Stormwater Management Options: Detention and Retention
The New York City sewage system is designed for a specified flowrate, and the NYC Department of Environmental Protection (DEP) establishes a flow limit for individual properties to avoid sewage saturation, which would cause it to overflow. The minimum stormwater storage capacity for new properties and major renovations was increased in 2012, as part of the NYC Green Infrastructure Plan.
By regulating the release of stormwater into the sewage system, the NYC Department of Environmental Protection can improve its reliability, while delaying expensive infrastructure upgrades. Designing sewage systems for the harshest storms is not cost-effective, since they are rare events – limiting 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 sleet also become water when they accumulate in buildings and melt. There are two ways in which individual properties can reduce the rate at which they release stormwater: retention and detention.
What is the Difference Between Stormwater Retention and Detention?
Although both retention and detention limit the release of stormwater into the sewage system, there is a key difference between both approaches:
- Detention consists on storing stormwater and releasing it at a controlled rate into the sewage systems.
- Retention consists on storing stormwater for on-site use or disposal. 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 deployed together, increasing abuilding’s stormwater management capacity
Stormwater management systems can also be classified by location: there are both rooftop systems and subsurface systems.
Rooftop Stormwater 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 stormwater.
- Blue roofs are designed to accumulate water, which is then released through roof drains into the sewage system at a controlled rate. To limit rainwater flow, each roof rain is equipped with a weir. Due to their operating mechanism, blue roofs are also called controlled flow roof drain systems. Blue roofs are better suited for buildings where the roof slope is below 2 percent.
- Green roofs use a layer of soil and vegetation, and they implement both detention and retention. Detention is accomplished by the soil, which can hold a certain amount of rainwater, while retention is achieved with plants, which consume rainwater as part of their biological functions. Green roofs have a higher tolerance for roof inclination, and can be deployed effectively with a slope of up to 5 percent. An added benefit of green roofs it that they improve rooftop 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 subsurface systems may be impossible or impractical. Of course, rooftop systems can also achieve synergy with subsurface systems in properties with ample outdoor space.
Subsurface Stormwater Management Systems
With respect to subsurface 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 cast concrete at the project site. They are available in various shapes, including culverts and rings. They are the most compact of all subsurface stormwater systems, but the soil infiltration rate is limited by their small contact area with the ground.
- Gravel beds are excavated areas filled with gravel, as implied by their name. They can accumulate stormwater in the spaces between gravel grains, which is then released into the sewer or allowed to infiltrate underground. Gravel beds perform better if a manifold is used to distribute rainwater evenly. These systems tend to occupy more space than the other three options, but also achieve the highest soil infiltration due to their large contact area.
- Perforated pipes use a combination of the two approaches above, where the pipes serve as storage tanks, and are embedded in gravel to enhance rainwater holding capacity. Perforated pipe systems can be a convenient option for projects that will be built in stages because they are modular. In terms of space footprint, perforated pipes rank third among the four types of subsurface systems.
- Stormwater chambers are prefabricated components that are embedded in gravel to hold rainwater, just like perforated pipes. They can have open bottoms to allow ground infiltration. Stormwater chambers are also modular, and a practical solution when future project expansions are planned. Stormwater chambers are the second most compact type of subsurface system, after storage tanks.
Subsurface stormwater systems are an effective solution when a property has plenty of outdoor space available, and they can also be deployed in basements if they are not below the sewage water level with enough clearance with respect to walls and foundations. An advantage of subsurface systems is that they are hidden from sight and can be covered with landscaping.
These systems must be located at least three feet above ground water level, and at least 10 feet away from the building foundation. If the infiltration rate of water in the soil is at least 0.5 inches per hour, the water storage requirements established by the NYC Department of Environmental Protection may be reduced.
The NYC Department of Environmental Protectio provides a series of recommendations to deploy subsurface stormwater management systems effectively:
- Test the soil and groundwater levels before anything else, to optimize the design process.
- Existing and projected utility lines should 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-absorbing capacity is decreased. Avoid a location that will be subject to heavy equipment traffic, since that can cause soil compaction as well.
- If dust or any other fine material accumulates in the excavated area, it must be removed before installing the stormwater management system.
Pretreatment structures have the goal of removing pollutants from stormwater before it reaches a subsurface management system. They are not mandatory, but if they are overlooked the system may lose performance over time due to the accumulation of debris and pollutants. There are two main ways in which pretreatment structures separate debris and pollutants from rainwater:
- Floating debris is separated with a baffle.
- Non-floating pollutants are removed with a filter.
Removing the accumulated debris and cleaning the filter in a pretreatment structure is much simpler and cheaper than having to service the entire subsurface stormwater management system. Therefore, including that system component is strongly advised, even though its use is not mandatory.
Obtaining LEED Credits with Stormwater Management Systems
If a building is applying for LEED certification by the US Green Building Council, it is possible to earn credits with stormwater management measures. The following measures are all eligible for LEED credits, and stormwater management systems can be used for their implementation:
- Minimizing runoff
- Recycling stormwater
- Green roofs
- Reduction of potable water consumption
- Water-efficient landscaping
- Stormwater quantity and quality control
- Innovative wastewater technologies
Stormwater management systems are subject to minimum performance requirements, established by NYC authorities, and they must pass an inspection just like all other building systems. There is 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 achieve the best results when deploying stormwater management systems, make sure you are working with qualified and licensed professionals. If you are applying for a LEED certification by the USGBC, a LEED accredited professional can help you maximize the credits achieved with effective stormwater use.