Mad (Flt) Scientist,
That central "wet" section appearance in contrast to the "contam" section is mainly due to the grooving in the central 20m. There is a central crown on the runway, and I think the crossfall drainage is at the lower limit of 1%. The grooving is very recent and part of an upgrade project. I’m guessing at the groove spacing and dimensions but it typically adds 1.1mm of macrotexture to create a macrotexture of 1.6mm. There is a very slight sheen in a few places in the central “wet” section, so it is fair to say that the runway surface macrotexture in the centre is holding the equivalent of maybe 1.5mm of water depth and leaving 0.0mm proud of the surface.
Plus we are seeing here the general poor/slow drainage of ANY wide (almost) flat pavement. The standing water at the sides is showing more prominently in the photo because (a) smooth/poor macrotexture of the ungrooved edges mean more water sitting above the asphalt, and (b) the “drainage length” affecting the runway at the edge is more the “drainage length” affecting the runway in the centre.
A model for the evaluation of the depth of rain water on a pavement is available and is shown in the following equation:
water film thickness or depth = fn (drainage length X rain intensity)^0.5/(drainage slope)^0.2.
The more the drainage length, the deeper the water film. For a runway of 45m width with the central 20m grooved, if the water film is 1.5mm at 10m perpendicular from the centreline (at the edge of the grooving), then it will be 2.3mm at the edge of the runway. However while the centre grooved “wet” section has no water sitting above the surface, the ungrooved outer edges have a texture depth of only say 0.6mm, meaning a water film on the surface of 2.3 – 0.6 = 1.7mm of water.
That is more than half-way to 3mm contaminated depth (on the edges) and the rain had stopped at the time of the photo.
I calculated the sort of depths of water that can occur on a runway in heavy rain, and did so for the YYZ overrun. At Toronto, their radar showed rainfall intensities of over 50mm/hour at the time the A340 ran off the runway, and the rainfall event was reportedly 36.2mm that day. For Australian rainfall patterns, that would be equivalent to a maximum short burst intensity of up to 120 mm/hour depending on burst duration. At that intensity, and for a 60m wide surfacing (45m runway plus 7.5m sealed shoulders), and crossfall of 1.5%, the depth of water on the runway could reach over 5mm and as high as 6.6mm.