Runoff comparison from extensive green roof: runoff measured by Stovin et Dunett [2007] and modelled runoff

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

0 100 200 300 400 500 600 700 800 900Time (min)

Rai

n (m

m/5

min

)

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0,2

Run

off (

mm

)

Rain

Measured runoff

Modelled runoff

Runoff quantity : May 1995, Ernage (Namur, Belgium)

Run

off q

uant

ity (m

m/h

)

05

10152025 Traditional roof

05

10152025 Intensive green roof

05

10152025 Semi-intensive green roof

05

10152025 Extensive green roof

05

10152025 Flat roof

28/5 29/5 Datum

1

012345 Traditional roof

012345 Semi-intensive green roof

012345 Intensive green roof

012345 Extensive green roof

012345 Flat roof

5/6

Runoff quantity : June 1989, Ernage (Namur, Belgium)

4/63/6 8/67/66/6 Datum

Run

off q

uant

ity (m

m/h

)2

Cumulative runoff comparison : May 1995, Ernage (Namur, Belgium)

0

5

10

15

20

25

30

35

40

28/5 29/5 30/5Datum

Cum

ulat

ive

runo

ff (m

m)

Traditional roofExtensive green roofSemi-intensive green roofIntensive green roofFlat roof

3Cumulative runoff comparison : June 1989, Ernage (Namur, Belgium)

0

5

10

15

20

25

30

35

40

3/6 4/6 5/6 6/6 7/6 8/6 9/6Datum

Cum

ulat

ive

runo

ff (m

m)

Traditional roofFlat roofSemi-intensive green roofExtensive green roofIntensive green roof

4

Roof storage systems: Modelling and performance’s comparison

E. Beckers1* and A. Degré1

1 Hydrology and Agricultural Eng., Environmental Science and Technology Department, Gembloux Agricultural University, 2 Passage des Déportés, 5030 Gembloux, Belgium (mail:beckers.e@fsagx.ac.be)

Introduction Rainwater runoff problems have become critical in large cities because of the increasing imperviousness of surfaces. Best practices to manage urban runoff improve water infiltration and evaporation through green areas establishment and, therefore, natural hydrological cycle restoration. Roofs, representing about 40–50% of the impermeable surfaces in urban areas, are a significant unused surface and could therefore represent a great tool for urban water management. The aim is to evaluate rainwater runoff from different types of roof storage systems and to conclude about their own interests to manage urban runoff production.

Model establishment

We established a model concerning three types of green roofs and a flat roof storage system based on a physical approach. The water balance’s terms were estimated: evapotranspiration depending on vegetation layer (sedums, grass, shrubs, trees…), runoff and interflows (via hydraulic conductivity, albedo)… The hourly meteorological data were recorded at Ernage (Namur, Belgium) and covered 7 years (1989-95). The

simulation gives the amount of runoff per hour for each roof and compares the result with a traditional roof. A first validation test compared results from the model and measurements on an extensive green roof in Sheffield, UK [Stovin and Dunett, 2007]. The results from our model coincide rather well with in-situ measurements.

Results

To illustrate existing differences between roof types, we compared each roof’s outlet during two rain events: a short intensive rainfall event (May 1995 [1]), and small successive rainfalls events (June 1989 [2]). Outcomes showed that the intensive and extensive green roofs caused some delays of the initial time of runoff and reduced or retained some rain events depending on the soil moisture level when it started raining. The semi-intensive green roof was less efficient. Flat roof reduced peak flows and distributed runoff over a long time period.

These behaviour differences appear more distinctly with the representation of the cumulative runoff along the time for both rain events. For May 1995 [3], differences between green roofs and flat roof storage systems are obvious. Flat roofs will generate runoff over the whole day while green roofs retain a relatively small quantity at the beginning of the rainfall. However, for the total quantity of runoff, intensive and extensive green roofs are the more efficient. They mainly reduce the runoff at the beginning of the rainfall event. For June 1989 [4] the curves tend to present a similar evolution. As a result, differences between roofs are less significant or even inexistent for runoff distribution. However, roofs are classified in the same order as before in the view of the total amount of runoff.

Conclusion

According to the model’s results, we can draw the first lines of a conclusion about peak flow reductions, initial times of runoff and annual runoff volumes reductions. Flat roof can reduce peaks flows and regulate rate of runoff over a long time period when the rain intensity exceeds its calibrated outlet. However, this roof type doesn’t delay the initial time of runoff and the annual runoff reduction is negligible compared to green roofs. Green roof’s performance is connected to water retention capacity of the substrate layer (porosity and depth) and rain event’s succession.

References: Allen R., Pereira L., Raes D., Smith M. (1998). Crop evapotranspiration - Guidelines for computing crop water requirements. F.A.O. Irrigation and drainage, paper 56. Dunnett, N., Kingsbury, N. (2004). Planting Green Roofs and LivingWalls. Timber Press, Portland. Guyon G. (1958). La méthode de Penman pour le calcul de l’évapo-transpiration. Ministère de l’Agriculture français, Direction Générale du Génie Rural et de l’Hydraulique Agricole, Section Technique de l’Hydraulique. Lazzarin R., Castellotti F., Busato F. (2005). Experimental measurements and numerical modelling of a green roof. Energy and Buildings, 37, 1260–1267. Maidment, D. [1993]. Handbook of Hydrology. MacGraw-Hill, New York. Mentens J., Raes D., Hermy M. (2005). Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscape and Urban Planning, 77 (3), 217-226. Revitt M., Ellis B., Scholes L. (2003). Review of the Use of stormwater BMPs in Europe. Middlesex University. Rezaei F., Jarret A. (2005). Measure and predict evapotranspiration rate from green roof. Transaction of the American Society of Agricultural and Biological Engineers, St. Joseph. Richards L.A. (1928). The usefulness of capillary potential to soil moisture and plant investigations. Journal of Agricultural Research, 37 (12), 719-742. Rosenzweig, C., Gaffi S., Parshall L. (2006). Green Roofs in the New York Metropolitan Region : Research Report. Columbia University Center for Climate Systems Research and NASA Goddard Institute for Space Studies. Stovin V., Dunett N., Hallam A. (2007). Green roofs – getting sustainable drainage off the ground. Techniques et stratégies durables pour la gestion des eaux urbaines par temps de pluie, 1, 11-18.