To people who have lived in hot climates, the idea of spraying the pavement to create a little fresh respite will sound like one of many familiar tricks to resist high temperatures. Nice is taking the idea a step further and testing how much of a cooling effect they can get with a special pavement that is soaked from below.

Buried pipes

Those familiar with gardening can imagine a system similar to drip irrigation, buried under the pavement and a layer of sand. When the system is active, the water is released and soaks the sand, which distributes it evenly across the surface, and finally evaporates at the surface of the porous pavement.

The team chose to use off-the-shelf material for the system, like pipes used for gardening or golf courses. They were also concerned with the sustainability of the solution and chose some circular options: they mixed the concrete base for the pavement with 30% crushed seashell and used raw water for the irrigation – water that is found in the environment such as rainwater, ground water, water from infiltration wells, or from lakes and rivers. The pavements were manufactured in the north of France, by the company Alkern.

All about precision

“The idea is to have a humid pavement,” explains Julien Grimaud, project manager at the Waste Management Department at SEURECA. “When the water goes through the pavement and evaporates, that’s what makes the temperature at the surface decrease. It’s better to have less water than too much. If you put too much water, it can be a drawback because it decreases the evaporation effect.”

This means that an important challenge for the testing of this solution is calibrating the water flow correctly. So, a big part of the job for the team in Nice is optimising the quantity of water to inject. “It’s a long process,” says Grimaud. “The water takes some time to get through all the layers. We are still in the process of learning and optimising.” Part of the challenge is that the needed volume of water depends on local climate conditions measured by the Universal Thermal Climate Index (UTCI) at a given time of the day.

Here is where monitoring becomes fundamental. In fact, a set of sensors collect data on temperature, air relative humidity, solar radiation, and wind speed and direction giving the team a direct overview on both the situation and the efficiency of the solution. It is based on this information that the team takes decisions on when to activate the system to have the most impact in terms of the cooling effect.

Remote control

“The idea was to have a completely stand-alone facility,” explains Grimaud. “We have climate sensors that enable us to calculate the UTCI, when the UTCI is above a certain value then the system starts sending water to the location. We have the capacity to have the system run 24/7 without any intervention from us.” The system can be triggered by the UTCI value – for example, Nice mostly set it to 30 or 35 degrees Celsius – or based on a programmed timeframe, or both.

Because the solution in Nice is still in its testing phase, the team can experiment to find the best balance between water volume consumption and the most efficient cooling results. Given the low number of users during night time hours, is keeping the system running during the night worth the extra water consumption, considering a lot of heat release happens during the night?

Careful perforation

Implemented in 2018 as part of the Covenant of Mayors project, the pavements in Nice were distributed along a 200-300m long avenue connecting the airport to the station, a strategic area for the city. “The whole district was planned to be renewed,” says Grimaud, “so the municipality and the urban planner added the urban heat island effect as part of the issues to tackle with the renovation. It’s also an avenue with a lot of passage, so a good test area.” Since this solution is underground and needs some construction work, it naturally helped to include it in a renovation project, saving on construction site costs.

“You have to be really careful when you implement the solution,” emphasises Grimaud. “Because all the hydraulic system is underneath the pavement, so you have to make sure the injection system is running correctly.”

Grimaud speaks from experience, as the testing area in Nice had a few mishaps, fortunately quickly identified thanks to the monitoring system. With a careful installation and a few precautions, like filtering the water to avoid residue clogging the pipes, the system will not need extra maintenance or operational costs, and the pavement itself is very robust, made to stand the test of time and many marching feet.

Crunching numbers

The frequency of passage was another reason for Nice to choose the airport avenue as a testing ground for their pavement. Based on simulations of thermal impact, wind, and impact of sunlight, the team selected five areas to fit with their special pavement. As waiting areas for pedestrians and public transport stops were found to be especially vulnerable to the heat, two areas of 100m² were installed under and around a bus and a tram stop along the avenue. Two more 100m² areas were created as a standalone and a bigger area of 600m² was also furnished with benches for people to rest on.

In 2019, Nice was able to paint an overview of the impact of the solution. The team recorded an estimated cooling effect between 3-5 UTCI degrees on certain days and a reduction of surface temperature between 5-10 degrees, with a water consumption of 2-3m³ per day distributed within the five areas. Considering that the system was, most of the time, set to trigger at the avenue’s busiest times – between 10:00 and 18:00 – and from 30-35 degrees UTCI.

Nice paired this innovative solution with a more common one too, spraying water on the road, as the team was interested to compare the performance of the two solutions. The response might be that cities might want to mix and match. “As the person stands directly on the pavement,” says Grimaud, “you have a more direct impact on the person’s temperature comfort, while spraying water on the road is more indirect, but it increases the fresh ‘bubble’ and the cooling effect overall.”

Out of testing, into the urban fabric

The next steps for the team in Nice is moving on from the testing phase and handing the solution over to the municipality to potentially scale it up in other areas of the city. And Grimaud has a last word of advice for cities working on similar solutions: “You should always, when you talk about urban cooling islands, think about a mix of solutions. It always has to be paired with green solutions, to really maximise the urban cooling effect.”