City wants to build watersquares

Following the example of Rotterdam, Montreal wants to investigate building watersquares, public areas used as retention basins during downpours. This would spare the sewer system and the river from the surge it normally gets at those times.

Not sure about this. Such a square would be dry concrete 99% of the time, no? How does this work with the other aim of adding green space and reducing heat islands? Doesn’t green space, with a variety of plants and trees on it, also soak up water and hold it? And how would this idea work with snow accumulation?