Annual and Individual-Storm Green Roof Stormwater Response Models Albert R. Jarrett Professor of Agricultural Engineering Penn State University arj@psu.edu www.abe.psu.edu/fac/jarrett.html D. J. Beattie, R. D. Berghage, F. Rezaei
Green Roof Types xtensive large areas Intensive roof gardens
Typical Extensive Green Roof Vegetation Growth Medium Drainage Layer Waterproof Membrane and Root Barrier
Drainage Layer ½-inch plastic wire grid. Geotextile glued to grid. Porosity = 78% Field Capacity = 5%
Media 12.5% sphagnum peat moss 12.5% coir 15% perlite 60%hydrolite 6.23 lb/in-ft 2 Saturated Porosity = 55% Field Capacity = 34%
Green Roof Plants Crassulacae Acid Metabolism (CAM) Plants Arid Region (Desert) Plants Stomates open at night (to reduce water loss). Stomates close during day (to conserve water). Plants get smaller as water becomes limiting.
Benefits Attenuate stormwater Runoff volume reduction. Peak runoff rate reduction. Delay the runoff event. Delay the peak runoff rate.
Impact of Development Flow Rate (Cfs) 200 180 160 140 120 100 80 60 40 20 0 Post-Development Runoff Pre-Development Runoff 0 2 4 6 8 Time (Hrs)
Green Roof Stormwater Management To understand how a Green Roof can help us manage stormwater, we must look at stormwater in three different ways: Individual storms (actual data collected). Annual rainfall basis. Extreme storm effects.
Individual Storm (Measured Data) Green roofs compared to non-green (traditional) roofs Stormwater runoff volume and flow rates
October 25, 2002 Rain Event Depth (in) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Retained = 0.18 in Rain Green Roof Runoff 0 6 12 18 24 Time (hr)
October 2002 Days Green Since Roof Delay to Storm Last Rainfall Runoff Start of Date Rain Depth Depth Retention Retention Runoff (Days) (in) (in) (in) (%) (hr) 16-Oct 3 1.15 0.79 0.36 31.3 1.0 25-Oct 8 0.94 0.76 0.18 19.1 4.0 Average = 1.045 0.78 0.27 25.23 2.50
November 2002 November 2002 7.75 42.58 0.31 0.50 0.81 Average = 2.0 34.6 0.54 1.02 1.56 4 16-Nov 4.0 33.3 0.15 0.30 0.45 1 12-Nov 18.0 69.0 0.29 0.13 0.42 5 10-Nov 7.0 33.3 0.27 0.54 0.81 4 5-Nov (hr) (%) (in) (in) (in) (Days) Runoff Retention Retention Depth Depth Rain Date Start of Runoff Rainfall Last Storm Delay to Roof Since Green Days
May 2003 May 2003 6.00 48.35 0.28 0.37 0.65 Average = 5.0 32.4 0.36 0.75 1.11 4.1 31-May 9.0 58.3 0.28 0.20 0.48 1.1 25-May 4.0 54.3 0.19 0.16 0.35 0.8 23-May (hr) (%) (in) (in) (in) (Days) Runoff Retention Retention Depth Depth Rain Date Start of Runoff Rainfall Last Storm Delay to Roof Since Green Days
June 2003 June 2003 2.88 53.13 0.20 0.24 0.44 Average = 0.5 11.1 0.02 0.16 0.18 1.4 21-Jun 0.5 69.2 0.18 0.08 0.26 1.4 19-Jun 12.0 49.0 0.49 0.51 1.00 3.0 17-Jun --- 100.0 0.19 0.00 0.19 2.0 14-Jun 1.0 85.7 0.12 0.02 0.14 3.4 12-Jun 0.5 60.9 0.14 0.09 0.23 1.4 8-Jun 3.0 39.7 0.27 0.41 0.68 2.2 7-Jun 0.5 33.3 0.05 0.10 0.15 0.5 4-Jun 5.0 29.2 0.33 0.80 1.13 2.0 3-Jun (hr) (%) (in) (in) (in) (Days) Runoff Retention Retention Depth Depth Rain Date Start of Runoff Rainfall Last Storm Delay to Roof Since Green Days
July 2003 July 2003 2.28 77.76 0.40 0.22 0.63 Average = 4.0 51.4 0.71 0.67 1.38 3.8 31-Jul 1.0 94.9 0.37 0.02 0.39 5.7 27-Jul 0.5 90.6 0.48 0.05 0.53 0.4 22-Jul 0.5 87.5 0.42 0.06 0.48 2.5 21-Jul 2.5 88.2 0.30 0.04 0.34 5.0 16-Jul 1.0 32.1 0.52 1.10 1.62 3.5 10-Jul 4.0 75.0 0.09 0.03 0.12 0.3 7-Jul 5.0 95.5 0.64 0.03 0.67 4.0 6-Jul 2.0 84.6 0.11 0.02 0.13 12.2 2-Jul (hr) (%) (in) (in) (in) (Days) Runoff Retention Retention Depth Depth Rain Date Start of Runoff Rainfall Last Storm Delay to Roof Since Green Days
Summary of Retention Results Retention (%) 90 80 70 60 50 40 30 20 10 0 77.8 53.1 48.4 42.6 25.2 Oct Nov May June July
Modeling Annual Green Roof Impact Assumed Simple ET curve; f(month) = daily ET Drainage layer, roof media, and plants had maximum of 1.582 inches of retention storage. For each daily rain amount Estimated water lost to atmosphere. Estimated water remaining in green roof. Added rain to green roof.
Schematic of Green Roof Rain Evaporation Transpiration Max. Retention = 1.582 in (3.5 in)
1986, GR Max Retention = 1.582 in 3.0 2.5 Rain Runoff Depth (in) 2.0 1.5 1.0 0.5 0.0 1/1 3/2 5/1 6/30 8/29 10/28 12/27 Date
1993, GR Max Retention = 1.582 in 3.5 3.0 2.5 Rain Runoff Depth (in) 2.0 1.5 1.0 0.5 0.0 1/1 3/2 5/1 6/30 8/29 10/28 12/27 Date
Modeling Results Based on 28 years (1976-2003) of rain data from State College, PA Roof maximum retention = 1.582 inches Average annual rain = 40.3 inches. 22.9 inches if average annual rain was retained on Green Roof Average annual rain retained = 65%. This captured water is returned to the atmosphere by Evaporation Transpiration
Extending the Annual Model If we assume the roof in question has maximum retention capacities different from our green roof, Retention values from 0.125 inches to 3.0 inches were evaluated.
70 Annual Rain Retained (%) 60 50 40 30 20 10 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Maximum Roof Retention (in)
Stormwater; ; Extreme Storm Response Stormwater Management is based on extreme hydrologic events. 2-year rainfall/runoff. 25-year rainfall/runoff. 100-year rainfall/runoff. How well can a Green Roof be expected to attenuate these large design storms?
Extreme Storm Response Modified Puls Routing Model Accounts for capillary & hygroscopic water stored in: drainage layer 3.5-in media CAM plants Accounts for 0.05 in interception. Blaney-Criddle ET Model applied on a daily basis to remove water from roof. Rain must fill all layers to field capacity before runoff starts.
Extreme Storm Response Model Validation. Inputted rainfall hyetographs for 16 storms measured at Rock Springs. 24-hour rains varied from: 0.14 in to 1.62 in Averaged 0.84 in Computed runoff from roof. Compared to measured runoff.
Green Roof Runoff, ver 5 1.4 Predicted (in) 1.2 1.0 0.8 0.6 0.4 0.2 R 2 = 0.930 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Observed (in)
Peak Runoff Rate, ver 5 0.4 Predicted (in/hr) 0.3 0.2 0.1 R 2 = 0.907 0.0 0.0 0.1 0.2 0.3 0.4 Observed (in/hr)
Start of Runoff, ver 5 Predicted (hrs) 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 R 2 = 0.938 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Observed (hrs)
Extreme Storm Response Applied synthetic rainfall to Green Roof. Type II Rainfall Distributions. Assumed State College, PA Assumed Green Roof had same characteristics as our research roofs. Simulated the Roof s runoff hydrographs for a 270- by 70-foot Roof: 1 day since last storm in February and July. 5 days since last storm in February and July. 2-Year Synthetic Rain. 100-Year Synthetic Rain.
2-Year Rain After 5 Dry July Days 2.5 2.0 Flow Rate (cfs) 1.5 1.0 0.5 2-yr Pre-Dev, CN = 79 Rain Roof Runoff 0.0 10 11 12 13 14 15 Time, t (hrs)
Cumulative Green Roof 2-Y After 5 Dry July Days Cumulative In & Out Flow (In) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Rain Roof Runoff 0 5 10 15 20 25 Time (Hrs)
100-Year Rain After 5 Dry July Days Flow Rate (cfs) 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 100-yr Pre-Dev, CN = 79 10 11 12 13 14 15 Time, t (hrs) Rain Roof Runoff
Cumulative Green Roof 100-Y After 5 Dry July Days 6.0 Cumulative In & Out Flow (In) 5.0 4.0 3.0 2.0 1.0 0.0 0 5 10 15 20 25 Time (Hrs) Rain Roof Runoff
100-Year Rain After 5 Dry Feb. Days Flow Rate (cfs) 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 100-yr Pre-Dev, CN = 79 10 11 12 13 14 15 Time, t (hrs) Rain Roof Runoff
Cumulative Green Roof 100-Y After 5 Dry Feb. Days 6.0 Cumulative In & Out Flow (In) 5.0 4.0 3.0 2.0 1.0 Rain Roof Runoff 0.0 0 5 10 15 20 25 Time (Hrs)
Summary of Extreme Storms 2-Year (2.6 inches in 24 hours)) July w/ 5 Dry Days = 85% reduction in peak. July w/ 5 Dry Days = 61% reduction in volume. 100-Year (5.3 inches in 24 hours) July w/ 5 Dry Days = 60% reduction in peak. July w/ 5 Dry Days = 30% reduction in volume. Rain water retained and detained Detained water released after storm
Thank You arj@psu.edu www.abe.psu.edu/fac/jarrett.html Questions?