A GIS METHODOLOGY FOR ESTIMATING THE POTENTIAL OF AVALIABLE ROOF SURFACE FOR IMPLEMENTING EXTENSIVE GREEN ROOFS IN URBAN

AREAS OF DEVELOPING COUNTRIES

G. PrezA, D.E. PezB and M.E. BernalB A BUniversidad de Los Andes, Colombia. E-mail: g.perez1404@uniandes.edu.co

ABSTRACT. In the past decades urbanization processes have reduced permeable areas in cities thus generating higher runoff volumes and increasing urban heat island effect. Conventional urban drainage design approach aims at collecting and transporting storm-water as soon as possible therefore leading to flooding problems and large expenditures on infrastructure regardless the preservation and improvement of the environment. Given the considerable amount of sub-used roof area available in urban areas, in the last decade green roofs have been promoted as a suitable alternative to reduce storm water volumes in developed countries. In the last years a number of green roofs have been implemented in some Colombian cites however there is little knowledge about the optimal location of these technologies within a specific area or the potential of implementation of green roofs for areas with serious stormwater management problems. This study assessed a new methodology to determine the optimal areas for the implementation of extensive green roofs based on high resolution aerial photography and the derivation of a digital surface model (DSM) for a specific urban area with problems of insufficient rainwater transport capacity in the city of Bogota D.C (Colombia). INTRODUCTION Urbanization and the increase in impervious surfaces typically associated with urban development have consistently been shown to result in degraded urban environmental conditions and increasing storm water management problems (Miltner et al., 2004; Wang et al., 2001). A number of policy tools have been implemented to reduce the impact that impervious surface has in urban watersheds. One strategy is to place a limit on the amount of total impervious area (TIA) in a given watershed (Carter & Jackson 2007). Local governments of developed countries commonly execute this standard of maximum allowed impermeable area based on the application of models for quantification of social and environmental benefits due to best management practices (BMPs) implementation. BMPs include, among others, green roofs, soak-ways, swales, infiltration basins and ponds. In highly urbanized areas of developed countries as Colombia there is no current legislation to guide or promote the implementation of BMPs to solve problems related to urbanization processes as temperature increasing, worsening of urban landscape and occurrence of flooding due to intense rainfall events. In city centres, where access to green space is negligible, green roofs systems offer the possibility to turn to account areas that would otherwise be useless and thereby create space where people can rest or enjoy as a part of the urban landscape (Teemusk & Mander 2007). Commonly construction of green roofs involves four layers: drainage material, filter, soil substrate and vegetation. The thickness and the composition of the layer material as well as the vegetation type show great variation along the world according to the specific climatic and structural conditions of the construction site (Stovin et al., 2011). Green roofs are typically divided into two main engineering categories depending on the type of vegetation used: intensive and extensive. Intensive green roofs are established with deep soil layers; they can support larger plants and bushes and typically require more maintenance and watering. Extensive vegetated roofs are established with thin soil layers. They are planted with smaller plants which in the final stage are expected to provide full coverage of the vegetated roof. Extensive vegetated roofs are most commonly aimed to be maintenance free (Luo et al., 2011). Extensive green roof have been chosen as a popular technology that mitigates urban runoff, decreases temperature and provides an ecofriendly space in high populated urban centres (Lee et al., 2013). The main limitation for the construction of a green roof is the slope and load capacity of the building

deck. According to the construction industry research and information association (CIRIA) standards, extensive green roofs can be installed and survive successfully in buildings with percentage slope between 0 and 30% (Snodgrass & McIntyre 2010). The lack of South American, and more specifically Colombian, validated data and modelling tools to enable green roofs to be evaluated against alternative stormwater management approaches and perhaps most significantly the current lack of any methodology for designing and assessing extensive green roofs is one of the major constraints to the implementation of this eco-friendly technology in Colombian cities. This article provides a simple methodology based on aerial photography to estimate the feasibility of green roofs taking into account the slope percentage of outstanding buildings in Cedritos, Bogota D.C (Colombia). MATERIALS AND METHODS 1. Study site The neighborhood of Cedritos in Bogota D.C, Colombia was selected as the test site for the proposed methodology due to its current problems as a result of urbanization processes. Cedritos is located at the north east of Bogota D.C in the eastern part of the Andes mountain chain in the equatorial South America. The city average elevation above sea level is approximately 2550 [m]. The neighborhood of Cedritos is mainly a residential area located in the north part of the city, near to the Monserrate minor mountain change. During the last twenty years Cedritos has suffered a major process of urban re-densification which has significantly reduced impervious surfaces in the area. According to the latest reports presented by the local waste water management company (Empresa de Acueducto y Alcantarillado de Bogota, EAAB) storm water network in the area has collapsed requiring major investments to update the infrastructure (Correa & Rodriguez., 2012). Cedritos neighborhood consists mainly of residential and commercial buildings with an average height of 3 floors per construction. Cedritos is located in an area of upper middle class and is one of the most valued areas in the circle of real estate, its comprises 27 blocks with a total area of 500 000 square meters and more than 25 000 square meters in roof area distributed over 3000 buildings, which offers great potential for the implementation of extensive green roofs.

2. Materials In this methodology the percent slope of the buildings that comprise the study area is calculated based on aerial photographs obtained from an unmanned aircraft or drone eBee manufactured by senseFly . The percent slope is estimated by deriving the Digital surface Model (DSM) from the aerial photography of the study area by using the GIS technology (ArcMap 10.1 & ArcScene 10.1). The DSM of the study area was constructed based on flight lines to ensure overlap between photos of more than 70% which resulted in a raster grid of 8 cm of resolution. 3. Slope percent estimation In highly populated cities of developing countries with a great number of different urban typologies, such as Bogota D.C, it is not so simple to determine the potentially useful areas for the application of a given low impact developed technology as extensive green roofs. In this case study a new methodology to estimate potentially useful area for implementation of extensive green roof is proposed. In this methodology slope percent and roof area are used as the main variables for the selection of areas suitable for the construction of extensive green roofs. Detailed cartographic model showing the procedure used to estimate the percentage slope of study area buildings is shown in Figure 1.

Figure 1. Flowchart of the procedure for calculating percentage slope. Blue boxes represent Input Layers, orange boxes represent internal result layers and filled box represents final output layer Once the vector layer with the information of slope type and roof area a visual validation of the result can be carried out using the study area ortho-photo mosaic and Google Earth and Street view imagery. - RESULTS The methodology proposed in the present study can easily provide a classification of potential applicability of extensive green roofs for a specified area when aerial photographs with adequate pixel resolution are available. In Figure 2 a map with the results roof classification based on slope is presented for the study site of Cedritos.

Figure 2. Extensive Green Roof Assessment based on GIS analysis and slope estimation for the study site (Cedritos, Bogota D.C, Colombia). Based on the geographic attributes of the vector output layer feasible areas can be estimated as those whose slope percent is less than 5%. In the same way impractical and unfeasible areas can be determined as those with slopes between 5 and 15% and higher than 15% respectively. These areas are shown in detail in Table 1 along with the percentages according to the total area of roofs in the neighbourhood of Cedritos. Table 1. Feasible Green Roof Areas resume table.

Type of Area Area [ha] % of Total

Feasible area 10.83 41 Impractical area 5.58 21 Unfeasible area 10.16 38

Total roof Area 26.67 100 DISCUSSION A methodology to estimate the potential area for implementation of green roofs has been proposed and validated to the studied site area of Cedritos. Based on the results presented in Table 1 it can be concluded that the study area has a high potential for implementation of extensive green roofs (more than 40% of the roof area can be vegetated), however it should be noted that this analysis does not take into account other important factors such as the roof material and the dead load capacity of each buildings deck. If conservative retention factors are used to conduct preliminary analysis of the economic benefits associated with the implementation of green roofs as a tool for runoff reduction it can be prove that it would reduce from 30 to 40 percent of annual runoff volumes (Mentens et al., 2005).

Figure 3. Detailed analysis of feasible areas for implementing

extensive green roofs. A: base aerial photography of Cedritos at 16Mp resolution. B: Slope grade polygons for Cedritos buildings, green: slope