Week 1 - TASK

10y , 20y, 50y and 100y hydrographs 
and associated flooded areas in the Var low valley

In order to achieve this task, we first had to :
- design the storm with IDF curve obtained through a local formula
- obtain the hydrographs from hydrological modelling
- compute the hydraulic modelling 
- analyse floodmaps given by the hydraulic modelling

Hydrological modelling : rainfall - runoff modelling

1. Storm design

In order to obtain hyetographs, we used a local formule given by Meteo France that gives us the IDF curve.

So we obtain different IDF curve for different return periods :

After obtaining the different IDF curves from the formula, we can obtain hyetographs with alternating blocks method with different peak values that are displayed in the table.


2. Hydrological modelling 

We decided to compute hydrographs with different softwares :

                                                                HEC-HMS            MIKE 11 + MIKE SHE

2.1 MIKE SHE coupled with MIKE 11 

Initially MIKE SHE coupled with MIKE 11 was chosen in order to obtain hydrographs for the hydraulic modelling.

For this particular investigation, the design storm was used to represent uniform rainfall over the whole catchment. The boundary conditions used were assigned for each stream according to annual average values reported by Banque Hydro, in order to consider interflow contribution. 

The other parameters used to input into MIKESHE were: 
  • A DEM of 75m was used representing grid, land-use and topography
  • The net rainfall fall fraction was 0.9
  • A simulation period of three days
  • Finite difference method
  • Initial Manning roughness (M): 20 m1/3/s
  • Initial interflow for the whole catchment: 300 m3.s-1
  • Simplified Overland Flow Routing. 


 The HEC-HMS model features :
- 5 sub basins
- 3 river reaches   
- 1 outlet

Differents methods have been applied :
  • to simulate loss, SCS Curve Number method has been used
  • for transforming, Clark Unit Hydrograph has been used
  • Kinematic Wave method has been used for routing process

Then, calibration has been made with the 1994 flood event with following parameters :

From this figure, it is possible to deduce that HEC-HMS provided a relatively acceptable result compared to the observed values, indicating that this calibration is a fair representation of reality. Of particular note, the time of the peak discharge of the simulated hydrograph occurred at the exact same time as the peak discharge recorded for the 1994 event. The peak value was obtained through HEC-HMS, this value was smaller than the observed flood event value, however, this is to be expected as the 1994 event was a particular and unique event. Moreover, such a peak discharge was recorded during the 1994 flood event as two weirs were destroyed during the event, contributing to a large flood wave and a large peak value seen in this figure.

Finally, we can obtain hydrographs :

Hydraulic modelling

For hydraulic modelling, we used IBER. 


10 years return period floodmap

20 years return period floodmap

50 years return period floodmap

100 years return period floodmap