# 4. Methodology and exposure results

This section explains the methodological details of how exposure was modelled based on the data sources described in the previous sections. Results will be presented only selectively as overview figures. For full results and a discussion of their significance, please refer to the main report (Rentschler et al. 2020).

The quantitative analyses described in this section were conducted using the programming language Python, and relied extensively on the free and open-source libraries Rasterio, Shapely, and GeoPandas.

## 4.1. Accounting for flood protection measures

As described in section 2.1, the flood maps used in this analysis do not incorporate information about existing flood protection infrastructure. In the event of heavy rain or a coastal surge, the underlying simulations assume that water flows freely in the natural terrain without being obstructed by any manmade structures such as dikes. However, in many provinces this is not the case as the Vietnamese government has invested significantly in flood protection infrastructure. Ignoring the existing protection systems and using the flood maps as they are would thus lead to an overestimation of flood exposure.

As part of the main report to which this paper contributes, a team of experts conducted a comprehensive assessment of flood protection measures in Vietnam. For all coastal provinces, the flood protection system was divided into stretches that were classified according to the flood events the infrastructure would be able to withstand, based on characteristics such as the type of the structure or its height.<sup>5</sup> For each province, the current state of existing flood protection measures is assessed. Based on these results, the share of modelled flood extent that existing flood protection infrastructure would stop has been determined per province, return period and flood type. The results hereof are shown in table 3.3 and 3.4, where the numbers represent the share of modelled flood area that would be flooded considering the existing protection infrastructure. Therefore, it decreases the flood extent in those places where dike systems and other protection infrastructure are in place. With this, knowledge based on a detailed qualitative protection infrastructure assessment is converted into a numerical offset. To illustrate this, consider 100 square kilometers of rice paddy in the province of Nghe An that are marked as flooded by a 5-year coastal flood according to the original flood maps that do not take into account protection infrastructure. The expert assessment here states that only 20 percent of the modelled coastal 5-year flood extent in this province would occur due to the protection offered by a dike along one part of the coast, reducing the actual area flooded to 20 square km. In the asset exposure analysis of this report, exposure estimates derived from the unprotected flood maps are treated this way to incorporate flood protection measures.

These protection offsets are applied uniformly per province and event after the flood exposure is estimated. The strongest assumption in the methodology lies in this uniformity – it assumes that the existing protection infrastructure protects all assets to the same degree, irrespective of their location within the province. Given this assumption this methodology cannot be used to identify affected areas within a province. At the national level, however, the methodology produces estimates that hold considerable advantages over the absence of any estimates, as well as over unprotected figures that dramatically overestimate the existing risk.

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