegal.)
[/size] Since you cannot prevent a flood, what we focused on is getting the water off the course as fast as possible. What we then did:[size=78%]1. Installed a system of pumps and cisterns and hoses at the 4 crucial areas on-course where the flooding occurs. As soon as the river is back within its banks, the pumps turn on and basically through the cisterns and vaults returns the standing water (on-course) to the river.
2. When we did "restoration" work in 2013 (under Jim Urbina's care), we installed 5-6 acres of "water wicking" in crucial fairway areas. It's somewhat spendy though. This basically helps the water quickly filter into the ground (I think) for MUCH better drainage. More info below:
https://www.youtube.com/watch?v=AUTUiKKRz10http://www.tomwaitgolf.com/the-water-wick-drainage-systemhttps://www.waterwick.com/The NET result now: our course drains better and, when there is a flood event, the back 9 may be closed for a day and sometimes not at all depending on bunker wash-outs and standing debris and dirt on fairways.
Hope this helps.
Cheers.