Plus, of course, the hills drain water that drops onto them down into the basin below - there are a few other spots on Mauritius that can flood like this, but in a big city, it makes a big mess....
I should add that rain here is very much tied to the geography - the great majority of rain falls in the hilly middle and south parts of the island. Often, one can see big dark clouds hanging over the peaks in the distance, but they never make it to the west coast at Albion. When there is rain here, it is more likely a system that is travelling out in the open ocean, and just happens to side-swipe the west coast.
The division of Réunion into a "dry" side (west) and a "wet" side (east) is in fact an official designation. The situation is somewhat similar on Mauritius, but because of the much lower topographical relief, the differences between east and west coast are generally less pronounced.
One does occasionally see some amazing cases of microclimate in the mountainous south of Mauritius, where heavy rain may be falling in one small area (at the inland end of a bay, closest to the mountains), while at the other end, particularly if there is a point going further out into the sea, it is completely dry and maybe even sunny.
Some of the rain left by cyclone Felleng last month, seen from the second story of my residence in Albion.
Just outside my front door here in Albion, a snail like I have never seen before. Moreover, there was a bunch of them off to the side, enjoying the vegetation after a bit of rain. Also, found them in the hills of the National Park in the south of the island - including a nice empty shell, which I took as a little souvenir.
Photos belong to Jaro Franta and local newspaper.