Whether it’s wildfires, coastal flooding, or torrential rains - the first step in any resiliency planning effort is to determine risk.

Weeks ago, prior to Ida, Regional Plan Association (RPA) did an analysis to determine the vulnerability of subway station entrances during high-intensity rainfall flooding. We also measured the vulnerability of the city’s housing stock, with a focus on small residential buildings.

In New York City, there are approximately 180,000 small residential buildings that remain vulnerable to future high-intensity rainfall flooding. This represents about 25% of all residential buildings in the city. The majority of these buildings have basements, most of which are below street level (out of 168,000 total basements, 123,000 are below grade). No one knows the actual number of how many of these basement spaces have been converted to unlicensed apartments. Based on previous estimates, there could be at least 30,000 unlicensed basement apartment conversions vulnerable to future flooding, with about 20,000 of them likely below street level.

There are about 295,000 homes in these small residential buildings vulnerable to future flooding - representing nine percent of the city’s entire housing stock. Although these buildings can be concentrated in certain areas (six out of the top ten council districts are in Queens), these tend to be widely dispersed throughout the city. This makes relocation strategies or managed ​“retreat” an unlikely - and extremely challenging - option.

After Hurricane Ida devastatingly impacted our area, the urgency to understand this kind of threat and determine the risk it poses became abundantly clear. Before we can fully define planning and design solutions, we must rely on improved and final versions of these high-intensity rainfall flood models. These improvements will help determine the magnitude of local risk and the vulnerability the city’s housing stock faces.

We learned the horrific price of not allocating sufficient attention or resources (like proactive resiliency planning) to address this risk. We can no longer ignore unsafe living conditions in basement apartments. With climate change accelerating, it is certain this will not be the last time flooding of this magnitude will occur.