Complex Extreme Sea Levels Prediction Analysis: Karachi Coast Case Study.

Faisal Ahmed Khan,Ahmed Sefelnasr,Ali Najah Ahmed,Mohsen Sherif,Ahmed El-Shafie,Tariq Masood Ali Khan,Haitham Abdulmohsin Afan

doi:10.3390/e22050549

Abstract

In this study, the analysis of the extreme sea level was carried out by using 10 years (2007–2016) of hourly tide gauge data of Karachi port station along the Pakistan coast. Observations revealed that the magnitudes of the tides usually exceeded the storm surges at this station. The main observation for this duration and the subsequent analysis showed that in June 2007 a tropical Cyclone “Yemyin” hit the Pakistan coast. The joint probability method (JPM) and the annual maximum method (AMM) were used for statistical analysis to find out the return periods of different extreme sea levels. According to the achieved results, the AMM and JPM methods erre compatible with each other for the Karachi coast and remained well within the range of 95% confidence. For the JPM method, the highest astronomical tide (HAT) of the Karachi coast was considered as the threshold and the sea levels above it were considered extreme sea levels. The 10 annual observed sea level maxima, in the recent past, showed an increasing trend for extreme sea levels. In the study period, the increment rates of 3.6 mm/year and 2.1 mm/year were observed for mean sea level and extreme sea level, respectively, along the Karachi coast. Tidal analysis, for the Karachi tide gauge data, showed less dependency of the extreme sea levels on the non-tidal residuals. By applying the Merrifield criteria of mean annual maximum water level ratio, it was found that the Karachi coast was tidally dominated and the non-tidal residual contribution was just 10%. The examination of the highest water level event (13 June 2014) during the study period, further favored the tidal dominance as compared to the non-tidal component along the Karachi coast.

Highlights

Many studies were made for extreme sea levels from several other regions, which helped the researchers conclude that, for tidally dominated coasts, most often the cause behind frequent extreme sea level events is sea level rise (SLR) [1,2,3,4]
It could be observed that the consistent levels of confidence both the annual maximum method (AMM) and joint probability method (JPM) approaches, which were more than the 95% confidence limits
At Karachi port, the SLR is contributing to increasing extreme sea levels along the Pakistan coast

Summary

Introduction

Many studies were made for extreme sea levels from several other regions, which helped the researchers conclude that, for tidally dominated coasts, most often the cause behind frequent extreme sea level events is sea level rise (SLR) [1,2,3,4]. The impact of SLR is expected to be intensified due to increased meteorological events, by shortening the return periods of extreme sea levels. The accuracy and reliability of any used technique for predicting extreme sea levels depend on the frequency and length of available observed sea data. The AMM involves the analysis of a series of annual maxima to predict extreme sea levels for the future Since this method uses only one value per year, the length of available data is very important. In terms of the accuracy of estimation of the return periods of an extreme event, the compromised to a few years, the execution of the method needs the tide gauge data and. The accuracy of estimation of the return periods of an extreme event, the JPM could be considered as more reliable than the AMM [27]

Study Area

Methodology

Results and Discussion

Summary

Conclusions