HAMBURGER WASSERBAU-SCHRIFTEN 13 13 13 Herausgegeben von Prof. Dr.-Ing. Erik Pasche 2010 RIFTEN H C S Dipl.-Ing.(FH), M.Sc. Sandra Hellmers Hydrological Impacts of Climate Change on Flood Probability in Small Urban Catchments and Possibilities of Flood Risk Mitigation HAMBURGER WASSERBAU- TUHH Technische Universität Hamburg-Harburg River and Coastal Engineering Sandra Hellmers Hydrological Impacts of Climate Change on Flood Probability in Small Urban Catchments and Possibilities of Flood Risk Mitigation Hydrologische Auswirkungen des Klimawandels auf die Hochwasserwahrscheinlichkeit und Maßnahmen zu deren Kompensation in kleinen städtischen Einzugsgebieten Hamburger Wasserbau-Schriften Band 13 Publicated by Prof. Dr.-Ing. Erik Pasche Hydrological Impacts of Climate Change on Flood Probability in Small Urban Catchments and Possibilities of Flood Risk Mitigation Master's Thesis at the Institute of River and Coastal Engineering by Sandra Hellmers Institute of River and Coastal Engineering, University of Technology Hamburg – Harburg 2010 All rights reserved. Reproduction in whole or in part without permission of the Institute is prohibited. Cover Design: Kerstin Schürmann, www.formlabor.de Cover Foto: Sandra Hellmers Editor: Sandra Hellmers ISBN 978-3-937693-13-2 ISSN 1612-8699 1. Edition, Institute of River and Coastal Engineering TUHH, Hamburg Preface Dear Reader, like many other disciplines the hydrological science community is strongly affected by the societal discussion about the consequences of a changing climate. Based on future economic and demographic development scenarios the progress of the climate system can be simulated with computer models. Further on the calculated climate variables are used in specific impact models such as detailed hydrological simulations, which use rainfall-runoff as well as water balance models to understand the consequences of these changing climate conditions on the hydrological cycle. So far the temporal and spatial resolution of the available climate data has not been sufficient enough in order to use it directly in hydrological models for flood analysis. However, within the German BMBF-project KLIMZUG-Nord (www.klimzug- nord.de) climate data has been provided with a spatial resolution of 1km² and a temporal resolution of 1 h which allows the analysis of discharge simulations in smaller river catchments for the first time. In her Master's thesis Sandra Hellmers developed a new method to derive extreme floods from this climate data and to classify them statistically. For these hydrological studies she used the non-linear semi-distributive rainfall-runoff model KALYPSO-Hydrology and demonstrated the improvements for flood analysis in rural and urban areas in the case study area of the river basin Krückau in Northern Germany. Furthermore she proved that extreme precipitations cannot be linearly transferred into extreme runoff. In order to resolve the various complex, interacting processes between the terrain, ground, river network and the urban drainage system very precise modelling instruments are needed. These model requirements are also necessary to quantify the effectiveness of non-structural measures such as SUDS (SUstainable Drainage System) and surface conveyance measures. Sandra Hellmers' research work is outstanding as her new method of climate impact assessment on the hydrological cycle is innovative, physically sound and so generic that it can be used as good guidance for hydrological impact studies of climate change. The new method to simulate the attenuation and retention effects of SUDS fully parametrises the components of SUDS thereby opening this method to scenario studies of urban drainage systems with various combinations and intensities of SUDS components. On this background I decided to publish her Master's thesis in the „Hamburger Wasserbau-Schriften“. I hope many practitioners and other researchers will benefit from this pioneering work and make use of this method for their own climate impact studies. They are invited to make use of the software KALYPSO-Hydrology which can be downloaded from http://kalypso.sourceforge.net. Hamburg, 08.11.2010 Prof. Dr.-Ing. Erik Pasche (Head of the Institute of River and Coastal Engineering at the TUHH) Abstract Impacts of climate change on the ecology, the human and the economy are already apparent and probably increase significantly in future. The magnitude and frequency of extreme rainfall is thereby assumed to change, which could affect the flood regime in river catchments substantially. Especially flooding in small urban catchments (SUCAs) is strongly dependent on intensive rainfall events which cause exceeding flow from small rivers, streams and storm water sewer systems. Developing a detailed and comprehensive methodology to quantify the hydrological impacts of climate change on flood probability in SUCAs is a required and forward- looking task, which has been worked out and described in this thesis. To cope with the impacts on flood risk in SUCAs, it is emergent to introduce and implement effective, flexible as well as adaptable possibilities of flood probability reduction, whereas sustainable drainage systems (SUDSs) have been identified as appropriate measures. To assess the effectiveness of these techniques, a software tool for simulating SUDS elements (namely: green roofs) on a catchment level has been programmed. The developed methodology in this thesis comprises the pre-processing of climate model as well as climate scenario data series, the processing of climate scenario results, the post-processing of calculated climate change impacts including the computation of climate change factors and the assessment of the effectiveness of SUDSs in post-impact studies. This methodology has been applied for climate change impact studies in one of the catchments in the region of the KLIMZUG-Nord project. An increase of the frequency and magnitude of extreme events has been calculated especially for summer periods, whereas for winter periods the average precipitation is computed to increase significantly. With the IPCC scenario A1B, in the climate period from 2040 to 2070, an increase of 13.3% for 100year summer rainfall intensities with durations of 1hour, as well as an increase of 22.5% for 100year peak discharges in summer periods has been calculated. Additionally, simulations for the IPCC scenarios B1 and A2 have been performed, but the results display lower changes in extreme events for the time period around 2050. The new developed software tool for simulating green roofs has been tested in adaptation scenario studies, along with the simulation of swales and swale-filter- drain systems. The appropriateness of the simulation results of hydrological processes in each SUDS element and the effectiveness of SUDSs on a catchment level has been verified. The compensation of climate change impacts on the flood probability in SUCAs has been achieved with the combination of different SUDS measures, which display larger effectiveness for events with higher probabilities of occurrence. Zusammenfassung Die Auswirkungen des Klimawandels auf die Ökologie, den Menschen und die Ökonomie sind bereits spürbar und werden voraussichtlich in Zukunft erheblich zunehmen. Veränderungen der Häufigkeit und Intensität von Starkniederschlägen sind dabei zu erwarten, die wiederum erhebliche Auswirkungen auf die Hochwasserverhältnisse in Flusseinzugsgebieten zur Folge haben können. Insbesondere Überschwemmungen von kleinen Einzugsgebieten in urbanen Räumen durch Gewässer und Entwässerungsnetze werden durch Starkniederschläge verursacht. Die Entwicklung einer umfassenden und detaillierten Methodik zur Quantifizierung der hydrologischen Auswirkungen des Klimawandels auf die Hochwasserwahrscheinlichkeit in kleinen städtischen Einzugsgebieten ist eine erforderliche und zukunftsweisende Aufgabe, die in dieser Arbeit erläutert und ausgearbeitet wurde. Um den Einflüssen des Klimawandels auf Hochwasser in städtischen Einzugsgebieten zu begegnen, ist es notwendig effektive und flexibel anpassbare Maßnahmen zur Reduktion der Hochwasserwahrscheinlichkeit zu ergreifen und umzusetzen. Nachhaltige Regenwasserbewirtschaftung (RWB) wurde hierfür als geeignete Vorgehensweise erkannt und für den Nachweis der Effektivität von diesen Maßnahmen wurde ein Software-Tool für die Simulation von RWB-Elementen (hier: Gründächer) auf Einzugsgebietsebene programmiert. Die entwickelte Methodik umfasst die Aufbereitung der Daten von Klimamodellen (Pre-Processing), die Berechnung sowie Analyse der Klimaszenarienergebnisse (Processing), die Nachbereitung der berechneten Auswirkungen (Post-Processing) einschließlich der Berechnung von Klimawandel- faktoren und den Nachweis der Effektivität von RWB-Maßnahmen. Für Studien über die Folgen des Klimawandels in einem der Einzugsgebiete des KLIMZUG-Nord Projektes wurde diese Methodik angewendet. Für das IPCC Szenario A1B der Klimaperiode von 2040 bis 2070 wurde eine Zunahme von 13,3% für 100jährliche Starkniederschläge im Sommer, sowie eine Erhöhung von 22,5% für 100jährliche Sommerhochwasserereignisse berechnet. Zusätzlich wurden Berechnungen der IPCC Szenarien B1 und A2 ausgeführt, die jedoch geringere Auswirkungen des Klimawandels für den Zeitraum um 2050 aufzeigen. Das entwickelte Software-Tool für die Simulation von Gründächern wurde zusammen mit der Modellierung von Mulden und Mulden-Filter-Rigolen Systemen getestet. Die Genauigkeit der Simulation der hydrologischen Prozesse in den jeweiligen RWB- Elementen und die Berechnung deren Effektivität