Flood Resilience Index

I.               Introduction

Nowadays in France, exceptional events are more and more frequent and the existing structures designed to protect cities from flooding can’t be seen as our only solutions to reduce flood impacts. The resilience is the capacity of a system to continue its activity after a hazard, here we will talk about flood resilience in the city of Nice, so the capacity of this city to come back to its normal activity after a flood event.

In this exercise we will assess the FRI (Flood Resilience Index) of different structures such as school, housing, working place, during the flood event of 1994. This index represents the resilience capacity of a building during a flood event. This method allows the decision makers to evaluate the area that need more protection or investment in resilience, one scenario of improvement will be tried. We will also assess the resilience at the macro scale, to have an overall idea of our scenario on the whole city.

II.            Creation of the scenario

To create an efficient flood resilience plan, we need to focus our results on a particular set of measurements. We chose to focus our scenario on the social, economical and institutional dimensions with a particular attention to the evacuation system, rescue system, community and transportation network that seem to be a priority during and after a flood event in this city. 

III.         Flood Resilience Index

In order to give a flood resilience index to each building we needed to know the function of all of them, for that we were given a map with all those functions (education, administration,…). The first step was to associate a flood resilience index to each function regarding external dependences (access to energy, water, evacuation of waste, communication, transport) and internal dependences (food, occupation, facility of access). We estimate this FRI for different water levels (0.1 m, 0.3 m, 0.5 m, 1 m) and when it was not flooded but in a flooded city, each function being affected differently regarding the water level. For example electric network are not much damaged for a water height of 0.1m so we can give a value of 5 for the energy resilience but when the water reach 0.3m the majority of the plug are submerged so the energy resilience will be reduced to 2.

 

The table below contains the flood resilience index values chosen for each urban function.

Figure 1 : Flood resilience index values for different function and water levels

Thanks to this table, the location of the buildings and there functions, we were able to assess the FRI of each building during the 1994 flood event with ArcGIS. We obtained the following map indicating the buildings FRI regarding the flood depth.

Figure 2 : Flood Resilience Index map

As we can see several houses located near the river bed have a low FRI (2 -3) due to the high level of water in the house (more than 1m). The airport is also with a low FRI due to its proximity with the river bed with a water level of 0.1 to 0.5 m. We can then assess the location that need to be prioritized such as the airport and the houses close to the river. The long area cover by the flood shows that a structural defense would be difficult to construct, flood resilience improvement should be investigate in order to reduce the consequence of such flood event.

IV.          Macro Scale

We want now to assess the FRI on the whole city scale. As done before, we focused our resilience approach on the social, economic and institutional flood resilience. We increased the weight of the variables that were linked to those dimensions such as the availability of insurance, the evacuation / rescue system, the flood risk education...

Figure 3 :  Macro scale FRI of Nice city

We obtained an overall flood resilience of 2.39 which is pretty low. This result means that a lot of variables need to be improved in order to increase the total FRI of the city to a good level. More information needs to be known about the city in order to lead the improvement of the flood resilience in a way that can be achievable regarding social, economical and environmental aspect. This tool is very useful but deep information are required in order to make a feasible scenario. We could then choose between those scenarios to find the more efficient one.

V.             Conclusion

Due to the augmentation of the frequency and the intensity of the rainfall events, cities need to become more resilient. Indeed, structural defense can’t be the only solution to flooding. It has to be assumed that those defenses can be overtopped and solutions need to be found to limit the consequences of those events. The FRI developed in the University of Nice Sophia Antipolis, is a tool that can be used in order to assess the resilience of buildings regarding its function and the intensity of the flood event. This can be very useful to obtain an overall idea of the resilience in the town and the location to focus on. The impact of improvement that could be done on a special domain or type of building can also be assessed. It can be useful for the decision maker to know if it’s better to invest money in resilience (and where) or in flood protection.

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