At the 1:32 point in this video of the canal in flood you can see the viaduct that I photographed that allows the water to spread out far and wide. While this clearly helps, still it was a torrent of water going down the canal. The video was taken in December 2016 and oddly I can't remember a serious lot of rain nor news of the canal flooding. The real problem is that it drains such a large area very quickly and with many more paved surfaces now as against say twenty years ago when there was a lot more homes with grass lawns and more gardens, and bluestone paving with gaps for water to soak into, the flooding is probably worse.
Video by Terry C. I hope I have the publish date and time right this time. I believe the chap towards the end is former Port Phillip Mayor Dick Gross, a known bicycle rider.