University of Twente Student Theses


Modelling of stormwater overland flow in urban areas: Assessment of WOLK as an overland flow modelling tool

Klok, T.M. (2012) Modelling of stormwater overland flow in urban areas: Assessment of WOLK as an overland flow modelling tool.

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Abstract:According to the Royal Dutch Meteorology Institute (KNMI) the rainfall intensity is likely to increase in the coming decades. The effects of this increase will be more severe in urban areas than elsewhere. The increase in precipitation intensity will likely cause the sewer capacity to be insufficient, resulting in flooded streets. In order to minimize these effects, measures have to be developed. Urban runoff models can aid in the design of alternative ways to deal with overland flow. There are several urban runoff models available with various degrees of complexity. One of such models is WOLK, developed by Tauw. WOLK is primarily developed to simulate overland flow in urban areas. WOLK is a grid-based model which computes surface runoff based on precipitation amounts and a Digital Elevation Model (DEM). GIS applications allow for a visual presentation of the output data, which is useful for stakeholder sessions in which different approaches and measures for dealing with overland flow can be discussed. A study was performed on overland flow models and in particular on the limitations and accuracy of WOLK. The goal of this research is therefore to assess the limitations and accuracy of WOLK and investigate alternative modelling procedures to complement for some of the limitations. An analysis of relevant aspects of urban Storm Water Management (SWM) shows that three parameters in the urban water cycle are relevant during extreme rainfall events for urban overland flow modelling. These parameters are: precipitation, infiltration and the sewer system. This information is used as input for the assessment of WOLK 2009. From the assessment of WOLK 2009, several limitations of the model have become apparent. These limitations are: runoff routing, the interaction between the sewer system and overland flow, interaction with open water, only information about the end situation of the simulation, the inconsistent working methods of the model users, the use of surface interpolation techniques and the availability of multiple versions of WOLK. The most significant of the limitations is the inconsequent working method. In order to deal with this issue a user guide has been developed, in which a different data handling method is advised, because of the increase in surface elevation accuracy. The user guide combines the available knowledge of data handling methods and presents a way to efficiently execute a WOLK, while minimizing the possibility of inconsistencies in working methods. The user guide forms the basis of WOLK 2011. In order to check the correctness of the newly developed guidelines, for a case the results of WOLK 2011 are compared with that of WOLK 2009. The assessment is based on user defined criteria, which have been established by interviewing several municipalities. Furthermore, photo and video material made by eyewitnesses is reviewed. To assess the importance of the other found limitations a SWOT analysis is conducted. The results of this analysis indicate that there is a need for alternative overland flow models, which differ in flow routing, simulation information, run time and presentation of results. To investigate the alternatives two alternative models have been developed in Matlab. The first model is a simple, intuitive based distribution model and the second model is based on Manning‟s flow equation for the computation of flow rates. Both alternative models have been calibrated on several test cases. In order to compare the alternative models with WOLK 2011, all models have been executed for the village of Uddel, part of the municipality of Apeldoorn, a fire fighting case and Deventer centre. The case results show that a stopping rule is recommended for the new models. The possibility of showing intermediate results during the computations is an advantage compared to WOLK 2011. The alternative flow routing procedures used showed that despite the change in computational methodology the intuitive distributed and Manning-based model show good agreement with the original WOLK. The final conclusion is that it depends on the need for a specific output which of the three models should be used. When only the final result of the simulation is relevant it is recommended to use WOLK. The alternative models are mostly suited for small research areas because of runtime, and are especially suited for a quantitative assessment of for instance maximum flood elevation levels as well as flood durations.
Item Type:Essay (Master)
Tauw bv. Water department
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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