While overwatering can be correctable, waterlogging and flooding are more of a problem. If they happen in winter, when most plants are dormant, the damage can be easily missed, but with summer floods, symptoms such as wilting, yellowing and browning of the leaves usually appear rapidly. Either way the roots suffocate, drown and rot, and the conditions are right for diseases such as Phytophthora.
Where waterlogging is known to be a problem, there are ways to improve the growing conditions. Using raised beds and planting trees and shrubs on mounds helps to lift roots above high water tables, and provides some protection against flooding.
Poor drainage on clay soils can be improved by cultivation. Similarly, water lying on lawns can be encouraged to soak in by spiking and topdressing with sand to create mini-drainage channels.
In the worst cases, perforated plastic drainage pipes can be laid in the ground to move water to soakaways and ditches.