As flood water covers grass and plants, it enters spaces between the soil where oxygen is stored. As the water enters these spaces in the soil, the oxygen escapes so there is none available for the microbes in the soil, or the plant roots to encourage growth. This in essence drowns the plants.
Before you begin clearing up after the flooding take a few photos of the damage the flooding has caused. Some insurance companies may request this as proof of damage, and some may even cover the cost of replacing any damaged plants.