Design of an Instrumented Model Green Roof

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Design of an Instrumented Model Green Roof Experiment Gregory O’Keeffe1, Kathryn Schulte2, Patricia Culligan3, Franco Montalto4 & Wade McGillis5 1

Graduate Student, Earth & Environmental Engineering, Columbia University, New York, NY Graduate Student & Student Member ASCE, Civil Engineering, Columbia University, New York, NY Professor, & Member ASCE, Civil Engineering, Columbia University, New York, NY 4 Post-Doctoral Fellow, Earth Institute, Columbia University, New York, NY 5 Doherty Research Scientist, Lamont Doherty Earth Observatory & Professor, Earth & Environmental Engineering, Columbia University, New York, NY 2 3

ABSTRACT: This paper describes the design, instrumentation, and data collection techniques developed for a research program to study model green roof performance as a pre-cursor to the design of a full-scale instrumented green roof. A model rubber roof and an instrumented non-vegetated model green roof were constructed and placed on the roof of the Pupin Building at Columbia University, New York City. Meteorological data and data pertaining to the storm water detention performance of the roofs were collected. Examples of these data, along with data describing the temperature and volumetric water content profile through the model green roof, are reported. INTRODUCTION Green roofs are roofs that have been modified to support plant growth. They typically consist of a waterproof membrane placed over the standard roof, on top of which are a drainage layer and several centimeters of lightweight (800-900 kg/m3 [1600-1825 lb/yd3]) growing medium in which diverse types of vegetation can be planted [Lazzarin et al. 2005). There are two main types of green roofs: extensive and intensive. Extensive green roofs are generally 10-16 cm (4�-6�) thick and planted with drought-resistant plants such as sedums. They are more common than intensive green roofs, which are deeper than 16 cm (6�) and can support more diverse plant life, including trees (which can be up to 10 m high if properly anchored), shrubs, and even some crops [Rosenzweig et al. 2005]. Extensive green roofs are more common than intensive green roofs due to their weight advantage; notably extensive roofs only have a maximum density of about 122 kg/m2 (25 lbs/ft2) when saturated with water [Rosenzweig et al. 2005]. Because intensive green roofs can only be used when the weight capacity of the roof structure is not exceeded, buildings must usually be designed to support an intensive green roof. Conversely, an extensive green roof can be applied to many existing buildings with few structural

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modifications. Extensive green roofs also require little maintenance, while intensive roofs require tending-to, and irrigation during periods of low rainfall [Lazzarin et al. 2005]. Green roofs are believed to provide numerous benefits, including mitigation of the urban heat island effect, improved building insulation and energy efficiency, increased biodiversity and aesthetic appeal, and storm water detention capacity. For cities with combined sewer systems, storm water detention by green roofs can lead to fewer combined sewer overflow events [VanWoert et al. 2005]. Although an estimated 15% of new flat roofs in Germany are green, the use of green roofs in the U.S. is quite recent. Hence, an understanding of green roof performance in cities like New York has yet to be developed. Even though several green roofs have been installed In New York City, the authors are not aware of any such roof that has been fully instrumented. This might be due to the fact that instrumentation for a full-scale green roof is expensive (upwards of $20,000.00 per instrument station), labor intensive, and timeconsuming. For this reason, the construction and instrumentation of scaled-down model roofs are an appealing way to (a) identify suitable instruments and calibration protocol for green roof monitoring and (b) obtain initial data to indicate how a full-scale green roof might perform. The work here involved the design and instrumentation of a model green roof and a model rubber roof, toward a goal of generating results that could be used to design an instrumentation system and data collection protocol for a full-scale, green roof. MODEL ROOF AND INSTRUMENTATION Two model roofs, with the same length to width ratio of a typical New York City brownstone roof, were constructed and instrumented. One roof is a model of a standard black rubber roof (Figure 1a); the other is a model extensive green roof without vegetation (Figure 1b). The decision to obtain data on the performance of a non-vegetated green roof, before gathering information on a vegetated system, was made because prior research had indicated that the performance of vegetated and non-vegetated green roofs were nearly identically with respect to storm water detention [VanWoert et al. 2005]. Hence, there was a desire to investigate whether this observation held true for New York City conditions.

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FIG. 1. (a) Model rubber roof ; (b) Non-vegetated model green roof .

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A HOBO® Weather Station was mounted between the two roof boxes. Several sensors, all manufactured by Onset Computer Corporation and described below, were mounted on the meteorological mast of the HOBO® Weather Station and placed in and around the green roof box. A more detailed description of the experimental setup follows. Location of Equipment Instrumentation The experimental setup is located on an approximately 2 m × 4 m (6′ × 12′) section of roof on the northwest corner of the Pupin building at West 120th street on Columbia University’s Morningside Campus in New York City. Data was collected beginning on October 24, 2006. The black rubber roof box, referred to as the “control” box because it models the behavior of a non-green roof, is lined with standard black roofing rubber and located to the west of the green roof box. A meteorological mast located between the boxes records ambient weather conditions. The sensors mounted on the mast are: (a) An anemometer recording wind speed, gust speed, and wind direction; (b) a Photosynthetically Active Radiation (PAR) sensor measuring solar radiation; (c) a tipping bucket rain gauge, and (d) a temperature/ Relative Humidity sensor Under the control box there is a second tipping bucket rain gauge recording runoff (i.e. outflow from the box) during rain events. A third tipping bucket rain gauge is mounted under the green roof box to record runoff from the model green roof. The decision to use of tipping buckets for measuring roof run-off was based on cost and compatibility with the HOBO® Weather Station data logging system. Other investigated alternatives, such as flow meters, were not only more expensive, their output signals were incompatible with the HOBO® setup. Two “temperature smart sensors” and two Echo volumetric water content sensors (termed “soil moisture smart sensors”) were placed within the green roof’s substrate. Data collected from these provide profiles of temperature and water content gradients through the growing medium. Figure 2 is a schematic cross-section through the nonvegetated model green roof box. The measurement interval for all instruments, including the tipping buckets, was set at one-minute.

FIG. 2. Schematic cross-section through non-vegetated model green roof: (1) Mesh to prevent wind-blown loss of substrate, (2) volumetric water content sensor (surface), (3) soil temperature sensor (surface), (4) volumetric water content sensor (base), (5) soil temperature sensor (base), (6) geotextile drainage liner, (7) rubber roof membrane, (8) plywood box.

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Model Roof Boxes Each box was constructed from 1.9 cm (¾�) thick pressure-treated plywood with rubber roofing material glued directly to the wood. The interior dimensions of the boxes are 122 cm × 61 cm (48� × 24�). The control box has sides that are 10.2 cm (4�) in height, while the green roof box has sides that are 19 cm (7½�) tall, so that it extends 10.2 cm (4�) above the surface of the green roof growing medium, which has a depth of 8.9 cm (3½�). There is a 5.1 cm (2�) interior diameter PVC drain, flush with the rubber roof surface, located in the corner of each box. The drains drop to 5.1 cm (2�) below the underside of the boxes. Each box slopes approximately 1% in the direction of the drain, which is the lowest point in each box. The performance of the model roofs is believed to be a good analogue for the performance of a full-scale roof provided that, for both model and full-scale roofs, flow through the green roof growing medium is predominately vertical, while flow through the underlying drainage layer is predominately horizontal and unrestricted. For a full-scale roof the latter criterion will depend on the roof design. Hence, it is difficult to generalize how well model roof data will indicate full system behavior. Flow Divider Due to the tipping bucket rain gauge capacity limitations (the gauge capacity is exceeded by flows greater than about 2.3 L/hr [78 oz/hr]), it was necessary to incorporate custom-built flow dividers in the roof drains to reduce the flow to the tipping buckets used to measure outflow from the control and green roof boxes. The flow divider was designed to divert a constant fraction of the outflow from a box away from the tipping bucket, and the remainder into it. The flow divider attaches directly to the outlet of the drain (Figure 3a). The runoff then passes through a runoff distribution medium to split the outflow evenly among the ten outlets, one of which flows into to the tipping bucket (Figure 3b). The flow dividers each divert about 10% of the flow into the rain gauge and 90% away from it. In the first version of the flow divider a sponge was used as the runoff distribution medium. However, it was found that the sponge clogged on a weekly basis. Hence, it was replaced by coarse aquarium gravel in February 2007.

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FIG. 3. (a) Flow divider cross-section: (1) PVC drain, (2) PVC flow divider, (3) runoff distribution medium, (4) brass outflow drains (receding from view); (b) Runoff flow divider on the Pupin building: (5) PVC drain, (6) PVC flow divider, (7) one outflow tube is directed into roof runoff gauge (gauge not pictured).

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Calibrations of Experimental Measurement Devices Calibrations for the surface and base temperature sensors in the green roof substrate were conducted in the following manner: A hot water bath was brought to the experiment site and set to cool for several minutes. The model green roof temperature sensors were then placed in the water bath alongside a factory-calibrated temperature sensor (“standard”). The sensors were left overnight while the water bath equilibrated with the ambient air temperature. The data for the surface and base temperature sensors were then plotted against the standard to obtain the necessary calibration curves. Calibration to verify the accuracy of the tipping bucket rain gauges used in the research was as follows: A known volume of tap water was introduced into each tipping bucket over a two-minute interval, and compared to the volume recorded by the system. The procedure was repeated three times for each bucket, and the values were averaged. The results are presented in Table 1. Note: the slight discrepancy between the introduced and recorded volumes is due to losses inherent to the tipping bucket rain gauge design (i.e. water sticking to surfaces). Such losses (