4501 Circle 75 Parkway Suite f-6320 Atlanta, ga 30339 (770) 953-4143 Table of Contents
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- Data Preparation
- Study Site Analysis
- Forest Health
- Energy Savings
- Air Quality
- Stormwater Management
Data CollectionAfter six suitable study sites were selected, preliminary inventory themes were created using the CITYgreen software. CITYgreen is an extension of ArcView, a GIS software program. Using the digital aerial photographs as a backdrop, an inventory theme for the following features was created for each study site: trees, grass areas, buildings, and impervious surfaces. The image and corresponding inventory themes for each study site were then printed and used to ground truth and update the data for each inventory theme. Data collected at each study site included the following: Tree data - species, diameter, height class, health class, ground cover, conflicts Grass data - land use, type, height, maintenance Building data - stories, roof color, building material, overhangs, locations of air conditioners and windows Impervious surface data - land use, type, color Additional data for features (trees, grass areas, buildings, etc.) not visible from the aerial photos was also collected. See Appendix A for additional descriptions of data collected. Data PreparationThe next step of the project involved compiling and entering the data collected from the six study sites into the CITYgreen software for analysis. Corrections to the preliminary inventory theme data were made based on the data collected during the ground truthing process. Study Site AnalysisAfter all of the data had been entered and verified for accuracy, the CITYgreen and ArcView analysis tools were utilized to assess the benefits of the trees in each of the study site areas. The analysis provided data about the following: Acreage of each study site Tree canopy area Average tree diameter Average tree height class Average tree health rating Total energy savings Total carbon storage and annual uptake Annual pollution removal Water runoff and peak flow reduction The following section discusses in detail the basic methodologies and techniques utilized in each of the CITYgreen analysis tools. Forest HealthDuring the data collection phase of the project, the health of every tree was classified on a scale of one to five. A rating of one indicated that the tree was in very poor health, was damaged, or could not sustain new growth. A rating of five indicated that the tree was in excellent health, was free from major damage, and had substantial growth in the last year. Using this data, CITYgreen calculated the arithmetic mean (average) health class for the trees in each study site. Energy SavingsStudies have shown that trees play an important role in reducing summer energy use in urban areas by providing shade to urban buildings and homes (Huang et al. 1992, McPherson et al. 1993). As a city that is a summer energy peaking community, Macon’s urban trees can save the city considerable amounts in summer air conditioning costs. To calculate these savings, CITYgreen analyzes a tree’s shade characteristics, height, and location to buildings. Each tree is then assigned an energy rating ranging from one to five, with five representing an optimally placed tree providing the most energy savings (American Forests 1996). These energy savings are then converted into economic values using research data from the USDA Forest Service of twelve U.S. cites. This research indicated that trees reduce air conditioner use between 5.5% and 22%, depending on the city (McPherson et al. 1993). A tree with an optimal energy rating of five is determined by applying the energy savings percent of a city with a similar climate with the annual cooling cost for a home in the city. In Macon, for example, USDA models estimate that an optimally placed tree can reduce cooling costs by 9.6%. Using an average annual cooling cost of $600, a tree with an energy rating of five can save a homeowner about $58 (9.6% of $600) during the course of a cooling season. The total energy savings for all trees are calculated by extrapolating the USDA Forest Service data to all trees in each energy rating class (American Forests 1996). Air QualityAn additional benefit of trees in the urban forest and across the entire landscape is the storage and sequestration of carbon. Trees convert carbon dioxide, a green house gas, to oxygen and stored carbon. The carbon is stored in the biomass of trees and other vegetation in various levels, depending on their size, distribution, and species type. Additionally, trees absorb carbon from the atmosphere at predictable rates, known as carbon sequestration. CITYgreen calculates the amount of carbon storage and annual sequestration rates of trees based on their average diameter class distribution and canopy coverage (American Forests 1996). The annual value of carbon sequestration was then determined by multiplying the average annual costs of carbon pollution control by the amount of carbon sequestered during a year (McPherson et al. 1994). The CITYgreen software program also assesses the air pollution removal capacity of urban forests for such pollutants as Nitrogen Dioxide (NO2), Sulfur Dioxide (SO2), Ozone (O3), Carbon Monoxide (CO), and particulate matter less than ten microns (PM10). Pollution removal estimates are based on USDA Forest Service research and models (McPherson et al. 1993). Stormwater ManagementAs in many areas of the country, stormwater management is a major concern in the Macon - Bibb County area. The flood of 1994 clearly demonstrated the problems of managing and controlling significant amounts of stormwater produced over a short period of time. Rivers were flooded beyond their capacities, buildings were damaged, and valuable riverfront property was lost. In the vicinity of the Spring Street bridge, the heavy rainfall and large amount of impervious surfaces in the downtown and Coliseum areas contributed to the loss of over four acres of riverfront property, as well as the flooding of Interstate 16 and Interstate 75. Effectively managing stormwater runoff is extremely expensive. But if left unmanaged, the costs of damage and cleanup from uncontrolled stormwater runoff can far exceed the preventative costs. After the 1994 flood, for example, millions of dollars were spent cleaning up areas along the Ocmulgee River and its tributaries. Trees have been shown to help control stormwater flow relatively inexpensively, especially when compared to the high costs of building and maintaining stormwater control systems. A moderately sized tree’s root system can absorb up to 400 gallons of water per day. A tree canopy also decreases the force at which water hits the ground, thus reducing the amount of erosion (particularly on slopes) during rainy periods (American Forests 1996). CITYgreen calculates stormwater management benefits using a hydrology model, commonly known as Technical Release 55 or TR 55, developed by the U.S. Natural Resource Conservation Service (NRCS). Using precipitation data, land cover percentages, hydrologic soil types, and slope information, TR 55 estimates runoff volume, percent change in time of concentration, and peak flow rates.
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