All modifications to street geometry should avoid unintended changes in the direction and disposition of stormwater runoff so as not to create ponding or flooding issues. Adequate roadway grades (i.e., 1.5% desired, with a minimum of 0.5%) are required to direct runoff to catch basins. Include planted areas and stormwater source controls within the roadway wherever feasible. For more information on green infrastructure treatments, review Stormwater Management Practices.
Stormwater control within the street network may offer opportunities for resiliency benefits in areas that experience frequent flooding.
Integral Curb and Gutter
In flatter, low-lying areas of New York City, such as Downtown Far Rockaway in Queens, where grading for proper drainage is challenging, implementing an integral curb and gutter system can provide more effective stormwater control and drainage. Review Integral Curb and Gutter.
Drainage at Blended Transitions and Pedestrian Ramps
Ensuring proper drainage at pedestrian ramps is critical to ensure the ramps are effective for people with disabilities. This is particularly true at blended transition ramps where grading can be especially challenging. Positive drainage must be provided, and the design must not hinder flow; gutter slopes must not be less than 0.5%.
Unified Stormwater Rule (USWR)
In accordance with the NYC Unified Stormwater Rule (USWR), DOT is actively working to reduce the amount of impervious surfaces in the city. Review Unified Stormwater Rule for more information.
One key initiative involves the introduction of more porous materials. Review Precast Porous Concrete Panels and Porous Asphalt. These materials allow water to permeate through the surface, enabling the collection and management of stormwater runoff. This improves street drainage and reduces pollution to local waterways. By incorporating these innovative solutions, DOT aims to enhance stormwater management and promote a more resilient urban environment.
DEP Cloudburst Design
A cloudburst is a sudden, intense rainfall event where a large amount of rain falls within a short period, typically causing significant flooding. New York City has seen an increase in the frequency and intensity of these events due to climate change. For example, storms like Hurricane Henri and Ida in 2021 demonstrated the city's vulnerability, with rainfall reaching record intensities. The changing climate, with more frequent and severe storms, has necessitated a comprehensive approach to managing stormwater to protect the city's infrastructure, property, and residents.
To manage and mitigate stormwater flood risks from cloudburst events, the City has developed planning and intervention strategies that aim to absorb, store, and transfer stormwater.
- Absorb: Focuses on capturing stormwater at its source and allowing it to infiltrate into the ground naturally. This is primarily achieved through green infrastructure, such as rain gardens and porous pavements, which facilitate the infiltration of stormwater into the soil. These practices not only reduce the volume of stormwater entering the sewer system but can also provide additional benefits such as cooling the urban environment, enhancing biodiversity, and improving air quality.
- Store: Involves temporarily holding excess stormwater to prevent immediate overloading of the sewer system. Storage solutions can be above ground, such as detention basins or retention ponds, or below ground, such as underground storage tanks. These solutions are designed to manage peak flows during intense rainfall events and release water slowly into the drainage system or natural water bodies.
- Transfer: Involves moving stormwater from areas where it collects to locations where it can be safely discharged or stored. This often requires enhancements to existing infrastructure like upgraded sewers and drains to handle higher volumes of water or the creation of new pathways for water movement such as stormwater conveyance channels that direct stormwater away from vulnerable areas. These strategies ensure that stormwater is efficiently managed, reducing the impact on populated and flood-prone areas.
The above principles can be applied together when choosing an appropriate strategy or a project can focus on individual principles if site constraints preclude other interventions.
Stormwater Management Intervention Strategies
To address stormwater management effectively, New York City employs a combination of grey, green, and blue infrastructure with reinforcing regulations and real-time flood monitoring and in-sewer flow sensors.
Green Infrastructure
Green infrastructure focuses on absorption strategies, utilizing installations like trees, rain gardens, and porous pavements.
Grey Infrastructure
Grey infrastructure includes traditional sewers and high-capacity drains designed to handle typical rainfall, as well as underground storage tanks that hold excess water temporarily.
Blue Infrastructure
Blue infrastructure encompasses natural systems such as bluebelts, which integrate wetlands and streams to manage stormwater and improve water quality, and daylighting streams to restore covered waterways.
Real-time monitoring systems, such as FloodNet, play a vital role in the City's stormwater management strategy (FloodNet can also play an important role in monitoring tidal flooding). FloodNet is an important step towards providing real-time information on street flooding across the city, which in time could enable proactive management and rapid response to stormwater events. These sensors also offer the future potential for engineers and planners to evaluate and calibrate modeled predictions with quantitative data on real-world outcomes. Interactive stormwater flood maps developed by the DEP help residents understand flood risks in their neighborhoods. These maps are updated with real-time data and are accessible to the public, allowing individuals and businesses to take proactive measures during storm events. Additionally, predictive analytics leverage historical data and real-time monitoring to forecast flooding scenarios and prepare accordingly, including pre-storm inspections of critical infrastructure, clearing debris from catch basins, and deploying resources to vulnerable areas.
By integrating these regulations and monitoring systems with stormwater infrastructure, New York City may develop improved toolkits for flood management. This multi-faceted approach not only addresses immediate flood risks but also contributes to long-term urban resilience and sustainability, ensuring the city can more effectively manage the challenges posed by climate change and increasingly intense rainfall events.