# 3. Methodology of the sea dike system assessment

Using the information presented in the previous section, a methodology — outlined below — was set out to preliminarily define potential investment levels for Vietnam’s sea dike system. The focus of this assessment is to define the difference between the actual and required crest height of each coastal stretch, and to translate this into an investment level using a common practice unit cost for upgrading dikes (per meter of crest height and per kilometer of dike length). This approach is used only for policy analysis, to assess investment needs and define priorities, and has been used globally in other countries, such as the Netherlands. The approach has several limitations, since it does not include any geotechnical or structural assessment of the dike system, inter alia. Nevertheless, this level of analysis is considered sufficiently accurate for gaining a better understanding of the order of magnitude and geographical distribution of investment needs in Vietnam’s coastal protection system.

The methodology consists of three steps. First, the hydraulic boundary conditions of the coastal stretch are defined. Second, the required crest height for each stretch is determined. Finally, based on the obtained information, the investment needs per dike stretch are determined. These steps are explained in detail below.

## Step 1: Defining the hydraulic boundary conditions for each coastal stretch

The design water levels for each stretch of the coastal protection system have been directly adopted from the MARD’s dike design standards. These numbers represent the best possible information available in Vietnam. Frequency curves are available for more than 120 coastal locations, providing water levels for return periods ranging from 1–100 years. In this assessment, each stretch of the coastal protection system has been assigned a design water level from the frequency curve of the nearest point. If necessary, the design water level has been interpolated or extrapolated from the frequency curve.

Apart from design water levels, wave information is also necessary to define required crest heights. The MARD’s dike design guidelines also provide detailed information about waves. However, design wave heights at the dike toe are not available and have to be calculated from offshore wave heights using wave transformation modelling on the most updated bed topography. The wave information used in this assessment has therefore been estimated in a simplified way, as described in the next two paragraphs.

During storms, wave conditions near the dike toe will be severe due to strong winds, and the depth will most likely limit certain wave characteristics such as the maximum wave height that can exist during these conditions. Therefore, a constant wave height (Hs)/water depth (d) ratio has been applied, herein: Hs/d = 0.6. This ratio is considered to be a good first-order estimate of the maximum wave height at a certain water depth, based on coastal engineering practice. This ratio has also been verified and confirmed by analyzing the detailed wave computation results in the current dike standards.

The toe of the embankments is set at -0.25 meters with respect to the reference level in Vietnam. This number has been estimated based on wave and water depth information near the toe from the current sea dike standards. Using the toe and design water levels, a design water depth has 7