ECOLE NATIONALE DU GENIE RURAL DES EAUX ET DES FORETS ENGREF

TECHNICAL SYNTHESIS

FLOOD DYNAMIC RETENTION SOME INTERNATIONAL EXAMPLES

Yvain BOUBEE E- mail : [email protected]

January 2006

ENGREF Centre de Montpellier

B.P. 44494 – 34093 MONTPELLIER CEDEX 5

Tél. (33) 4 67 04 71 00

Fax (33) 4 67 04 71 01 ABSTRACT

The “ Dynamic Retention ” of flooding is a recent French concept, developed at the national level by the CEMAGREF, following the invitation to tender carried out by the Ministry for Ecology and the Sustainable Development (MEDD). The publication by the MEDD of a Guide on the “ Dynamic Retention ” had like function to give a technical support to water stakeholders, project designers and local communities wanting to integrate in their urbanism plans infrastructures in relation with the “ Dynamic Retention ” of flooding. Concretely, this results in the development of constructive and river works at the catchment area scale, in order to slowdown as much as possible the runoff speed of water during rainy events of strong importance. The experience feedbacks start to emerge in France, but it would be useful to know how foreign countries carry out their water policy in their fight against the floods. Which is their guideline? In which measurements the Dynamic flood retention is known, or perceived? Are there examples of implementations?

Key words: flood management, floodplain areas, dynamic flood retention, dry dams, Ralentissent Dynamique, protection crues, laminage crues, bassins rétention, barrages écrêteurs

INTRODUCTION

The national call to projects named « Plans Bachelot » or « PAPI » for « Plans d’Action de Prévention des Inondations » gave a new dynamic in the search for alternative approaches to flood protection. It aimed at supporting more specifically concepts that are different from techniques used until now, which recommend first of all accelerated discharge of water. Indeed, even if they locally improve protection, these traditional techniques only transfer the risk towards towns located downstream.

Among these concepts, that of the Flood Dynamic Retention has been particularly highlighted. The CEMAGREF has been working since the 90s on this concept which aims at designing works that decrease or store flows in order to reach optimal control. In September 2004, the French Ministry for the Environment and the Sustainable Development published a book of guidelines for flood prevention based on RDC, in order to support local communities, designers and owners.

On the technical side, this strategy is based on the realization of small works that are spread across the zone concerned and this requires a wide range of techniques. The objective is to detain water before it reaches significant speed flow and to distribute in time volumes arriving at the rivers. Compartmental retention would be an ideal solution, but it is easy to see why that it’s difficult to carry out. Moreover, combined utilization of “traditional” installations for flow calibration and of storage works and river works for flow deceleration should not to be excluded and can even be strongly considered in some cases.

In addition, this concept is also based on a concern over durability and protection of the hydrosystems. For instance, the use of former wetlands as overflow areas can go in this direction. The construction of Flood Dynamic Retention projects results from the motivation to protect human lives on the one hand, and the ecosystems in the other.

Consequently The Flood Dynamic Retention appears to be the new tool for “hazard” monitoring. But it is today advisable to combine it with legal measures aiming at reducing the vulnerability of some areas against floods (zoning, chart of risks...), provisions of flood management, calling upon operational measurements judiciously answering the catastrophes, as well as compensation systems having the function to repair and/or to compensate for damage. Today, all these criteria take part of a new policy known as « Flood Risk Management », policy which is slowly adding to « Flood Risk Control ».

The goal of this synthesis is therefore to grasp how well Flood Dynamic Retention is understood and perceived abroad, through some international examples.

RIVER WORKS TYPOLOGY [Cemagref, 2004]

A construction work is a realization the size of which is low compared to the size of the watershed in which it is located. However, its association to other constructions of the same type can lead to a visible effect on the discharge. Moreover, river works imply a certain spatial extension and gathers several works, in the catchment area or in the river beds, as well as soil use techniques.

These river works can be featured as following, depending on their spatial localization in the catchment area.

 IN VERSANT

Actions on existing draining networks Draining networks and agricultural ditches can have a strong influence on a flood regime. The installation of buzzards can allow to attenuate the flows into the ditches, taking care to avoid the possible obstructions.

Vegetal growth In order to limit runoff, vegetal invasion actions are undertaken on the watershed flood prone areas. Grassed meadows are preferred to cultivated fields and their maintenance can be the object of a contract between their owner and the community.

 IN THE STREAMBED

Bank maintenance The restoration of the overflow area requires works on the river bed banks. Effective maintenance of the banks contributes to the restoration of flow conditions in prone areas and supports water speed reduction. In addition, this bank maintenance minimizes the risk of encountering sediment deposits coming from the banks into the constructive works.

 IN THE FLOOD PLAIN

Overflow dams At the same time Overflow dams are concerned with the main channel and the flood plain. They intercept the rivers transversely. An overflow outlet at the bottom of the construction ensures the transparency for weak floods, but allows the drainage of the retention area once it’s full, that is to say after huge storms. A safe overflow spillway, located on the upper part allows quick discharges of water during exceptional floods, and a freeboard (rising of the fill equal to the difference between the crest line and the higher water level, fixed for the selected project flood) prevents the whole immersion of the installation.

Use of existing works The question is to know how a road construction or rail work can contribute to the Dynamic Retention. According to the CEMAGREF, such work isn’t able to carry out this function, unless hydraulic and mechanic stability studies of the fills are satisfactory. The installation of a waterproof facing can be considered on the existing fill, as well as the realisation of drains that would confer to the fill hydraulic work aptitudes.

Transversal levees (realization of new works) This technique concerns open trench dams. Transversal fills in the flood plain force water to discharge into the streambed, causing a rise of the upstream waterline and mobilizing the overflow areas. In this case, the operation has to be designed for a flood exceeding the protection objective. As far as the streambed is concerned, it must be protected by rockfill in

order to avoid possible erosion than can occur due to water acceleration at the narrowing point.

Storage works Basins can be positioned beside a river for temporary storages. For instance, in former overflow areas. In other cases, beyond a determined flow in the main channel, the side basin is filled by the river thanks to a specific channel. It will empty in the same river with drains located at its lowest point. Basins are as much as possible natural and obtained by the construction of girdling fills, thus avoiding the clearing operations.

Specific protections Dike construction is not a specific tool for Dynamic Retention, contrary to their destruction, their lowering or their attenuation. However, their use can be justified to ensure the defence of specific sites, although it leads to the local rise of the water.

Nowadays, it is recommended that such a realization is complemented with the construction of a spillway allowing fixing in which point the discharge of the exceeding water will be executed [DEGOUTTE, 2005]. However, the volume occulted by the dike has to be compensated by an equivalent volume obtained by clearing another place.

Mitigation Storage Ditch flow limiters In Versant Afforestation, vegetal invasion Detention basins Slopes, transversal hedges Overflow dams Direct intake storage basins In Flood Plain Open trench dams Diversion storage basins Spreading field polders Banks maintenance Bridges and culverts In Streambed Transversal storage basins Buzzard passages

THE RALENTISSEMENT DYNAMIQUE ABROAD

 RIVER WORKS IN CATCHMENT AREAS

Detention Basins, Limbourg Province, Belgium [E.U, 2005]

The development of detention basins has mainly a local and specific interest, but can also contribute to the mitigation of the water levels in the hydrographical network. This local decrease of the watershed level is a consequence of the use of detention basins, but does not take part of the Dynamic Retention in its first sense. In France, Wallonia and Flanders, the number of detention basins is growing, due to the national subventions. In the wet and dry valleys of the “Heuvelland”, in the south of Limbourg, floods and erosion are frequent because of strong storms. At the present time, about 200 detention basins are in step, and another 50 are planned for 2010.

Detention Basins, Chester Country, the United States [EMERSON, 2005]

Valley Creek watershed, located in suburban Philadelphia, in the United States, has experienced more frequent and severe flooding during last decades, with a parallel growth of the erosion. Since 1970, more than 100 storm water detention basins have been constructed in the watershed to control runoff and to attenuate on-site peak flow rate for large storms of 2-to-100-year return periods. Recently, models were run in order to determinate the efficiency of whole basins and their influence on the runoff regime. The results show that the existing system of detention basins in Valley Creek watershed provided only a slightly attenuation to the watershed's storm water flow regime, in average a mitigation of 0.3 %. Moreover, during the storm of 24th August 2002, the computed peak outflow was slightly increased by the presence of detention basins. This phenomenon occurred because the peak flow rate of the unattenuated original nine basins (those not connected with detention basins) occurred during a receding limb of the flow from the areas that are equipped with detention basins. The two flows can merge to produce a higher peak flow than the one that would have been measured without detention basins. According to the authors of the program, the efficiency of a detention basin should systematically be evaluated at the watershed scale, and not only at the local scale.

Small sills in ditches, Flanders and Netherlands [E.U, 2005]

Thanks to ditches, runoff is accelerated during wet periods, whereas water is extracted in summer to irrigate fields. To fight against floods and the drying-up of soils, some projects are being executed. One of them has just finished, in the transboundary region between Flanders and The Netherlands with the aim of better water management thanks the development of small sills in the ditches. These sills have a double vocation: to keep water for summer periods and mitigate the discharge speeds.

 RIVER WORKS GENERALISED IN FLOOD PLAIN

As soon as catchment surface areas reach a large size, a new requirement appears, that is to say the taking into account of multiple objectives, in various fields but which can be linked to each other (overflow areas, erosion, river dynamics…). Thus, the same work can be multi functional.

Unique function river works

a) Basins along the Elbe River [JRC, 2004]

The JRC (Joint Research Center) is the service of European Commission in charge of providing customer-driven scientific and technical support to Member States for the conception and the implementation/monitoring of EU policies.

Following the disastrous floods in the Elbe and Danube Rivers in August 2002, the European Commission announced in the communication (COM(2002)-481) the development of a European Flood Alert System (EFAS).

Since the beginning of 2003, the JRC has contacted Germany and Czech Republic in order to get necessary high resolution data such as observed daily/or hourly discharge data, river geometric data and data about reservoirs and retention polders, as well as river bed enlargement information ( thanks to dyke movement or floodplain enlargement).

The international river conventions like the IKSO/MKOO (Oder/Odra), the IKSE/MKOL (Elbe/Labe) and the IKSD/ICPDR/ (Danube) are the main structures that deal with transboundary items.

One of the EFAS and JRC missions consists of an evaluation of the possibilities of using new potential flood retention areas to calculate time delay and prolongation of the flood wave along the Elbe River as well as how polders eventually contribute to mitigate the flood peak in Elbe River. The Aim is to work out a decision base for planning and realization of measures to build controlled flood polders.

It is important to underline that Czech feedback from Elbe upstream polders will be gathered with German results in order to determine a joint and organized program.

Relating to the planned time schedule, scenario calculations should be worked out by the end of 2005.

Fig 1 and 2: Polder localisations along the Elbe River [JRC, 2004]

Multi functional developments

a) Erstein polder, Germany [VNF, 2004]

The franco-german program aims at reinforcing the protection against flooding in the upper Rhine plain. Its main objective is to reduce peak flows of bi- century floods of about 12%, in order to diminish 70 cm of the water line downstream of Iffezheim.

This development program suggests the creation of 18 polders (2 in France), which could be quickly overflowed when the Rhine flow exceeds a critical threshold.

Erstein Polder takes part in this program. Located in Erstein, Plobsheim and Nordhouse towns, the polder is like a peninsula, it is completely surrounded by dykes and girdled by water. Its 600 ha are principally wooded.

The program’s interest is double:  create a water retention basin which will have an influence on flood protection.  restore the ecological value of this natural space.

When it is filled, the Erstein polder is like a gigantic basin which can receive up to 7.8 million cubic meters, that represents an average height of 1.20m on the 600ha of the basin. Intake structures located in the Rhine’s dike allow its filling. Moreover, two constructive drainage works located in the low points are used to empty it.

Although of important capacity, this basin only represents 3% of the 270 million m3 of storage estimated by the program. The retention basin filling starts as soon as the river flow reaches 3600 m 3/s in Strasbourg.

To accustom fauna and flora to attenuation floods and to restore alluvial ecosystems close to those that existed formerly, the polder is regularly filled.

Indeed, the successive river works realized along River Rhine, mainly to avoid floods, have little by little modified the supplying system of the alluvial environments. The Giessen (water harms) disappear, the ground water decreases and the forest is drained. The basin’s water supply in water will contribute to save the Rhine alluvial environment.

The natural hydrographical red of the polder, constituted by Giessen, is supplied by the Rhine as soon as the river flow exceeds 1500 m 3/s, that is to say about 60 days per year.

In addition, the basin is partially filled, to an average height of 75cm once a year, in order to accustom animal and vegetable species to these overflow conditions.

The realisation of the polder highlights the will of the different actors that took part in the project to combine effective inhabitant protection downstream against floods and restoration of the ecosystem in the Rhine’s alluvial plain.

The polder’s equipment has required ten years of design and five years of works. It has been realized by Voies Navigables de France and the whole cost of the realisation reached 25 billion euros.

Remark: Contrary to Dutch polders, the Erstein polder isn’t a piece of ground won on the water to permit agriculture or breeding, first sense of the word, but a space intended to be overflowed in crisis situation.

b) Alliance overflow areas / sedimentation, Denmark [JRBM, 2005]

The project presented here has the particularity of combining restoration in the Flood Expansion Areas, and a restoration of the natural river dynamics.

The “Danish Watercourse act” is the national directive defining the orientations for the river management. The Danish parliament signed this decree after having heard that only 900km of 30 000km of rivers still have their natural bed.

Since 1982, more than 1000 projects of natural river restoration have been lead in Denmark. During this period, the development of the action plan n°2 has included the restoration of 16 000ha of humid areas with the double objective of reducing the nitrogenize concentration present as nitrates in the surface’s waters, and of rebuilding the flood expansion areas.

In August 2003, 16 projects representing a surface of 1 021 ha, had been defined, and a total of 111 pre-projects have been studied. The main part of these projects is to restore the original bed of the river and their goal is to support the natural flood of meadows. Thus, permitting the overflow of some areas in the flood plain, the damage risk downstream has been reduced.

As a consequence, the Dynamic Retention appears implicitly as a multi function tool, for flood prevention on the one hand, and on the other for the restoration of ecological environments.

c) IRMA Project on the Rhine and Meuse Rivers [EU, 2005]

In 1993 and 1995, the rivers Rhine and Meuse flooded their banks twice within a thirteen- month period. Catchment areas having no borders, flood control became a matter of co- operation between the relevant authorities of each country (France, Flanders, Wallonia, Germany, Luxembourg, and Netherlands) in order to define a joint flood control plan to the European Commission within the framework of its INTERREG II-C initiative. This program was adopted on 18th December 1997 under the name IRMA (Interreg Rhine-Meuse Activities). It constitutes a combination of three important elements, water management, spatial planning and damage prevention. All of which are stressed by the European Commission:

The projects to carry out are the following:

In the Catchment Area:  retain the water the most upstream as possible, thanks to wooded areas and thus support infiltration. In addition, try to contain urbanized areas and reduce the runoff rate;  give more space to rivers, restore its natural discharging areas and reduce runoff’ speed.

In the flood plain, a lot of changes that were made over the last century must be reversed:  for instance, relocalize dikes that used to produce immersions upstream;  create parallel channels and broaden the river bed.

Project feedback: more than 153 projects were carried out in the flood plain of the Rhine and Meuse rivers between 1997 and 2003.

The program has brought an important contribution to water management. Technical approach has been replaced by a spatial approach using technology.

The results expected for a century flood and once all the projects are finished are the following. “It is more a qualitative estimation than a quantitative estimation, resulting from models, and the implementation has not been yet appreciated.” [WEBER, 2005]

 the peak flow in some areas will be reduced by 20%  the limit watershed will be lowered to 120cm in average  the bed’s area of both rivers will be increased to 125km²  100km of brooks and affluents will have their ecosystem and environment restored  the retention areas will reach a storage capacity of 215 billion of m 3 .

d) Dyke relocalisation, Lent, Netherlands [Ecoflood, 2005]

A river widening project is in progress near the city of Nijmegen, Netherlands, at a point in the River Waal that forms a bottleneck during periods of high water level. The winter bed of the Waal River is narrow at Nijmegen. On the left bank is located the city centre and on the right bank is a narrow flood plain. Behind the dike on the right bank is a relatively open rural area, including the village of Lent. Relocating the dike at Lent would allow to reduce considerably the risk of flooding and would conform to the objectives of the Dutch government’s national policy on river management, “Room for River”. The effects of this measure would be felt over dozens of kilometres.

According to Dirk Van Der Graaf, the Ministry’s project leader since 2002, “the project is examining two alternatives”:

 “the first alternative is to relocate the dike in the near future. This would result in a major, permanent enlargement of the river”;  “the second alternative is to create a channel in the narrow flood plain and keep the area enclosed by the dike in reserve”.

The projects for relocating the dike have caused strong emotions in Lent, because they would involve the disappearance of some 50 houses. The final decision will be taken this year.

e) Polders, Netherlands [Ecoflood, 2005]

In 2001, the expected flow during extreme storms was set at 16000 m3/s, but the directives will be reviewed by 2015 and will contemplate developments for 18000 m3/s flows.

The runoff capacity of the river will be increased thanks to the combined use of several bed enlargement measures, relocating some dikes or creating by-pass for example, as well as cutting the river side areas in compartments.

The Ooij basin is a proposed emergency area. It’s a low lying polder with villages at one two meters above the surroundings fields. It should provide a safe and controlled means of water storage in order to avoid flooding in the downstream areas both sides of the Waal River. However, because of high costs, limited benefits and risks, the government has not implemented the plan.

 OPTIMIZATION OF EXISTING WORKS

Use of hydropower plants as storage areas, Switzerland [Jordan, 2005]

For the development of a flood prediction and management model in the upper Rhone river, an hydrological/hydraulical modelling system for the optimization of existing hydropower plants is in project. Indeed, among the hundred hydropower plants located on this catchment area, only about thirty can have a negative or positive influence on the flood flow from the Rhone. The optimization of water storage in the dams, as well as the derivations management would influence the peak flow rate en the Rhone River and some of its tributaries in order to mitigate the damages during a flooding event.

Fig 3: Overview of the Rhone River catchment area including the major hydropower plants. [Jordan, 2005]

The mean analysis criteria are the storage volume, the derivation capacity and the drained areas. They allow a wide evaluation of the potential of a development for food protection, taking into account flows resulting from snow melting.

This analyse allowed to underline the eight more effective installations a priori for the flood rolling, as well as the more effective eight installations for their derivation potential. Il appears for instance that the Grande Dixence dam owns more than 20% of the total capacity of all the installations located in Valais (1203 million m3), whereas it concerns about 35% of the drained areas of the region (1430 km²). So this installation will be considered with a specific attention by flood protection projects.

o SUMMARY TABLE OF THE LISTED ACTIONS

Belgium Netherlands Germany Czech Republic Denmark Switzerland United-States TOTAL Overflow areas X X X X 4 Storage Basins X X X X 4 Detention Basins X X 2 Diversion Channels X X 2 Dike relocalization X 1 Sills in ditches X X 2 Ecological Interest X X 2 Existing Works Optimization X X 2 TOTAL 4 5 3 2 2 2 1 This table quickly recapitulates natures of river and constructive works presented in this part. It is significant to note that this enumeration is not exhaustive. The absence of cross in a cell does not mean therefore the inexistence of such type of river works in the country concerned. It just means that little information was collected during the research or that they were not described in this study. Moreover, the Netherlands, in accordance with their historical past in the fight against floods, gather in their action the broadest range of topics. The figures on the right-hand side can be indicative of the frequency of return of a topic given in the projects of Dynamic Retention.

CONCLUSION

Flood Dynamic Retention is a recent concept, of which we only start to collect feedback of implementation, at least in France.

It is difficult to find applications of this concept abroad, mainly owing to the fact that the Dynamic Retention is a French approach to flood protection, and there is no exact literal translation in other languages.

Moreover, little information is available about local initiatives, such as slopes, hedges or farming practices. It may be due to the fact that the documents available in libraries or on the Internet mainly deal with adjustments of big scale.

We saw that a lot of projects dealing with Dynamic Retention are being realized on the Member States territory, thanks to the means put at the disposition of the governments by organisations financed by the European Commission (JRC, EFAS). One of the principal observations is that the Dynamic Retention applies more to watershed plains than to catchment areas featured by big slopes and prone to very quick floods. This is illustrated by the different examples of alluvial plains of big transboundary rivers mentioned previously.

What’s more, it is difficult to compare implemented projects in various countries owing to the disparities in scale. Indeed, each realization is specific to a basin, even if it is part of a more general plan. Thus, it is easy to understand that the installation projects of the Yangtze River, in China, can’t be considered from the same perspective as those of small French basins.

Finally, a last notion too difficult to apprehend at the present time but that should not be forgotten is that of the social acceptability of such projects. The realization of Flood Expansion Zones or of overflow areas, with only a hydraulic function, an ecologic function or an agricultural function, should be systematically accompanied by procedures of compensation or of expropriation for the fields intended for overflowing. Compensation systems can thus be set up, under the form of global insurance or step by step compensation.

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