One of the last things you do about after a disaster like a flood is worry about the front lawn. After the foundation, the basement and the appliances are squared away, however, that mess out front will have to be rehabilitated before you can grow anything but plantains and bulrushes. The first steps in reestablishing a healthy lawn require stabilizing your transformed environment.
Flooding suffocates your lawn. If the flooding is the result of a Hurricane storm surge, salt water deposits a load of sodium on the lawn grass that can kill it. If water sits on the soil for longer than a week or two, it can become "hydric", meaning that all of the air is squeezed out of it, making it fit only for aquatic plants and mosses. Most flooding is not so severe, however, and it is possible to rehabilitate the lawn. Your objective will be to get the air back into the soil and "mitigate", or relieve, any toxicity in it.
Clear drainage ditches or swales to carry off flood waters. Remove fallen trees and debris to allow sunlight to get through to grass roots. Stay off the lawn until the ground is solid enough to support movement. Heavy equipment and excessive foot traffic can tear up plants and create a muddy, silty mess in lawns that would otherwise be lightly damaged. Core aerification, slicing and verticutting can be used to open up heavily damaged lawns to dry.
If the soil is heavily silted, the fine particles will lock out air and light. Remove silt that is more than a few inches deep with a flat shovel and streams of fresh water. Use heavy irrigation to leech sodium left by saltwater through soil; sodium allowed to dry will form a hard crust and completely dehydrate the soil.
Bacteria and chemicals carried by floodwaters must be removed before re-planting can proceed. Soil composition may also be severely altered. Fertilizer will certainly be washed away. The best way to analyze your soil's needs is to contact your local state university extension or USDA field office for a soil test. In the meantime, aerate the soil to get air and sunlight to struggling roots.
Spread gypsum over aerated soil with saltwater-damage not cured by irrigation alone; it will react with gypsum to form sodium sulfate and wash through the soil with more irrigation, leaving harmless calcium precipitate in its place.
Once soil tests are complete, add lime or sulphur to correct pH and replace only essential nutrients---usually phosphate and phosphorus---according to recommendations; a heavy nitrogen feeding will shock lawn grasses into rapid growth in soil that is unable to support it. Lay down top dressing of an inch or two of manure or clean compost to establish a new healthy layer of topsoil, particularly if the turf is very thin.
Methods of reestablishing lawn grass will depend on the degree and type of damage done by flooding. Cool-season lawns in northern states may be temporarily over-seeded with perennial ryegrass or tall fescue. Overseed with ryegrass or bentgrass in warm-season areas. Recovering lawns can be fertilized and overseeded in fall. Heavily damaged lawns should be cultivated, fertilized, rolled and re-seeded---or sodded--in fall. Mow frequently and water deeply to encourage re-establishment of lawn grasses and discourage weed growth.