Undergraduate Capstone Project (Presentation) [UC Berkeley]
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Rainwater Savings Potential of Prototypical Green Roofs @ UC Berkeley Green roofs are vegetated roof installations comprised of various tiers that provide plant support, drainage, and waterproofing. Extensive green roofs may provide environmental and economic benefits for UC Berkeley and help achieve UC system-wide and campus-specific environmental standards and goals, particularly through water conservation. To assess their potential, this study compares the rainwater savings of prototypical modular extensive green roofs with that of a regular gravel roof. The measures used for comparison are water detention and retention. Simulated precipitation patterns modeled after a local 25-year rainfall event were applied to roof treatments, and runoff rates and volumes were collected and analyzed. Environmental Sciences provides broad, comprehensive education in the fundamentals of biology, chemistry, math, physics, and social sciences. The discipline involves the study of interactions between human activities and biological and physical environments on all scales, from the local to global.
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Undergraduate Capstone Project (Presentation) [UC Berkeley]
- 1. Rainwater Savings Potential of Prototypical Green Roofs at UC Berkeley David Shen 4/30/07 ES 196 Research Project
- 7. Green Roofs @ UCB vegetation - substrate - drainage - insulation - support - - separation - waterproof - separation introduction methods/ results conclusion what | green roofs/ form/ structure <epa.gov>
- 24. P < .0001
- 25. P < .0001
- 26. P = .087
- 27. P < .0001
- 28. P = .052
- 29. P < .0001
- 32. Rainwater Savings Potential of Prototypical Green Roofs at UC Berkeley David Shen 4/30/07 ES 196 Research Project
Editor's Notes
- Thanks… Good Afternoon. For my research project, I investigated the rainwater savings potential of prototypical green roofs at UC Berkeley.
- To begin: The what and why of this project.
- Green building is an innovative… By using a… green building confers more… than those of conventional building
- The practice is federally endorsed, promoted by NGOs, and adopted by local and state institutions. However, green building is still a fledgling enterprise due to a few barriers including insufficient research. My project contributes to the limited scientific knowledge of green buildings by examining an environmental benefit provided by a novel green technology in the context of a state institution. …
- Having multifaceted applications, that technology is green roofs.
- A green roof is a vegetated roof installation that is comprised of multiple layers that provide plant nourishment and support, drainage, and waterproofing--the existence and make of individual layers (notably the substrate) depend on various factors including plant species, roof stability, and on which of the following green roof types is being applied:
- There are two extreme types of green roofs, and they are distinguished by their apparent emphasis on either form or function, underlying design, construction, and operation costs; as well as primary benefits. They are either loose laid on rooftops by pre-fabricated and planted mats, or pieced together with modules. Generally, intensive green roofs are AKA “roof gardens”, while their extensive counterparts can be called “green/ brown roofs.” A modular extensive green roof was the focus of my project (as will be shown)
- Either type of green roof imparts more or less some or all these benefits.
- Architecturally and socially, green roofs provide aesthetically pleasing and eco-therapeutic places to look at and be in.
- Economically, green roofs help extend roof life and conserve energy. Environmentally, they help mitigate the urban heat island effect, restore wildlife habitat; and, purify, retain, and detain rainwater Green roofs’ rainwater detention and retention capabilities (specifically those of an extensive green roof) were the environmental benefits of interest in my research.
- They are defined as follows: Detention: ability to retard, or slow, runoff--measured as a rate of volume over time Retention: ability to withhold, or contain, water--measured as a percentage of total inflow (or proportion of net flow to total inflow)
- Why investigate those features? A national non-profit organization…the USGBC has created… LEED as a set of guidelines for the design, construction, and operation of green buildings, rating performance in five areas of human and environmental health (including): Sustainable site development, energy efficiency, materials selection, indoor environmental quality, and (water savings). A state educational institution, UC Berkeley has adopted these standards for its New Century Plan as a Strategic Goal for a Sustainable Campus.
- In its 2005 Campus Sustainability Assessment, UC Berkeley recommended the use of extensive green roofs to improve the campus’s water conservation--to reduce runoff from its increasing amount of impervious surfaces due to continual development. Why not?
- Though limited, recent research mostly focuses on: Rainwater savings of green roofs, because it is their most quantifiable and impactful environmental benefit and are better than that of standard gravel roofs However most studies have been done @… What about here?
- In an effort to judge its feasibility on campus, I quantified…vs. that of a standard gravel roof Furthermore, I hypothesized that... The extensive green roof will demonstrate slower runoff rates and withhold more water than the gravel roof during and after any given rainfall event characteristic of this region. That translates to how a green roof will produce more rainwater savings than a conventional roof (planted or not)
- Now, the when/ where, how, and what of my project.
- To meet my objectives and test my hypothesis, I conducted my study at the… for 2 months.
- During the experiment, 3 roof treatments (gravel, soil, and plant) were each subjected to 3 rainfall events (5, 10, and 15-minute 25-year rains) for (totals of 12 replicates per treatment, 12 replicates per rainfall event, and) a grand total of 36 replicates. The soil and plant treatments were alternated and rotated every other day to dry.
- All 3 roof treatments shared the common characteristics of area, drainage, and slope: Gravel treatment contained: ASTM C-33 compliant all-purpose gravel The other two treatments were contained within GreenGrid extensive green roof modules by Weston Solutions, Inc. and had one of the following: Soil treatment- a proprietary organic and inorganic media mixture Plant treatment- the mixture in addition to select sedum and native species supplied by Mountain Crest Gardens
- The rainfall was delivered… based on hydrological data. frequency: 25-year rain or a rainfall event that has a probability of happening once every 25 years--more years mean more intensity; buildings should withstand at least a 10-year rain; a 25-year rain was chosen as an overestimate (and provided suitable test conditions in terms of producible and measurable rainfall depth) durations...
- durations: selected based on a hydrologic time-of-travel equation relating time of concentration (duration) with hydraulic length and velocity; velocity held constant for paved slopes, hydraulic lengths, and hence durations, roughly reflect the range of roof dimensions on campus
- After setting the parameters for each replicate, …during and after rainfall with graduated cylinders and a stopwatch. computed runoff rates and % water withheld by a series of formulas in Excel… And analyzed significance using one-way ANOVA tests applied to these results Time-series, Box-Pierce, and Ljung-Box are more suitable, but ANOVA to simplify
- (The data points were the averages of replicate results.) As expected during and after the 5-minute rain, only the gravel treatment exhibited runoff - it started about halfway through the rainfall duration when the gravel treatment became saturated. However, both the soil and plant green roof modules did not produce runoff.
- Therefore, they were able to retain all of the rainfall while the gravel roof model did not. Over half of its rainfall ran off.
- Yet, an additional 5 minutes of rain generated runoff from all treatments. As for the 5-minute duration, the gravel treatment produced runoff once it reached saturation early on, but the soil and plant treatments did not do so until later on near the end of the rainfall. Even then, their runoff rates were lower than the gravel treatment’s. The plant runoff rate was slightly lower than the soil’s--but this relation was reversed during the 15-minute rain. After the rain, all treatments’ runoff rates decreased at similar rates and eventually converged to barely anything--and this trend was apparent after the 15-minute rain.
- Looking at retention during and after the 10-minute rainfall, both green roof treatments withheld more water than the gravel roof.
- Back to detention, the addition of another 5 minutes to rainfall duration produced similar but less pronounced results. All treatments still became saturated and produced runoff at the same point in time, but the plant treatment had higher runoff rates than the soil and even the gravel after the rain.
- Regardless, both green roof treatments still retained more water than the gravel one--though the soil treatment more so than the plant.
- Generally, my hypothesis was supported: green roofs save more rainwater than gravel roofs in terms of reduced runoff rates and decreased amount of water discharge An environmental benefit of green roofs has been proven but the economic cost of applying them is unaccounted and may or may not outweigh the benefits Study limited by: Small sample area and low amount -> exercise cautious when extrapolating results to actual roof sizes due to hydrological concerns that are beyond the scope of this project The daily watering regime might have caused temporal autocorrelation and biased the results Essentially, my study was a preliminary one at best. More in-depth research including economic analysis would further help justify the use of extensive green roofs to conserve water on campus and make it greener.
- Finally, I thank: a couple of faculty for their academic expertise, a couple of staff for their technical assistance, a contact at the green roof company, the ES instructors and GSIs for their guidance, and a photo-sharing website. Thank you.
- Questions?
- 2 special Sedum species/ 1 CA native General grid arrangement w/ native triangle @ center