From Drs. Roch Gaussoin and Bill Kreuser, University of Nebraska via their TurfiNfo blog:
The recent spring floods have been devastating for many communities across Nebraska and Western Iowa. As floodwaters recede, priorities shift from survival to recovery. Many are finding inches to feet of sand, silt and ice covering their turfgrass surfaces. We have received numerous inquiries from turf managers and others wondering what to expect and what to do next.
As submersion depth increases, the potential for injury increases. Turf in flooded conditions declines from lack of oxygen and light. For actively growing turf, substantial loss can be expected after 4 days of continued submersion. While minimal data for winter dormant turf is available, testimonials indicate most turf at this stage of dormancy can survive considerably longer than 4 days. If the leaf tissue is above the water line, even just a little bit, the turf has a greater potential for survival.
Read it all here