A hazard report tells you what the maps say. A site visit during heavy rain tells you what the ground does. Here is the list of things worth looking at while it is actually raining, and the questions to ask after.
Most people see a property on a good day. The seller schedules it, the weather cooperates, and the lot looks like every other lot. An hour on the same site during heavy rain is worth more than three dry-day visits.
Run the hazard screening before you go. It takes about a minute, and it turns a vague walk into a targeted one. If the report flags mapped flood exposure, you go look for where water enters and leaves. If it flags slope or landslide susceptibility, you go look at the cut behind the house. If it puts a waterway nearby, you go find it.
Walking a lot without that list is how people miss things that were visible the whole time.
Where does water enter the property, and where does it leave? Follow it. Water takes the same route every time, and in heavy rain it shows you the route for free.
Is anything ponding? Standing water on part of the lot after an hour of rain tells you about drainage and about the ground underneath.
What is the road doing? A road that sheets water toward the property is a design problem you inherit. A road that floods at a low point is an access problem.
Where does the neighbour's water go? Roofs, driveways, and walls redirect runoff. Yours will receive some of it.
Is the drainage flowing or blocked? Look at the canal, the culvert, and the outlet. Blocked drainage is a fixable problem, but only if you know about it before you buy.
On sloping ground, look at the cut and the wall. Water seeping out of a slope face, fresh soil at the base, or a retaining wall with no weep holes are all things to ask about.
How high is the nearest channel running? Compare it against how high it can go. Neighbours will show you the mark.
While it is fresh, ask the people who live there:
The full set of seller questions is in what to ask the seller about flooding, and it works just as well on neighbours, who have nothing to sell you.
Come back to the screening with what you saw. Three outcomes, all useful:
You will not remember the details later, and details are the entire value of the visit. Bring:
Photograph the same spots you would want to compare later: the gate, the lowest corner of the lot, the road at its low point, and the nearest channel.
The rainy visit tells you what happens. A dry visit tells you what was done about it.
On a dry day you can see the things water hides: the condition of the canal, the size of the culvert, whether the drainage outlet is actually connected to anything, whether the retaining wall has weep holes, and whether the ground has been filled recently. Ask the questions that occurred to you in the rain, when there is someone around to answer them.
Two visits in different conditions is the cheapest due diligence available on any property, and almost nobody does it. If you are choosing between candidates, doing it for both and comparing the results is the method described in comparing two lots.
A hazard screening reads national datasets at your coordinates and will never see the neighbour's new wall or last month's blocked culvert. A rainy-day visit sees exactly those things and cannot tell you about the rarer, bigger event that has not happened lately. Use both, and pay a professional before you build on the one you choose.
CheckHazard does not replace a professional geotechnical or engineering survey.