When the preventative evacuation of Orléans can be decided on before a flood?

When the preventative evacuation of Orléans can be decided on before a flood? GOUTX David (et al.) Loire flood-forecasting service 1/25

Major floods: a major issue for the Loire valley 2/25

Major floods: a major issue for the Loire valley Potential flood victims in the Loire valley : 300,000 residents 013,500 companies 070,000 employees 3/25

Major floods: a major issue for the Loire valley Potential flood victimes in the Val d Orléans : 65,000 residents 02,000 companies 10,000 employees 4/25

Major floods: a major issue for the Loire valley Directly and slowly flooded area: 9,000 residents Directly threatened by deadly flows if a levee breaches: 22,000 residents - if a breach occurs, 22 000 residents have less than 1 hour to save themselves which is quite impossible! - the evacuation must be decided on well in advance, as a preventative evacuation 5/25

The necessity of a preventative evacuation plan - the stability of this levee is threatened above a certain water level (corresponding to a 4200 m 3 /s stage-discharge at Orléans) - so the evacuation have to begin at least in the 2 days prior to the water reaching that critical level 6/25

The heavy cost of a false alarm - about 1200 hospitalized patients will have to be evacuated but: - 1 ambulance can evacuate only 1 to 4 patients per hour - about 5 to 10% of the patients will die when evacuated - about 40,000 employees will be barred from their place of work for 2 to 5 days and the whole local economy will be paralyzed for an entire week - some sectors of the economy will suffer from interruptions in the supply chain : farming, industries - many public and urban services will not function correctly - the requisitions of shelter areas and transportation means will need a special budget - the Préfet will be accused of excessive precaution 7/25

The genesis of a Loire flood - sudden flood of the upper Loire, reduced by Villerest dam, which represents 1/3 rd of the peak discharge at Orléans - the Morvan increases the volume of flow, representing 1/5 th of the peak discharge at Orléans - flood on the Allier basin, for about 1/4 th of the peak discharge at Orléans - both the Loire & Allier sending higher volume of water simultaneously at the Bec d Allier - propagation to the Middle Loire river 8/25

Flood Forecasting models Orléans Decision about a preventative evacuation Middle Loire 1D-Saint-Venant propagation Burgundian Loire 1D-Saint-Venant propagation Sioule Allier basin Discharge propagation models Dore Bec d Allier Loire & Allier peak simultaneously discharge Villerest (Loire) Discharge reducing dam Morvan Rainfall Discharge Allier Alagnon Upper Loire Rainfall Discharge 9/25

Basic Uncertainties - discharge measurement - the extrapolated value of the discharge, which is used, will fall between [-10% +60%] the actual value +100% True value -100% - this can be found to within [-10% +10%] by direct gauging during the early times of the flood (as hydrometric teams are guided by the flood-forecasters during the event) +100% True value -100% 10/25

Basic Uncertainties - hydrological process modelling (when rainfall is perfectly known) +100% - for moderate floods: the hydrological models are calibrated well enough to give a value which is between [-20% +20%] of the true value True value -100% - for extreme floods: the uncertainties are much higher; as there are no well-measured event for a correct calibration, the range becomes about [-50% +50%] the true value +100% True value -100% 11/25

Basic Uncertainties - hydrometerological forecasts and hydrological consequences +100% - for the next 36 hours, Météo-France provides the flood-forecasters with forecasted rainfall in 3-hrs segments, with their forecasted value falling between [-50% -10%] the true value True value -100% - between 36 and 54 hours, Météo-France provides the flood-forecasters with forecasted rainfall in 24-hrs segments, with their forecasted value falling between [-90% -30%] the true value +100% True value -100% 12/25

Basic Uncertainties - hydraulic propagation and attenuation - forecasted peak flowing out from Villerest - for the next 24 hrs, using the measurement of the volume of water going in the dam, it is possible to estimate, for the next 24-hrs, the flow water going out of the dam; this estimation will be between [-25% +25%] the true value - for the 24 hrs after that, the value of the outgoing flow water from the dam is calculated from measured rainfall, which will be within [-75% +0%] the true value - for the 24 hrs after that, the value of the outgoing flow water from the dam is calculated from forecasted rainfall, which will be within [-100% -30%] the true value +100% True value -100% +100% True value -100% +100% True value -100% 13/25

Basic Uncertainties - hydraulic propagation and attenuation - the real value of the flow going out from the Bec d Allier will be within [-16% +0%] of the sum of the Loire and Allier peak flows - what about the breaches? - if there are any breaches in the small dikes between Givry and Gien, then the true value of the flow arriving at Orléans will be between [-8% +0%] of the forecasted value - if there are any breaches in the high dikes between Gien and Orléans, then the true value of the flow arriving at Orléans will be between [-20% +0%] of the forecasted value +100% True value -100% +100% True value -100% +100% True value -100% 14/25

Combined Uncertainties 1 day before the D-day - discharge measured at Gien - breach between Gien and Orléans 15/25

Combined Uncertainties 2 days before the D-day - discharge measured at Bec d Allier - hydraulic propagation on the Middle Loire river - beaches between Gibry and Orléans 16/25

Combined Uncertainties 3 days before the D-day - discharge measured on the Loire river at Decize - discharge measured on the Allier river at Moulins - simultaneous peak flows at Bec d Allier - hydraulic propagation on the Middle Loire river 17/25

Combined Uncertainties 4 days before the D-day - discharge measured on the Loire river (upstream Digoin) - discharge measured and forecasted from measured rainfall on Morvan tributaries - discharge measured on the Allier river (Chatel-de-Neuvre) and on the Sioule river - simultaneous peak flows at Bec d Allier - hydraulic propagation on the Middle Loire river - breaches 18/25

Combined Uncertainties 5 days before the D-day - discharge measured on the Loire river (Villerest) - discharges forecasted on the Morvan tributaries, from measured and forecasted rainfalls - discharge measured on the Allier river (Saint-Yorre) and on the Dore river - discharges forecasted on the Sioule river, from measured rainfalls - simultaneous peak flows at Bec d Allier, hydraulic prop. 19/25

Combined Uncertainties 6 days before the D-day -discharge forecasted on the Loire river (Villerest) from measured discharges upstream -discharges forecasted on the Morvan tributaries, from forecasted rainfalls (36 hrs) -discharge forecasted on the Allier and Dore rivers from measured discharges upstream -discharges forecasted on the Sioule river, from forecasted rainfalls - simultaneous peak flows at Bec d Allier, hydraulic prop. 20/25

Combined Uncertainties 7 days before the D-day - discharge forecasted on the Loire river (Villerest) from forecasted discharges upstream - discharges forecasted on the Morvan tributaries, from forecasted rainfalls (48-72h) - discharge forecasted on the Allier river and on the Dore river from forecasted discharges on the upper basin - discharges forecasted on the Sioule river, from forecasted rainfalls (72h) 21/25

Typical uncertainties while forecasting a minor flood D (m 3 /s) - Peak flow discharge at Orléans: 3800 m 3 /s (if no breach occurs) Level at which a preventative evacuation of the Val d Orléans is activated Level at which evacuation plan is put on stand-by Possible spread of the forecasted flow value Median forecast In case of a breach between Nevers and Orléans 22/25

Typical uncertainties while forecasting a moderate flood D (m 3 /s) - Peak flow discharge at Orléans: 4800 m 3 /s (if no breach occurs) Level at which a preventative evacuation of the Val d Orléans is activated Level at which evacuation plan is put on stand-by Possible spread of the forecasted flow value Median forecast In case of a breach between Nevers and Orléans 23/25

Typical uncertainties while forecasting a major flood D (m 3 /s) - Peak flow discharge at Orléans: 5800 m 3 /s (if no breach occurs) Level at which a preventative evacuation of the Val d Orléans is activated Level at which evacuation plan is put on stand-by Possible spread of the forecasted flow value Median forecast In case of a breach between Nevers and Orléans 24/25

As a conclusion - the preventative evacuation before a major flood is possible with low uncertainties with up to 4 to 6 days of anticipation of warning - a needless evacuation might happen for moderate floods (between 3900 and 4900 m 3 /s), especially in case of dike failure between Bec d Allier and Orléans - as it is impossible to predict the breach, or to estimate properly the discharges through the breaches, it still would not be safe to suspend a preventative evacuation in case of breaches upstream 25/25

Thanks for your attention! 26/25