been estimated for this assessment. For example, a design water level of 2 meters results in a total water depth of 2.25 meters. The wave height/water depth ratio is applied to determine the design wave height for the dike stretch. In this example, the design wave height is then equal to 0.6 x 2.25 meters = 1.35 meters.

# Step 2: Defining the required crest height for each dike segment

The required embankment crest height is defined by meeting an allowable/design wave overtopping, using the dike geometry and hydraulic boundary conditions. The allowable overtopping rate is not a constant and can vary depending on the quality of the dike’s landward slope. The landward side of dikes in Vietnam is often unprotected but has grass cover. Hence, a maximum allowable overtopping rate of 10 liters per second per meter has been used as an initial estimate for this assessment. This maximum allowable overtopping rate originates from the current sea dike standards. Actual overtopping depends on many factors, including the magnitude of hydraulic boundary conditions, the obliqueness of the waves, dike geometry, and the use of friction elements such as blocks on the seaward-facing slope. The presence of a wave berm — an additional step on the seaward-facing side of dike that dissipates some of the wave energy — can be included in the design geometry to reduce the required crest height.

A simplified approach has been chosen to define the required crest height, by estimating the required freeboard — that is, the difference between water level and crest height — to meet the overtopping criterion mentioned above. This freeboard is generally expressed relative to the wave height at the dike toe. The assumption is made that a wave berm is generally applied for most of Vietnam’s sea dikes and some friction elements are in place at the seaward-facing slope to reduce wave overtopping. The seaward-facing slope is generally built at a 1:3–1:4 gradient with a wave berm present. The typical freeboard required is then about equal to the wave height to limit the overtopping below 10 liters per second per meter. This approximation of the required freeboard is applied throughout the assessment.

The required crest level also includes a safety height increment and provision for sea level rise and settlement. The safety height increment, established based on guidance from the MARD’s sea dike design standards, varies from 0.3–0.5 meters, depending on the grade level. A provision for sea level has been included in the crest height in accordance with the design guidance, calculated by using the lifetime of the construction (which depends on grade level) and the projected average sea level rise during that lifetime, which is around 5–6 millimeters per year using a medium climate change and sea level rise scenario (MONRE 2016).<sup>5</sup> The provision for sea level rise in the required crest height is 0.2–0.7 meters. No allowance for settlement has been included in this assessment.

# Step 3: Defining the dike system’s investment needs

Potential investment needs are determined using a dike raise unit price. For this purpose, the difference between the required and actual crest height for each dike stretch (determined using Steps 1 and 2) gives a potential deficiency in crest height. This is then translated into an investment cost using a unit price for dike reinforcements from literature reviews of $0.7–1.2 million per kilometer dike stretch and per meter dike raise (Hillen 2008; see also Mai et al). Based on reviews of dike reinforcement programs in Hai Phong and Nam Dinh, this unit price is set at $1 million per kilometer dike stretch and per meter dike raise for the time being. It is noted that 8