Coastal flooding in the Maldives: an assessment of historic events and their implications

Abstract

With many inhabited islands only at about 1 m above mean sea level, the Maldives is among the nations most threatened by coastal flooding and sea level rise. However, the understanding of recent coastal flood events in the Maldives is limited and is important to understanding future flood threats. This paper assesses (1) the sea level and wave climate of the Maldives, (2) the sea level and wave conditions during recent coastal flood events, and (3) the implications for flood management and future research. The analysis uses observed still water levels (1987–2015) and hindcast wave conditions (1979–2015). Two significant flood events on 10–13 April 1987 and 15–17 May 2007 are examined in detail. This shows that coastal flooding in the Maldives occurs due to multiple interacting sources. These include long-period (up to 20 s) energetic waves generated in the Southern Ocean combined with spring tides. Wave run-up (mainly wave set-up) appears an essential mechanism for a flood, but is currently poorly quantified. However, as sea levels continue to rise the conditions that produce a flood will occur more frequently, suggesting that flooding will become common in the Maldives. This analysis is a starting point for future research and highlights the need to continue research on flood sources, pathways and receptors, and plan adaptation measures. Priorities include monitoring of waves, sea levels and flood events, and a better understanding of set-up (and other shallow water processes over reefs).

Appendices

Appendix 1: Selection of sources on land height in the Maldives

BBC (2004): ‘80% of its [the Maldives] 1200 islands are no more than 1 m above sea level’

Pernetta (1992): ‘The highest point on Male is “just over” 2 m above sea level; over 85% of the original land area is <1 m above SL. No island stands more than 3 m above mean sea level and most are less than 1 m high’.

Harangozo (1992): The islands are ‘mostly lying less than 2 m above MSL’ and that during 1987 and 1988 experienced floods of up to 1 m in places.

Woodworth (2005): ‘The islands have typical elevations of only 1–2 m above MSL’

Titus (1989): Male is ‘generally about 2 m above sea level, although some of the reclaimed areas are somewhat lower’. Maldives: ‘virtually the entire nation is within 4 m of sea level’.

Kench et al. (2015): ‘The maximum natural land levels approach 6 m above mean sea level (MSL) in places; the mean elevation of land is approximately 1 m above MSL’.

Khan et al. (2002): ‘Male: >85% total land area is ≤1 m above SL (including most reclaimed land), max height ~2 m. Maldives: 80% of the land area <1 m, max height above sea level ~3 m’.

Evans (2013): Average height of the Maldives is 4 ft above sea level; the highest point (entire nation) is <8 ft (2.4 m).

Wikipedia: ‘With an average ground-level elevation of 1.5 m above sea level, it is the planet’s lowest country. It is also the country with the lowest natural highest point in the world, at 2.4 m’.

Henley (2008): This is the citation used within the Wikipedia post, and actually states: ‘nowhere on the Maldives does the natural ground level exceed 2.3 m. Most of its land mass, which totals roughly one-fifth of Greater London, is a great deal lower than that, averaging around 1.5 m’.

UTNE (2011): ‘Our islands are on average just 1.5 m above the ocean’.

UCS (2010): ‘80% of the Maldives are less than 1 m above sea level’.

DIRAM Team and UNDP Maldives (2007): Some islands (e.g. Viligilli which is south of Gan) are as low as 0.7 m above MSL, whilst others (e.g. Kulhudhuffushi: a northern island with >7000 inhabitants) have average elevations of 1.4 m above MSL, and with 2.5 m above MSL natural ridges. Most islands appear to be around 1 m above MSL.

Appendix 2: Methodology for wave power calculations

It is well known that wave set-up on the reef top increases with the off-reef wave power P₀ (or energy flux) (e.g. Munk and Sargent 1948). This can be expressed as follows:

$$P_{0} = \frac{{\rho g^{2} }}{32\pi }H_{o}^{2} T$$

(1)

Appendix 3: WaveWatch III grids and location of tide gauges

See Figs. 7, 8, 9.

Fig. 7

Tide gauge locations (red markers) for which wave time series were extracted from WaveWatch III. Example image is given for significant wave heights during the 17 May 2007 event

Fig. 8

Bathymetry around the Maldives and location of the tide gauge locations (black dot markers) (plotted from GEBCO08 grid: http://www.gebco.net/)

Fig. 9

Significant wave heights (in metres) in the Indian Ocean prior to and during the April 1987 and May 2007 flood events (wave period is plotted in Fig. 4 to indicate the path of the swells)