Retenční nádrž Interlov

“Lost rivers whisper tales,” an old Prague proverb claims. This proverb perfectly describes the story of Retenční nádrž Interlov. Retenční nádrž Interlov sits at the edge of Kunratický Forest. It was originally built in 1980 as a dry retention reservoir. This means it was designed to hold floodwaters, not maintain a permanent pool. Its purpose was to protect the lower part of Kunratický potok and Braník from flooding. Retenční nádrž Interlov was a key part of the rainwater drainage system for Jižní Město II. This included a network of rainwater sewers leading to a sedimentation reservoir. Here, the water was cleansed of larger impurities before flowing into the retention reservoir. The dam, 4.5 meters high and 83 meters long, could hold an impressive 16,399 cubic meters of water during floods, covering an area of 12,559 square meters. The reservoir was created by damming the valley of Roztylský potok. The original springs were already controlled by the rainwater sewer system. Therefore, the stream now begins just above the reservoir. Retenční nádrž Interlov underwent a major revitalization in 2016. The goal was not only to maintain flood protection but also to enhance water retention in the urban landscape. The project aimed to integrate Retenční nádrž Interlov into the Kunratický Forest complex. It also sought to create an additional water area. So, Retenční nádrž Interlov is no longer a dry reservoir. It has been transformed into a classic retention reservoir, a permanent body of water. The revitalization involved removing the original concrete channel lining and an asphalt road with a concrete base up to 1 meter thick. To maintain the reservoir’s retention volume while creating a water area, the space for the new water surface was excavated. Unexpectedly, a large quantity of hidden concrete objects were unearthed, remnants of the dry reservoir’s construction. Due to the pervious subsoil, the bottom of the future water area required extensive sealing. A sealing trench of clay material was constructed around the perimeter. The bottom was sealed with separation geotextile and layers of compacted clay, then covered with gravel. The combined structure of the safety spillway and outlet also received attention. The old, unsuitable steel parts were removed, and the structure was surrounded by quarry stone. A new stone staircase replaced the original steel bridge, providing access to the safety spillway from the crest of the dam. In 2021, the left abutment was repaired to prevent undermining the dam’s body. In 2023, the outlet was repaired and faced with stone. A new railing was added. Damaged concrete structures in the outlet were removed, and the entire outlet was rebuilt with boulder riprap set in concrete. Roztylský potok was revitalized between the sedimentation reservoir and the retention reservoir dam. The original straight channel, lined with unsightly concrete panels, was transformed into a more natural streambed stabilized with boulder riprap. Several small thresholds were created to vary the flow of water. Downstream from the dam, where the stream flows through mature forest, the channel remained in its original course but was stabilized with stone sills. The revitalization improved the stream’s ecological significance. The Retenční nádrž Interlov stands today as a testament to Prague’s dedication to balancing urban development with natural preservation.