Non return valves are an important aspect of flood mitigation. Often drains can back up and flood properties from the inside if these are not installed. There are basically 3 options to stop water backing up the system, a Bung, a @push fit' nrv or a mains unit NRV. Bungs and push fit will stop water backing up past the man hole but if you are doing boundary protection the manhole will still 'blow' so a main unit are often better before the first manhole inside your boundary.