Climate change, as well as increasing urbanization, lead to an increase in urban flooding events around the world. Accurate urban flood models are an established tool to predict flooding areas in urban as well as peri-urban catchments, to derive suitable measures to increase resilience against urban flooding. The high computational cost and complex processes of urban flooding with numerous subprocesses are the reason why many studies ignore the discussion of model uncertainties as well as model calibration and validation. In addition, the influence of steep surface (hillside) conditions on calibration parameters such as surface roughness are frequently left out of consideration. This study applies a variance-based approach to analyze the impact of three uncertainty sources on the two variables—flow and water depth—in a steep peri-urban catchment: (i) impact of DEM validation; (ii) calibration of the model parameter; (iii) differences between 1D/2D and 2D models. The results demonstrate the importance of optimizing sensitive model parameters, especially surface roughness, in steep catchments. Additional findings of this work indicate that the sewer system cannot be disregarded in the context of urban flood modeling. Further research with real heavy storm events is to be pursued to confirm the main results of this study.
<p>Urban drainage is subject to a variety of external influencing factors that can have a negative impact on hydraulic system performance. These include changing precipitation characteristics due to climate change [1], an increase in sealed surfaces due to advancing urbanisation [2], but also further failures and malfunctions [3] in the technical grey and green infrastructure. With an increasing share of decentralised urban stormwater measures and uncertainties regarding responsibility, care and maintenance of these facilities, an increase in malfunctions can be assumed [4]. In this work, we are investigating common malfunctions in urban drainage systems with 1D/2D urban flood model in a virtual urban study site. The goal is to highlight differences between the failures and malfunctions in both grey and blue-green infrastructures for different design rainfall events (Type Euler II) and compare them to other possible scenarios like climate change and urbanization.</p><p>For the research, a model of a small virtual urban study site (1.5ha) is developed with the commercial software PCSWMM2D [5], which represents a small part of an urban catchment. It includes the following sub-structures and assets: i) combined sewer system, ii) urban stream, iii) urban structures including buildings, marketplaces, streets, bridges, pathways, and underbridges and iv) four sustainable urban drainage system (SUDS) structures (green roofs, permeable pavements, swales and bioretention cell). Connected to the combined sewer system are three border areas (30ha, 10ha, 10ha), representing inflows from outside. The urban drainage infrastructures and SUDS were designed based on a design rainfall (Euler Typ II) event with a 5-year return period and 1-hour event duration.</p><p>In total 12 different scenarios were designed for the virtual urban study site i) the SUDS-base-scenario which includes four different green infrastructure assets, ii) three reference scenarios with climate change, urbanisation and wet preconditions and iii) the malfunction-scenarios with seven single malfunction scenarios and one worst case which is all of the single scenarios combined. Each scenario was run with design rainfall events with an Euler II distribution and interval lengths between 15 minutes and 24 hours as well as return periods between 1 and 100 years.</p><p>To compare the different scenarios and assess their severity for the urban area we used 3 different objective values. i) The maximum water depth in the vulnerable infrastructure (underbridge), ii) the flooded area with water depths > 10cm and iii) the total combined sewer overflow emissions released into the urban stream.</p><p>Results show a clear difference between the different malfunction scenarios, with a higher influence of malfunctions in grey than in green infrastructures. In most cases, the reference scenarios climate change, urbanisation and wet preconditions show higher values than the malfunctions scenario. All scenarios are highly dependent on the rainfall event characteristics, with no differences in the objective values compared to the base case for low return periods and rising differences for medium to high return periods.</p>
ZusammenfassungNiederschlagsinduzierte Überflutungen auf einer lokalen Skala erfahren in den letzten Jahren eine vermehrte Aufmerksamkeit. Vor allem im urbanen Raum ist in den letzten Jahren eine Vielzahl von Ereignissen mit erheblichen Schadenssummen aufgetreten. In der Fachdiskussion wird durch Klimawandel und Urbanisierung eine weitere Zunahme bezüglich Häufigkeit und Ausmaß dieser Ereignisse prognostiziert. Im deutschsprachigen Raum werden unterschiedliche Begriffe, teilweise in Abhängigkeit von den beteiligten Prozessen, benutzt. Neben der kurzen Darstellung der geläufigsten Fachbegriffe wird eine mögliche Klassifizierung vorgeschlagen. Für eine Überflutungsanalyse steht mittlerweile eine Vielzahl an ausgereiften Werkzeugen zur Verfügung. Eine hierarchische Darstellung der Modellansätze erlaubt eine der jeweiligen Aufgabe und dem jeweiligen Untersuchungsgebiet angepasste Auswahl der Modellansätze und eventuelle Kombination dieser. Dabei wird auf GIS-basierte Methoden zur Bestimmung der Fließwege an der Oberfläche, Zelluläre Automaten zur Bestimmung von Überflutungsflächen, 1D-hydrodynamische Kanalnetzmodelle, 2D-hydrodynamische Oberflächenabflussmodelle und gekoppelte 1D-2D-Überflutungsmodelle eingegangen.
<p>A city-wide approach to reduce the uncertainty regarding the spatial variability of urban flooding events is required in urban catchments. The goal of this study is the development of a modelling framework independent of the spatial scale to address the most hazardous areas in the current state and the future. The framework starts with the definition of the study objectives (e.g. reducing flood risk), which have a direct impact on the spatial and temporal scale, the used model approach, the data requirement and the level of detail. Furthermore, potentially hazardous areas will be identified with the potential flood risk index (PFRI<sub>i</sub>). The determination of this is a risk-based approach (R=E*V*H) which combines the exposition (E) with the vulnerability (V) and the hazard (H). The population density of each object and the total number of persons in the catchment will quantify the exposition. The vulnerability includes the number of past damage events and the object use. How accurate the modelled hazard is considered, depends on the used model approach: i) GIS-based; ii) only 1D; iii) only 2D; iv) 1D/2D models. The combination of H and V resulted in the risk factor (RF<sub>i</sub>) in four levels of detail depending on the used model approach. This allows both, the quantification of hazardous areas at the current state and the change of the PFRI<sub>i</sub> by future scenarios such as climate change and urbanization.</p> <p>PFRI<sub>i</sub> = n<sub>P,k</sub> * RF<sub>i</sub> / (A<sub>k</sub> * &#8721;P)&#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160;</p> <p>PFRI<sub>i</sub>=Potential Flood Risk Index; n<sub>P,k</sub>= number of Persons on a private ground k; A<sub>k</sub>=total object area; =total number of persons in the catchment; Rf<sub>i</sub>= risk factor depending on the used model approach k</p> <p>The GIS-based flow path analysis as the first level of detail can be used to identify the urban flooding hot spots. This allows the identification of hazardous sub-catchments in a city or high-risk private ground in a catchment quantified by the PFRI<sub>GIS</sub>. This is useful for further detailed analysis with other model approaches (e.g. 1D/2D model). The next steps are the implementation of the demonstrated framework for each level of detail in the city of Graz in Austria. Furthermore, the framework will integrate different climate scenarios based on a high-resolution climate model to address the impact of climate change on the urban drainage system quantified by the PFRI<sub>i</sub>.</p> <p><br />Keywords: urban flooding, urban flood modelling, risk assessment, future changes</p>
ZusammenfassungJährlich verursachen zahlreiche konvektive Starkniederschlagsereignisse enorme Schäden und Kosten in den österreichischen Kommunen. Aufgrund ihrer komplexen räumlichen Strukturen und des von der Urbanisierung hervorgerufenen steigenden Versiegelungsgrads ist das Risiko einer Überflutung in urbanen Einzugsgebieten besonders hoch. Daher bedarf es Methoden und Werkzeuge, um mit dem steigenden Überflutungsrisiko in urbanen Einzugsgebieten umgehen zu können. Die Ausweisung von Überflutungsflächen innerhalb eines Einzugsgebiets gilt dabei als wesentliche Grundlage für die Ableitung geeigneter Maßnahmen zur Reduktion des Überflutungsrisikos. Integrierte 1D-2D-Modelle, welche die hydrologische Abflussbildung einerseits sowie den Oberflächenabfluss und den Kanalabfluss hydrodynamisch andererseits berücksichtigen, sind ein zuverlässiges Werkzeug, um die benötigten Zielgrößen – Wasserstand und Fließgeschwindigkeit – zur Ermittlung dieser Überflutungsflächen bestimmen und im Anschluss daran eine Risikoanalyse durchführen zu können. Das Ergebnis sind Überflutungsrisikokarten, die eine wertvolle Planungsgrundlage für die Kommunen darstellen. Die in diesem Beitrag vorgestellte Methodik beschreibt ein Schritt-für-Schritt-Vorgehen, wie Risikokarten in urbanen Einzugsgebieten von Planungsbüros und Kommunen erstellt werden können. Die Methodik ist gegliedert in eine Voruntersuchung zur Identifizierung der neuralgisch wichtigen Stellen, eine Detailanalyse für den Aufbau des integrierten 1D-2D-Modells sowie eine Risikoanalyse zur Erstellung der Überflutungsrisikokarten.
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