In the past couple days, I’ve read a couple articles on how to manage flood risk in cities (New York City, in particular). In my opinion, and to oversimplify, a lot of it is about managing elevations in building codes for private property and in design standards for public property, and avoiding or carefully managing development in floodplains.

From MIT Technology Review:

• “more permeable architecture, like green roofs and rain gardens” – I think this is a great idea, and full disclosure, it is part of what I do for a living. But it doesn’t help that much in really enormous storms, or in flooding of major rivers and coastlines. It helps to manage small- to medium- storms, which cause a lot of inconvenience and damage over time, and it helps to manage water quality.
• Also, “less concrete”. Amen to this, although one idea of a city is to build at a high density in one spot so you can leave a lot of other spots undeveloped. We don’t do this well in the U.S. because of political fragmentation and the car/highway/oil industry propaganda we are bombarded with on Monday Night Football.
• “upgraded pumps and drainage pipes” – well, yes. Figure out what you think the peak flows are going to be 50-100 years from now, and then modify your building codes and design standards to move or temporarily store that amount of water. Then, as your long-lived infrastructure gradually wears out, upgrade to the new standards, always keeping an eye on changes in projections and changes in technology.
• “sea barriers and coastal protections” – a no-brainer, but not much help in a storm like Ida which was a rainfall-runoff and river flooding event in the Northeast. If anything, you want to get the water to the ocean quicker so you don’t want anything in the way! Of course, sea level rise and storm surges can come from the ocean side at the same time, so you have to take all of this into account based on your risk tolerance and the value of property you are trying to protect.
• “proposed solutions ranging from social strategies, like educating local city councils on flood risks” – because political fragmentation, you can only ask nicely and hope other jurisdictions do something. You would also like homeowners/businesses to minimize runoff where practical and have insurance to cover their losses.
• “green infrastructure like floodable park walkways, as well as a basketball court designed to hold water during major flooding.” – good idea, this is like an engineered floodplain, which you can dry out, hose off, and use for something else most of the time when it is not raining. It’s hard for these measures to deal with truly enormous quantities of water, but they can help in more localized urban flash flooding events.
• Legalize basement apartments, because people who live in illegal ones tend to be ineligible or afraid to get help.
• The story also references a flood risk study done for NYC by the Danish. This is always a good idea – collect data, map vulnerable areas, have computer models up and running to assess future risks (again, full disclosure, you can pay me to do this…) The Danish are good at this. So are the Dutch, and yes, my fellow geographically challenged Americans, the Danish and Dutch are different (but either will do).

Another article in Slate lists a couple more ideas for NYC:

• “expand upon the modeling completed for this effort and continue developing a citywide hydrologic and hydraulic (H&H) model to better estimate runoff flow for various climate scenarios to be included in the drainage planning process.” Slate calls this “policy gibberish”. Okay Slate author, just leave it to the experts if you don’t want to try to understand it.
• “Plant more trees” – I love trees. Again, mostly helpful in smaller to medium size storms, and for water quality. Also great for cooling, habitat and biodiversity, carbon sequestration, and mental health among many other things. During big storms they will actually cause some damage and even deaths. But the benefits of trees far outweigh the costs. They need to be cared for.
• “Pick up the trash”. There was a lot of talk in Philadelphia too about storm drains clogged with trash. This is absolutely an issue. I am not sure it is a decisive issue in a massive storm like Ida, when all the pipes are full whether storm drains are open or not. But it would help during the 99.99% of the time we are not experiencing the remnants of a major tropical storm. Source controls and modernizing trash collection are also a big deal for getting the plastic out of the ocean and for quality of life in cities. The only losers are the rats, so let’s get this one done!
• “Protect the subway” – I saw this done well in Singapore. Every subway entrance, and every building with an underground parking garage (which is most there), has a “crest elevation”, which is basically a little ramp you have to walk or drive up before you go back down underground. This works. It actually pushes flash flooding onto streets, which the public and politicians don’t like very much, but it is a practical way to deal with very large events. In civil engineering we call the streets the “major drainage system”, acknowledging that every once in awhile they are a good place to park water temporarily.

The one major thing not listed here is managing (avoiding where possible) flood plain development. You might think major cities don’t have much space left to develop in floodplains. But in Philadelphia, a lot of the flooding that made national news during Ida was flooding of recently built developments in floodplains. You want to leave those as park land, natural land, or agricultural land when you can. When you do allow development in the flood plain or you are dealing with historical floodplain development, you need to think about the elevations of entrances as mentioned previously.

Even with all these measures, disaster planning and response will still be needed. We are going to be doing more of this so let’s have plans in place and get good at it.