EU-PROJECT

INTEGRATED FLOOD RISK ANALYSIS AND MANAGEMENT METHODOLOGIES

Task 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

objective Identification of flash flood hazards Estimation of extremes (river, estuarine and coastal extremes, incl. marginal extremes, joint probabilities, temporal and spatial variability) Contribution to European Flood Hazard Atlas Understanding and predicting failure modes Predicting morphological changes in rivers, estuaries and coasts Modelling breach initiation and growth Reliability analysis of flood defence structures and systems Flood inundation modelling / methodologies Guidelines for socio-economic flood damage evaluation Socio-economic evaluation and modelling methodologies Risk perception, community behaviour and social resilience Identification and ex-post evaluation of existing flood mitigation and defence measures Investigation of integrated strategies considering planning and communicative instruments (to increase preparedness) Designing and ex-ante evaluation of innovative strategies for flood risk management Radar and satellite observation of storm rainfall for flash-flood forecasting in small and medium size basins Real-time guidance for flash-flood risk management Emergency flood management - evacuation planning Framework for long-term planning in flood risk management Framework for flood event management planning Development of framework for the influence and impact of uncertainty Pilot Study of the River Elbe Pilot Study of the River Tisza Flash flood basins – monitoring and validation Pilot study of the River Thames Estuary Pilot study of the Scheldt Estuary Pilot study of Ebro Delta Pilot study of a coastal site in the German Bight Integrated information management Text-based knowledge transfer Web-based knowledge transfer Face-to-face knowledge transfer Networking and harmonisation Assessment and review Financial Audit Project co-ordination

Weitere Informationen unter: www.floodsite.net

Task 27 – Micro-scale Assessment of Coastal Flood Risks – Pilot Study of a Coastal Site in the German Bight SUMMARY St. Peter-Ording is a large community at the Schleswig-Holstein North Sea coast with the character of a tourist seaside resort. Since 1997 three detailed studies have been carried out on the flood risk situation of this community (Reese 1997; Mertsch 2002; Reese et al. 2003) Therefore, the information aspects and data which describe the community’s vulnerability to flooding during a major North Sea storm flood are considered to be very good. In particular, St. Peter-Ording was one of five case study sites in the state of Schleswig-Holstein (S-H) where extensive research on risk assessment was carried out in the project MERK (= micro-scale evaluation of risks in flood-prone coastal areas). This 3-year project just finished at the end of 2002 and has produced a comprehensive data set (on top of preexisting information) with which most of the key elements for flood risk analysis can be described quantitatively and in GIS format. Thus the following information is currently available in a coastal GIS data base: 9 Physiographic setting of the St. Peter-Ording coastal segment: topography above and below MWL; beach ridge and dune structures and heights; progress of morphologic changes; erosion rates etc. 9 Hydrographical parameters such as tidal range, extreme water levels (height and frequency since 1900), mean and maximum wave heights, etc. 9 Coastal protection situation: dike lengths, heights and profiles; design water levels; previous dike failures, heights of wave run-up during major storm floods; dune sand volumes etc. 9 Socio-economic setting: size of (populated) community area; population (permanent and seasonal); aerial distribution of population; type of housing, community infrastructure aside from housing such as businesses & industries, agriculture, jobs, gross production over all economic sectors ( = damage potential); 9 Parameters describing St. Peter-Ording’s risk situation: probability of extreme flood event; quantified socio-economic risk; dike breach and inundation scenarios; scenario-based damage calculation; existing risk management arrangements; risk awareness and perception by local population . The community is located on the west (=exposed) coast of Eiderstedt peninsula. The size of the study area is approximately 6000 ha; from these about 4000 ha are considered to be flood-prone with the respective height distribution (NN = Ordinance Datum = regional Mean Water Level). The territory of the community amounts to 2800 ha. In this area the irregular topography with intermittent small hills and dunes makes it difficult to draw flood-distance boundaries. Presently, flood protection is provided by a major dike (12,5 km) as well as dune structures (>2,5 km), surrounding the community on 3 sides over a length of more than 15 km. First objective of the study is to combine the failure probability of coastal defences with the detailed socio-economic damage evaluation data, especially on micro-scale level. A comparison of the microscale study with the results of an earlier meso-scale study, also carried out for the Schleswig-Holstein region, showed significant differences. Both under- and overestimations have been identified in the range of factors from 1 : 2.9 to 1 : 0.5. This showed evidence that a micro-scale approach is mandatory for concrete planning/measures. Since a detailed risk analysis is extremely cost and time consuming, a transferable method which still meets the detailed information requirements has to be developed. Hence the second objective is the development of an integrated and transferable micro-scale risk assessment and management methodology for future planning in the coastal zone based on riskanalysis under consideration of public perception and participation.

Within risk analysis some categories are difficult to assess and beyond that intangible and indirect damages are often neglected. The micro-scale method applied in the previous project provides an excellent basis for the risk analysis but shall be also optimised. As a pilot application site one major objective will be the assignment of other methods to the study area.

Background Description and data available St. Peter-Ording is a large community at the Schleswig-Holstein North Sea coast with the character of a tourist seaside resort. The community is located on the west (=exposed) coast of Eiderstedt peninsula (Figure 1). The size of the study area is approximately 6000 ha; from these about 4000 ha are considered to be flood-prone with the respective height distribution (NN = Ordinance Datum = regional Mean Water Level).

N

Baltic Sea

Sc hl es wi gHo lst ei n

North Sea

30

Figure 1:

0

30

60 Kilometers

Map of the pilot site „German Bight“

The territory of the community amounts to 2800 ha with about 6300 inhabitants. In this area the irregular topography with intermittent small hills and dunes makes it difficult to draw flood-distance boundaries. Presently, flood protection is provided by a major dike (12,5 km long, about 8,0 m high) as well as dune structures (>2,5 km, about 10 m and up to 18,0 m high), surrounding the community on 3 sides over a length of more than 15 km. The major threats from the sea result from storm surges which may occur several times a year. Big storm surges have occurred in 1962 and 1976 where during the latter the highest storm surge water levels were recorded with a water level up to 5,6 m. Three other storm surges in 1962, 1981, and 1999 have come over the 5,0 m margin with an increase of storminess over the last decades.

Figure 2 shows the pilot site and a mapping of the heights in the area. It can be seen that the area is very flat, except of dune belt hardly any areas being higher than 8,0 – 10,0 m. The coastal defence line can also be seen from the Figure as a dark solid line in the area.

Figure 2:

Map of heights in the pilot site „German Bight“

Figure 3 shows two locations along the dike line of St. Peter-Ording where the left photo was taken at the very North of the pilot site whereas the second shows the ‘overtopping’ dike made of asphalt at the western part of the site. ‘Overtopping’ dike is a dike where wave overtopping is tolerated.

Figure 3:

Photos of the pilot site showing the coastal dike at the North of the pilot site (left picture) and the overtopping dike in the West (right picture)

The area in front of the sea defences can best be seen in Figure 4 where a view from the dike in the direction of the North Sea is shown on the left photo, and a similar view a bit more in the South is shown on the right Photo.

Figure 4:

Photos of the pilot site showing the foreland in front of the coastal defence line (West part of pilot site on left picture, more in the South on right picture)

Studies performed Since 1997 four detailed studies have been carried out on the flood risk situation of this community (Reese 1997; Mertsch 2002; Reese et al. 2003, Kaiser et al, 2003). - Assessment of the damage potential for the flood prone coastal lowlands in SchleswigHolstein (Research project - 3 years, 1995-1998; funded by the Ministry of rural affairs) - Micro-scale risk evaluation of flood-prone coastal lowlands (MERK – research project - 3 years, 2000-2002, funded by the Federal Ministry of Education and Research and the Ministry of Rural areas. - Risk management as a concept for risk mitigation. Case study of a flood prone coastal municipality in Schleswig-Holstein (Diploma thesis within the MERK project) - COMRISK subproject 3; “Public Perception of coastal flood defence and participation in coastal flood defence planning” (Ongoing research project - 18 months - Schleswig-Holstein State Ministry of the Interior; Coastal Defence Division - Project will be finished 09/2004) - Probabilistic design methods of sea dikes (PRODEICH - research project - 3 years, 19992001, funded by the Federal Ministry of Education and Research and the German coastal research council KFKI) - COMRISK subproject 7, “Risk assessment for the Wadden sea” - research study on probabilistic dike failure analysis of the Ribe sea defences (Ongoing research project – to be finalised within 2004 - Schleswig-Holstein State Ministry of the Interior; Coastal Defence Division) Main objective of the first project was to evaluate the damage potential on a meso-scale level for the flood prone coastal lowlands of the northernmost federal state. St. Peter-Ording was one of more than 280 municipalities potentially threatened by storm surges respectively flooding. Due to the fact that some of these (meso-scale) results were doubtable the former Ministry of rural affairs financed another project, where the damage potential, the damages and the risk was assessed on a much more detailed level, the so called micro-scale. St. Peter-Ording was on of five case study sites in the state of S-H where this MERK project (= micro-scale risk evaluation of flood-prone coastal lowlands) was carried out.

Scales and Assessment Levels Degree of detail

Size of the study area

Local

high

Micro

Regional

Meso

Medium

Macro

(Inter)National

low

Expenses, Effort

Low

Medium

High

Fig. 1: Scales and assessment levels Within the EU-COMRISK project St. Peter-Ording was selected as the German study area. The aim of this project is an improved risk management for coastal flood prone areas and to give recommendations for methods to improve public perception of the risks of coastal flooding as well as public participation in coastal flood defence. Hence for the municipality of St. Peter-Ording a complete risk assessment (risk analysis, risk evaluation, risk management) has been carried out even on different scales. In a parallel project LWI carried out a basic research study to compute the probability of dike failures which has been applied to typical North Sea dikes.

Purpose and objectives Risk analysis Risk analysis involves both the determination of hazard and of the vulnerability. The hazard determination estimates the probability of flooding or dike failure. The objective of the vulnerability assessment is to calculate the potential damages and costs of a disastrous event on the basis of the determined overall damage potential. Both have already been conducted for the pilot site. Since risk analysis is a very complex procedure there are still refinements and improvements to do. The first objective within Task 27 is to combine the probabilistic dike failures with the detailed risk analysis data, especially the micro-scale socio-economic damage potential respectively damages. This can generate new insights how the damages and the risk will vary according to different approaches. The already in this area conducted risk analysis on different scales showed evidence that a micro-scale approach is mandatory for concrete planning/measures. Since a detailed risk analysis is extremely cost and time consuming, a transferable method which still meets the detailed information requirements has to be developed. Hence, one objective is the development of an integrated and transferable micro-scale risk assessment tool. The second objective is to provide data and information for the risk analysis methods and tools which will be developed within FLOODsite. New methods which are foreseen from Theme 1 and 2 of FLOODsite have to be applied to the pilot sites to demonstrate their applicability and show potential advantages and weaknesses. Task 27 has a lot of data available for this objective right now but will also be able to provide more information if developed tools should require this.

Risk evaluation As introductorily mentioned there has already been some research on this topic. Within an EUInterreg IIIB COMRISK (sub-)project the perception and participation has been investigated. We also conducted two further interviews in flood prone coastal lowlands. Due to the evaluated risk perception and participation procedures in this area, the dissemination of the information can be well directed to the communities and management strategies can be elaborated according to the specific needs. In the federal state of Schleswig-Holstein and also within the district of the pilot site several projects are on the way. Due to the identified lack of information and risk awareness it is intended to built up a mixture of information instruments (internet, leaflets, exhibitions, etc) and test their effectiveness. A succession of COMRISK is also planned, where information and participation instruments also play an important role. Hence the pilot site St. Peter-Ording not only provides data and experience concerning risk evaluation but also offers numerous possibilities to apply the FLOODsite tools and instruments to the practise. Risk mitigation The coastal defence authorities in the Federal State of Schleswig-Holstein follow the objective to establish an integrated coastal defence management scheme. Thus new approaches, tools and ideas are necessary. Besides national activities the FLOODsite project may contribute to this objective. The management methodology for the future planning in the coastal zone shall probably be based on riskanalysis under consideration of public perception and participation. Hence it is of great interest to contribute with practical requirements and maybe test or implement selective management or mitigation concepts or tools.