Preventing Floods After Urbanization

How can cities counter the higher incidence of flood in urbanized areas?

December 26, 2019 04:04 AM

The following question was recently posted on Quora:

“Does urbanization increase flooding? If so, how?”

When areas become urbanized, hard materials are used to create roads, buildings and other infrastructure. These materials do not absorb water and the general shape of city infrastructure tends to accelerate the speed of water. Therefore, flooding is more prevalent in urbanized areas.

To counter this problem, Mike Barnard offers several proven suggestions:

There are a variety of techniques that can be used to minimize this problem:

Green roofs - “In summer, depending on the plants and depth of growing medium, green roofs retain 70-90% of the precipitation that falls on them; in winter they retain between 25-40%. For example, a grass roof with a 4-20 cm (1.6 - 7.9 inches) layer of growing medium can hold 10-15 cm (3.9 - 5.9 inches) of water.” Green Roof Benefits - GRHC WEBSITE
Porous roads - “Porous asphalt, also known as pervious, permeable, “popcorn,” or open-graded asphalt, is standard hot-mix asphalt with reduced sand or fines and allows water to drain through it. Porous asphalt over an aggregate storage bed will reduce stormwater runoff volume, rate, and pollutants. " Porous Asphalt Pavement
Sustainable drainage systems - “Examples of this type of system are basins (shallow landscape depressions that are dry most of the time when it’s not raining), rain-gardens (shallow landscape depressions with shrub or herbaceous planting), swales (shallow normally-dry, wide-based ditches), filter drains (gravel filled trench drain), bioretention basins (shallow depressions with gravel and/or sand filtration layers beneath the growing medium), reed beds and other wetland habitats that collect, store, and filter dirty water along with providing a habitat for wildlife.”Sustainable drainage system
And of course really serious storm drain systems - Storm drain

Furthermore, roads tend to be straighter than anything in nature. That’s amenable to some change as well, but less so than the porousness question for building materials.

Another common problem are the presence of urban underways. Living in Calgary as I do now, I offer this collage of pictures from the 2013 floods of the Elbow and Bow Rivers which run through and around downtown Calgary.

These rivers run out of the mountains 100 km away, were already running high with snow melt and across ground which had become sodden with five days of rain. Then there was a lot of rain for 24 hours and the rivers rose to engulf much of the downtown core and the Beltline neighbourhood. Basically, the city is built on a floodplain, as are many prairie cities, there being very little that isn’t floodplain on the prairies. And when floods happen, there isn’t much of anywhere for the water to go except over the banks and into the cities.

This of course happened long before there were cities on rivers in floodplains. But urbanization leads to built infrastructure, more structures beside the rivers than optimal and often more urban discharges into the rivers due to lack of the mitigations mentioned above. So normal flooding does a lot more damage and is often exacerbated by bad urban design.