Title: Storm Water Mass Balance Study of Green Roofs for CSO abatement
INTRODUCTION
In its basic definition, a green roof is a rooftop that is vegetated. Green roofs are emerging as a very effective means of addressing many of the environmental concerns that exist in today’s urban centers. In studies, they have shown great promise in reducing the urban heat island effect, improving air and water quality, and increasing the amount of plant life in an urban area. The objectives of this study are to demonstrate how green roofs can play a dramatic role in confronting the problems of storm water runoff. In cities such as New York, where the sewage and storm water systems are combined, Combined Sewage Overflows (CSOs) are a real threat to the surrounding marine and coastal environment any time there is a large rain event.
COMBINED SEWAGE OVERFLOWS
In many of the older cities throughout the country, the sewage systems carry both the sanitary sewage of everyday city life and also take on runoff during rainstorms. A major problem that combined sewage systems have had throughout their history has been that they are unable to handle the full amount of runoff from large rain events and the residential and industrial sewage that is produced by the surrounding city. A Combined Sewer Overflow (CSO) occurs when there is a rain event and the combined runoff and sewage levels are greater than the capacity of the sewage system. The excess sewage and rain water is discharged without treatment into surrounding bodies of water. CSOs pose a great threat to the environmental and sanitary conditions in many urban areas. In some cities, a CSO occurs almost every time it rains.
OBJECTIVES
In this study, we are constructing an extensive green roof section on Steinman Hall, City College’s engineering building.
Figure 1 Project location: Roof Section on Steinman Hall, CCNY
Our primary objective is to quantify the green roof section’s ability to retain rain water. We will test the ability of water retention in different green roof designs using a mass balance approach. Specifically, we will measure rainfall, soil moisture and drainage from the sections over a calendar year. This is important to reduce the amount of water that gets into the sewage system during large rain events. By holding this water, the frequency and magnitude of CSOs will be reduced. Below is an illustration of our experimental design.
First we will determine the mass of water retained by the green roof (hence not entering the sewers) as:
M retained =
∫ time of storm (m in,rain – m out,overflow ) dt
Figure 2 Quantifying Water Mass Balance
Further, we will quantify the partitioning of water within the green roof system between the soil and plants. A series of soil moisture probes will provide a spatial distribution of the amount of water in the soil, m soil (z,t). The water stored in the plants and lost to evapotranspiration can then be determined as:
M plants =
M retained - A xy ∫ depth∫ time of storm m soil dtdz
(A xy is the area of the roof surface.)
We will also collect temperature data to quantify the insulating capabilities of the different designs. The last aspect of the research will be a cost/benefit analysis for green roofs versus more traditional CSO abatement measures. In this part of the research, we will evaluate the projected costs that the city and state use and are planning to use in CSO abatement and the added amount of storm water that could be processed following these projects. We will compare these figures to costs if green roofs were installed throughout the city, comparing them also with the amount of storm water retention that can be attained from all of these green roofs combined based from the study of our individual green roof system. These findings will demonstrate whether green roofs provide a definite solution or partial solution to the CSO abatement in the City.
The results of this study may yield further evidence that in cities, green roofs can be one of the most prudent methods to resolve CSOs environmentally and economically.
OUR GREEN ROOF
The decision to use an extensive green roof system as opposed to an intensive green roof is based upon the use of the existing surface available on Steinman Hall. Although intensive green roof systems allow for larger plants that can retain more water, they are by far more expensive. In many cases, intensive green roofs also need to be designed into the building during initial construction because they are considerably heavier than the extensive roof and require additional structural support. Extensive green roofs, like the section we are building are far more cost effective at least initially and no additional structural support is required in the installation of the green roof. The following figure on the next page is a section showing the layers in a typical extensive green roof:
Figure 3 Section of Green Roof Layers (from www.greenroofplants.com)
In the green roof systems that we are building, we will use several varieties of hearty sedums. These plants are succulents, meaning that they are able to retain a relatively large amount of water considering their small sizes. Because of the ability to retain water well, succulent plants in green roof systems lower the amount of maintenance necessary compared with other types of plants. In many cases, sedums are even frost resistant in the winter months. More than one species was chosen to insure that even if a specific species is not able to adapt to the roof environment, there will still be several others that will still be there. Also, several species allows for a more aesthetic looking roof that does not generally increase the cost of the roof system. Each species of sedums used in this study have very fibrous root systems that do not reach deep into the soil layer. The soil layer in our green roof system will only be from four to six inches. Having plants with short but fibrous roots allows for more water to be retained and hold the soil in place.
Figure 4 Example of a Sedum Variety (from
http://www.virginia.edu/blandy/sedum.jpg )