European Water Association Expert Meeting on Pluvial Flooding 28 October 2009, Brussels

Working Group F Floods27 October 2010, GhentFeedback from Questionnaireon Pluvial Flooding in EuropeRonnie FalconerEuropean Water Association

European Water Association Expert Meeting on Pluvial Flooding 28 October 2009, Brussels Member State representation and invited experts involved in pluvial flooding and urban flood management Italy, Hungary, Poland, Netherlands, Ireland, Portugal, Spain, UK, Commission, JRC and EWA Draft report circulated to interested MS including France, Germany and Sweden Report tabled at WGF meeting April 2010 Key outcome – circulate Questionnaire to determine extent of problem across Europe and how this type of flooding being managed.

Pluvial Flooding Pluvial Flooding is flooding as a result of heavy rainfall when water which does not infiltrate the ground pondsin natural or artificial hollows or flowsover the ground as overland flow, beforeit enters a natural or man-made drainage system or watercourse …… or when it cannot enter because the system is already full to capacity. Usually associated with short duration high intensity rainfall but can also occur with lower intensity rainfall over longer periods, or melting snow, and can be worse when the ground is saturated, frozen, compacted, developed or otherwise has low permeability. High velocity overland flow and deep ponding and pose a particular hazard.

Questionnaire Responses from Member States and FloodResilienCity project partners • Initial questionnaire on pluvial flooding circulated to Interreg IVB FloodResilienCity (FRC) project partners - Nijmegen, Flanders, Mainz, Orleans, Paris, Bradford and Dublin • Combined questionnaire on flash floods and pluvial flooding circulated to Member States in advance of the Working Group F Workshop on Flash Floods and Pluvial Flooding hosted by Italy and held in Sardinia in May. • Combined questionnaire responses reviewed at the Sardinia workshop • Responses relating to pluvial flooding abstracted and combined with those from FRC project partners

Questionnaire Responses from Member States and FloodResilienCity project partners • Themes covered by the questionnaire included: • - Characterisation • - Analysis • - Scale of the problem and institutional management • - Data and records available on flood events • - Protection measures • Approximately two-thirds of Member States (MS) responded through their representative organisations

Characterisation Q1.1 - Can you easily classify flash floods and pluvial flooding events occurring in your country?The majority of MS responded “yes” but for some both types of flooding occur together and the distinction is not always clear. Q1.2 - Are there any flood events circumscribed in time and space that do not properly fit with the definitions provided? The majority of MS responded “no” but it was pointed out that other characteristics can be relevant. Q1.3 - Which historical events can be included in the proposed typologies? / What are the causes of those events? / What are the effects and damages? All MS were able to refer to specific historic flood events and many to specific pluvial events. All are concerned to various degrees. The relevance of topography, drainage capacity and soil saturation was highlighted as well as rainfall. Damages are mostly economic but, in some cases, human causalities are reported (more often associated with flash floods)

Characterisation Q1.4 - Have you detected an increase in flood event occurrence frequency? Responses varied - some reported an apparent increase in frequency – others no significant trend. Some MS were able to report an increase in damages but this may be due to change in land use. apparent increase in frequency or damagesno significant trend or evidence inconclusive

Analysis Q2.1 - Which flood monitoring and early-warning systems are in place in your country?All MS appear to have implemented some form of flood monitoring/early warning system relevant to either or both types of flooding. Q2.2 - What kind of statistical analysis is performed to assess hazard? Which return periods do you consider? A range of approaches were reported. Only UK and Ireland appear to have performed or are undertaking mapping of pluvial/surface water susceptibility at national level. Reported return periods ranged from 1 in 25 up to 1 in 1000. Q2.3 - Which parameters are used to assess magnitude of flood event?A range of parameters were reported including extent, depth, duration, discharge, precipitation intensity, source, flow velocity, damages, costs and risk to life. Q2.4 - When assessing the significance of flash and pluvial floods, do you refer to causes or to the effects of a flood event (or a combination)? For example, rainfall intensity, extent and damage, etc. The majority of MS consider both causes and effects. Both Ireland and UK reported that the criteria for significance are likely to include both probability and consequence.

Member State responses FRC Partner responses Q3.2 - Are these types of flood recognised as a potential flooding problem? The majority of MS recognise that there is a potential problem with both types of flooding (flash floods and pluvial flooding). Some MS consider both as potentially serious – others consider one more significant than the other. FRC partners generally reported a higher degree of recognition of pluvial flooding as a potential problem.pluvial flooding more of a problem

Scale of the Problem Q3.6 - Are there particular areas that are considered to be particularly vulnerable to these types of flooding or is it considered to be a risk in all areas of the country?There was a wide range of answers from MS and difficult to draw a distinction for pluvial floods. Those that could do so indicated that pluvial flooding can be a potential hazard across the whole country and in some countries work is in hand to identify areas that may be more vulnerable. FRC partners generally considered pluvial flooding can be a potential hazard in all areas. Q3.7 - Are flash flood and/or pluvial flooding more of a problem in urban areas or rural areas or both?Pluvial flooding is considered by a number MS as more of problem in urban areas possibly due to the greater potential for damage and higher vulnerability. Impermeable ground can increase the risk in urban areas. FRC partners generally considered that both urban and rural areas are at risk although greater damage could occur in urban areas.

Institutional Management Q3.1 - Are flash floods and/or pluvial flooding defined in national legislation or guidance? If so, please provide details. The majority of MS appear not to have specific definitions for pluvial floods in national guidance or legislation as yet. There are however some exceptions including Hungary and UK. Some FRC partners were able to refer to specific guidelines. Q3.3 - Which authority is responsible for performing the Preliminary Flood Risk Assessment under Art. 4 of the Floods Directive? MS responses varied - can be either a national authority or regional / local authority, sometimes in combination. Most FRC partners reported collaboration between national and regional / local authorities.

Institutional Management Q3.4 - Is there an active participation of local authorities to manage and assess risks from these types of floods? If so, how does it happen? In general this takes place in a majority of MS although there are a few exceptions. FRC partners indicated that participation takes place in dealing with pluvial flooding although the level of participation can vary. Q3.5 - Are any institutional changes in this respect anticipated by the transposition of the Floods Directive (2007/60/EC)? For most MS there will be no change although for a small number some changes are being made or are under consideration. Similar response from FRC partners.

Data and Records Available on Flood Events Member State responses FRC Partner responses Q4.1 - Are records of floods and/or pluvial flooding gathered?Responses from MS varied – most gather records and a number gather records of pluvial flooding for significant events. FRC partners responded that records for pluvial flooding are generally available – in some cases combined with records of other types of flooding. specific records of pluvial flooding records of flash and pluvial flooding but combined partial records (both types)

Data and Records Available on Flood Events Q4.2 - Which is the body in charge of collecting data relating to these floods? For many Member States this function is centralised by a national body. In the UK local authorities have a key role in gathering data on surface water flooding. FRC partners also highlighted the important role that local authorities can play in this. Q4.3 - What is the content of records? (Type of flooding / causes / date & time /duration / hydro-metorological data / flood extent / depths / flows / damages and loss of life) There was a broad range of responses from MS on the specific content of information gathered. Many indicated that most of the information listed is gathered but were not specific on any differences for flash floods and pluvial flooding. A number of the FRC partners indicated that much of the information listed is gathered for pluvial flooding.

Data and Records Available on Flood Events Q4.4 - What is the period covered by available record? There was a wide range of periods in the responses from MS – some have long records (some over 100years) although it was difficult to specifically single out record periods for pluvial flooding. The majority of FRC partners indicated good records of pluvial flooding in recent years – in one case up to 40years. Q4.5 - In what formats are data available? There appears to be a range of formats amongst MS but a significant number now have digital and GIS databases. A similar response was provided by FRC partners.

Protection Measures Q5.1 - Is national guidance available which is relevant to managing the risk of these kinds of floods? If so, please provide details.Responses from MS varied - some already havenational guidance and for some it is under preparation. For those that do have guidance no clear distinction could be made between flash floods and pluvial flooding guidance but some do or will have specific guidance for pluvial flood management. Several FRC partners also referred to guidance being available for pluvial flooding. Q5.2 - Is other guidance used? If so, please provide details. Some specific examples of related guidance were given although several MS responded “no”. A similar response was provided by FRC partners.

Protection Measures Q5.3 - Are measures already applied to deal with flash floods and/or pluvial flooding? If so, please provide information on the types of measures applied under the broad categories of (i) Infrastructure/conveyance, (ii) Source control and overland flow routing/storage; (iii) Non structural measures; (iv) Other measures. Many MS provided specific examples of the types of measure that are being applied although it was difficult to distinguish between measures applied to deal with flash flood risk and pluvial flood risk. Some did highlight specific measures to address pluvial flood risk including measures based on early warning, source control, controlling overland flow and local flood retention. A number of FRC partners responded that specific measures are being applied or are being developed to manage pluvial flood risk and provided examples.

Overview Characteristics and Analysis: • There needs to be common understanding of the nature and characteristics of pluvial and surface water flooding • An apparent increase in frequency has been noted in several countries • Most countries have some form of flood monitoring/early warning system relevant to either or both types of flooding Scale of the Pluvial Flooding Problem: • Most countries recognise that there is a potentially serious problem with both flash floods and pluvial flooding - some consider one more significant than the other • Pluvial flooding appears to be recognised as agrowing flooding threatmore inWestern and Northern Europebut progressively in other parts of Europe • Pluvial flooding a potential hazard in both urban and rural areas but the risk in urban areas possibly greater due to grater potential damage

Overview Institutional Capacity to Respond: • Institutional mechanismsto deal with this type of flooding are largely already in place • Local authorities have an important role to play – collaboration is generally good • There appears to be a need to develop best management practice for pluvial flooding • Data and Records: • Several countrieshave good recordsof pluvial flooding available and some over a considerable period though often combined with other flood records Protection Measures: • Some countries already have relevant national guidance available or under preparation • Specific measures to manage pluvial flood risk are already being applied in some countries – some good practice examples are available • Forecasting and mapping techniques are rapidly evolving - together these can be used to inform flood emergency planning

Next steps? • Build on outcomes fromthequestionnaire responses • Raise awareness of pluvial flooding as a potentially serious flood risk which can affect all areas including those which would not normally be expected to be at risk from flooding • Learn from currentEuropean research programmesrelevant to pluvial flooding e.g. FloodResilienCity / IMPRINTS and others • Identify further research needs • Develop best management practice and guidanceto help manage pluvial and surface water flood risk across Europe

Discussion

European Water Association Expert Meeting on Pluvial Flooding 28 October 2009, Brussels