Graduation Year


Document Type




Degree Granting Department

Civil and Environmental Engineering

Major Professor

Kalanithy Vairavamoorthy


adaptation, flexibility options, robustness, stormwater management, sustainable urban drainage systems


Urban drainage systems are influenced by several future drivers that affect the performance as well as the costs of the systems. The uncertainties associated with future drivers and their impact creates difficulties in designing urban drainage systems sustainably. A review of the different future drivers for urban drainage systems illustrates that no sufficient future predictions for the long operational life spans of the systems are possible. This dissertation contends that to deal with future uncertainties, flexibility in urban drainage systems is necessary.

At present, profound insights about defining, measuring, and generating flexible urban drainage systems do not exist. This research systematically approaches these issues. First, a clear definition of flexibility and an approach for the measurement and optimization of flexibility is operationalized. Based on the generic definitions of flexibility used in other disciplines, a definition tailored for urban drainage systems is generated. As such, flexibility in sustainable urban drainage systems is defined as `the ability of urban drainage systems to use their active capacity to act and respond to relevant alterations during operation in a performance-efficient, timely, and cost-effective way'. Next, a method for measuring flexibility is provided based on the developed definition of flexibility including the metrics, 'range of change', 'life-cycle performance' and 'effort of change'. These metrics are integrated into a framework for the measurement of flexibility based on a comparison of performance and effort in different alternative solution with respect to different future states. In addition the metrics are the core components for optimizing flexible design of urban drainage system. The measurement method is successfully applied in two case studies in Tuttle Hill, UK and Hamburg-Wilhelmsburg, Germany. Using the developed definition and method for the measurement of flexibility, this dissertation illustrates that a transfer of the general theoretical background of flexibility to the field of urban drainage is possible.

It is currently unclear how the flexible design of urban drainage systems can be executed. Based on a review, this research identifies nine potential principles of flexible design, described by the indicators of modularity, platform design, flexible elements, cost efficiency, decentralized design, real time control, low degree of specialization, scalability, and a combination of these principles. A case study of Hamburg-Boberg is then presented to analyze which of these principles of flexible design can be verified. For each alternative solution in the sample, the indicators for the different potential principles of flexible design as well as the flexibility provided by the design are calculated. Testing is done to determine if there is a significant correlation between the potential principles of flexible design and the measured flexibility using a chi-square-test and F-test. Two principles are verified with a high degree of confidence, 'platform design' and `flexible elements'. The `platform design' principle provides high flexibility, in which urban drainage system elements with high change costs are designed robustly with huge tolerance margins, whereas elements with low change costs are designed with flexibility options. The 'flexible elements' principle aims to include as many component elements as possible, which provides high individual flexibility in the design of the urban drainage system.

These design principles and associated static indicators enable a quick screening of huge number alternative solutions and provide guidance for the development and optimization of flexible urban drainage system. Within the framework for optimization of flexibility, the design principles can help identify the most promising alternative solutions for the design of urban drainage systems. The optimization framework includes the following steps: identification of the required flexibility, generation of alternative solutions for the design of urban drainage systems, screening of the most promising alternative solutions, detailed measurement of flexibility provided by the alternative solutions; and selection of optimal solution. Hence out of a sample of different design approaches, the solutions with the highest flexibility could be identified.

The successful application of flexible design in three case studies illustrates that the concept provides a suitable strategy for dealing with the challenges associated with future uncertainties. For urban drainage systems, flexible design guarantees high levels of performance in uncertain future states while reducing the effort required to adapt the system to changing future conditions. This study contends that flexibility allows for profound decision making for urban drainage design despite future uncertainties.