§ 4.5 Policy Frameworks And Strategies
S PROJECT (VERTICAL) synergetic development
M CITY (HORIZONTAL) connectivity EXTRACT XL / I+I / S / M 6. KARLSRUHE ATLAS RETENTION POLDERS
Figure 35
Overview of retention polders along the Upper and Lower Rhine (Rhine map based on ICPR Atlas of river typologies, urbanization Corine Land Cover 2003, map of retention areas from state water authorities)
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Figure 36
Figure 37
Hydro-Power Plant / Side Channel / Rhine km 174.
Land use: hydro power plants / Conw volume: capacities of all hydro-power plants 45 million m3 / Building period: n.i. / Status: in use
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Figure 38
Weil Breisach Section 1+2 / Rhine km 174.3-218.8.
Land use: free dev’t of vegetation on gravel banks / Context: excavation of flood plain by 6 m / Flood frequency: 15 days /year / Floodable area: 596 hectares / Flood volume: 25 million m3 / Building period: n.i. / Status: land use planning approval (Raumordnungsbeschluss) / divided into 4 sections: section 1 plan approval, section 3+4 plan-approval procedure / Building costs: n.i. / (www.rp.baden- wuerttemberg.de)
Figure 39
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Figure 40
Figure 41
Kulturwehr Breisach / 1965 / Rhine km 242.8.
Land use: mainly forestry, in small parts agriculture flooded until 1970, barrage erected to support ground water / Context: surrounded by Flügeldamm, main dike III and French dike and Franzosenweg Rhine km 219 to the South the retention polder matches the flood prone area / Flood frequency: n.i. / Floodable area: 505 hectares / Flood volume: 9.3 million m3 / Building period: 1960-1965 / Status: in use since 1988 / Building costs: n.i. / (www.rp.baden-wuerttemberg.de)
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Figure 42
Breisach Burkheim / Rhine km 228.2-236.
Land use: mainly forestry / Context: barrage Markolsheim, main dike III, outflow area below main barrage / Flood frequency: n.i. / Floodable area: 605 hectares / Flood volume: 6.5 million m3 / Building period: n.i. / Status: land use planning approval (Raumordnungsbeschluss) / Building costs: n.i. / (www.rp.baden-wuerttemberg.de)
Figure 43
Wyhl Weisweil / Rhine km 241.5-253
Land use : mainly forestry context Rheinau, main dike IV, part of free-flooding area during special use of barrages south of canal / Flood frequency: n.i. / Floodable area: 595 hectares / Flood volume: 7.7 million m3 / Building period: n.i. / Status: land use planning approval, preparation of plan approval procedure / Building costs: xx million Euro co-financed by EU Interreg Rhine-Meuse / (www. rp.baden-wuerttemberg.de)
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Figure 44
Elz Aperture Rhine km 260-268.5
Land use: forestry and agriculture, nature protection area / Context: barrage Gerstheim, main dikes VI and VII, below barrage Gerstheim free / Floodable area: n.i. Flood frequency: n.i. / Floodable area: 469 hectares / Flood volume: 5.3 million m3 / Building period: n.i. / Status: plan approval procedure / project currently on hold by court decision until demanded improvements are made / Building costs: n.i. / (www.rp.baden-wuerttemberg.de)
Figure 45
Ichenheim/Meissenheim/Ottenheim Rhine km 272-276.0
Land use: mainly forestry / area flooded unit 1970 / Context: barrage Strasbourg , main dike IX, retention polder Altenheim / Flood frequency: n.i. / Floodable area: 390 hectares / Flood volume: 5.8 million m3 / Building period: n.i. / Status: preparation of plan approval procedure / Building costs: n.i. (www.rp.baden-wuerttemberg.de)
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Figure 46
Erstein / Rhine km 276
Land use : forest / Context: n.i. / Flood frequency: 1/10 years (3600 m3/s) / floodable area: 600 hectares / Flood volume: 7.8 million m3 / Building period: 10 years of planning 5 years realization / Status: ready for use / Building costs: 25 million Euro / (Guide pratique du risque dìnondation das la schèma de cohérence territoriale de la région Strasbourg, 2002 and http://www.vnf.fr/vnf/img/cms/ Domaine_public_fluvial/hidden/dp_200411191502.pdf)
Figure 47
Altenheim 1+2 / Rhine km 278.3-284
Land use: forestry and agriculture flooded until 1970 supportive infrastructural measures to protect Altenheim, Goldscheuer und Marlen / Context: barrage Strasbourg, main dike X / Flood frequency: 1/x years (03/1988, 02/1990, 02/1999, 05/1999) / Floodable area: 520 hectares (350 hectares and 170 hectares) / Flood volume: 17.6 million m3 / Building period: 1977-1985 / Status: in use since 1988, plan aproval in 1977, ecological floodings at 1550 m3/s since 1989 (by 8/2006 about 100 times) / Building costs: n.i. / (www.rp.baden-wuerttemberg.de)
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Figure 48
Kulturwehr Kehl/Strasbourg / Rhine km 284-290.2
Land use: permanent water storage to level ground water and retention, supportive infrastructural measures to protect Altenheim, Goldscheuer und Marlen / Context: in the area of the Strasbourg Rhine sling, barrage Gambsheim, main dikes XIII, XIV, XV / Flood frequency: n.i. / Floodable area: 700 hectares / Flood volume: 37 million m3 / Building period: 1977-1985 / Status: in use as flood retention basin since 1985 / Building costs: n.i. / (www.rp.baden-wuerttemberg.de)
Figure 49
Freistett / Rhine km 302-309
Land use: fluvial forest and retention, forestry and gravel excavation, agriculture, after the main dike was built in 1974 / Context: barrage Gambsheim and main dikes XIII, XIV, XV / Flood frequency: n.i. / Floodable area: 475 hectares / Flood volume: 9 million m3 / Building period: n.i. / Status: preparatory research completed / Building costs: n.i. / (www.rp.baden-wuerttemberg.de)
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Figure 50
Söllingen/Greffern / Rhine km 317.4 - 329.5
Land use: former flood plain, cut off from the Rhine when the barrage Iffezheim was built in (1974- 1978) / Flood frequency: 1/30 years / Floodable area: 580 hectares / Flood volume: 12 million m3 Building period: 6 years / Plan approval procedure 1998, ready for use in 2006 / Building costs: 67.5 million Euro / (Gewässerdirektion Nördlicher Oberrhein / www.4gwd.de/karlsruhe)
Figure 51
Moder / Rhine km 330
Land use: n-i. /Context: former Moder Delta / Flood frequency: n.i. / Floodable area: 40 hectares Flood volume: 5.6 million m3 /floods at 3500 m3/s / Building period: completed in 1992 Status: built / Building costs: 16.7 million Euro / (http://www.vnf.fr/vnf/img/cms/Domaine_ public_fluvial/hidden/dp_200411191502.pdf)
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Figure 52
Bellenkopf Rappenwört / Rhine km 353.8-359.3
Land use: forest with fluvial remnants, agriculture and water, flood plain cut off from river in 1934/5 Context: highly heterogeneous urbanized landscape / Flood frequency: 1/20 years / Floodable area: 510 hectares / Flood volume: 14 million m3 / Max. speed of flooding: 445 m3/s / Elevation: 116-122 m above sea level / Max flood level: 2.5 meters / Building period: 6 years / Status: preparation of plan approval procedure / Building costs: 67.5 million Euro / (www.rp.baden-wuerttemberg.de; http:// www.rp.baden-wuerttemberg.de/servlet/PB/menu/1158654/index.html)
§ 6.1
XL / The River Scale
In the following section, retention polders along the Rhine (see fig. 35) are evaluated in terms of their global relationship to the river. The atlas will mirror the paradigms regarding their development. The atlas documents all retention polders, planned and realized, along the Rhine in Baden-Württemberg, Rheinland-Pfalz and Nordrhein- Westfalen. This has never been done before beyond the scale of the individual state programs of the federal states. A coherent overview with a focus on the specific typology aims to give information about the heterogeneous contexts and the sectoral development of this specific flood mitigation typology.
Typology
Retention polders are installed by perforating and/or setting back the main dike and reinforcing old dike lines on the land side, many of which outline old Rhine meanders (see Atlas fig. 36-52 and 63-79). In steered versions, they are filled end emptied via in- and outlet gates. They are installed to lower water tables by ‘shaving’ the flood wave’s peak. This is achieved by opening the gates directly when the peak flood reaches the height of the polder. As a steered measure, the impact on water levels is generally more effective than that of non-steered polders or dike set-backs, yet open measures are preferred by ecologists. In some cases, retention polders and dike setbacks are combined (see for example fig. 64 Neupotz-Wörth). Steered retention polders appear along the Upper Rhine and the Lower Rhine only. Along the Mid-Rhine, the steep and narrow valley does not provide the spatial capacities to accommodate polders. For the Rhine branches in the Netherlands, retention polders were not included in the measures of the Ruimte voor de Rivier program, since they produced severe local resistance and were not considered the right strategy within the Dutch context. Noodoverloopgebieden (controlled flooding areas) were suggested in ‘Anders omgaan met water’ in 2000 as a possible emergency measure. If, during an extreme river flood, dike breaks would seem likely, it should be possible to control the flooding by directing it to previously designated areas (of course these would be less densely populated, and with less capital invested). The Commissie Noodoverloopgebieden also known as the Commissie-Luteijn was created in 2002 to advise this strategy, suggesting nine potential areas. Under normal circumstances, controlled flooding areas are considered to be as protected against floods as other areas. Yet, in case of an emergency (exceeding the 1250 year return level) they may be sacrificed to avoid worse floods elsewhere (Helpdesk Water, 2011). After some years of discussion, it was concluded that the costs of developing controlled flooding areas would be very high, and the benefits questionable. The idea was dropped altogether (Tussenbesluit Rampenbeheersingsstrategie overstromingen Rijn en Maas, 2005).
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In expectance of a potential increase in extreme floods as a consequence of climate change, space for events beyond a 1/200 year flood have also been reserved along the German Rhine. Additional emergency retention polders are foreseen in Rheinland-Pfalz and Nordrhein-Westfalen. In Nordrhein-Westfalen the retention polders Islicher Bruch and Bylerward are part of the original Hochwasserschutzkonzept. Their realisation, however, depends on the results of the research currently conducted by the ICPR to assess the impact of climate change on water levels for the German Rhine. Results are expected in 2012 (Umweltbericht NRW, 2009). In Rheinland-Pfalz specific plans have been made for the emergency polders Hördt (max. 36 million cubic meters) and Eich- Guntersblum (max. 28 million cubic meters) as emergency polders for extreme events in addition to the planned measures (LUWG, 2011).
Locations
The retention polders along the Upper Rhine were positioned in the former flood plains, the areas that were free to flood before the channelling of the Rhine in the 19th and 20th century. They were positioned according to the available land and the respective flood impact (Webler, 2009). For the most part they are projected on sites of the former alluvial forests and to a lesser degree on agricultural land. These sites are then empoldered and flooded to mitigate peak discharges, and more frequently flooded for ecological reasons. The aim of ecological flooding is to redevelop the flora and fauna of the alluvial forest. As water levels can not be stored at the same height, the decision to include the rejuvenation of the former flood plain implied an increase from 5 to 13 retention polders in Baden-Württemberg. Upon completion, the retention polders in Baden-Württemberg will total an area of 7000 hectares of which about 70% are forest, 12% are agricultural land and 15% are surface waters (old Rhine arms, gravel pits, fishing ponds, streams, etc.) The remaining three percent are roads and buildings (Pfarr, 2010:146).
Scale
Retention polders require extensive capacities to be effective and considering their scale, the impact on water tables is very moderate - only decimetres. During the floods in February and May 1999 the activation of the installed polders, Kulturwehr Kehl and the polder Altenheim, lowered water tables by 33 and 24 cm at Maxau (Karlsruhe) with measures lasting four to five days (Homagk, 2010). Along the Upper Rhine, the storage capacity of these measures varies between 3.6 million cubic meters (Mechtersheim, fig. 48) and 37 million cubic meters (Kulturwehr Kehl, fig.67). The retention volume depends on the elevation of the site and the available area. For ecological reasons water levels in the retention polders will reach a max. level of 2.50 meters and have a continuous current during a flood event. During ecological flooding, due to topographical variation, not the entire area is flooded nor do water levels exceed the range of decimetres. (Pfarr, 2010)
Constructed Ecologies
In- and outlet structures are needed to steer the retaining process through the main dike at the waterfront and the adaptation of a secondary dike on the land side. Supportive infrastructure is needed to manage seepage and rising ground water levels to avoid damage to the built environment in the vicinity of the polder. Until a critical water level is reached, the in- and outlet gates are kept open to enable ecological flooding. While space for flooding is reserved for events lasting several days (although the entire flood wave along the Rhine takes up to two to three weeks), other functions have fundamentally different time patterns, e.g. ecological flooding requires an annual occurrence. Annual flooding is necessary to redevelop an alluvial forest habitat while flood management will only involve flooding at a discharge beyond 5000 cubic meters for the retention polders before and 6000 cubic meters for those located after the Neckar tributary. Ecological flooding is initiated to recreate the alluvial forest and its inherent biodiversity. For agricultural land use it can be problematic. In Germany, most farmers accept the additional use of their fields as retention areas with a statistical risk of being flooded every 100 to 200 years. However, with the exception of corn, which in turn is too obstructive in the context of, for example, dike set backs, ecological flooding destroys crops (Ebner, 2010).
Seepage Problematic and Local Resistance
By perforating or setting back the main dike, the polder and its inherent seepage problematic move closer to existing urban developments. Conflict potential and thus the pressure not to flood the designated polders during high water levels arises when measures to protect adjoining areas and to ensure other programs are not taken - escape mounds for animals (also demanded by hunters) or wells and drainage systems to reduce the negative impacts of the dammed up water for example. Therefore, to manage rising ground water tables and seepage in the vicinity of the polder, supportive measures are necessary. Ground water wells, drainage systems and pumping stations can be installed between the dike and the endangered developments to avoid the flooding of basements, etc. (RPK, 2008). Local resistance against the polders is rooted in their potentially negative effects for the adjoining areas. For the retention polder Langel in Cologne, 165 property-related objections against the plan decree were filed (Hartmann, 2011). Also against three polder projects along the Upper Rhine files were sued (Homagk, 2010). Homagk sees the cause in a lacking willingness by the municipalities upriver who are largely protected up to a flood level of 1/1000 to provide flood mitigation measures for areas downriver. But also a local threat can be part of the argument. Jürgen Jacob, mayor of Altrip, one of the municipalities that is taking their case to federal court is convinced: “Already today our municipality is affected by floods. From our point of view, the erection of the polder bears incalculable risks for our inhabitants. We hope that now the Federal Administrative Court will finally ensure safety for our municipality” (Baumann, 2010). Ironically, the negative effects rise with load increase. Enlarging retention areas to lower the stored water levels would reduce the effects on ground water levels and the inherent seepage issues. The area affected by the water pressure may be considered as
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an amphibious zone with (re-)development potential. Steering water levels by expanding the retention area could provide an alternative approach.
Global Relation to River Basin
The retention polders are part of ”Rhine 2020,” initiated by the International Commission for the Protection of the Rhine to recreate retention capacities.
Specifically, the construction of barrages between the towns of Maerkt and Iffezheim from 1928 to 1977 has led to a reduction of wetlands and thus a reduced discharge capacity for floods - increasing the risk of dike exceedance and breaches (Homagk, 2010:38). Reduced to a discharge capacity of 1/60 as a consequence of the barrage- construction series, the aim of the High Water Action Plan as a binational agreement between France and Germany in 1982 was to reinstall retention capacities to the 1/200 level as they were before barrage construction (Griesbaum et al, 2007). The program was expanded to include the redevelopment of near-natural wetlands in the projects in the 1990s. This demanded a severe increase in the polders required as their water level was limited to 2.5 meters and required a constant current (Pfarr, 1996). At present, discharge capacities have reached a level of approximately 1/120 with 53% of the retention measures in place (Homagk, 2010). As an accumulative measure, the retention polders along the Upper Rhine aim to lower water tables by 70 cm in total upon completion. In the Treaty of Versailles, the use of water energy along the Upper Rhine was granted exclusively to France. However, when the Rhine discharge exceeds 1500 cubic meters (capacity of barrages) France and Germany are allowed to use excess water to equal terms. Within the Integrated Rhine program, this water is used for ecological flooding. (Pfarr, 2010).
Forms and Quantity of Appearance
Although the agreement between France and Germany on the HWAP was signed in 1982, only three of thirteen measures have been realized along the Upper Rhine in Baden-Württemberg as the Integrated Rhine Program (IRP) (Pfarr, 2010) and in Rheinland-Pfalz it is five of ten (RLP, 2007). Hessen, as the third adjoining federal state along the Upper Rhine, is not realizing polders, but co-financing measures in Rheinland-Pfalz. Along the Lower Rhine, the first retention polder of eleven measures was completed in Cologne in 2009. In France, the two polders Erstein and Moder have long been realized, but so far only Erstein has been used once in August 2007, lowering flood levels by 7 cm at Karlsruhe (Homagk, 2010:40). One reason may be the missing infrastructural support to cope with seepage (Baumgärtner, 2011). In Germany, the difficulty of installing these measures is rooted in the extensive plan approval procedure (Homagk, 2010) and politically supported local opposition. Negative effects in the vicinity of the retention polder prevail and the flood threat remains abstract to the inhabitants, as they have been protected successfully by the installed flood defence system in the past. This 19th-century defensive system not only provided protection against floods, but also enabled land uses behind the defence line to generate growth and prosperity for this previously poor region.
Paradigm Regarding Development
The restoration of the flood plain by steered retention as a concept of proactive flood mitigation is driven by two goals - flood protection (infrastructure) and wetland development (landscape). As a program to reinstall retention capacities as they were before the channelling of the Upper Rhine, the Integrated Rhine program does not involve urban development, but foresees the installation of a series of infrastructural landscapes. Between a highly technical approach to lower water tables and the idea of redeveloping the former wetlands on these sites, peak shaving and ecological flooding are combined with the sites’ programs during the rest of the year (usually agriculture and/or forestry in combination with recreational concepts). German environmental law only permits flood retention measures in former flood plains cut off from the river. Ecological flooding is a precondition for a system of steered retention (Pfarr, 1996). Whereas a monofunctional approach shaped the border between river and land during industrialization (channelling), today the demanded efficiency of retaining measures and an ecological perception of the landscape are combined. This leads to a hybridization of the retention polder as an infrastructural measure to lower flood levels and to remediate the former flood plain with the dynamics of the river landscape. Yet, the synergetic approach is limited to the project scale and reaffirms the system introduced by channelling, which determines the flood plain as a flood- free area. Regarding the heterogeneous programs that have evolved in the former flood plain, neither emergent potentials between them nor those in combination with the polder are considered. The polder projections are therefore just as much sectoral developments. While infrastructural support measures manage potential negative side effects, further capacities of the polder in the context of the urban landscape are not taken into account. Even as expansive measures, retention polders both rely on and stabilize the defensive system.
For their lack of innovation regarding sectoral development in the flood plain as well as their dependency on and support of the defensive system, retention polders may be considered path-dependent.
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§ 6.2
I+I / Initiation and Installation
Initiation
The definition of goals by the ICPR to binational agreements between Germany and France, the coordination and co-financing by the state and the execution by regional administrative authorities make the installation of retention polders along the Rhine a top-down process. France and the participating German states assigned the sites