11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
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: pasche@tu-harburg.de
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.
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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 dryproofing 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 (http://www.urbanesturzfluten.de/), 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 (http://suca.wb.tu-harbug.de). 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
MEASURES (NSM)
ABOUT
NON
STRUCTURAL
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. Risk 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).
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Table 1 Flood Resilience Measures (FRM) covering the 4 A’s of receptor control
FRM
Type of measure
NS Responses
Scale
Emergent
Intermediate
Emergent
Intermediate
Emergent
Catchment
Emergent
Local
Traditional
Local
Insurance of residual risk
Reserve funds
Emergency Response:
Emergent
Catchment
Evacuation and rescue plans
Forecasting and warning services
Traditional
Catchment
Control Emergency Operations
Emergent
Intermediate
Traditional
Intermediate
Traditional
Intermediate
Emergent
Intermediate
Information
Flood maps (Inundation and Risk)
Info material (brochures, public
Capacity building of
presentations, internet portals etc
human resources
Education - Communication
A1: Awareness of flood risk Face-to-face learning
Web-based learning
Training
Collaborative platforms
Spatial Planning
Land use control
Flood risk adapted landuse
Building regulations
A2: Avoidance of the risk
Building codes
where possible
Zoning ordinances
Flood Resistant buildings
Flood preparedness
Wet-proofing
Dry-proofing
A3: Alleviation of the effects
Flood action plan (local scale)
of the flood
Infrastructure maintenance
Financial Preparedness
Contingency measures
Providence of emergency
response staff
A4: Assistance in the event
Emergency infrastructure
of difficulties
Allocation of temporary
containment structures
Telecommunications network
Transportation and evacuation
facilities
Recovery:
Disaster recovery plans, pecuniary
provisions of government
In this context both, FPRM and FRM are 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
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
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 socioeconomic, 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,
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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.
RESULTS AND DISCUSSION
According to this methodological framework the current results of this study are as follows:
Phase 1: Key requirements for successful implementation of NSM in SUCAs:
As an outcome of the expert panels all partners of the Crue Era-Net project SUCA agreed on
the Manchester agenda for a successful implementation of NSM in SUCAs, which contains
the following key statements:
Lemma 1: Empowering the public
The paradigm shift in flood risk management, which is the driving force for NSM, demands
public involvement. Without risk awareness of the public and understanding its role in flood
management, flood resiliency will not be accomplished in urban catchments.
Lemma 2: Making institutions work and the role of planning in minimising flood risk
Application of NSM in SUCAs requires integrative flood risk management. This can only be
accomplished by integrating spatial planners, ecologists and hydraulic engineers in the
planning process. A coherent system of regulations and responsibilities between water
utilities, local and regional authorities, environmental agencies and spatial planning is needed.
Lemma 3: Development of professional attitudes and cultures to non-structural measures
Proper application of NSM needs good knowledge about the emergent components of NSM,
an understanding of the resilience principle in urban flood management.
Lemma 4: High performance of warning and emergency responses
Some measures of NSM require logistical activities prior to a hazard event. As the response
time in SUCAs in short, a high performance is requested for warning and response.
Lemma 5: Flood preparedness by individuals
NSM is not only based on the involvement of the public in the planning process but also
during the hazard event in SUCAs. Residents have to build their capacity to attain flood
resiliency of their properties.
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Lemma 6: Make hydrological system and efficiency of FPRM in SUCAs transparent to
spatial planners via rapid assessment tools such as Hydrological Sensitivity Matrix (HSM)
Spatial planners run at the front line of implementing NSM, which have to be considered
already in the planning process. As in general FPRM require more space than traditional
methods and their performance varies in dependence on local conditions, those new methods
are required already in the initial planning phase.
Lemma 7:Requirements related to decision-making and implementation
Flood Management of SUCAs is to be analysed on a catchment level as in case of large rivers
and decisions are to be comprehensible and transparent for all stakeholders in order to be
accepted and supported.
Phase 2: Main obstacles of present social, institutional and legislative situation opposing
application and successful operation of NSM
In Germany the main problems related to the key requirements have been identified as:
 Rather low awareness and ignorance of the flood problem by residents (Lemma1)
Residents have in general poor understanding of the complexity of urban flood management
They are not informed about the changes in German Water Act driven by the new European
Flood Directive. Thus they are not aware of their responsibilities in flood risk management.
For any mismanagement, the authorities are blamed.
 Insufficient communication lines between public and relevant authorities (Lemma1)
Due to surfeit in administrations dealing with urban flood issues public does not have a “one
stop shop” to contact. The public has poor trust in authorities’ readiness to help, but they
respect and acknowledge their competence. Administrations do see the need for better
information and interaction with the public, however they do not have sufficient resources.
 Rigid legislation and administrative structures (Lemma 2)
The German Water Act, 2005 establishes a legal framework that supports and prioritises the
implementation of NSM. The application of SUDS is regulated by law and obligatory for all
new urban developments in most German states. ATV 138, 2002 standards set the
requirements for their design. Similar to the storm water pipes, the SUDS are not designed to
convey extreme floodwater. No national law or regulation in Germany requires the proof of
stormwater discharge into watercourses for flood situation above the design flood of the
pipes. As the regulations of the water tariffs do not include expenditures others than for the
maintenance and renewal of the pipes, the storm water utility (SWU) cannot use their fees for
financing SUDS. Distributed responsibility for flood risk management between the SWU for
the sewerage system and the authorities for watercourses creates mistrust and dissociation.
The responsible water administration has not sufficient financial and human resources to
provide sufficient maintenance of the urban watercourses. Any extension of their
responsibilities is impossible opposing the implementation of NSM due to lack of resources.
 Ambiguity in responsibilities, insufficient cross-disciplinary cooperation and transparency
between institutions, above all between spatial planners water authorities (Lemma 2)
The interface between responsible agencies and authorities is often ambiguous leaving room
for misinterpretation. For example, in Hamburg, although the boroughs have been given more
responsibilities regarding the management in SUCAs since January 2007, the city authorities
can assert a claim on a development plan and push it forward even without consent of the
relevant borough. Current practice shows that there is still lack of cooperation and common
language between the spatial planners and water managers on the local level. Whereby the
spatial planners call for more urban development and further compaction of urbanised areas
even in SUCAs (e.g. in Hamburg within the “growing city” concept), the water authorities
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call for reduction in volume and peak in the urban catchments. An alarming example is the
Kollau catchment in Hamburg that has urbanisation rate highly above the average. (20,63% to
Hamburg’s 10,30%) (Gätgens, 2007)
 Insurance companies have rather poor interest in SUCAs (Lemma3)
In Germany, the insurance of buildings in the flood prone areas of small urban catchments is
in its initial phase. At present insurance companies define without proof a zone of 100 m
adjacent to the urban watercourses as risk zone in which no insurance of the buildings is
given. However the insurance companies have started to determine flood prone areas based on
mathematical models, for which they rely on their own methods. The „Zonierungssystem für
Überschwemmung, Rückstau und Starkregen“ (ZÜRS) is a zoning system of the German
insurers for estimation of potential risk due to overflow and storm events, which is unique for
the whole Germany. However, the interviewed insurance companies showed low interest in
fostering the adaptation of the built environment to flood through financial incentives or a
reduction of the prime rate in SUCAs.
 Ambiguities in emergency management; responsibility split among different institutions
usually without coordination (Lemma 4)
Although for the studied areas and cities the term “hazard” as well as the procedures in case
of emergency are defined, it is in practice not applicable for the small catchments. A lack of
systematic approach has been observed. In general case, the responsibility is split among the
fire brigade, the storm water utilities and the local authorities and agencies. This distribution
can cause operational problems, as in Hamburg, where no superior body is in charge of the
coordination. Slightly more centralised structure of the city of Cologne simplifies the
emergency management to a certain extent. There are neither flood action plans for small
watercourses nor additional services (e.g. hot line, sms, alarming) provided to the residents.
The extreme thunderstorms in Hamburg July 2002 (damage €15M) triggered an initiative for
more systematic approach when dealing with SUCAs, reflected in designation of so-called,
hot spots i.e. locations that were or could be considerably affected by pluvial flooding. The
hot spots are mapped and explained in detail and serve as a basis for action plans of fire
brigade. At the moment 6000 of those hotspots have been identified.
 Lack of systematic approach to improve built environment (Lemma 5)
People are mostly aware of the flood problem, but at the same time the measures were mostly
taken on an ad-hoc basis. The inadequate measures as well as their wrong implementation
show lack of knowledge and core understanding of the problems in small catchments of both
residents and involved professionals. Although some good examples were identified, in
general there is no systematic approach in selection of appropriate measures. Additionally, the
question who is bearing the costs of such measures has to be raised. Although the residents
are by law obliged to take the responsibility for their properties, the idea of incentives given
by the authorities combined with the insurance should not be abandoned, so that social and
economic differences among the residents can be reduced. Innovative concepts for
automatisation or systems for rapid response for dry proofing measures are to be developed.
 Drivers for FPRM are ecological improvement and not flood mitigation (Lemma 6)
The interviews with the relevant agencies from the three cities showed that the notion of
“decentralised systems” or urban drainages is regarded as something ecologic or sustainable
and as such positive, but the current practice in SUCAs shows rather fragmented planning and
practice on ad-hock basis. Hydrological characteristics of catchments are rarely considered
when deciding on FPRM. In the studied areas of Dresden and Hamburg some SUDS are
implemented, but there are no proofs of their hydrological effectiveness in case of flood
events. Also, the drivers for reopening of watercourses in the studied areas are ecological
improvements and not flood mitigation.
 Underestimation of flood management in SUCAs in decision making process (Lemma 7)
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In all three cities the flood risk in SUCAs is playing inferior role in comparison to flood
management of large rivers passing these cities, e.g. the river Rhine in Cologne, the River
Elbe in Dresden and Hamburg. However the extreme storm event of 2002, which has caused
extreme flood damage in SUCAs of the cities of Hamburg and Dresden, triggered some
changes in the stakeholder’ attitudes.
Phase 3: Supporting the transition process by development of methods to overcome them
In the transition process from current to resilient approach in urban flood management the key
to initialising this transition stage is capacity building (Ashley, 2007). The key stakeholders
should identify and accept their responsibilities, and learn why and how to be proactive in an
adequate way. It should be done by initiating learning process and trainings for individuals
and organisations tailored to their role and requirements for their involvement in flood
management of SUCAs. Interactive Learning Groups (ILGs) have been applied both, to assess
the potential of the stakeholders-residents to build their capacity and to initiate such a learning
process. The learning process, originally based on the Kolb’s cycle (Kolb, 1975) has been
modified based on the previous experience gained within the FLOWS (www.flows.nu)
project, where this concept has been applied on residents and spatial planners. According to
(Kolb, 1975) learning process is divided in four steps from concrete experience trough
reflection followed by the abstraction of the concepts learnt, to testing the acquainted
knowledge in new situations. As additional step to the original Kolb’s concept, an inspection
on site with the objective to get a better insight on the current situation and identify weak
points has been introduced (Figure 1). Based on the results from the workshops,
questionnaires and oral feed back from the participants, the possibilities to initiate the learning
process within the capacity building has been assessed as follows:
Figure 1 Extended Kolb’s cycle as basis for ILGs and Flood Animation Centre (FAC)



8
A shift from “blaming the authorities” to “acceptance of self responsibility” by the
residents has been observed throughout the workshop series. It was initiated by creating
open atmosphere for discussion between the residents and the authorities and by bringing
more transparency into decision-making process on authority level. After the workshops
the residents showed more understanding for possibilities and limitations of the
institutions, mostly related to lack of financial and human resources
For the residents the feeling that they are not left alone with their flood problems plays an
important role. In order to reach them, one should appeal to their emotions and relate the
general problem (such as flooding) to local conditions and their own concerns.
A slight improvement in understanding of the processes and complexity of FM has been
observed. It mostly implies the resilience measures to attain built environment achieved
through hands on approach within the practical part of ILGs (phase 4). For the participants
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
flood problems have local context. In order to improve the general understanding and
forming of abstract concepts, appropriate learning techniques have to be developed.
The outcomes of the ILGs showed that a systematic approach is necessary in “educating” the
public. Such workshops can initiate learning process and cannot be accomplished within one
session. As such it is quite time intensive and requires corresponding infrastructure and
resources. One of the alternative ways to reach the general public and people directly affected
by flooding is to make use of the flood animation centre (FAC) (Pasche at al, 2007), which
creates virtual flooding situations by various flood symbols and animated flood events (Figure
1). Such tools should be tailored to SUCAs specific problems (e.g. short time for response).
Phase 4: Developing methods to support the decision making process (hydrological sensitivity
matrix for pre-screening, flood-damage modelling, cost-benefit-comparison)
For decision making on FPRM and planning procedures in urban environments, a substantial
understanding of the hydrologic processes in a catchment is of crucial importance, however
due to their complexity or low awareness of decision makers, they are usually neglected.
Also, for any thorough hydrologic analysis based on hydrological modelling, data of high
resolution are required, which is often not available. New methods and tools have to be
developed, by which a rapid pre-screening of FPRM already in the initial planning phase can
be performed. Hydrological sensitivity of a catchment has to be assessed, by which the key
hydrological, climatological and geomorphological characteristics are to be analysed and
correlated. Such analysis should be scalable, i.e. be adjusted to different level of detail of
available data. Assessment of hydrological sensitivity of a catchment encompasses several
steps starting with the GIS based characterisation of catchments in terms of their
geomorphological features (Step 1). In addition to standard catchment parameters such as
intensity of relief or hydrogeology, an analysis of the urban morphology has been introduced.
Distribution of different urban structures and green spaces has a decisive influence on design
and performance of FPRM (Gill at al, 2006). For verification of geomorphological parameters
of the HSM, the experiential and standard values has been researched and considered (e.g.
ATV 138, 2002 or Sieker, 2006). The hydrologic characterisation of a catchment and
sensitivity analysis on the catchment response (Step 2) assesses the flood situation by
analysing the flood events of different return probabilities and correlating them to
geomorphological characteristics. Accurate description of processes and influence of
parameter on flood wave is possible only by applying mathematical simulations with rainfallrunoff-models and hydrodynamic fluvial models. As the models are usually not available, GIS
based approaches have to be applied in which similar to the method of (Wackermann, 1981)
the geomorphological parameters are correlated to characteristic parameters of observed flood
hydrograph (such as, discharge of the flood peak per unit area or the runoff coefficient of the
flood wave). By comparison with natural catchments with little urban developments the
impact of urbanisation on the runoff characteristics could be demonstrated. Scenario analysis
of different FPRM (Step 3) gives a preliminary assessment of their efficiency. By varying the
parameters depending on the selected FPRM the influence on the flood wave can be analysed.
Again as mathematical models are usually not available, new tools should be developed and
tailored to the needs of spatial planners such as a tool implemented in the KALYPSO
Enterprise, an open source software for flood risk modelling (www.kalypso.wb.tu-harburg.de,
Pasche et al, 2007). For the study areas, thorough analysis is being performed only for the
areas where mathematical models were already available. Building new models was beyond
the scope of the project.
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Discharge [m3/s]
SQ:status quo
NC: natural conditions
Sc 1: Swales
Time [h]
Figure 2 a) urban morphology and b)scenario analysis in the Kollau catchment
Preliminary results in the Kollau catchment, shows predominantly low land topography (slope
<1% is occupied by 86% of the area) that in general gives favourable conditions for SUDS,
Sieker, 2006. The analysis of urban morphology (Step 1) shows distribution of green spaces
from matrix (upper catchment-1), green corridors (middle part-2) to green patches
(downstream area-3) (Figure 2a.) that indicates potential for SUDS on allotment scale (1),
conveyance systems and multifunctional spaces (2) to SUDS such as swales and filters (3).
The influence of anthropogenic factors resulting in 40% of impervious surfaces, has been
simulated by the Kalypso model, (Figure 2- NC) showing three times higher peak discharge
than in natural conditions (Step 2) and at the same time showing considerable influence of
topography (low land) to flood discharge. The efficiency of different FPRM such swales has
been assessed for different probabilities of occurrence (Figure 2b-Sc 1) (Step 3).
Improvements could be identified such as peak reduction of 50% and peak- SQ to peak-Sc1
lag time of 6 hrs in case of one-year flood, which might vary for the events with lower
probability of return. The results of all studied catchments are to be compared and transferred
to HSM giving the basis for decision making, which is at the moment work in progress.
CONCLUSIONS
The obstacles of rather fragmented and unstructured flood management in SUCAs, due to
their underestimation in the current practice can be overcome by systematic application of
NSM, following the key requirements postulated in the Manchester Agenda. As a
compulsory step in flood management of SUCAs their hydrological sensitivity has to be
analysed and should serve as a basis for planning of FPRM. A Hydrological Sensitivity
Matrix (HSM) has been introduced by which rapid assessment of SUCAs can be performed.
Key factor for successful implementation of NSM and initiation of transition process is the
capacity building of stakeholders that should be tailored to special requirements of their
involvement in SUCAs.
ACKNOWLEDGEMENT
This research work is part of the research initiative Crue Era-Net Funding Initiative on Flood
Risk Management Research and is funded by the German Federal Ministry of Education and
Research. The results presented within this paper were based on the interviews with the
representative authorities and agencies of the City of Hamburg, Cologne and Dresden.
10
Floods in Small Urban Catchments
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
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