University of Twente Student Theses


Comparison of the effects of nature based solutions on urban runoff in Kigali using different parametrisations

Zaag, A.R. van der (2022) Comparison of the effects of nature based solutions on urban runoff in Kigali using different parametrisations.

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Abstract:Many cities in the world face rapid and unsustainable urban developments, increasing the prevalence of paved surfaces. The combined effect of urbanisation and climate change cause the rainwater peak discharges and thus the number of flooding events to increase over time. Therefore, there is a need for applying Nature-Based Solutions (NBSs) which complement the more traditional grey flood reduction measures - because of their ability to adapt to changing environmental conditions. The capital city of Rwanda, Kigali, is a city facing unsustainable urban developments. As a results, numerous urban flood events have occurred over the past few decades. The World Bank is actively investigating the implementation of NBSs in the city. Previously, the hydrological wflow Simple Bucket Model (SBM) has been used to estimate the effects of NBSs on the urban runoff in Kigali. This approach is referred to as the old implementation in this study. The effects computed by the old approach are uncertain, because of the lack of quantitative data and challenges to parametrise NBSs. Hence, the aim of this research is to compare the effects of NBSs on urban runoff simulated with the old NBS implementation into wflow-SBM and a more physically based NBS implementation, such that it also benefits NBS studies in other catchments where different hydrological models are used. Almost thirty NBSs are included in this study. First, interviews are conducted to discover the strengths and possible improvements of the old implementation. The expected effects of NBSs on hydrological processes are described, to cluster all NBSs into groups that can be implemented using the same model parameters. Next, sensitivity analyses of the model parameters and fluxes are used to derive a suitable implementation for each NBS-cluster. All individual NBSs are then parametrised by translating hydrological changes to model parameter value changes. Some limitations of the old implementation, that are improved, include the use of unrealistic Manning values, the omission of infiltration, and the parametrisation of surface runoff storage (e.g., ponds and retention areas) and small-scale NBSs. The sensitivity analyses of the model parameters and fluxes show that infiltration is a hydrological process that is relevant for the peak runoff and should be included when determining the effects of NBSs. The parametrisation of surface runoff storage NBSs is improved by using the function for paddy areas instead of increasing the branch trunk storage (Swood). After the implementation of NBSs, the individual and combined effects are obtained in terms of the delay of the peak runoff, and reductions of the maximum runoff and peak runoff volume. The unit effects of NBSs in Kigali are updated and the possibilities of the wflow-SBM concept for implementing NBSs are assessed. In addition, more insight is gained in the effectiveness of NBSs in different types of urban catchments. The most important conclusion is the old implementation is likely to underestimate the effects of NBSs. This is suggested because infiltration is included in the new implementation and the parametrisation of surface runoff storage NBSs is improved. In addition, it is shown that the location of a NBS mainly determines its effects. Individual NBSs can increase or decrease the maximum runoff depending on where they are implemented in both approaches. NBSs are especially effective in preventing flood hazards in Mpazi, which is a sub-catchment consisting of steep slopes and paved areas. In this sub-catchment, combining all NBSs could lead to a 90% reduction of the maximum runoff in the new approach. To improve the accuracy of the obtained NBS effects, it is recommended to obtain local measurements of the runoff effects of NBSs in Kigali and to do further research to the negative effects of individual NBSs. Since the results are not validated and location specific, the results of this study should not be used as generalised effects of NBSs. However, the methodology can be used to assess NBSs in other catchments using different physically based hydrological models.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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