Evaluation of two stormwater infiltration trenches in central Copenhagen after 15 years of operation.

Two stormwater infiltration trenches were installed in 1993 in an area in central Copenhagen. The system was monitored continuously for almost three years after establishment, and a small reduction in performance over that time, possibly due to clogging, was noted. A new study was conducted in 2009 to see whether the reduction in performance has continued and to determine how the system performs today. Water levels in the trenches were monitored for almost 4 months, and from this period seven events were selected to analyse the infiltration rate. A comparison with similar analyses on storm sequences from the first 3 years of operation shows that the infiltration has decreased since the establishment of the system 15 years ago. The decrease is statistically significant (p<0.01). A clogging model was fitted to the data and predictions were made for future performance. The results show that the system will discharge around 10 times more annual overflow to the sewers after 100 years of operation compared to the initial volumes, if clogging continues at current rates. This corresponds to 60% of the total runoff from the area. The results show that clogging and proper maintenance are important factors to consider when implementing stormwater infiltration trenches.