Abstract
The rising urbanization causes more runoff of storm water and also a further increase of 3 to 6
% in precipitation is predicted. The local authorities of Amsterdam planned green roofs in the
city to reduce the water runoff. Green roofs are roofs covered with vegetation. Green roofs
could store and reduce the rainfall runoff and could therefore play an important role in urban
hydrology. The Polder roof contains an extra water reservoir to store more water. The aim of
this research is to establish the effect of roofs with different ages, slopes and designs on the
runoff. Normal roofs, traditional green roofs and Polder roofs with 0°, 0.9° and 5° slopes and
with young and old vegetation mats are tested in a rainfall simulator. The surface runoff,
drainage runoff and water level in the water reservoir are measured during the experiments.
The results show that the traditional green roofs without slope have a delaying effect on the
peak runoff by an average of 46.8%. The Polder roof seems to be an even better design of the
green roof concerning the buffering of rain water. The Polder roof ensures the most delayed
peak and a decrease in the peak flow of 28.5 % compared to the normal roofs. This was
expected, because the Polder roof has the largest capacity to store the water. Also expected
was that the old vegetation could store more water. The results of the runoff of the older
vegetation are lower than the young vegetation runoff except for one measurement, due to a
shortcoming in the method.