Interpolation of mass points (contours) to create a continuous surface

To gain an understanding of the reef-island morphology, a series of transects across the reef island were derived from the constructed DTM. The profiles extracted did not provide much elevation detail therefore a second TIN model was constructed by converting the 0.1 m contours to mass elevation points. Visual analysis of the 0.1 m contour dataset in some low-gradient areas showed that contours were jagged introducing “stepping” artefacts developed during their generation (Smith and Clark, 2005). The 1 m contours did not show these issues as they were spaced further apart producing a smoother surface. In some areas, the contours did not match the spot heights as shown in Figure 4.7.

Figure 4.7. Example of a rare situation where contours (yellow) do not match the spot height (white). Also note the jaggedness of the contours which is an artefact of the generation of contours.

The area where the foot of the beach meets the solid reef flat was obtained by using the 1998 base of the beach (BB). The beach meets the reef flat, and it was assumed that the elevation was 1.166 m above SEAFRAME datum, similar to that of the reef flat (see 4.2.2 for more details).

111 Using the ArcToolbox, the TIN model features were edited. This was done by adding mass points developed from contour and the base of the beach data. The model shows elevation variation in greater detail compared to the constructed DTM. This TIN model has been used to show the elevation across the reef islands as well as showing the risks to extreme water levels.

4.2.7 Future tidal and extreme water levels

The projected tidal and extreme water levels, which can result in inundation and flooding, depend on the selected emission scenario (Cyan et al., 2008; Ramsay et al., 2010). To develop extreme water level projections would require the selection of a timeframe and a Climate Change Scenario that would be realistic for Tarawa’s situation. The probability that the Low IPCC Scenario B2 will occur within the timeframe Te Tibu (2012-2036) is almost certain, compared to the Te Tibu-

mwamwanu (2060-2084) and higher emission scenario (A1FI) as applied by Elrick

and Kay (2009), as it is closer to the average sea-level record of 33 years provided by Ramsay et al. (2010). Extreme water levels rarely occur and vary in their frequency. The Annual Exceedance Probability (AEP) is defined as the probability that a given extreme water level is exceeded within a year (Ramsay et al., 2010). These are provided as 10%, 2% and 1%, with 10% having a higher occurrence probability. Based on these assumptions and the likelihood that these events would occur, extreme water levels were generated from the coastal calculator using the Te Tibu timeframe (2012 – 2036), emission scenario B2 and a 1980-1999 average of level of the sea for present day as the baseline year relative to the SEAFRAME datum. This is the average level of the sea water that has been measured during the period 1980 to 1999. The generated tidal and extreme water levels for Tarawa are shown in Table 4.5.

Tables 4.5 and 4.6 show that elevation of extreme water levels does not vary significantly around the atoll. The magnitudes of extreme water levels (storm surge) in descending order occur on the ocean beaches in the northern part of the atoll (3.28 m) followed by south ocean beaches (3.17 m) and finally the ocean beaches on the western end of the atoll (3.14 m). The ocean areas facing the east experience waves that are 0.9 m higher than those on the southern area, and 0.14 m higher than those

112 on the western end. This difference is related to the exposure of the areas to the trade winds, and in order of decreasing exposure is the ocean area of North Tarawa (east), which is directly exposed, followed by the southern end and lastly the western end. Table 4.6 shows the insignificant differences in the storm tides for different locations on the lagoon side ranging from 3.09 m to 3.10 m. The storm tide levels, in descending order, are as follows: Eita to Tabiteuea (3.10 m), Betio to Eita and Abaokoro to Buariki (3.09 m). The results show a slight difference of 0.01 m between those three corners of the atoll which is within the error of analysis and is probably related to the changes in tide and storm surge characteristics (Ramsay et al., 2010).

Table 4.5. Generated tidal and extreme water levels relative to SEAFRAME datum for Tarawa Atoll developed from the Te Tibu (2012 – 2036) and low emission scenario B2. Water Levels relative to SEAFRAME

datum (m) Lagoon (m) Ocean North (east) coast (m) Ocean South coast (m) Ocean West coast (m)

Mean Level of the Sea 1.70 1.70 1.70 1.70

Mean High Water Spring (MHWS) 2.58 2.58 2.58 2.58

Mean High Water Perigean Spring (MHWPS) 2.71 2.71 2.71 2.71

Storm Tides (10% AEP) 3.09 3.09 3.09 3.09

Storm surges (Storm tides and wave set up

(10% AEP) 3.28 3.17 3.14

Table 4.6. Generated tidal and extreme water levels for Tarawa Atoll lagoon shores relative to SEAFRAME datum developed from the Te Tibu (2012 – 2036) and low emission scenario B2.

Water Levels relative to SEAFRAME datum (m)

Betio-Eita (west) Eita (central) - Tabiteuea Abaokoro - Buariki

Storm Tides (10% AEP) 3.09 3.10 3.09

A high tide exceedance curve has been generated for the next 100 yrs (by 2090) to show the frequency in which each astronomical tide is exceeded (Figure 4.8). The figure shows four curves representing the simulated MLOS relative to present day MLOS, using three different scenarios of sea-level rise: 0.18 m, 0.58 m and 0.78 m. Percentage exceedance of a present day MHWSP (2.71 m) was calculated to be 29% and 96% of all high tides for sea level rises of 0.18 m and 0.78 m respectively (Ramsay et al., 2012). This demonstrates that the Higher Astronomical Tides (HAT)

113 at present day will be exceeded by high tides under differing projected sea-level rise within the next 100 yrs.

Figure 4.8. A high tide exceedance curve simulated from tide predictions for Tarawa Atoll for the next 100 years (by 2090). The dotted black lines represent the mean Higher Astronomical Tides relative to MLOS. In ascending order the dotted black lines represent MHWN, MHWS and MHWPS. The solid black curve shows the MLOS at present day. The other curves running parallel to this curve show the high tide exceedance for different sea level rises of 0.18 m (green), 0.59 m (blue) and 0.79 m (red). The red solid dots show the frequency to which the present day is exceeded MHPWS is exceeded under different sea level rises, for example, under a sea-level rise of 0.79 m, the MHWPS will be exceeded by 96% of all high tides (source: Ramsay et al. 2010, p. 34).

To determine what elevations on land these extreme water levels may reach, transects were extracted from the TIN model and plotted with the different extreme water levels. Three reef islands were selected on the basis that they have a high percentage of low-lying areas, < 2 m above MSL, and that they are inhabited; those with higher population densities were considered a priority, as the risk from extreme water levels would be greater. Reef islands that were uninhabited (see Table 3.3) were not used in this part of the study. The reef islands selected for this analysis are:

a) Bonriki-Taborio with 78% of its area low-lying (Table 4.9) and a population density of 23 persons/ha (Table 3.3),

b) Betio with 25% of its area low-lying (Table 4.8) and a population density of 94 person/ha (Table 3.3), and

c) Taratai with 81% of its area low-lying (Table 4.8) and a population density of 1 persons/ha (Table 3.3).

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4.3 Results