Webinar: Nature-based solutions for flood management

Louise Hartley, Senior Programme Manager at Earthwater Europe opened up the webinar today. She set the scene, stating how in the UK, heavy rainfall represents about one third of all flood risk. There are challenges in urban areas with impermeable surface areas such as tarmac and paved areas. Plus, climate change risk is likely to provide more frequent and intense extreme rainfall events, alongside population growth pressures.

We are looking for solutions, and there are growing acknowledgements for a combination of grey and green infrastructure, such as nature-based solutions, Sustainable urban Drainage systems (SuDs), and planting trees to manage growing risk of flooding. Trees, wetlands, bio-swales and rain gardens all provide opportunities for incorporating nature.

Earthwatch’s project on nature-based solutions for flood risk, involved engaging with volunteers and growing citizen science. They collaborated with 28 local research partners in 17 cities around the world looking at urban nature-based solutions.

Louise introduced Professor Wouter Buytaert from Imperial College London, who is one of the researchers Earthwatch collaborated with on the project. Wouter began by explaining the hydrological pathways of precipitation as it hits the surface or is intercepted, and the relation with surface runoff and flood risk in an urban landscape. Green elements can help minimise and slow runoff and flood risk. These include planting trees and vegetation to both directly intercept rainfall, as well as to take water out of the soils and contribute to evaporation of moisture. Secondly, soil can be viewed as a major reservoir to store water temporarily and allow percolation through to recharge ground aquifers. We are interested in the functioning of this conceptual store and how quickly it fills up. This is complex and will depend on the properties including soil moisture storage, porosity and rainfall event magnitude. This type of research is necessary to provide evidence into urban flood project design.

Wouter provided a sneak peek into the results of the soil storage capacity project, using 3 examples of urban green spaces, in London, Birmingham and Paris, France. He talked about how different management activities such as tree planting can influence soil properties and storage capacity. They statistically tested for differences in water storage capacity, and the ability to avoid surface runoff. Overall, unmanaged samples tended to have a higher water storage capacity than managed samples, indicative of less surface runoff. Reduced compaction of soil, and leaving leaf litter material in place, improved roughness and soil structure.

Next, Wouter presented results from some work on green roof technologies. Compared to conventional roofs that generate higher runoff levels, all 3 test green roof thicknesses showed a considerable reduction of runoff rates. However, he pointed out how although thicker green roof layers are better from a hydrological perspective as they store more water, we should consider the weight of these techniques and not create heavy features which could impact upon the building structure.

Green roofs have many benefits including reducing water runoff and helping reduce flood risk, but also help with thermal performance. They can act to cool down roofs during hot days and provide insulation capacity on cooler days.

The next presenter was Charlotte Markey, Green Urbanisation Innovation Manager at Polypipe. She mentioned how we should look to develop technologies that optimise our assets, including source control SuDs, smart technologies and climate change resilience.

Charlotte introduced the Polypipe capillary irrigation system, which is a series of cones inserted within the ground to wick water up towards green infrastructure features. This means water is stored in a below-ground system, providing a unique system to continue to irrigate below ground, reducing the need for above ground irrigation.

New technologies allow us to add green infrastructure to linear assets through retrofitting. We can model how water flows through a site, and remotely control and move water throughout SuDS features using smart technology.

Continuing from Wouter’s presentation on green roofs, Charlotte talked about the benefits of blue or green roofs, including enhancing biodiversity at rooftop level, storing water, rainwater harvesting, health and wellbeing. One of her projects, ‘Project Smartroof’ used basic, conventional roofs, installed permavoid (permeable systems), and showed how biodiversity improved, as well as doubling the attenuation capacity of the roof feature.

However, we must consider how maintenance can be complex, including access issues and upkeep. These issues such as engaging with local stakeholders and maintenance responsibilities need to be outlined in the design process of a project. If properly implemented, green roofs offer a chance for adaptive irrigation, and the opportunity to use water when we need it i.e. in summer to stop vegetation drying out. As well as this, Charlotte mentioned how sports pitches can be implemented on roofs. The use of blue/green roofs does not have to be fully planted, but can incorporate human use within nature-based solutions.

Charlotte finally talked about trees and implementing tree pits. Research was carried out to determine what soil type is vital to integrate with specific tree species. Then, they used the permavoid system for SuDs tree pits. This incorporates the needs of the tree, by making sure the soil is not compacted, and integrates a passive pipe system which can store up to 4 week’s worth of water the tree can use on demand. Tree pits act as a visual amenity as well as storing large quantities of storm water.

Flexible modular systems are key. It is importance to engage and integrate with other infrastructure and highways practices, and incorporate other construction which might be taking place at a similar time. This avoids implementing a nature-based feature that may become impacted or compromised by other construction in the near future. Furthermore, there may not always be appropriate space for interventions. We would benefit from more innovation and reduced maintenance needs.