In one of Copenhagen’s western municipalities (Brøndby), climate adaptation is carried out with bright ideas. In the urban area “Horsedammen” a dual purpose carpark is part of the city’s climate adaptation. The project concerns one of several solutions, where 2,800 m3 of basin capacity is established, of which 800 m3 is located on terrain and in the permeable asphalt foundation. The retention basin receives water from two existing rainwater pipes. When these stormwater pipes are half full, overflow is directed into the basin. Thus it is not in operation for any rain event and a significant part of the particulate matter in the stormwater pipe is bypassed. The basin is in operation approx. 45 times annually.

The retention basin is built using plastic tunnel arches which achieve their strength through ballast stones around and above the arches. The ballast stones have a cavity of 30% and contribute to the storage volume. Below the entire construction a bentonite membrane preventing leakage to the ground water.

The uniqueness of this solution is the operational development of the plastic tunnel concept. Particulate matter is buried in a 25 m long concrete sand trap structure, dimensions 3 m wide and 2 m high. Furthermore, the sand trap structure acts as a manifold distributing the water into 18 rows of plastic arches. At the bottom of each arch a plastic base makes it possible to flush the basin. This ensures that it is possible to CCTV inspect the basin from the inside and clean it of particulate matter. The lightweight plastic arches and the plastic bottoms can be handled manually, providing an uncomplicated, easy and low-cost construction phase.

The ballast stones are conveyed and distributed by machines in a continuous process concurrently with placing of the arches/plastic base. The ballast stones between the arches and the NCC DrænStabil ® are two different textures which work well together. This ensures that the system has the intended bearing capacity and utilizes the cavity in the stone structure around both the plastic arches and in the subbase for the retention of water. The parking lot is lowered 10 cm and established as a flat area. It allows for further storage of water on the surface, without affecting cars parked on the area.

The solution is possible because permeable asphalt is selected to ensure the daily drainage of the area. The retention basin operates without mechanical installations as it gravitates to one of the existing stormwater pipes.