Global secular changes in different tidal high water, low water and range levels

Robert J Mawdsley,Neil C Wells,Ivan D Haigh

doi:10.1002/2014ef000282

Robert J Mawdsley, Neil C Wells + Show 1 more

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https://doi.org/10.1002/2014ef000282

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Abstract

AbstractTides exert a major control on the coastal zone by influencing high sea levels and coastal flooding, navigation, sediment dynamics, and ecology. Therefore, any changes to tides have wide ranging and important implications. In this paper, we uniquely assess secular changes in 15 regularly used tidal levels (five high water, five low water and five tidal ranges), which have direct practical applications. Using sea level data from 220 tide gauge sites, we found changes have occured in all analyzed tidal levels in many parts of the world. For the tidal levels assessed, between 36% and 63% of sites had trends significantly different (at 95% confidence level) from zero. At certain locations, the magnitude of the trends in tidal levels were similar to trends in mean sea level over the last century, with observed changes in tidal range and high water levels of over 5 mm yr−1 and 2 mm yr−1, respectively. More positive than negative trends were observed in tidal ranges and high water levels, and vice versa for low water levels. However we found no significant correlation between trends in mean sea level (MSL) and any tidal levels. Spatially coherent trends were observed in some regions, including the north‐east Pacific, German Bight and Australasia, and we also found that differences in trends occur between different tidal levels. This implies that analyzing different tidal levels is important. Because changes in the tide are widespread and of similar magnitude to MSL rise at a number sites, changes in tides should be considered in coastal risk assessments.

Highlights

Tides are a major control on the coastal zone
Results from our analysis show that significant (95% confidence) secular changes have occurred in all tidal levels, and at sites on every continent and around every ocean basin, over the time span of the observations
On the north-west American coast, most sites have significant positive trends in Greater Diurnal Tidal Range (GDTR) and Mean Higher High Water (MHHW) and significant negative trends in Mean Lower Low Water (MLLW). This is observed in the German Bight (Figure 5b), Australia and southeast Asia (Figure 5c) and South Africa; these last two regions have lower data coverage and may not capture all the changes occurring in the tide

Summary

Introduction

Tides are a major control on the coastal zone. Coastal communities are vulnerable to extreme high sea levels and coastal flooding [Nicholls et al, 2007], in which tides play an important role along the majority of the world’s coastlines [Haigh et al, 2011; Pugh and Woodworth, 2014]. Tidal range influences the spatial extent of species in coastal ecosystems [Stumpf and Haines, 1998], while tidal currents control sediment transport [Allen et al, 1980] and the tidal energy potential [Mackay, 2008]. Tidal levels relative to chart datum are used as the legal basis for many national and international boundaries [Pugh and Woodworth, 2014]. Any changes to tides have wide ranging and important practical and scientific implications

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