Stormwater management options

By regulating the release of stormwater into the sewer system, the New York Department of Environmental Protection can improve its reliability while delaying costly infrastructure upgrades. Designing sewer systems for the most severe storms is not cost-effective since they are rare events – limiting the stormwater flow rate under demanding rainfall conditions is a much smarter solution.

It is important to note that rain itself is not the only source of stormwater. Snow, ice and hail also turn to water when they accumulate on buildings and melt. There are two ways individual properties can reduce the rate of stormwater release: retention and detention.

What is the difference between stormwater retention and detention?

Although both retention and detention limit the release of stormwater into the sewer system, there is a fundamental difference between the two approaches:

• Detention consists of storing rainwater and releasing it in a controlled manner into the sewage networks.
• Retention consists of storing rainwater for use or disposal on site. Two common disposal methods are evaporation and soil infiltration.

It is important to note that stormwater detention and retention are not mutually exclusive: they can achieve synergy when implemented together, increasing a building's stormwater management capacity.

Stormwater management systems can also be classified by location: there are roof systems and underground systems.

Rooftop rainwater management systems

There are two main types of rooftop stormwater management systems: blue roofs and green roofs. In both cases, the structural design of the roof must consider the weight of the system when fully loaded with rainwater.

• Blue roofs are designed to accumulate water, which is then released through roof drains into the sewer system at a controlled rate. To limit the flow of rainwater, each rain cover is equipped with a weir. Due to their operating mechanism, blue roofs are also called controlled-flow roof drainage systems. Blue roofs are best suited for buildings where the roof slope is less than 2%.
• Green roofs utilize a layer of soil and vegetation and implement both detention and retention. Retention is done by the soil, which can retain a certain amount of rainwater, while retention is done by plants, which consume rainwater as part of their biological functions. Green roofs have a greater tolerance to roof slope and can be implemented effectively with a slope of up to 5%. An additional benefit of green roofs is that they improve roof insulation, preventing heat gain in the summer and heat loss in the winter.

These systems are an excellent stormwater management solution in buildings with limited outdoor space, where the use of underground systems may be impossible or impractical. Of course, roof systems can also achieve synergy with underground systems on properties with ample outdoor space.

Underground Stormwater Management Systems

When it comes to underground stormwater management, there are four main options: storage tanks, gravel beds, perforated pipes, and stormwater chambers. All four options provide stormwater detention, but can be designed without a bottom slab to provide combined detention and retention.

• Storage tanks are typically made from prefabricated concrete or corrugated metal pipe, and can also be made from concrete cast at the project site. They are available in various shapes, including culverts and rings. They are the most compact of all underground stormwater systems, but the rate of soil infiltration is limited by their small ground contact area.
• Gravel beds are excavated areas filled with gravel, as their name implies. They can accumulate rainwater in the spaces between the grains of gravel, which is then released into the sewer or infiltrated underground. Gravel beds perform best if a collector is used to evenly distribute rainwater. These systems tend to take up more space than the other three options, but they also achieve the greatest infiltration into the soil due to their large contact area.
• Perforated pipes utilize a combination of the two approaches above, where the pipes serve as storage tanks and are embedded in gravel to increase rainwater retention capacity. Perforated pipe systems can be a convenient option for projects that will be built in stages as they are modular. In terms of space occupation, perforated pipes rank third among the four types of underground systems.
• Stormwater chambers are prefabricated components embedded in gravel to retain rainwater, much like perforated pipes. They may have open bottoms to allow infiltration into the soil. Stormwater chambers are also modular and are a practical solution when future project expansions are planned. Stormwater chambers are the second most compact type of underground system, after storage tanks.

Underground rainwater systems are an effective solution when a property has plenty of outdoor space available, and can also be installed in basements if they are not below the sewer water level with sufficient clearance from walls and foundations. An advantage of underground systems is that they are hidden and can be covered with landscaping.

These systems must be located at least one meter above the water table and at least 3 meters away from the building's foundation. If the rate of water infiltration into the ground is at least 0.5 inches per hour, the water storage requirements established by the New York Department of Environmental Protection may be reduced.

The New York Department of Environmental Protection provides a series of recommendations for implementing underground stormwater management systems effectively:

• Test soil and groundwater levels first to optimize the design process.
• Existing and projected utility lines must be considered when specifying the system layout.
• If the system uses an open bottom to facilitate infiltration, the subgrade should not be compacted because its water absorption capacity decreases. Avoid locations subject to heavy equipment traffic, as this can also cause soil compaction.
• If dust or any other fine material accumulates in the excavated area, it must be removed before installing the stormwater management system.

Stormwater pretreatment

Pretreatment structures are intended to remove pollutants from stormwater before it reaches the underground management system. They are not mandatory, but if they are neglected the system can lose performance over time due to the accumulation of debris and pollutants. There are two main ways pretreatment structures separate debris and pollutants from rainwater:

• Floating debris is separated by a baffle.
• Non-floating pollutants are removed with a filter.

Removing accumulated debris and cleaning the filter in a pretreatment structure is much simpler and less expensive than having to maintain the entire underground stormwater management system. Therefore, the inclusion of this system component is strongly recommended, even though its use is not mandatory.

Earning LEED Credits with Stormwater Management Systems

If a building is applying for LEED certification from the US Green Building Council, credits can be earned for stormwater management measures. The following measures are eligible for LEED credits, and stormwater management systems can be used to implement them:

• Minimizing runoff
• Rainwater recycling
• Green roofs
• Reducing drinking water consumption
• Water-Efficient Landscaping
• Rainwater quantity and quality control
• Innovative wastewater technologies

Final Recommendations

Stormwater management systems are subject to minimum performance requirements established by New York authorities and must undergo an inspection, just like all other building systems. There are a wide range of options available, and the best type of system for a specific project is determined by site conditions and building characteristics.

To get the best results when implementing stormwater management systems, be sure to work with qualified, licensed professionals. If you are applying for a LEED certification from USGBC, a LEED-accredited professional can help you maximize the credits you earn through effective stormwater use.