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FINAL Green Roof Performance Study Seattle Public Utilities

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FINAL Green Roof Performance Study Seattle Public Utilities FINAL Green Roof Performance Study Seattle Public Utilities June 2012 Prepared by Cardno TEC, Inc. 2825 Eastlake Avenue East, Suite 300 Seattle, Washington 98102 This publication, methodology, and associated monitoring activities were funded in part through a King Conservation District Member Jurisdiction & WRIA Forum Grant. FINAL REPORT Green Roof Performance Study June 2012 EXECUTIVE SUMMARY Over the past ten years, Seattle Public Utilities (SPU) has considered green roofs one of its promising sustainable strategies for stormwater management within Seattle’s (City) highly urban areas. Given this, numerous questions have been posed regarding the benefit of green roofs as a stormwater management strategy in Seattle’s climate. For example, how effective are green roofs at reducing overall runoff volume and peak flow rates given our climate’s low-intensity rainfall patterns? And specifically, during the rainy season (November through March), can green roofs effectively reduce stormwater runoff volumes and peak flows entering into the City’s combined sewer system? To answer such questions, SPU managers identified the need to better understand how green roofs perform for stormwater control in the Seattle region. In turn, the City could further develop stormwater codes and appropriately focus incentive programs based on the stormwater benefit received through increasing green roof coverage across the City. In 2005, SPU initiated discussions to answer such questions. At that time, there were very few green roofs on public or commercial buildings in Seattle. In addition, regional research quickly revealed that green roof performance data for Seattle, as well as the Pacific Northwest region, was limited. Thus, SPU, in conjunction with the City of Seattle’s Office of Sustainability and Environment (OSE) embarked on the Green Roof Performance Study (GRPS). This study focused on developing a high-quality, continuous data set that can be used to understand the hydrologic performance of green roofs within Seattle. The first study objective was to identify monitoring guidelines to ensure consistent data collection for all hydrologic parameters of interest across all green roof monitoring sites. Both SPU and OSE worked closely with Cardno TEC, Inc. (Cardno TEC [formerly Taylor Associates, Inc.])1 to develop design guidelines for monitoring the performance of green roofs. The monitoring guidelines (Taylor Assoc. 2006) recommend a mass balance approach to measure the hydrologic (water quantity) performance of green roofs. In support of this approach, data parameters monitored included flow (runoff), irrigation input, and weather parameters including rainfall, air temperature, relative humidity, solar radiation, and wind. In addition to hydrologic parameters, the monitoring guidelines called for collecting flow-weighted storm event composite samples to characterize the water quality of green roof runoff. While writing the green roof monitoring guidelines, site selection was initiated through a rapid assessment process. Proposed monitoring sites were visited and site plans reviewed to determine the most feasible sites to collect cost-effective, quality data to meet the overall study goal. At the completion of the rapid assessments, five buildings were selected [Seattle Parks and Recreation’s Ross Playfield (Park) Shelterhouse, Woodland Park Zoo’s Zoomazium (Zoomazium), Seattle Public Library’s Ballard Branch (Ballard Library), City of Seattle’s Fire Station 10 (FS10), and City of Seattle’s Emergency Operations Center (EOC)]. Plans for adding a sixth building (Seattle Center’s 5th Avenue N. Parking Garage) were abandoned due to funding limitations. Upon selection, each site’s conceptual monitoring design (also developed 1 The Green Roof Performance Study was completed by Taylor Associates, Inc., which became the Water Resources Division of Cardno TEC, Inc., in October 2011. ES-i FINAL REPORT Green Roof Performance Study June 2012 during the rapid assessment process) was then realized by the Cardno TEC project team through collaboration with each site’s building owner, mechanical and electrical engineers, and architects and landscape architects. Data collection for the first of the five buildings began at the Zoomazium in early 2007. The monitoring system began testing in February 2007 and was officially brought on-line the following month. Two more green roof monitoring sites came on-line late in 2007 (Ballard Library in May 2007, Ross Park Shelterhouse in October 2007) and the fourth and fifth (FS10 and EOC) in May 2008. A total of five separate green roofs and two conventional (control) roofs were monitored across the five buildings. The monitored conventional roofs were located on the Ross Park Shelterhouse and EOC building. At the start of the study, the objective was to collect three years of data for each monitoring site and perform quality assurance and quality control (QA/QC) for each data set. In late 2008, project tasks had to be reprioritized based on changes in available project funding. At the end of the study, the following data collection and QA/QC was completed: A continuous, three-year data set was collected and QA/QC’d for three green roofs across three buildings; the Zoomazium, FS10, and EOC. A continuous, three-year data set was collected for one conventional roof at the EOC. (Note: Due to budget constraints, this study included QA/QC for only two quarters of the conventional roof flow data set.) Continuous, one-year data sets were collected for two green roofs and one conventional roof across two buildings; Ballard Library (one green roof) and Ross Park Shelterhouse (one green roof and one conventional roof). Data QA/QC was completed for three months of the 12-month Ross Park Shelterhouse data set. Data QA/QC was completed for three months of the 12-month Ballard Library data set. Thus, the data analysis results presented in this report includes data collected and QA/QC’d for the Zoomazium (one green roof), FS10 (one green roof), and EOC (one green roof and one conventional roof) monitoring sites. The SPU green roof modeling effort is currently in progress and expected to be complete later in 2012. Therefore, modeling results are currently not available for this report. Study Results The data analyses performed as part of this study provides a basic understanding of the hydrologic performance of the monitored green roofs in terms of runoff volume reduction and peak flow reduction. The methods for quantitative analyses are described and the results of those analyses are presented herein. A qualitative analysis discussion on (1) the significance of increased runoff travel time (lag time) and (2) the effects of antecedent moisture and rainfall event patterns on green roof runoff is also presented. ES-ii FINAL REPORT Green Roof Performance Study June 2012 Selected event rainfall and corresponding unit area peak runoff data from the EOC conventional roof were used to evaluate the effectiveness of the green roofs in reducing peak runoff rates. The median estimated peak flow reduction percentages are 53.3% (SD±7.8), 28.6% (SD±9.1) and 15.0% (SD±28.4), for the EOC, FS10, and Zoomazium green roofs, respectively. Runoff volume reduction on each of the greens roofs was evaluated on the quarterly basis and for selected events. This analysis was completed by calculating runoff volume as a percentage of rainfall volume over each of the green roof areas. The percentage of rainfall volume represented in the runoff volume varies between the green roofs. Generally, for all three green roofs, the runoff percentages increase during the wetter seasons approaching 100% and decrease during the dryer seasons to approximately 30% or lower. Each of the roofs produces a rainfall-runoff response reflective of their unique physical, hydrologic, and hydraulic characteristics and processes. The differences in performance are attributable to several variables including: differences in size, shape, and slope on the roofs, antecedent soil moisture conditions, magnitude and distribution of rainfall events, vegetative conditions, and runoff travel path. The effectiveness of green roofs for stormwater runoff control in the Seattle climate is mixed. The capacity of these systems to provide long term (seasonal) runoff volume reduction varies significantly depending on the season. During extended rainy periods, runoff volume control provided by these green roofs is limited due to the relatively thin soil profile, reduced evapotranspiration and the direct connection of the underlying drainage layer to the roof drain system. Effectiveness in peak flow reduction, however, can be significant even during extended rainy periods due to the increased runoff travel time compared to that of a conventional roof. Also, substantial recovery (drying out) of the soil profile even during the wet season can occur over a dry period of only a few hours. Therefore, peak runoff rate reduction for intermittent rainfall events is significant year around. The study findings presented reflect the stated purpose in the project scope of work. Additional future work that could be performed using and building on these findings include: Producing hydrologic model calibrations for each of the green roof data sets. Applying calibrated green roof hydrologic models in evaluating the usefulness of green roof systems in reducing basin specific confined sewer overflow occurrences. Using data and models to optimize green roof design parameters including size, shape, slope, soil depth,
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