University of Twente Student Theses


The influence of green roofs on the rainwater management system in an urban, tropical and undeveloped environment

Bent, H.S. van der (2009) The influence of green roofs on the rainwater management system in an urban, tropical and undeveloped environment.

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Abstract:The main research question was if green roofs are a good design solution to improve the water management system in an urban, tropical and undeveloped environment. To answer this question the water management capacities of an inexpensive extensive green roof design have been examined in the tropical city of San Pedro Sula. Due to the fact of a short research period the investigation has been approached from a water balance point of view. The boundary conditions tell us the total amount of water which can be handled by a green roof. For example the water handling capacity is about 32 mm of water for an extensive green roof with a 7.6 cm thick soil layer and the local used soil mix. This 32 mm of water is divided into three areas: The bottom capacity, the retention capacity and the drainage capacity. Out of these boundary conditions two main issues have been extracted. 1. The evaporation rate per day. This is the amount of water which leaves a green roof and therefore equals the retention capability when a new rain event approaches. The retention capability is limited by the fact that an aggressive irrigation strategy is used which waters the green roofs after one day without precipitation. The maximum amount of water which can be retained is therefore maximized by the evaporation rate of one day. This is about 5 mm of water. If a better plant type can be found which survives with a less frequent irrigation strategy the retention capacity can be a bit higher. 2. The second issue is the ability of green roofs to delay the discharge of water. This gives the sewer system more time to handle large amounts of precipitation. The boundary conditions maximize this drain capacity at 3.5 to 8 mm, depending on the size and slope of the roof. Half of this amount is delayed more than between 2 and 6 minutes depending on the slope of the roof. Other not examined configurations of green roofs leave space for different times of delay. These rainwater management capacities have been compared to the precipitation data of ten years in San Pedro Sula. These data were organized in small, middle and heavy rain categories. Small 0-20 mm/day 99 days per year Middle 20-40 mm/day 12 days per year Heavy 40 -200 mm/day 6 days per year The influence of green roofs with their maximum of 5 mm retention and the ability to slow down 2 to 4 mm for 2 to 6 minutes is a positive number, but certainly on the heavy rain events the influence is limited. It should not be considered that green roofs can totally solve problems with the discharge of precipitation. A well maintained sewer system stays a governing design solution.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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