A Preliminary study of the Water RetentionPerformance of an ExtensiveVegetated Roof Using a Scaled Model at Lafayette College
How did this get started? Spring 2008: Kney! can we have a roof garden?? Pleasee?? Summer 2008:Excel research to identify possible roofs. Attend GRHC workshop “Green Roofs 101” Fall 2008: Select Acopian roof for the project, bring Spillman Farmer Architects into the project. Spring 2009: Select vegetated roof material based on architects recommendation. Preliminary design for the model in CE 351. Summer 2009: WOOT! We have a vegetated roof! Fall 2009: Preliminary data collection
Why Vegetated Roof? Filters water/air Retain water (reduce sewage system loads) Act as a rain water buffer Extend the life of the roof: UV and acid rain protection Lowers heating and cooling cost (acts as an insulator) Increased property value Stores carbon Creates a habitat Helps maintain healthy ecosystem commitment to sustainability/reducing footprint A pilot for future vegetated roofs new route for research which could combine different discipline: geo, bio, eng, econ. Easton has combined sanitary and storm water system, green roofs reduce run off into the system Increases the life of the roof Energy cost reduction due to better insulation Increased property value In General: For Lafayette:
Vegetative Roof System Vegetation Growing Medium Slab/Sheet rock Retention Fleece Waterproofing membrane Drainage Mat Insulation
The roof garden system used is a pre-vegetated mat from XeroFlor. The vegetation is primarily sedum and moss, which are both low lying durable plants. This is an ultra lightweight system, weighing approximately 12 lb/SF and only 3” thick. The waterproofing membrane will be a modified bitumen membrane. The system costs approximately $37/per square foot.
Preliminary Study Goals Get model up and running properly Gather data from at least 3 storm events Determine whether data seems reasonable Estimate retention for large roof based off data or other vegetated roof studies
Hypothesis Average Vegetated Roof Runoff < Average Precipitation By how much?? Other Expected Outcomes:
Runoff from green roof will form a smooth hydrograph
Runoff Peak is delayed slightly from precipitation peak
Methods: Construct model of roof Collected data from 10/14/09 - 11/14/09 (32 days) (however ..data collection is on going) Used RL-Loader 2.2.1 (a RainWise program) to export raw data into excel for analysis. Adjusted vegetated roof runoff based on an area ratio. The area of the vegetated roof = 16SF or 2304 in2 compared to the gauge 4 in diameter (50.265 in2). A ratio of .0218 was multiplied to the vegetated roof runoff data to normalize it. Compared runoff data and precipitation data with graphs and
Model A vegetated roof model was used to study the main roof because it was difficult to access and control the drainage on the main roof. Vegetated Roof runoff capture system with RainWise tipping bucket Adjacent rainfall Gauge RainWise tipping bucket
Model is made from recycled wood.
Model is sloped towards the 2 inch PVC drainage pipe
PVC pipe penetrates just above the waterproofing layer
Tipping buckets record how many tips there are in 10 second intervals (1 tip = .01 inches)
Data Summary There were 5 days with 100% retained The 17 days with no rainfall were not included in Vegetated roof runoff was less than precipitation in all rain storms recorded.
Data Summary Weather.com precipitation is on the same order as the rain gauge, the difference might be accounted for in the distanced between the gauges. Vegetated roof runoff is consistently lower than the precipitation
Comparing Our Results Lawerence Technological University: 1,000 SF green roof. retained an average of 68.24%over the course of 21 storm events. 9 were small, 8 were medium sized and 3 were large. Penn State University: detention and retention abilities of green roofs between July and October of 2002. 100% of storms less than 4mm(~.157in) were completely retained by the green roof. 20-25% of runoff is retained for storms around 25mm (~.984 in.) North Carolina State University: two different sites 2003-2004. The Goldsboro site had a 63% retention rate, while Raleigh had a 55% retention rate.
Conclusions Did the model meet expectations? Average Vegetated Roof Runoff < Average Precipitation
The difference in Lafayette’s results most likely due to the size of the storms being observed. It may also be possible that water is leaking.
Runoff Peak would require further study to determined if it is delayed slightly from precipitation peak
Due to issues setting up a procedure and constant adjustment to the model runoff collection system, not much data was taken. It is difficult to draw any conclusions with statistical confidence without further study. Is the roof reducing runoff? If the vegetated roof retained 58% of the total rainfall, I estimate it could hold 106 gallons for small (.5in) storms and up to 200 gallons for medium storms (1in).