Statistical Modelling of Significant Wave Height: Inshore and Offshore Bonny River Estuary, Nigeria

Itolima Ologhadien

doi:10.7176/cer/11-4-08

Abstract

The selection of best – fit probability distribution is of fundamental importance in the design, planning, and operability studies for harbours, coastal and offshore structures and exploitation of coastal resources. A wrong choice of probability distribution could lead to either under – or over – estimation of design loads, which may have detrimental impacts on safety and project economy. Consequently the Inshore and Offshore wave data of Bonny River estuary, Nigeria was studied by fitting Rayleigh, 2- parameter Lognormal, Fisher – Tippett Type 1(FT-1), 2-parameter and 3-parameter Weibull probability distributions using probability paper method. The probability distributions were subjected to five model evaluation metrics; coefficient of determination (R 2 ), demeaned RMSE, normalizedRMSE, Scatter index (SI) and Performance scores index(d 1 ) and a scoring scheme was adopted on the basis of which the best – fit probability distribution for either Inshore of Offshore was selected. The results show that 2-parameter Lognormal distribution is the best –fit probability distribution for the Inshore station, seconded closely by Fisher – Tippett Type 1 and 3- parameter Weibull distributions. The results also show that Fisher – Tippett Type 1 is the best – fit probability distribution for the offshore station seconded closely by 3 – parameter Weibull distribution. This study will contribute to the development of the coastal zone and also a recipe for Integrated Coastal Zone Management of the Nigerian Gulf of Guinea. Key words : Bonny river estuary, goodness of fit tests, integrated coastal zone management, significant wave height, probability distribution models. DOI : 10.7176/CER/11-4-08 Publication date :May 31 st 2019

Highlights

Ocean waves may be generated by wind stresses from the atmosphere, earthquakes, gravity of the earth and celestial bodies, cosmic forces, and surface tensions
The RMSE is corrected for bias resulting in RMSE and the other forms of RMSE which includes components of variance and bias as normalized (NRMSE)
The inshore and offshore wave climates were scored separately in order to account for the different wave height attenuation processes

Summary

Introduction

Ocean waves may be generated by wind stresses from the atmosphere, earthquakes, gravity of the earth and celestial bodies, cosmic forces, and surface tensions. These forces can impose enormous impacts upon coastal and offshore structures (WMO – No 72). The calculation of these forces in extreme is done statistically using the linear wave theory to predict the height and periods of the extreme waves. Various probability distributions are fitted to the significant wave height data and the one giving the best visual fit is accepted. The distributions that have received particular application to coastal and offshore engineering are Rayleigh, 2- and 3- parameter log-normal, 2- and 3- parameter Weibull and Extreme value distribution

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Conclusion