Recover Your Lawn From Waterlogging
While there is not a great deal you can do for a lawn that is currently under water, once the water has subsided then you can go about trying to stop it happening again. While it is never completely avoidable then there are measures you can take to help alleviate it.
When a lawn is waterlogged then there is a great deal of downward pressure on the soil structure and so one of the first things that you need to do is try to improve this by removing the compaction. In order to do this then one of the first things you can do is aerate the lawn. This is a process of spiking multiple holes into the surface and this will break through the compacted barrier on the surface and therefore allow the dose of rain to drain through and into the subsoil. It also allows air to flow back through the soil and around the roots of the grass which is vital for grass growth and recovery from the waterlogging. There are 2 main types of aeration, solid tine and hollow core. Solid tining uses, as the name suggests, solid tines that stab a narrow hole into the ground whereas hollow coring uses a hollow tine that will remove a plug of soil and leaves a larger hole, usually about 10-20mm across. Both systems will work up to about 10-15cm down into the soil. If you do not have access to one of these machines or are not able to hire one, then the same effect can be gained by using a garden fork.
Once you have created these air pockets in the soil then you want to make sure that they remain open for as long as possible and the best way to do this is to top dress the lawn with a 70/30 top dressing. This is a mix of soil and sand and while it will fill the holes up, it is open enough to allow air and water to still pass through it. To do this job properly then you need to roughly spread the top dressing on the lawn and then use a lute, drag mat, stiff brush or something similar to work it down into the grass and also into the holes you have created.
While you would only top dress a lawn once or twice a year, there is no harm in regularly spiking your lawn throughout the year; once a month would help to minimise the effects of waterlogging the following Autumn.
If the lawn has had water sat on it for any length of time then you will undoubtedly have bare patches in the sward and so you will need to look at applying both iron sulphate to deter any ingress of moss and also fertiliser to help the grass to recover. A combined fertiliser such as High-Fe or MossTop will help massively with this. Then as we get more towards Spring and some warmer temperatures then you will need to be applying higher Nitrogen fertilisers such as Phased Release or Spring Rise.
In very extreme cases then you might even consider installing some drainage. This can usually be done with a French drain where you dig a trench across the lawn to a pond/ditch, line it with a water permeable membrane , fill it with large stones or shingle and then wrap the membrane over the top of it to stop soil and silt from infiltrating it. You can then cover this with a shallow depth of top soil and turf or seed over that so that you cannot actually see the drain.