Hiddenowl:
If rainfall is sufficiently intense, water does not drain away quickly and cause appreciable depths to build up and cause accidents.
This sounds to me like something that would be much better studied in a laboratory. The concept seems simple enough. Deliver 100 litres (or 1000 litres) of water onto an area over a specified time period and see how quickly it disappears down the drain. If the surface it falls on is level and flat it can surely be assumed that the water will have a uniform depth for all practical purposes. The difference between the mass of water in the tank and the mass of water in the sump will be the mass of water on the surface - from which the depth can be calculated.
Trying to get useful data from a test outside the laboratory seems to me a waste of time because the data would be affected to such an extent by the local ground and weather conditions that it could not be assumed to apply at another location. For example wind blowing against the water flow will cause the water depth to build up - not to mention its effect in causing ripples or even waves.
...R