Exceptional Flood of 2002, September 8Th and 9Th in Gard River, South France

Exceptional Flood of 2002, September 8Th and 9Th in Gard River, South France

Exceptional flood of 2002, September 8th and 9th in Gard river, South France MENBO, Marrakech, May 2005 Hydrological network of the Gard basin MENBO, Marrakech, May 2005 TOTAL RAINFALL IN THE GARD BASIN Remoulins Cumulative rainfall on the Gard for the whole event 08/09/02 09:00 to 09/09/02 16:00 MENBO, Marrakech, May 2005 The famous roman aqueduct Pont du Gard Normal conditions Monday 9th September 2002: The water level is closed to top of the first arch’s level. Build 2000 years ago, it was not damaged. MENBO, Marrakech, May 2005 Some damages In the city of Remoulins The village of Goudargues Maximum level reached Bridge of Ners (Gard). Collias on the Gardon The bridge of the railway has totally disappeared… Here was the village MENBO, Marrakech, May 2005 Station CNR MENBO, Marrakech, May 2005 Typical hydrograph of the event Water level recorded at the station of Mialet (upper Gardon, 240 km2) Peak discharge is 730 m3 s-1. MENBO, Marrakech, May 2005 Field data collected • Maximum level recorded (from AWLR): 26.72 m • Staff gauge reference: 16.06 m • Level of the balcony: 25.0 m • Max level on the house: 26.7 m • Slope: 1.7 m km-1 • Surface velocity: 5 – 6 m s-1 (CNR estimation) Maximum level reached MENBO, Marrakech, May 2005 General situation Bridge downstream the village is a good control station MENBO, Marrakech, May 2005 First estimation of discharge Mean velocity Um / Mean surface velocity Vs 1,2 1,0 0,8 0,6 K=Um/Vs 0,4 0,2 0,0 18,00 18,50 19,00 19,50 20,00 20,50 21,00 NGF MENBO, Marrakech, May 2005 Second estimation of discharge Ratio Max surface velocity Vmax / Mean velocity Um 0,9 0,8 0,7 0,6 0,5 0,4 Vmax/Um 0,3 0,2 0,1 0,0 18,00 18,50 19,00 19,50 20,00 20,50 21,00 NGF Mean velocity / water level 6,00 5,00 4,00 m s-1 3,00 2,00 1,00 0,00 18,00 20,00 22,00 24,00 26,00 NGF MENBO, Marrakech, May 2005 Rating curve Rating curve Q=f(h) (from 1900 to 2001) 4000 3000 -1 s 3 2000 m 1000 0 18 20 22 24 26 28 30 NGF MENBO, Marrakech, May 2005 Rating curve Q=f(h) Estimation 09/09/2002 until 1976 7000 6000 5000 -1 4000 s 3 m 3000 2000 1000 0 18 20 22 24 26 28 NGF MENBO, Marrakech, May 2005 Q=f(h) in log-log until 1976 10000 Extrapolated discharge = 9000 m3 s-1 1000 Log-log graph showing the change in the 100 m3 s-1 slope 10 1 10NGF 100 MENBO, Marrakech, May 2005 As conclusion • Simple tools (topography, velocity correlation…) give a good estimation of the peak discharge; • Results can be produced quickly; • Allow a better estimation of the damages (volume of sediments,…); • Estimations by models are in the same range : – Specific discharges for upper basin range from 20 to 30 m3 s-1 km-2 – Specific Peak discharges reached more than 35 m3 s-1 km-2. • But still estimation and not measurement. • Last point: the cost of rehabilitation (cleaning, restoration…) is approximately 5 millions euros. MENBO, Marrakech, May 2005.

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