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Greening the city: The role of green infrastructure in the urban water cycle

Boek - Dissertatie

The rapid growth of cities around the world poses a lot of challenges to the environment. An important issue that comes with this growing trend of urbanization is the alteration of the hydrological cycle. Because of the high level of impervious surfaces in urban areas, processes such as infiltration, runoff, evaporation and interception of rainwater undergo drastic changes. An increase in runoff is observed, while evaporation, infiltration and interception processes are reduced. This not only causes floods and other water problems in the cities themselves but also to downstream areas located outside the city limits that cannot handle the excess of incoming rainwater. This PhD dissertation wants to contribute to a greater understanding of the role of vegetation in an urban context. Its main focus is on the role of green infrastructure (trees, shrubs, grass, …) in the urban hydrological cycle. Green infrastructure provides a wide range of ecosystem services, among which water retention is one of the more prominent ones. By intercepting rainwater on their leaf- and stem surfaces, the precipitation that reaches the ground is reduced. Because peak flows are buffered, drainage systems experience less pressure which results in fewer floods and other water related problems. Moreover, areas planted with green infrastructure provide pervious spots where overland runoff, produced on impervious surfaces, can infiltrate and reduce the often depleted groundwater table underneath cities. Green infrastructure consists of a mixture of green types. In the first part of this dissertation, the focus lies on small vegetation types, an often overlooked component of the urban green spectrum. Despite being smaller than trees, their influence on the urban hydrological balance can be substantial. To determine their impact on water retention, an ex-situ greenhouse experiment with a rainfall simulator was set up. Four small urban green species were saturated with rain and their interception storage capacity was measured. These findings were then correlated with vegetation characteristics to create multiple linear regression models. Biomass was found to be a good predictor of interception storage capacity potential, both for individuals of one species as for individuals of different species. Moreover, the rainwater interception potential of these small urban green species was found to be of similar order than the rainwater interception potential of trees, emphasizing their importance in the urban hydrological cycle. The second part of this dissertation centers around open-grown individual trees. In a rainfall catchment experiment spanning three consecutive years (2015-2017), a large V-catchment setup was built under a Norway maple and a small-leaved lime. The interception, throughfall and stemflow of both trees for rainfall events of different sizes and intensities were quantified. The created dataset was used as input for several hydrological models. The performance in predicting total interception storage was compared between the commonly used interception models of Gash and Rutter (Gash and Morton, 1978; a. J. Rutter et al., 1971) on one hand, and an adapted version of the integrated water balance model WetSpa on the other hand. Our findings suggest that the whole water balance model WetSpa performs similar to the interception only models of Gash and Rutter. Due to its holistic nature, WetSpa is recommended to make predictions about the impact of trees on the whole urban water balance. The final part of this dissertation looks at green infrastructure from an urban landscape scale. Instead of focusing on individual green elements, the effect of different spatial patterns of urban green in a landscape are investigated on the functional runoff connectivity, which is a measure of the ease of which water flows through a landscape. Through the use of a virtual runoff experiment featuring 100's of artificially created landscapes, the influence of 17 different landscape metrics on the functional runoff connectivity is assessed. The effective impervious surface area and the average impervious cluster size were found to be the most important variables that influence the functional runoff connectivity. The relationships of these variables with the functional runoff connectivity were modelled for urban areas with different drainage grid densities and percentages of green infrastructure abundance. The results showed that to reduce the amount of runoff, a decentralization of the water system is paramount. By creating small pervious spots with green infrastructure in the landscape, runoff can be intercepted near its point of origin. This way less pressure is put on the drainage system and the intercepted water can be used for other purposes. To conclude this part, a small case study on a subcatchment of the Watermaalbeek in Brussels was done and its results showed that the relationships found in the virtual experiment also hold true for a real world landscape. Overall, this dissertation answers several questions regarding the potential effect of green infrastructure on the urban hydrological balance. We conclude that by implementing more green infrastructure in the city landscape, substantial improvements in water management can be made. The hope of the author is that the information in this dissertation will help to create more awareness among the general population about the benefits of green infrastructure and stimulate adoption by policymakers and urban planners.
Jaar van publicatie:2020
Toegankelijkheid:Closed