Parks’ potential for flood mitigation in a warming climate
Updated: May 19, 2021
Hyde park, one of Royal Parks in London, has always been one of my favourite places for enjoying nature in the city during weekends. I love walking through its green paths, discovering over 100 species of vegetation, and admiring its 3,174 trees (though I haven’t personally counted them all). As well as its widely appreciated amenities, it is also incredibly valuable for a lesser-known function - making London more resilient to flooding. In fact, Hyde Park alone can save up to £5,430 per year by reducing pressure on the local sewer system caused by stormwater runoff or, in other words, the water being drained away from non-permeable surfaces such as streets and buildings.
This is because leaves and branches can catch partial rainwater while green cover (i.e. grass lawn) in the park allows more water to seep into the ground. In the time between rainfalls, vegetation can restore the soil’s water retention capacity through a process called evapotranspiration, which refers to the total amount of water transferred back to the atmosphere from tree transpiration and from evaporation.
Figure 1. On a rainy day, part of the rainwater in a park (right) can be intercepted by leaves and branches, infiltrated into the soil, or transported through roots for plants’ growth and later moved back to atmosphere via evapotranspiration. In a typical urban (impervious) site (left), most rainwater flows into local sewer systems, which may overwhelm the drainage capacity and lead to urban flooding.
So if we could double Hyde Park by planting more trees or increasing green covers, resilience to surface water flooding would theoretically improve. But can this alone be an effective measure in London under a changing climate? In future climate projections under the highest emissions scenario (RCP8.5), the biggest issue faced by vegetation will be the potential severe drought during summer months, which is also the typical growing season for deciduous trees. Tree transpiration can reduce runoff by sending water back into the air as vapour, but if a plant continuously loses water through this process during hot and dry summers, this can lead to high plant water stress. Without proper management, this can result in drought-induced mortality, damaged plant growth, early leaf shedding (also known as leaf senescence), and consequently reduce vegetation’s role in runoff mitigation.
In my research within CAMELLIA, we identified that summer resilience under climate change can be markedly improved if urban green operates in an online fashion with the drainage system. If the surface runoff from the surrounding impervious area, such as roads, is conveyed directly into the green spaces before entering the local sewer system, the collected water irrigates the plants reducing its water deficit, especially during late summer. With less drought-induced stress, early autumn leaf senescence in a changing climate can be mitigated, in turn reducing the negative impact on plant transpiration and runoff mitigation performance.
Figure 2. Sufficient water is crucial for vegetation growth. In an environment under high vapour pressure deficit (i.e. less available water in the air in the form of vapour), a plant is likely to face a drought-induced problem – plant water stress. This can harm both stomata (physiological) and growing season and foliage density (phenological) activities, and finally limit plant evapotranspiration and parks’ overall performance in urban flood mitigation.
As one of the greenest capital cities in the world, London has great opportunities in utilising its current and potential green spaces as a sustainable measure in stormwater control. For the current existing green spaces such as Hyde Park, redirecting otherwise ‘wasted’ water into the green spaces before reaching the sewers would offer a plausible measure to sustain the park’s performance to collect surface runoff and to support plants’ water requirements during dry summers. We also recommend that planting high drought-resistance species of trees, implementing leaf cutting practices and improving the soil substrates would bring additional benefits to the integrated drainage approach, making Royal Parks and new green spaces cost-effective, efficient and amenable strategies for flood mitigation.
Do you still think Hyde Park is just a park?
You can read our paper on this topic at https://www.sciencedirect.com/science/article/pii/S0022169421001736?via%3Dihub.
Written by Ziyan Zhang