This might be a better question for Engineering.SE.
I know that flash flooding to the extent and intensity that is seen in the Midwest or South is more rare in the Pacific Northwest, because it was the subject of a question I asked on ES.SE. However, areal and river flooding is not uncommon in the fall and winter, such as at the Squamish River in October 2003, Stillaguamish River in March 2014 and the Thanksgiving 1990 floods.
Right now the Sumas Prairie neighbourhood of Abbotsford, BC, near the US border, is undergoing catastrophic areal and river flooding as the dikes keeping back the Nooksack River have failed. This river also flooded in the Thanksgiving 1990 floods, but I digress.
What kind of protections does the Fraser River have, particularly in the Lower Mainland (including the north and south arms)? Are there probable scenarios where the Fraser River bursts its banks and causes epic areal/river flooding in areas like Marpole, Richmond, the Fraser Foreshore area of Burnaby, or North Surrey? Or is the Lower Mainland sheltered from river flooding events and rapid snowmelt?