Sustainabilityof Water Resources under Increasing Uncertainty (Proceedings of the Rabat Symposium SI, April 1997). IAHS Publ. no. 240, 1997. 487

Assessment of the balance between environmental demands and water resources

PHILIPPE GOURBESVILLE1 European Centre on Geomorphological Hazards, Louis Pasteur University, ¥-67070 Strasbourg,

Abstract The Cotentin and Bessin marshlands (, France) are an integral part of the European network of natural habitats. The quality of this ancient environment is based on the balance between environmental demands and agricultural practices. The main difficulty is to maintain a water level that satisfies agricultural needs and sustains the natural fauna and flora. The winter floods are important for the flora and the reproduction of birds and fishes while the summer water level must remain high enough to preserve the mechanical characteristics of the boggy soils. The major problem is that the difference between a flood level and a dryness level is about 0.4 m. Since 1992 the marsh­ lands have been integrated into a National Park. A strategy to organize the land use and to manage the water levels has been defined for the different valleys. Two elements of the plan are to generate winter flooding and to destroy some embankments to reduce the water levels in the high valleys.

INTRODUCTION

Located at the cross-roads of the route taken by birds migrating from northern to southern Europe, the Cotentin and Bessin marshlands (Normandy, France) provide a key habitat. This must be protected because it is an important area for food, reproduction and wintering for nesting and migratory birds. The marshlands are an integral part of the European network of natural areas. The quality of the marshlands also affects the flora and fauna. For many centuries up to the present day people have succeeded in handing down this rich but fragile heritage and it is important that it continues to be preserved.

AN HISTORICAL "MANMADE" ENVIRONMENT

The Cotentin and Bessin marshlands are located in the low valleys of the Douve, Taute, and Rivers which drain into Veys Bay (Fig. 1). The area covered by these four basins is about 4000 km2 and the marshlands occupy more than 45 000 hectares. The nature and quality of the environment are determined by water and its management. The hydrological conditions are fundamental to the creation of this environment. Until the seventeenth and eighteenth centuries, runoff accumulated in the valleys which were tidal in their lower areas. This hydrology created the Cotentin refuge area.

Also at: SOGETI, 13 rue Karl Probst, F-14000 Caen, France. 488 Philippe Gourbesville

Fig. 1 Location map of the marshlands of the Cotentin and Bessin.

In the tenth century the Normans built the first mills. During this period the low valleys were flooded for 8 to 9 months. Some embankment works started in the twelfth century to create agricultural lands. These projects were continued by the Anglo-Normans during the Hundred Years War. The first drainage projects began in the seventeenth century and were elaborated to fight the endemic fevers. In 1610, King Louis XIII granted a land concession to the Comte of Soisson and agricultural exploitation started. The real modification started at the end of the century with the construction of several sea gates under a bridge designed to protect the area from the sea. The gates prevent sea water penetrating inland during high tides and resulted in runoff accumulating upstream. The construction of the bridge for the Douve - the largest valley - started in 1720 and finished in 1731. The bridge is composed of eight arches with sea gates. A similar solution was applied to the Taute and the Aure valleys. For the Vire, a different solution was chosen; embankments were built on both sides of the river in the whole low valley. The banks protect land from the sea water flooding during high tides. The direct runoff in the flood plain is drained by a series of valves when the water level in the river decreases. In 1826 the sea defences were completed by building sea gates. These works have reduced the flooded period from several months to several weeks every year. The saline area of rivers has been limited to the downstream course. In 1754, a royal decision instituted the owners' federations which managed the maintenance of rivers and hydraulic works. From that time a series of ditches were excavated in the marshlands to allow quick and easy drainage at the end of the flood period. The ditches Assessment of the balance between environmental demands and water resources 489

were also used to irrigate fields during summer. With these works, the maritime charac­ teristics of the marshlands progressively disappeared and allowed a new ecology to develop. From the eighteenth century, the marshlands were used as grazing grounds and for extensive farming of cows and horses. This agricultural practice is very important because it is responsible for the maintenance of herbaceous vegetation. This new balance has been threatened by agricultural development. Farmers realized the high fertility of the fields in the marshlands and wanted to obtain a more efficient production. To cultivate the land, inundations have to be controlled and reduced to a minimum duration. After the Second World War, projects were designed to totally dry the marshlands by pumping. However, these projects were rejected because of the high investment and running costs. During these studies, the importance of soil moisture was underlined by the irreversible alteration of mechanical properties of boggy soils. The water level decrease induces soil drying which can then break down the structure of the soil. The land reverted to its traditional agricultural use with extensive livestock farming. This land use is very important because it is the only way to preserve and maintain the quality of the environment. All these manmade impacts have contributed to the development of an original environment, associated with a strong ecological habitat. In 1986 a survey was carried out in all the municipalities of the Douve, Taute, Vire and Aure valleys to organize a National Park for the marshlands area. This process was initiated by the European Directive of 2 April 1979 which established the marshlands in the Cotentin and "baie des Veys" isthmus as a major ecological area. In 1992 the National Park of the Cotentin and Bessin Marshlands was created, uniting more than 110 municipalities in the and regions. The aim of this structure is to preserve and to valorize the specific characteristics of the marshlands.

FLOODS: CHANCE OR CALAMITY?

One of the major characteristics of the hydraulic system in the Cotentin marshlands is the seasonal flooding of large areas. The sea gates located downstream of the four low valleys determine the discharge conditions. During the summer more than 20 km of the Douve low valley is influenced, while during floods, the influence is limited to approxi­ mately 12 cm for several kilometres upstream of the sea gates when the water level is over the height of 1.10-1.20 m NGF (normal altitude in France). At this level 40% of the flood plains are submerged. The extent of the flooding progresses and more than 95 % of the low valleys are flooded when the water level reaches 1.60-1.80 m NGF. The limit of the flooded area is not modified and only the water depth increases for floods with a return period of between 2 and 30 years. Floods have a major ecological function. The creation of a submerged winter area is necessary to maintain the fodder quality of grasslands. The floods also favour the aquatic fauna and bird population, which are unique in Europe. During winter the two aims — agricultural and ecological — are in harmony. Outside this season, the needs of the different users in the marshlands diverge. The optimum agricultural use of the grass­ lands imposes the absence of flooding which can compromise the quality of the fodder harvests. To achieve this the water must be maintained at the lowest level. To do this the 490 Philippe Gourbesville water level must be progressively reduced during the spring while leaving open the sea gates. However the level must be maintained at a correct height to maintain soil wetness. These particular conditions impose a rigorous management and a precise regulation of several centimetres. During summer the downstream works allow the water to be maintained at a height of 0-0.20 m NGF. The first overflow occurs between 0.40 and 0.45 m NGF for the lowest areas, generally located several kilometres upstream of the sea gates. During summer and with this water management the river has an optimum storage capacity. For the Douve River, the available volume in the river is about 1 190 028 m3. The variation of 0.40 m to reach the overflow limit is equivalent to a volume of 279 711 m3. An hydrological analysis (Gourbesville, 1993a, 1993b) has shown that this volume could be filled up with an effective rainfall of 0.9 mm on only the upstream area of the basin. The rainfalls which supply these volumes can occur during the whole year. The sea gates are an efficient means of regulating water levels during the dry period. However the works have a limited function when the flood plains are submerged. The floods are essential to the evolution of the low valley morphology. During high water periods important sediment transport takes place in the high valleys. In general, clay and limon (silt) particles are mobilized and transported to the downstream area. In the low valleys, during summer levels (controlled by valves in the sea gates), the mobilization and transport of the clay and limon (silt) particles are only possible a few kilometres upstream of the marshlands so a sedimentation area develops downstream. The suspended particles move downstream where transport and mobilization are possible with the sea gates open. A silting phenomenon appears for several months every year. High waters during autumn and winter permit a natural cleaning out of the river. The mobilization of clay and limon (silt) particles eliminates a large part of the sediment. So the longitudinal profile of the four rivers does not show a very spectacular evolution. Furthermore the water level management by the owners' federations reduces the deposition areas by limiting the size of winter floods. Downstream of the sea gates a large quantity of maritime sediment accumulates in the summer when the discharge is reduced. The quantity of sediments is sufficient to disturb the gate opening when the continental water level is over 0.20 m NGF. For cohesive sediments such as silt the transport capacity necessary to erode the deposit is a function of consolidation and soil/water complex. The diameter and the density of the particles are not sufficient to determine the beginning of the erosion and the initial rigidity must be taken into account. Freshly settled silts can be considered as a dense fluid. At the water/silt interface, wave formation leads to particle suspension. The silts, which are consolidated, are resistant to the flow. Rigidity and suspended sediment concentration have been measured on sediments downstream of the sea gates. All the obtained rigidity data are included between 200 to 300 N m"2. The particles can be mobilized at a critical speed of 0.22-0.27 m s"1. General transport is achieved when the mean speed is greater than 0.42-0.50 m s"1. The silt concentrations exceed 500 g H and are close to 850 g l"1 for all the samples. Values for mobilized particles possess a similar concentration. During summer the water velocity downstream of the sea gates is just sufficient to mobilize the particles brought by the sea. However no massive mobilization can occur. A high water period cleans the sediments that arrived during the low water period. Values obtained for the water level 1.80 m NGF - one year return period flood - are Assessment of the balance between environmental demands and water resources 491 just sufficient to realize an effective evacuation of the silts. From 1988 to 1992 there were no floods with a level equal to or higher than 1.80 m NGF; this has allowed the silts to consolidate. The sediments threaten the functioning of the works located in the downstream sections of the Douve, the Taute, the Vire and the Aure Rivers. In order to avoid silting phenomena, a high water period is necessary. This occurred during the floods of January 1993. If this situation does not occur naturally, the continental waters should be retained.

THE VIRE LOW VALLEY SITUATION

The Vire River is restricted by embankments which are designed to preserve the agricul­ tural fields from floods. These embankments - 6 m wide, 3 m high over a length of 35 km - are completed by sea gates located downstream. Water levels lower than a 30-year return period flood are restrained by the embankments. The water level is then higher in the river than in the flood plain. However, the submersion of the flood plain occurs because the ditches are unable to drain water. Direct overflowing occurs with a discharge associated with a return period greater than 30 years. The embankment system maintains a high river level especially in the upstream area of the low valley. This level induces important submersions which directly threaten urban areas. The flood intensity has increased during recent years with the extension of impervious surfaces. The marshland users have analysed the flood dynamics and the possibilities of reducing flood intensity. The most efficient technical solution consists of destroying a large part of the embankments to favour earlier flooding onto the important storage of the flood plain. To evaluate the impact and the efficacy of the technical solution, an hydrodynamic model has been built on the whole low valley area with the modelling system MIKE 11 from the Danish Hydraulic Institute (SOGETI, 1996). This model is calibrated using observed morphology, discharges and water level data. The approach has simulated the removal of the embankments (Fig. 2) and the consequences for different specific floods. The water level decreases strongly - 0.85 m - particularly for the lower intensity floods. The removal of the embankment allows the decennial flood to be reduced to a level less than the annual event. The technical solution shows a clear reduction in flood hazards in the upstream area, however the direct flooding in the low valley is increased. To be acceptable this solution must still allow agricultural uses in the marshlands because these guarantee the ecological quality. Overtopping starts when water levels reach 2-2.2 m NGF at the sea gates and for an input discharge in the low valley of 60-65 m3 s"1. Without embankments, a direct over­ topping occurs with a level of 1-1.25 m NGF and a discharge of 20-25 m3 s"1. In an average year, the discharges are over 60 m3 s1 on 38 days, that is 10% of time. Discharges over 65 m3 s"1 occur on 26 days, that is 7% of the year. The destruction of the embankments is critical at discharges between 20 and 25 m3 s4. The number of days associated with a discharge of over 20 m3 s"1 is 149, that is 41 % of the year. Discharges of over 25 mrV1 represent a cumulative length of 119 days, that is 33% of the year. The modification of the embankments provokes a significant change in flood duration. How­ ever in the low valley the groundwater near the soil surface is present for an annual period that is above 38 days. 492 Philippe Gourbesville

Fig. 2 Location of the embankment destruction area in Vire low valley.

Removal of the embankments increases the flood risk during most of the year. How­ ever this increase is not equal in all seasons. The most significant flood risk is during April, May, June and October. In these periods, the flood hazard is higher and is similar to the situation observed in the low valleys of Douve, Taute and Aure Rivers. An estimate of flood hazard has been made from the information recorded at the sea gates. Table 1 shows the return period of floods caused by direct overflowing with and without the embankments. Removal of the embankments creates a new low valley mor­ phology and imposes a new management of the flow dynamics especially in the spring and summer. The water level in the Vire River will have to be maintained to ensure the irrigation of marshland fields (necessary for the mechanical stability of soils), and to supply drinking water for cattle. However this level must not be too close to the over­ flow level in order to allow dynamic storage during flood periods with heavy rainfalls. Rules for water management that unite the different partners are necessary to establish management methods that guarantee the agricultural exploitation of the marshlands as grasslands, and maintain the ecological quality of the natural environment. The direct link between the ditches and the river would improve flows in the ditch network which has largely degraded today. The restoration and increase in flow would remove a large part of the accumulated sediments. The environment quality — fauna and flora - will be obviously improved in these conditions.

Table 1 Hazards of flooding: the different return periods (in years) for flooding with and without embankments for each month.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

(1) 2 3 5 15 >30 >30 >30 >30 >30 20 5 3 (2) <1 1 1 4 9 15 >30 >30 >30 5 2 1

(1): actual situation with embankments; (2): future situation without embankments. Assessment of the balance between environmental demands and water resources 493

The general reduction in the water level could require rigorous management of the water levels outside the flood period. If the lowest water levels are not modified by the removal of embankments, the spring levels in particular may restrict the management of upstream marshlands and agricultural uses. In this situation simple hydraulic works could be designed to manage the water level. Figure 3 shows the different extent of floods. The minimum occurs just after the flood overflow and is maintained after the passage of the flood hydrograph. In the case of removal of embankments, these areas will be flooded for between 115 and 150 days per year. The intermediate extension corresponds to a phase of flood increase or decrease with a duration of 80-100 days per year. Finally the perimeter of the maximum extension which is similar with or without embankment destruction should occur for no more than 30 days per year.

Fig. 3 Evolution of the flooded areas in the Vire low valley.

WATER MANAGEMENT: AN OBVIOUS CONCLUSION

In the context of the National Park, a dialogue with the different marshland users has been implemented. This procedure was necessary to describe and elaborate the optimal use which is dedicated to preserve the environment with the land use and the water management. The following principles have been adopted: - from December to March, voluntary floods can be organized in the low valleys to preserve the environmental quality and to eliminate the sediment accumulations located downstream of the sea gates; and - during the end of the spring and the summer, the water level is maintained at a reduced height to create an optimal flood storage capacity. The definition of management methods has been realized with an hydrological and hydraulic modelling approach (Gourbesville & Lecluse, 1994). This analysis has 494 Philippe Gourbesville permitted understanding of the hydrology of the marshlands and simulation of different management strategies. With this information, the different users have chosen to restrict the land use to grassland. This strategy associated with the water management allows the ecological quality of the environment to be maintained while optimizing the agricultural production.

REFERENCES

Gourbesville, P. (1993a) Analyse systémique d'une situation hydrologique complexe. Procédures multimodèles appliquées au bassin versant de la Douve et aux marais de l'Isthme du Cotentin. CEREG UA 95 CNRS, Strasbourg. Gourbesville, P. (1993b) Modélisation numérique d'un écoulement transitoire à surface libre en lits composés. XXV Congress IAHR, Tokyo. Gourbesville P. & Lecluse Y. (1994) STORAGE: a STochastic multimOdel pRocedure for dischArGe forEcasting. In: HYDROINFORMATICS'94. Balkema, Rotterdam. SOGETI (1996) Water management in the low valley of the Vire river, Caen.