Most of NYC's sewers are combined sewers, meaning they collect both sewage from buildings and stormwater runoff from the streets in a single pipe and carry it all to a wastewater treatment plant. During times of significant rainfall or snowmelt, however, the increased volume of runoff entering the sewer can exceed the capacity of the system. In order to prevent a backup, the excess (including untreated sewage) is dumped directly into area waterways.
To help mitigate this problem, the city recently embarked on a plan to build $2.4 billion worth of "green infrastructure" that will "use or mimic natural systems" to collect stormwater runoff before it reaches the sewer. Right-of-way bioswales, like the one above, capture water flowing down the gutter and allow it to seep into the ground through the soil, keeping it out of the sewer system altogether.