Building structures in Rochester, New York have historically been redundantly designed to account for high snow load, and recalling that 40 to 50% of a city’s impervious surfaces are roofs, green roofs are a promising opportunity for greening the city’s stormwater management systems (6). However, green roofs are often more expensive than other green infrastructure or conventional stormwater retention systems, which often makes them difficult to promote (3). This project introduces a preliminary survey of how green roofs may be integrated into Rochester’s urban landscape, seeking first to address which roofs are fundamentally viable for green roof retrofits, and second to develop a hierarchy of locations where green roofs may have the highest environmental impact. The study window selected samples Rochester’s downtown area and sections of the neighborhoods immediately surrounding it, suggesting how roofs might perform in the greater metropolitan area.
1. ESRI’s extract roof forms for municipal development tool was used with DEM and DSM rasters and building footprints from April 2017 Monroe County data to determine building heights and roof geometry
2. Viable buildings were defined as below 11 stories (110 feet) (1) with roof pitches below 20 degrees from horizontal (7)
3. The local annual runoff was calculated by modifying 2010 NOAA CCAP Regional Land Cover data to reflect curve values of each land use class and subsequent annual runoff depths for the city of Rochester
4. Percent vegetation cover was derived from the Landsat 8 red and near infared bands using the positive values from the formula NDVI=(NIR-R)/(NIR+R)
5. Percent vegetation cover and annual runoff depth data was attatched to each building using extract values to points using the inside centerpoint of each footprint polygon
6. Local runoff and vegetation cover values were weighted equally in a suitability analysis to compare buildings’ potential for environmental improvement, producing a gradient map
7. Life cycle cost comparisons were made using medium performance averages for costs and lifespans of green roofs and conventional subsurface retention adveraged over the 40 year lifespan of a green roof (3)
From the 2668 buildings in the study area, 1744 (65.4%) of roofs appear to be viable for green roof renovations, and 91% of the nonviable roofs were removed due to their pitch.
Considering figure 1, while small-roofed buildings vary widely in potential, most large roofs show a high potential for retrofit. Combining this with polygon data and figure 4, most of this potential centers in the downtown area on large, flat-roofed buildings.
By developing every viable roof, the city’s annual runoff would decrease by 517743 cubic meters (94.6%) This slightly exceeds the maximum reduction of 90% found in research (6).
It is important to note that combining the design assumptions for Rochester with mid-range national price estimates, the annual cost per cubic meter of stormwater retained by installing green roofing is $0.22, compared with the cost for conventional stormwater storage of $0.492 per cubic meter.
Considering figure 1, while small-roofed buildings vary widely in potential, most large roofs show a high potential for retrofit. Combining this with polygon data and figure 4, most of this potential centers in the downtown area on large, flat-roofed buildings.
By developing every viable roof, the city’s annual runoff would decrease by 517743 cubic meters (94.6%) This slightly exceeds the maximum reduction of 90% found in research (6).
It is important to note that combining the design assumptions for Rochester with mid-range national price estimates, the annual cost per cubic meter of stormwater retained by installing green roofing is $0.22, compared with the cost for conventional stormwater storage of $0.492 per cubic meter.
Without considering the structural demands of a green roof, the majority of buildings throughout Rochester meet simple viability for a retrofit. However, more detailed analysis will be necessary to determine a more concrete viability model. By these results, planning efforts should focus on developing the larger flat roofs in the city center, as they generally have the highest priority rankings and stormwater interception potential. Most high priority buildings are also along the Genesee river, and therefore have a greater opportunity to reduce the amount of runoff directly entering the river. This project does not indicate a strong potential for residential roofs to greatly impact their surrounding region, but may still have the possibility to improve the thermal performance of individual buildings.
Acknowledgements
1. Center for Neighborhood Technology. (June 29, 2009). Green Values Stormwater Calculator Methodology. Retrieved from http://greenvalues.cnt.org/calculator/downloads/methodology.pdf.
2. Lamsal, M. (2008). Green Roof Adoption: a GIS-Integrated Cost-Benefit Analysis in Atlanta Incorporating a Positive Externality. Retrieved from https://getd.libs.uga.edu/pdfs/lamsal_madhur_201208_ms.pdf.
3. Monroe County (July 12, 2016). City of Rochester & Monroe County Green Infrastructure Retrofit Manual. Retrieved from https://www2.monroecounty.gov/files/DES/Stormwater/Rochester%20%26%20Monroe%20Cty%20Green%20Infrastructure%20Manual.pdf.
4. NOAA Coastal Services Center (2014). Technical Guide for OpenNSPECT, Version 1.2. Retrieved From https://coast.noaa.gov/data/digitalcoast/pdf/opennspect-technical-guide.pdf
5. NYSDEC. (2015). New York State Stormwater Management Design Manual Chapter 5: Green Infrastructure Practices. Retrieved from https://www.dec.ny.gov/docs/water_pdf/swdm2015chptr05.pdf.
6. Wilkinson, S., Rose, C., Glenis, V., & Lamond, J. Modelling a green roof retrofit in the Melbourne Central Business District. Retrieved from https://www.researchgate.net/publication/269031117_Modelling_green_roof_retrofit_in_the_Melbourne_Central_Business_District
7. Vinnova. (2017). Swedish guidelines for green roofs. Retrieved from https://greenroof.se/gr-16/wp-content/uploads/2017/04/Swedish-handbook.-Translated-short-version.pdf.