Bachelor Assignment
The influence of green roofs on the rainwater management system in an urban, tropical and undeveloped environment Herman van der Bent
Heavy rain events can have a big influence on urban environments due to the fact that cities have large impermeable areas. These impermeable areas collect rain water rapidly with, in some cases, flooding as a result.
h.s.vanderbent@student.utwente.nl
Period April-August 2009 Place San Pedro Sula, Honduras Supervisors Arcitect A. Stassano Plaza Comercial Bioclimatica, Honduras Ir. J.E. Avendano Castillo University of Twente
Solutions, besides a well maintained sewer system, can be to lower the amount of impermeable areas in the urban environment or use the impermeable areas more efficient. A design concept which uses this idea to use impermeable area’s in a city more efficient is the green roof engineering concept. A substrate layer is placed on top of a roof and plants are able to grow in it. This green solution has already proved to be a good design concept to lower temperatures inside a building in a tropical climate. It is said that the concept also has a water retaining capability which can help cities to prevent them from flooding. In this research this effect has been investigated for the tropical city of San Pedro Sula, Honduras. An inexpensive green roof design is used to assure a possible large scale implementation in this undeveloped urban environment. The research has been approached from a water content balance point of view, instead of using an empirical model, because of a relative short research period of only three months. Tests have been performed on four small test green roofs to be able to determine the water handling capacities of the green roof concept. The capability of green roofs to retain rain water is maximized by the evaporation rate and the used irrigation strategy. The maximum retention is determined at 5 mm of rain water because of a evaporation rate of 5 mm per day and an aggressive irrigation strategy which waters the plants after one day without precipitation. A second capability of green roofs to delay an amount of water after a rain event (so the sewer system has more time to manage the water) is also examined. Results show a capacity to delay an amount of water between 3 to 7 mm for several minutes. These capabilities have been compared with the data of precipitation rates between 1997 to 2006 in San Pedro Sula. 95% of the rain events has less than 40 mm of precipitation. These already deliver problems with the discharge, but the 5% of rain events with more than 40 mm of precipitation (and as shown in the table this can be way over 100 mm) have a really big social and economical impact. Comparing the rainwater handling capabilities of green roofs (5 mm retention, 3 to 7 mm delaying) with the heavy rain events result in the conclusion that the influence of green roofs on the water management system is quite small. Therefore a well designed sewer system stays a governing design solution to prevent cities from flooding.
Water content balance approach
Used test setup
Precipitation in heavy rain events