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Abstract— Urbanisation has changed the vegetated pervious areas into impervious urban roof areas and increase water production and urban heat island ...
2012 IEEE Symposium on Business, Engineering and Industrial Applications

Green Roof As a Potential Sustainable Structure For Runoff Reduction

H. Kasmin and S. Musa Department of Civil Engineering and Environmental Universiti Tun Hussein Onn, Malaysia Batu Pahat, Johor, Malaysia

be installed on existing or new structures [3]. These roofs not only become a mitigation strategy of stromwater runoff but they also give other various benefits [5]. However, many consider storm water runoff mitigation to be the primary benefit of green roofs due to the prevalence of impervious surfaces in urban and commercial areas and failing storm water management infrastructure [6]. An estimated 14% of all flat roofs are green in Germany, a nation widely considered the leader in green roof research, technology, and usage [7]. Hence, focus of this study is to use green roof suitably for storm water management in Malaysia before it can proceed further.

Abstract— Urbanisation has changed the vegetated pervious areas into impervious urban roof areas and increase water production and urban heat island effects. In attempt to solve excessive water problems in urban area, green roof or vegetated roof structure as a part of Sustainable Urban Drainage System (SUDS) structure; has been introduced to replicate the pervious vegetated area on vacant urban roofs with role to control water quantity at source. This study examined the potential of green roof at retaining rainfall with less storage available than its maximum water capacity based on the antecedent dry weather period (ADWP) of less than 24 hours. The soil composition has permeability of 0.00011 cm/s with porosity of 55.1% that is equivalent to 44.1 mm available maximum storage from its total substrate depth of 80 mm. Result shows that events with ADWP more than 19 hours were 100% retained while events with ADWP less than 15 hours has shown various retention from 50 – 100% retention depends on total rainfall. Therefore this study has shown that one of the main factors that affected the retention percentage was the available storage within the green roof itself due to loss activities during dry days. Keywords-Green roof, water quantity; urban storm water management; sustainable urban drainage

I.

Green roof is a living vegetated ecosystem of lightweight soil and self-sustaining vegetation. It is biologically ‘alive’ and as such provides a protective cover on the building by using the natural elements of sun, wind, and rain to sustain itself [8]. Green roof consist of several layer that have its own specified functions. Green roof main components include protection layer, drainage layer, filtration layer, substrate or growth media layer and vegetation layer [9]. Selection of vegetation or plant materials can range from mosses, lichens and ferns, to sedums and other succulents, to grasses, herbs and ground covers [8]. Green roof have recently been recognized as potentially useful for local management of storm water [10]. One of the significant benefits of green roofs is reducing the volume of runoff thereby possibly reducing pollutant loadings. Studies have shown that green roofs can absorb water and release it slowly over a period of time as opposed to a conventional roof where storm water is immediately discharged [5].

INTRODUCTION

In developing countries such as Malaysia, the level of urbanization is still rising and expected to reach 83% in 2030 [1]. Cropland, grassland and forests are displaced by the impervious surfaces of streets, driveways and buildings greatly intensifying storm water runoff, diminishing groundwater recharge and enhancing stream channel and river erosion [2]. As additional impervious surfaces are created, there is an increase in storm water runoff and anthropogenic pollutants that are responsible for urban aquatic environmental problems [3]. Quantity and quality control at source in urban is one of main approach in stormwater management [4]. A stormwater management program consists of structural and non structural stormwater controls [3].

Any small reduction in runoff quantity from a green roof could still improve storm water runoff and surface water quality. In urban areas, rooftops provide large percentage of total impervious surfaces and provide unique opportunities for storm water management. Garden rooftops or green roofs have been used for many decades in Western countries as landscape areas and recently has been quite a number of studies done for storm water mitigation; however this idea on green roof for water mitigation is still new in Malaysia. Therefore in this study,

In urban areas, space is limited and full with buildings. Vegetated or green roofs use engineered growing media, drought-tolerant plants, and special roofing materials that can

Universiti Tun Hussein Onn Malaysia, Parit Raja 86400 Batu Pahat Johor

978-1-4577-1634-8/12/$26.00 ©2012 IEEE

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monitoring on rainfall runoff from a small scale green roof was done; to determine the potential of this structure for storm water management under Malaysian climates in term of quantity aspects. II.

METHODOLOGY

Data collection from two test rigs; the green roof rig and another empty test rig as a control was done within Universiti Tun Hussien Onn Malaysia area using two rain gauges (Figure 1); one as the rainfall data (from the control rig) and another one as the runoff data (from the green roof rig). Comparison was made between control roof and green roof.

Figure 4. The view of empty test rig and the green rig during plantation

(b) Figure 1. (a) Rain gauge; (b) Data logger

The roofs were constructed in 2009 with dimension of 0.6 m (W) × 0.7 m (L) × 0.15 m (H) following [11]. The roofs were used as a normal roof (control roof) and green roof with substrate and vegetation (Figure 2 & 3).

Figure 5. Details of the layers used in the green roof [11]

Figure 5 shows the layers used in the green roof model, using the simple basic layers such as ground cover vegetation (Pearl Grass), soil or substrate layer, drainage layer (geosynthetic and geogrid) and roof deck (concrete). It is expected that by using this simple basic layers, this green roof still has a good potential in retaining rainfall. The materials and layers in the green roof model are: i. ii. iii.

Figure 2. Plan view for the roofs [11]

iv.

Figure 3. Side view for the roofs [11]

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Roof deck – Concrete Drainage layer – 3 mm of geosynthetic Soil layer –was changed from 150 mm to 80 mm of plant soil depth Vegetation layer – Pearl Grass

delays of time between the start of rainfall to the start of runoff. TABLE I.

Figure 6. Pearl Grass

Figure 6 shows the vegetation for the green roof, Pearl Grass that easy to grow and has low maintenance. Pearl Grass can grow very fast and has been widely used for landscape purposes. In addition, Pearl Grass does not required special care, not susceptible to pests and also easy available locally. Whilst for the 80 mm depth of substrate layer, soil composition of 1: 3: 5 of sand: burn soil: and red soil was used. The water permeability of the soil composition is 0.00011 cm/s and porosity is 55.1%. For the drainage layer, there were two layers of geotextile used in the green roof. The first layer is (geosynthetic) non woven geotextile and the second layer is the Modular Atlantis 3 mm. The geotextile performs as drainage layer besides prevents the clogging and soil erosion. III.

RAINFALL RUNOFF DATA

Date

ADWP (day)

Rainfall (mm)

Runoff (mm)

% of retention

Lag time (min)

08/01/2011

3.2

3.2

0

100

0

12/01/2011

2.4

0.2

0

100

0

13/01/2011

0.9

0.2

0

100

0

14/01/2011

0.9

87.4

0.2

100

10

15/01/2011

0.9

24.8

1.6

94

5

17/01/2011

1.7

41.4

0

100

0

23/01/2011

3.0

2.8

0

100

0

24/01/2011

0.8

11.6

0

100

0

25/01/2011

0.9

0.4

0

100

0

26/01/2011

0.6

58.6

15.6

73

125

27/01/2011

0.5

2.0

0

100

0

28/01/2011

1.1

11.0

0.4

96

325

29/01/2011

0.4

190.6

128.2

33

990

02/02/2011

1.6

1.2

0

100

0

In the next comparison (Figure 7 and Figure 8), 24 hours of ADWP is chose based on the characteristics of percentage of retention within 6 weeks of monitoring. Although [13] and [14] suggested that 48 hours of ADWP could retained 5% and 27% of rainfall depth at their location, however [15] also suggested that these could be varied due to rainfall time series, evaporation rates of different climatic condition, the roof’s maximum retention depth and the relevant storage routing parameters. Therefore in this comparison, due to higher evaporation rates and temperature under Malaysian climatic condition, 24 hours of ADWP is initially chosen. However, further analysis is needed.

RESULT AND DISCUSSION

The experiment is still in a preliminary stage; hence ADWP is the main focus of this initial study on these two basic test rigs before further experiment could be done on the variation of parameters such as soil depth, types of vegetation and substrates. Table 1 describes all the storm events during the 36-days of study period. There were 15 storm events occurred within the 6 weeks of observation period. Storm event on 9th January was neglected due to data error from rain gauges. ADWP represents the length of time since the last recorded rainfall in a rainfall event with a mean intensity in excess of 0.6mm/hr [12], however in real practice, event start and end times for ADWP are not straightforward to define [13], total rainfall and runoff were collected using rain gauges, percentage of retention (%) is the percentage differences between total rainfall and total runoff and lag time is the

Figure 7. Intensity rainfall and runoff for storm events that has ADWP less than 24 hours

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Figure 7 shows the difference between total rainfall and total runoff. The difference in percentage describes the retention between rainfall runoff respectively. Almost all events occurred within ADWP less than 1 day had 100% retention except event 6 and 8 due to increment in total rainfall. Event 6 (26/01/2011) and event 8 (29/01/2011) show that the percentages of retention decrease due to less available storage less than 14.4 hours and 9.6 hours ADWP respectively. Although Event 2 has higher total rainfall intensity of 3.6 mm/hr than Event 6 of 2.4 mm/hr but with more available storage based on 21.6 hours ADWP for Event 2, green roof could effectively retained 100% of runoff. Event 8 also shows that green roof potentially has the capability to retain water more than 33% for higher total rainfall of 7.9 mm/hr (that is equivalent to 190.6 mm rainfall depth) if green roof is left about 9.6 hours without any rainfall events; considering the moisture has lost to higher evapotranspiration during dry days. Lag time of 990 minutes which equivalent to 16.5 hours may due to shower type of rainfall in the early of event, therefore this describes on how this green roof still have 33% of available storage though it has the highest of 190.6 total rainfall.

temperature. It could be suggested that with lower potential ET could reduce the available storage of the green roof.

Figure 9. Total runoff was positively correlated with total rainfall

Figure 10. The % volume retained decreased as a function of total depth of rainfall. Figure 8. Intensity rainfall and runoff for storm events that has ADWP more than 24 hours

These few examples of events show that the test rigs’s rainfall runoff performance was most likely influenced by ADWP (Figure 9 & 10). Assuming that within 24 hours is the most duration for the test rig to be completely drained off after any heavy rainfall considering Malaysia climatic condition with high temperature, all the 15 storm events were divided into two types of data; the events with less than 1 dry day (< 1 day) and the event with more than 1 dry day (> 1 day). Figure 9 and Figure 10 show the dependency performance of rainfall runoff on ADWP.

Rainfall events in Figure 8 suggested that for all events that occur on green roof after 24 hours of dry days has highly potential of 100% runoff retention. However, longer continuous monitoring is needed for significant results. Therefore this study has shown that one of the main potential factors that affected the retention percentage of a green roof could be the available storage within the green roof itself. Under Malaysian climatic condition which experiences equatorial climate with high temperature around 27-33ºC and wet months alternately all year has provided estimation of potential evapotranspiration (ET) using Thornthwaite formula at 4 mm/day. This potential evapotranspiration could be varied in different climatic condition depends on its

Figure 9 and Figure 10 describe positive correlation between total rainfall and total runoff where this relationship suggests that with ADWP less than 1 day, higher total rainfall will produce higher total runoff (with less retention) for the green roof while for ADWP more than 1 day, all runoff

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REFERENCES

production will 100% retain. This suggest that in any relationship between total rainfall and the volume of retention will shows that the % of volume retained decreased as a function of total depth of rainfall.

[1] [2]

In general, the relationship shows that as the total rainfall increasing, the percentage of the rainfall retention is decreasing. This is reasonable considering that the system may have saturated with the previous rainfall event. As indicated by the dashed lines in both figures, the ADWP for > 1 day under Malaysian climatic condition are predicted to have some trend lines as events of < 1 days ADWP once the storage capacity of the system has been exceeded. However, longer monitoring is needed for higher storm events to be collected, therefore could provide significant data for higher total rainfall with ADWP more than 1 day. IV.

[3]

[4] [5]

[6]

CONCLUSION AND RECOMMENDATION

[7]

Results showed that green roof test rig was satisfying retained storm water during the period of study. For all rainfall events occurred, green roof test rig had retained more than 50% of the runoff in which it suggested that green roof potentially could serve as effective tools for storm water management if proper design and planning is executed. Green roofs could be one of the structures that need to be considered when developing storm water plans in urban area using Sustainable Urban Drainage System that limit impervious cover and maintain the natural hydrology, water quality, water quantity and amenity of a site.

[8]

[9]

[10]

[11]

Lots of recommendation could be considered to overcome and improve the results of the green roof efficiency in storm water management for further studies; 1) longer data collection should be done continuously for similar total rainfall with similar ADWP could be compared; 2) lighter substrate should be used; 3) better rainfall runoff simulation model with additional of initial loss data on evapotranspiration should be considered therefore precise and effective results could be applied for more comprehensive and systematic tools; 4) understanding on how green roof could affect the climatic changes is needed; where replicating the natural green on wasted roof area might reduce the heat of the current urban roof for better future in urban area .

[12]

[13]

[14]

[15]

[16]

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