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 0 100 200 300 400 500 600 700 800 900 Time (min) Ra in ( m m/ 5 m in ) 0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2 Ru no ff ( m m) Rain Measured runoff Modelled runoff
Runoff quantity : May 1995, Ernage (Namur, Belgium)
R un off qua nt ity ( mm/ h) 0 5 10 15 20 25 Traditional roof 0 5 10 15 20
25 Intensive green roof
0 5 10 15 20
25 Semi-intensive green roof
0 5 10 15 20
25 Extensive green roof
0 5 10 15 20 25 Flat roof 28/5 29/5 Datum 1 0 1 2 3 4 5 Traditional roof 0 1 2 3 4
5 Semi-intensive green roof
0 1 2 3 4
5 Intensive green roof
0 1 2 3 4
5 Extensive green roof
0 1 2 3 4 5 Flat roof 5/6
Runoff quantity : June 1989, Ernage (Namur, Belgium)
4/6 3/6 6/6 Datum 7/6 8/6 Ru n of f qua nt ity ( mm /h ) 2
Cumulative runoff comparison : May 1995, Ernage (Namur, Belgium)
0 5 10 15 20 25 30 35 40 28/5 29/5 30/5 Datum Cu mu la tiv e ru no ff (m m) Traditional roof Extensive green roof Semi-intensive green roof Intensive green roof Flat roof 3
Cumulative 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 9/6 Datum C um ul at iv e ru no ff (m m ) Traditional roof Flat roof Semi-intensive green roof Extensive green roof Intensive 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.
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