A Comprehensive analysis of hydrological benefits of low impact development techniques: experimental investigation and numerical modeling

Abstract

Urban floods, recently increasing due to the combine effect of climate change and urbanization, represent a potential risk to human life, economic assets and environment. In this context, the traditional urban drainage techniques seem to be inadequate for the purpose, therefore a transition towards an innovative sustainable and resilient urban stormwater management is a valid solution. One promising strategy is the implementation of decentralized stormwater controls, also known as Low Impact Development (LID) systems that provide several benefits at multiple scales. Despite several studies demonstrated the LIDs’ capability in terms of surface runoff reduction, the transition towards a sustainable urban drainage system, which includes these techniques, seems to be very slow. One of the key scientific limiting factors can be found in the lack of comprehensive analyses able to highlight the hydrological performance and the physical processes involved in LID systems at multiple spatial scale and by considering long-term experimental data. The complexity of the physical processes, involved in each specific LIDs stratigraphy, requires modeling tools able to accurately interpret their hydraulic behaviour, as well as to correlate their hydrologic efficiency with the management of stormwater in the surrounding urban area. For these reasons, so far different empirical, conceptual and mechanistic models have been proposed, however in many of these studies, the hydrological parameters, as well as the physical ones were not properly investigated, limiting the analysis only to specific factors, or by considering literature values for the numerical modeling. Thus, principal aim of this thesis is to present a comprehensive analysis of the hydrological benefits of LID techniques by experimental investigation and numerical modeling. To achieve this goal, several analyses were carried out by considering different: LID systems, spatial scales, weather conditions, modeling investigation, as well as mathematical optimization approaches. Monitored data at the full scale implementation and laboratory measurements were used to support the numerical modeling. More in detail, first a global sensitivity analysis (GSA), based on the Elementary Effect Test (EET) was applied to a PCSWMM hydrodynamic model of the University Campus Innsbruck, which combines traditional drainage infrastructures and low impact development techniques, as Rain Gardens. In this regard, main findings have showed that soil hydraulic parameters considered in the model, (i.e., principally Soil Hydraulic Conductivity and Seepage Rate) were the most sensitive parameters. Therefore, the identification of these properties for LID systems is crucial in order to correctly evaluate their hydraulic performance. Starting from this finding the analysis of the hydrological efficiency of a full-scale extensive green roof, located at University of Calabria in Mediterranean Climate was assessed, by considering field monitored hydrological data, as well as soil hydraulic properties evaluated in lab, and a modeling analysis. Thus, first a field monitoring campaign for one year was carried out, and then hydrological performance indices on an event scale were evaluated. The findings have revealed the optimal behaviour of the specific green roof in Mediterranean climate, which presents an average value of Subsurface Runoff Coefficient of 50.4% for the rainfall events with a precipitation depth more than 8 mm. Later, to evaluate the influence of increasing values of substrate depths (6 cm, 9 cm, 12 cm, 15 cm) on green roof retention capacity, the hydraulic properties of the soil materials were first investigated in Laboratory, by the simplified evaporation method, and then considered for the implementation of the mechanistic model HYDRUS 1D. The results obtained in this phase have showed how the considered substrate depths were able to achieve a runoff volume reduction of 22% to 24%. Thus, as the outflow volume reduction achieved by increasing the soil depth was not significant, the ideal depth for specific soil substrate would be 6 centimetres. Following this study, and based on the findings obtained at building scale, next phase was focused on the analysis of hydrological effectiveness of Low Impact development solutions at largeurban scale in a south Italian case study. This investigation was carried out by considering different LID conversion scenarios by a predictive conceptual model (PCSWMM). In this regards, a specific permeable pavement and green roof, developed and installed at University of Calabria, were considered for the model implementation. Globally, modeling results have confirmed the suitability of these LID solutions to reduce surface runoff even if just a small percentage (30%) of the impervious surfaces is converted. By considering all of the findings, previous achieved by experimental and modelig investigation, it emerged that many aspects related to LIDs design and operation, as well as the choice of the facility and its location can affect the results in terms of hydraulic efficiency. In this regard, a mathematical optimization approach to consider several aspects together could be a suitable tool for designers of LID systems and experts in the field. Therefore, in the last part of the work, new Mathematical Optimization Approaches for LID techniques were evaluated. More in detail, the optimization of rainwater harvesting systems, by using TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) and Rough Set method as Multi-Objective Optimization approaches, was carried out. The results have demonstrated that these approaches could provide an additional tool to identify the ideal system. In conclusion, main findings of this thesis confirm the suitability of LID systems for urban stormwater management providing useful suggestions for their design and tools for assessing their hydrological effectiveness, analysing physical and hydrological parameters that affect their operation, introducing advanced concepts for the optimization of LID systems, therefore providing a significant and innovative contribution for the improvement of scientific research in the field and the spread of these sustainable techniques.