With rapid urbanization, Low Impact Development (LID) is promoted as an alternative to Conventional Drainage (CD), seeking a natural solution for current urban water problems. The positive effects of LID were the main theme of recent LID researches, but this project aims to deeply explore the hidden troubles about the extreme peak runoffs influenced by LID city on catchment scale.
In this research, the SUPERFLEX conceptual model was adapted to a rural-urban semi-distributed model to simulate the current rainfall-runoff relationship of the catchment where San Antonio city is located in. Besides, the model expressions of 4 representative LID practices (bioretention cells, vegetated swales, green roofs, and permeable pavements) were devised under SUPERFLEX framework. To deal with the prediction uncertainty, three urban development scenarios in 2040 and five LID implementation scenarios were designed for San Antonio city. And their influences on the basin peak runoffs will be quantitively studied.
Research result shows that, the urban runoff tended to swing between extreme flood and extreme drought in the reference situation; And next, the infill urban development strategy, which means developing the vacant or undeveloped land within an existing community, was more helpful on peak runoff and total runoff volume control than sprawl urban development strategy with the same population growth; Thirdly, the bioretention cells, vegetated swales, and permeable pavements had similar good performance on peak runoff reduction, which can be mainly ascribe to the stormwater infiltration process. As for the retention of total runoff volume, the bioretention cells, permeable pavements, and green roofs perform better than vegetated swales since the rapid water transportation character of vegetated swales decrease the water residence time for infiltration;
The runoff reduction function of LID practices performs effective on the large peaks in dry and normal seasons, but it will be restrained significantly in flood season. According to model result, the rural peak runoffs happened 6.5 to 15.5 hours after the urban peaks. And for 4 LID implementation scenarios in which 15% of urban grey areas is covered by LID practices, the urban peaks are delayed between 0.5 and 2.5 hours. And for the scenario with the LID cover areas as 50% of the urban grey areas, the time lag of urban peaks varies from 0.5 to 6.5 hours. For this scenario, since the obvious time delay of urban peaks, more stack of urban and rural peaks is caused by the time approaching of urban and rural peaks, which causes the increases of two total basin peaks in flood season from 3.57 to 3.65 mm/d and from 6.35 to 6.47 mm/d respectively.
In conclusion we may say that the stacking effect of LID implementation on total basin runoff is limited in the case of San Antonio basin, partly due to the fact that only a small part of this basin is urbanized.
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The report starts in Chapter 1 with an introduction to the Sponge City Programme (SCP) in China and the project area which is the ErQi International Business District in Wuhan. In this chapter, the problem statement, our collaboration with Arcadis and our project goals are also introduced. Chapter 2 delves into our methodology to tackle the brief. Starting from how we shaped our interdisciplinary approach, we explain our approach towards the project and our decision to include resiliency with the Sponge City concept as an objective. We continue by providing background information on ErQi area in Chapter 3 to get an overall understanding of the planned urban design and potential urban flooding. To provide a thorough analysis and recommendations for the selection process of adaptation measures to mitigate excess rainfall as part of the SCP in the context of ErQi area, an assessment of the Wuhan Sponge City criteria, a stakeholder analysis complemented by a spatial assessment was performed and described in Chapter 4. Setting the context allows understanding the complexity of the system and its constraints in the implementation of the SCP. Thus, we decided to first focus on the implementation of low-impact development (LID) measures using a multi-criteria analysis (MCA) presented in Chapter 4 and then developed an integrated and resilient system design later in the report. As the Sponge City is not sufficient to cope with high precipitation events (Arcadis, 2017), the project combines sponge city and resiliency principles in an integrated system approach.
The guiding resilient design principles of the Sponge City are further described and explained in Chapter 5 and applied in the opportunistic design process in Chapter 6, bridging the research with the designs. Here the designs of the MengQiao Bridge and the Water Road are presented along with their proposed effects on the urban flooding. Chapter 7 serves to assess the designs through the criteria of the integrated sponge city to improve flood resilience. The following chapter serves to share our conclusions on the challenges for implementing a functioning of the SCP that includes the concept of resilience. It also touches upon the difficulty of implementing the value-based design in a profit-based context. The final chapter is composed of five parts, all of which is our recommendations. It starts with our recommendations to improve the Sponge City criteria to make them more effective in reaching the goals of the programme. Then we give our recommendations for the selection process of LID followed by what we have learned of this interdisciplinary approach. That includes what we consider to be crucial to achieving a genuinely interdisciplinary process resulting in an integrated design. The final part of the chapter is dedicated to what we believe should be researched further. We believe a more in-depth assessment of the designs with the Sponge City criteria and input of the stakeholders is required for a final design. Further, the working definitions and approach of the Wuhan city government need to be considered, and an approach that assesses the necessary maintenance protocols is necessary.
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