Week 2

Work plan 2 Week 1 Week 2 Hydrological models Hydraulic models 1D quasi 2D 2D (25x25 m) Flood resilience 2D (5x5 m) Structural measures

Flood resilience 3 City scale FRI Parcel scale FRI FRI mapping at parcel scale, Nice

Weekly question 4 Structural measures for flood reduction: management of slope & management of weirs including weir removal Strategy Indicator → water level What is flood reduction? Type of model? Scenarios? 1D river model Sensitivity analysis

Flood reduction 5 Our approach: ■a method which is valid for any flood event (not only 1994) ■simple indicator to compare numerous scenarios ■location to compare water level → “highest” flood risk 1D model is suitable ! Flood reduction = water depth reduction Threshold Flood reduction

Type of model 6 2D model ❏ Identify specific locations water begins to overflow ❏ 2D flooding starts at point ❏ This point is similar in 2D and 1D ❏ 2D computational time is not necessary Napoleon Bridge 1D river model

1D model 7 ■Model setup from Week 1 ■Representing the situation in 1994 ■Length: 24 km ■Structures: 9 weirs (2 and 3 destroyed) ■Boundary: 1994 (Qmax= 3000 m3/s - without dam break) ■Location for comparison: 600 m upstream of Napoleon III left levee max. water level

Scenarios 8 Positive or negative impact of weirs Adding additional weirs upstream of the indicator cross-section (representing the situation before dam break) Removing all weirs ➔ without modifying the river bed ➔ with a modified slope Removing weirs one by one to identify the individual impact

Flood reduction indicator Weir 1 Weirs Weir 16 Scenarios Situation in m 8

Scenarios 10 Slope management & weir removal Initial slope + 9 weirs Baseline water level Reference for flood reduction Slope smoothing due to weir removal Irregularities reduction Cross section interpolation Structure removal Steeper slope

Scenarios 11 Location: Branch ,33 mWater levelWater depthDifference in water depth NoScenario Peak Water level (m) Peak Water depth (m)(m)(%) First study - which weir is important? 0Baseline No weirs, initial bed No weirs, new bed additional weirs additional weirs Removing weir Removing weir Removing weir Removing weir Removing weir Removing weir Removing weir Removing weir Removing weir Optimization of weir 1 5Crest level 0 m Crest level 0.5 m Crest level 1 m Crest level 1.5 m Crest level 2 m

First study results 12 Slope + weir management Weir management Impact of weir 1 Impact of weir 10 ?? Results: ➔ Upstream weirs → no or negligible impact on the selected location ➔ Backwater effect of downstream weir (Weir 1) is important! ➔ Further investigation of Weir 1  removing → feasible ??  optimizing the crest level 30 cm 43 cm 27 cm 4 cm

Management of weir 1 13 Actual depth: 4.2 m ➔ Purpose: blocking seawater intrusion to the aquifer ➔ Management options:  lowering (0 or 0.5 m)  1 m → keep actual crest level  increasing (1.5, 2.0 m) ➔ Decision makers: Higher flood risk Water quality problems flood safetyintrusion

Findings 14 Question: Structural measures for flood reduction: management of slope & management of weirs including weir removal Answer: ➢ Flood reduction is possible up to ~43 cm ➢ Upstream weir management has low impact ➢ Backwater effect of weir no. 1 is the most important: ~27 cm ■remove ■optimize ➢ Slope management after removing weirs can help but low impact: ~13 cm

Thank you for your attention 15 Questions to be discussed / future investigation: ➢ Is our chosen criteria enough to evaluate flood reduction? ➢ Consequences of removing/adding weirs on groundwater? ➢ Is there need for 2D modelling in the floodplain? ➢ Impact of the sedimentation accumulation upstream of the weir? Discussion