11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
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These instructions have been prepared in the format that should be used for the final manuscripts submitted to the 11th International Conference on Urban Drainage, Edinburgh, Scotland, 31st of August to the 5th of September 2008, and are designed to help authors provide manuscripts that will comply with the defined paper format. Papers should not exceed ten (10) A4 pages or 5000 words (including text, figures, tables and references). An up to 200 words abstract and 3-6 keywords are mandatory. The main text should be logically subdivided into sections (e.g. abstract, keywords, introduction, methods, results and discussion, conclusions, and references). Format using 2.5 cm margins, 12pt Times Roman typeface and single line spaced. Include a running heading at the top of each page, 1.25 cm from the edge of the paper, with the conference title, place and date extending from margin to margin. Include a running footer at the bottom of each page, 1.25 cm from the edge of the paper, with page numbers, author’s name and paper title. A paper template with detailed instructions for authors can be downloaded from the conference website (this is it!). All papers accepted for the 11th ICUD conference, for both oral and poster presentation, will be distributed to the participants at the conference on a CD-ROM and possibly in hardcopy or via the internet. The organisers retain the right to distribute submitted papers after the event and use them to promote the conference.
Floods in Small Urban Catchments: Hydrological Sensitivity, Risk Assessment and Efficient Integrative Strategies of Mitigation
E. Pasche1, N. Manojlovic1*, N. Behzadnia1
1 Institute for River and Coastal Engineering, Hamburg University of Technology, Denickestrasse 22, 21073 Hamburg, Germany
*Corresponding author, e-mal:
Abstract
As a consequence of ad hoc solutions in urban drainage management and spatial planning in the past, many large cities are affected by floods from small rivers. These rivers produce rapidly rising flood waves that are result of both, pluvial and fluvial flooding. Main drivers of future development such as climate change (IPCC, 2007) and rapid urbanisation (EPA, 2005) favour even more extreme situations. In spite of the importance of this problem, the current practice in urban flood management in Germany and Europe-wide shows lack of systematic approach to cope with this type of flooding. It is the objective of this paper to focus on the flood risk management of small urban catchments (SUCAs). In order to develop a sustainable approach for SUCAs and cope with their flooding in an efficient and effective manner, it is necessary to consider their socio-economic environment, existing legal framework as well as their physiogeographic characteristics (hydrological sensitivity). Based on the case studies in three German cities (Cologne, Dresden and Hamburg), the possibilities of non-structural measures (NSM) and integrative strategies of risk mitigation are assessed. Ideas to implement more holistic approaches to urban catchment management are given.
Keywords
Hydrologic sensitivity, Flood Probability Reduction Measures (FPRM), Flood Resilience Measures (FRM), Non-structural measures (NSM), Small urban catchments (SUCAs), SUDS
Introduction
Considering the uncertainty of future conditions shaped by the main drivers of future development such as climate change (IPCC, 2006) and rapid urbanisation (EEA, 2005) flood problem will gain momentum in urban environments in the coming years. While policy and water experts are aware of the flood risk along rivers and large streams and have developed future oriented and sustainable strategies for their management (EU Flood Directive, German Flood Control Act -FCA, 2005, PPS25, 2006, etc), pluvial floods caused by exceeding flow in the storm water drainage system and small urban watercourses have been hardly considered so far and efficient strategies are still missing. Characteristic of these pluvial floods is their rapid growth in reaction of an extreme local storm event that they are often compared to flash floods. This fast and intensive reaction is due to small drainage areas with high degree of impervious surface and a limited conveyance in a dense pipe network. At the end of these drainage areas small open watercourses receive this overflow, which due to encroachment in small compact channels and culverting act like a bottleneck causing unexpected and underestimated flooding of urban environment. This problem should be treated on the Small Urban Catchment (SUCA) level. Instead of restricting the drainage of urban developments to a pipe network designed only for small floods of 2-10 year return probability (general practice all over Europe), the flood capacity of the recipient watercourses and the management of the exceeding flow of the pipe network need to be included in the storm water management plan. This integrative flood management approach however faces many obstacles such as water management at administrative units, insufficient risk awareness of stakeholders and the entrapment at obsolete practices (Ashley at al, 2007). Especially the uncertainty of future developments calls for adaptive measures of flood mitigation, which are seen in the application of Non-Structural-Measures (NSM) comprising flood resiliency measures (FRM) and flood probability reduction measures (FPRM). But due to insufficient understanding of the hydrological system in urban areas and the contribution of FPRM as part of NSM to retain and attenuate flood flows, a considerable uncertainty exists about their effectiveness. Latest research on hydrodynamic modelling of combined sewer overland flow such as (Djordjevic, 2007; Ettrich; 2007; El Khadi 2007) have shown the complexity of the hydrological cycle in urban environments and that still considerable progress is necessary to quantify the effect of FPRM on the flood flow in small urban watercourses. Also resilience measures such as dry-proofing of buildings through movable flood abatement systems need review and extension for being able to cope with the short response time to get them into operation. Some first ideas, outlined in the RIMAX-project URBAS ( e.g. the use of radar based warning systems and the integration of emergency forces in the maintenance of drainage infrastructure need further consideration and extension by developing strategies of capacity building of stakeholders as described by (Ashley at al, 2007) and (Pasche et al, 2007).
It is the objective of this paper to demonstrate and to provide solutions to overcome the main obstacles of integrated flood risk management of SUCAs. Necessary adaptations of the legal framework and institutional organization as well as ways to improve public involvement are described and their potential to support capacity building of stakeholders for efficient application of NSM is demonstrated. The concept of a hydrological sensitive matrix (HSM) is introduced as an instrument to support the decision process of professional stakeholders by enabling a rapid and robust assessment of the effectiveness of FPRM to retain and attenuate pluvial floods in dependence on geomorphological conditions. The research results are extracted from the Crue Era-Net project Risk Assessment and Risk Management in Small Urban Catchments ( They represent the findings of a case study carried out at SUCAs in the German cities of Hamburg, Cologne and Dresden. Their comparison with the results of the other project partners from France (ENPC), England (University of Manchester) and Scotland (University of Sheffield) and their projection into a European guidance document is still at consideration and thus will be only outlined in its perspectives.
Theoretical discourse about non structural measures (NSM)
The risk of flooding is regarded as the product of the intensity and probability of flooding (source), the vulnerability of the flooded area (receptor) and the pathway of flood (exposure). According to the EU policy (EC 2003, EC 2007/60) risk management is the appropriate strategy to cope with the increasing flood risk due to climate change and other anthropogenic drivers. As a consequence of this paradigm change structural measures, which only act on the pathway of the flood through defence systems and the enforcement of the conveyance capacity of pipes and watercourses, are no longer regarded as the best solution to mitigate flood, as these large scale structures do not have the flexibility to adapt efficiently to changes in the future projections and to reduce the detention capacity of the hydrological system leading to higher flood risks downstream. Thus, theRisk management in the sense of the EU addresses all components of the Source-Pathway-Receptor-Consequence model and prioritises flood probability reduction measures (FPRM) and flood resiliency measures(FRM).
Table 1Flood Resilience Measures (FRM) covering the 4 A’s of receptor control
FRM
/Type of measure
/ NS Responses / ScaleCapacity building of
human resources
A1: Awareness of flood risk / Information / Emergent / Intermediate
Flood maps (Inundation and Risk)
Info material (brochures, public presentations, internet portals etc
Education - Communication / Emergent / Intermediate
Face-to-face learning
Web-based learning
Training
Collaborative platforms
Land use control
A2: Avoidance of the risk where possible / Spatial Planning / Emergent / Catchment
Flood risk adapted landuse
Building regulations
Building codes
Zoning ordinances
Flood preparedness
A3: Alleviation of the effects of the flood / Flood Resistant buildings / Emergent / Local
Wet-proofing
Dry-proofing
Flood action plan (local scale) / Traditional / Local
Infrastructure maintenance
Contingency measures
A4: Assistance in the event of difficulties / Financial Preparedness / Emergent / Catchment
Insurance of residual risk
Reserve funds
Emergency Response: / Traditional / Catchment
Evacuation and rescue plans
Forecasting and warning services
Control Emergency Operations / Emergent / Intermediate
Providence of emergency response staff / Traditional / Intermediate
Emergency infrastructure / Traditional / Intermediate
Allocation of temporary containment structures
Telecommunications network
Transportation and evacuation facilities
Recovery:
Disaster recovery plans, pecuniary provisions of government / Emergent / Intermediate
In this context both, FPRM and FRMare referred to as opposition to structural measures, that they are denoted here as Non-Structural Measures (NSM). FRM are regarded as sustainable as they reduce the vulnerability of the receptor and/or reduce its exposure without negative impact on the hydrological system. They support the recovery of society after an extreme flood in SUCA’s and thus stand for the improvement of resiliency of the whole system. They can be categorised in the 4 A’s of the safety chain of flood resiliency: Alleviation, Avoidance, Awareness and Assistance (Biemans, 2006) as depicted in Table 1. According to (Ashley et al, 2007) some of these measures can be regarded as traditional or understood, as they are based on legacy, current understanding of systems and good practice. However most of them need to be denoted emergent as they refer to a process of transfer, which can be adjusted to the dynamics of the system (e.g. climate change, stakeholder capacity, urban development, knowledge of society) leading to greater effectiveness than the tangible measures of traditional response (Ashley et al, 2007) as they help to overcome the entrapment in obsolete practices which are adherent to most social systems and support continuous adaptation to changing flood risk with flexible response. But emergent stands also for “new” and thus need capacity building of stakeholders for accepting them and applying them in a most effective way. The technology of flood resistant buildings through dry- and wet-proofing is already well established and latest research studies such as (Defra, 2007) give good guidance to assess the suitability and cost effectiveness of a variety of these measures. On the other hand, this publication is inconsistent with the earlier publications terming dry-proofing measures as resistant measures whereby the wet-proofing measures are referred to as resilience measures. Due to their innovative potential, they are here classified as emergent. The second group of NSM, the Flood Probability Reduction Measures (FPRM) encompass those measures which restore the retention potential of the natural hydrological system or even enhance the detainment of rain water through small retention basins distributed all over the SUCA’s. On a local scale (property, allotment), this includes sustainable drainage systems (SUDS), which are already regulated by law for the drainage of new urban development in most German states. On an intermediate level FPRM include:
a)controlled surface conveyance of flood water exceeding the drainage pipe network and its temporary storage in public spaces (e.g. green areas) designed as multi-functional spaces,
b)the reopening of culverts and the restoration of natural elements (flood plains, meandering river bed and wooden vegetation) for they enhance the retention of flood water and
c)small retention reservoirs, retaining the flood water in a semi-distributive way, by receiving flood water from a central stormwater pipe or a small watercourse in SUCA’s.
Obviously FPRM are related to the development of new hydraulic structures in SUCA’s. Thus its denomination as non-structural seems to be inconsistent and contradictory. But in the context of flood risk management structural stands for the conventional flood defence strategy (such as increasing the flow capacity of the storm water network) and thus the listed flood probability measures (FPRM) are in this sense non-structural as they represent an alternative to those conventional structural approaches.
Methodology
The basis of this research study has been a case study at small urban catchments within the European countries England, Scotland, Germany and France. The selection criteria for the catchments have been their representativeness, availability of data and occurrence of pluvial floods within the last years. Studied areas are either totally urbanised or divided into a rural/natural upstream and an urban downstream part and their size is approx. 50 km2. The urban areas were drained either by a stormwater pipe network or a combined sewage system which releases the drained water into a central water course which used to be a small natural river or brook, but has been now turned into an encroached and artificial channel bed. The channel reaches might flow through a public green spaces, can be culverted and pass under shopping centers and highways or are fenced off and flow behind the private gardens. Although the catchments are considered to be small, they might cross administrative borders with different institutional and political responsibilities. As flood dynamics considerably differs between flat and steep areas, catchments have been chosen ranging from the lowlands to the Mid-range Mountains. The four German catchments in the cities of Hamburg, Cologne and Dresden cover these different topographic regions and cover a wide range of socio-economic, hydrological and geo-morphological conditions. As pointed out by the discourse, NSM interacts with the hydrological regime and has major implications with the social, institutional and legislative system, which raises the question about the necessary conditions, under which NSM can develop its potential of flood mitigation in SUCAs at all. Through expert panels between the Crue Era-Net partners from France England, Scotland and Germany and a review of documented work on flood risk management, a general framework for successful application of NSM has been defined in the first phase of this study. On this basis, each partner surveyed the main obstacles of present social, institutional and legislative situation in their case study areas opposing the implementation of this framework (phase 2). Most of the Flood Resilient Measures (FRM) are emergent and need to be accompanied with capacity building of public and professional stakeholders. In phase 3 new learning tools has been developed and tested at private and professional stakeholders giving indices about their readiness and capability to accomplish this transition process and demonstrating efficient ways of communication and raising awareness. Spatial planners have neither access nor the expertise to use complex hydrological models. Thus this study was looking for simple methods of assessing the hydrological efficiency (phase 4). Through a GIS-based data analysis at the three case study areas and a sensitivity study with a rainfall-runoff model at catchments in Hamburg the main dependencies ought to be derived and transformed into a Hydrological Sensitivity Matrix (HSM). Additionally, literature has been reviewed to complete and verify the parameterisation of this matrix (phase 5). In the end, a final assessment of the effectiveness and efficiency of NSM in SUCAs will be given by merging the results of all Crue Era-Net partners of this project leading to a guidance document for the application of NSM in SUCAs, which is still in process.