A pilot flood retention area in La Bassée

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Diane Atkinson
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1 A pilot flood retention area in La Bassée Institution interdépartementale des barrages-réservoirs du bassin de la Seine 8, rue Villiot Paris - Tél. : Télécopie :

2 Flood risk in the Paris region

3 The 1910 flood in Paris

without electricity 2,7 M without water supply 170 000 businesses

4 Flood risk in the Paris Region Flooded area for the 1910 flood 140 km of metro lines would be interrupted people flooded 2 M without electricity 2,7 M without water supply businesses would be affected A 1910 flood would cost more or less 30 to 40 billions

5 The 2016 june flood in Paris

The necessity of a new storage reservoir The existing storage reservoirs control only 17% of the river Seine catchement The river Yonne, one of the tributaries of the river Seine, gives quick and

6 The necessity of a new storage reservoir The existing storage reservoirs control only 17% of the river Seine catchement The river Yonne, one of the tributaries of the river Seine, gives quick and high flow peaks leading to high flood risks in Paris The dam located on the river Yonne controls only 2% of the Yonne catchment Need to mitigate the impact of the river Yonne on the flood risks downstream The only area available for an efficient mitigation of the Yonne flow peaks is the Bassée area, located at the confluence between the Seine and the Yonne rivers

7 La Bassée project

8 The downstream Bassée area o The downstream Bassée area is located at the confluence between the Seine and the Yonne Rivers in the Ile de France region La Bassée o The Bassée area used to be a flood plain for the river Seine before the 1960 s (flooded every 1-2 years)

The downstream Bassée area The Bassée area used to be a floodplain for the Seine river before the 1980 s (flooded every 1-2 years) but two major projects deeply altered its structure and

9 The downstream Bassée area The Bassée area used to be a floodplain for the Seine river before the 1980 s (flooded every 1-2 years) but two major projects deeply altered its structure and hydrodynamics on its downstream part: 1-The local transformation of the river Seine into a shipping canal: o local decrease of the river level with no more sollicitation of the area as a floodplain o increase of the flood risks downstream o drainage of the area leading to a poorer ecosystem 2- Sand and gravel excavation: o random-located extraction sites o resulting holes filled with rain and groundwater

11 Two Objectives - hydraulic aspects : flood alleviation - ecologic s aspects 1

12 Concept: retaining part of the Seine river flow during Yonne flood peaks Pumping from the Seine river into the retention areas Flow decrease downstream from the confluence High flood event on the Yonne river

Features and layout of the global project o 10 retention areas built by the means of shallow dykes (up to 4,70 m high) o a potential retention capacity of 55 Mm 3 ranged over 23 km 2 and 12 districts

13 Features and layout of the global project o 10 retention areas built by the means of shallow dykes (up to 4,70 m high) o a potential retention capacity of 55 Mm 3 ranged over 23 km 2 and 12 districts o 50 pumps with an overall capacity of 230 cumecs (7 pumping stations) o gravitary emptying into the river once the level has decreased (13 sluice gates) Estimated decrease in the water level of the river Seine: between 20 and 50 cm according to the cities location and the type of floods

A second goal: wetland restoration o The downstream Bassée area used to be a frequently

The area is Natura 2000 and Ramsar classified o The second goal of the project is to

restore the former flora and fauna biodiversity by : creation of spawning grounds,

14 A second goal: wetland restoration o The downstream Bassée area used to be a frequently flooded wetland. The area is Natura 2000 and Ramsar classified o The second goal of the project is to restore its dynamics of alluvial wetland o In particular the ecological project aims to restore the former flora and fauna biodiversity by : creation of spawning grounds, combat invasive species transform poplar plantations in wetlands monitoring of the species living in the area: evolution, number other actions of biodiversity improvement

15 La Bassée: a multi-functional area Flood alleviation 55 Mm3 Wetland classified Ramsar, Natura Mineral extraction Archeology and cultural heritage Water resources Agriculture and recreation green-blue

16 A first step : a pilot project

Global project How choose the pilot area - 10 retention area - 58 km of dikes - 2 300 ha of surface area - pumping capacity of

environnemental and uses approach), have been discused with stakehloder during a progressing process during 18 month Pilot

17 Global project How choose the pilot area - 10 retention area - 58 km of dikes ha of surface area - pumping capacity of 230 m 3 /s - capacity of 55 Millions m3 - Global cost : 600 Millions The choice d based on multi-factory analysis (technical, environnemental and uses approach), have been discused with stakehloder during a progressing process during 18 month Pilot project - 7,8 km of dikes - Capacity of 8,8 M. de m3-372 ha of surface area - pumping capacity of 34 m 3 /s - Global cost : 100 Millions

18 Principe de fonctionnement Management principle Pumping : 3 days Storage : 5 d approx. Draining : 15 d-1,5 month Water level : 2,5 m Frequency : 7 to 10 year in average

cumecs) In the 80 s the river Seine was transformed into a great template channel

19 Why filling in the storage compartments by pumping? La Bassée floodplain used to be floodable every 1-2 years before the 80 s (Q>150 cumecs) In the 80 s the river Seine was transformed into a great template channel (Q>400 cumecs corresponding to the 1910 flood) Pumping station Storage compartment Pumping station Up to max. level

Modèle amont : filaire 2D - casiers 35 000 mailles couvrant le lit majeur

20 Modèle amont : filaire 2D - casiers mailles couvrant le lit majeur gravières modélisées par des casiers de stockage Lit mineur : modélisation 1D

21 4. Construction du modèle casier pilote Taille moyenne d une maille : 20 m

22 Réseau de mesures Le réseau de mesures en

Suivi continu: 27 piézomètres Suivi de manière

gravières : 6 Suivi de la qualité des eaux : Seine,

23 Suivi continu: 27 piézomètres Suivi de manière ponctuelle: 69 piézomètres Suivi des niveaux d eau des rivières : 3 dans l Auxence, 1 dans la Seine, 1 canal de Bray-la- Tombe Suivi des niveaux des gravières : 6 Suivi de la qualité des eaux : Seine, Auxence, gravière, eau de nappe alluviale et nappe de la craie

06/12/2016 25/01/2017 16/03/2017 05/05/2017 24/06/2017 13/08/2017 Premières observations Cote d'eau (m NGF) Suivi

24 06/12/ /01/ /03/ /05/ /06/ /08/2017 Premières observations Cote d'eau (m NGF) Suivi hydrogéologique 51,00 Comparaison des niveaux piézométriques et de l'auxence 50,50 50,00 49,50 26-EPTB-AA AUXINT 9-EPTB-CR 49,00 48,50

25 /2017 > Carte piézométrique d Octobre 2016 (piézométrie de calage en régime permanent) :

26 /2017 >dratec Modèle conceptuel : structure géologique Seine Auxence

Further steps The pilot storage area would be operated once every 7 to 10 years on average Water level decreased by 5 to 10 cm in Montereau and Paris 1 invested

27 Further steps The pilot storage area would be operated once every 7 to 10 years on average Water level decreased by 5 to 10 cm in Montereau and Paris 1 invested 2,3 avoided damages the project enables to save 171 M in 50 years time Design studies Environnemental impact studies Public enquiry Construction works

28 Thank you for your attention

The Lower Prut Floodplain Natural Park (Romania)

The Lower Prut Floodplain Natural Park (Romania) Gabriela Costea Natural Sciences Museum Complex Galati (Romania) & member of the Scientific Council of the Lower Prut Floodplain Natural Park The Prut The