How retention basins saved Milan
The basin in Parco Nord is the first of four planned basins to be completed as part of a plan to contain flooding and overflows of the Seveso River, which have been flooding the Milanese neighborhoods of Niguarda, Pratocentenaro, Ca' Granda, Istria, Zara, Maggiolina and sometimes even Isola
On Sunday, March 12, 2023, at around 1:00 PM, the retention basin in Parco Nord was activated for the first time to contain excess water from the Seveso River. The basin, which was completed a few months ago, was opened due to the "dangerous" rise in the level of the river, which otherwise risked overflowing into the streets of the Niguarda district of Milan. When the basin was opened, the water level on Via Valfurva had reached 1.88 meters. As soon as it was activated, the water level stopped and then began to recede.
The Seveso River had been monitored since morning. The possible opening of the basin had been announced by the Councilor for Security of the Municipality of Milan, Marco Granelli. At around 9:30 AM, the river had reached the alert threshold of almost 2 meters (it overflows at around 3 meters). "In any case, the basin in Milan is ready to be activated, and this reassures us all," Granelli assured. "For the Seveso, the basin is ready to be activated, and when we get close to two meters in Milan, we will activate it. Unfortunately," Granelli added, "the basins upstream are not yet there and it is up to Milan to protect itself. We hope that the Lombardy Region and Aipo will move."
How the Seveso flood basin works
The basin in Parco Nord is the first of four planned basins to be completed as part of a plan to contain flooding and overflows of the Seveso River, which have been flooding the Milanese neighborhoods of Niguarda, Pratocentenaro, Ca' Granda, Istria, Zara, Maggiolina and sometimes even Isola.
In case of a weather alert, according to the procedures defined between the competent authorities, the lake is emptied in about three hours. If there is a risk of flooding, the lake is then filled with the excess water of the Seveso by opening the sluice gates, in a time varying between 3 and 10 hours. Before entering the basin, the river water is cleaned of branches and other materials thanks to a system of grids. The water will remain in the basin for the duration of the flood and will then be released back into the river, once the alert is over, by means of a pump system.
The basin, once emptied of river water, is cleaned of flood debris and refilled with clean groundwater, which is constantly agitated to oxygenate the water, prevent stagnation and the spread of algae. The entire cycle of emptying, cleaning the basin and refilling with clean water can take between four and six days, depending on the severity of the weather event. "For most of the year," the City of Milan assured, "the basin is a real lake suitable for the nesting and roosting of water birds, thanks to the positioning of special planted rafts, which are hospitable to wildlife. There will be green areas around the lake and along the banks."
The lake is adjacent to a forest that has been expanded with new trees. Cycle paths have been built around it, connecting it to Parco Nord and the city. All access to the lake is video-monitored to ensure the safety of citizens. In case of flooding, the administration concluded, a system of automatic barriers prevents access to pedestrians and bicycles, and provides an alert via variable message signs.
The Lambro River is also at risk of flooding. The communities present in the park have been evacuated. At around 1:00 PM, the Civil Protection of the Municipality of Milan warned the residents of the flood-prone areas, inviting them to be cautious, because the river had reached the alert threshold.
The weather forecast predicts that the rain will last the entire day. There could be a break in the afternoon, followed by more rain in the evening. The Natural Risk Monitoring Center of the Lombardy Region has activated a yellow weather alert for hydrogeological risk on the Milan hydraulic node. The alert will be in effect for 24 hours, from midnight on March 10 to midnight on March 11.
The successful activation of the retention basin in Parco Nord is a significant step forward in Milan's efforts to protect itself from flooding. The basin is a cost-effective and sustainable way to improve flood protection and provide a number of other benefits, such as creating a new habitat for wildlife and providing recreational opportunities for people.
