Advanced

A simple model for wave generation under varying wind condition

Shanko, Alemayehu Dula LU (2016) VVRM01 20161
Division of Water Resources Engineering
Abstract
The Hanson Larson evolution model assumes that wave height grows or decays from an initial condition towards an equilibrium wave height through a simple exponential growth. The model uses wind input data, fetch length and depth measurements. Using this input parameters, it both determines the wave height and period values, thus predicts wave climate. The model is developed using deep water conditions where group velocity is assumed be constant.

The model is validated using measured wave data from the Baltic Sea. The wave and input wind data are retrieved from the Swedish meteorological and hydraulic institute’s (SMHI) data base. The wind stations used in the analysis have a correlation coefficient of 0.72. The wind inputs were also... (More)
The Hanson Larson evolution model assumes that wave height grows or decays from an initial condition towards an equilibrium wave height through a simple exponential growth. The model uses wind input data, fetch length and depth measurements. Using this input parameters, it both determines the wave height and period values, thus predicts wave climate. The model is developed using deep water conditions where group velocity is assumed be constant.

The model is validated using measured wave data from the Baltic Sea. The wave and input wind data are retrieved from the Swedish meteorological and hydraulic institute’s (SMHI) data base. The wind stations used in the analysis have a correlation coefficient of 0.72. The wind inputs were also checked for their reliability to represent the wind condition at the wave buoy. This was done by comparing the wind speed and wind direction measured at the wind stations. Whenever the difference between speeds was less than 5 m/s and wind direction difference less than 45 degrees, the wind input was taken as reliable.

The wave heights calculated were in a fair agreement with observed values for most of the trial runs, except when wind direction was abruptly changing. The error observed was more for wave periods as compared to wave height, but the growth and decay pattern calculated by the model was similar as the observed pattern.
Other two more models based on wave energy analysis are also tested as to predict wave height and wave period, from wind input data. In comparison to the Hanson Larson evolution model, these models resulted in more disagreement between calculated and observed values, rendering them less useful. (Less)
Popular Abstract
It is common to hear weather forecasts and talk of the climate these days everywhere. Most of these forecasts, especially the ones made available to the public are rainfall and temperature measurements. And they most of the time concern the atmosphere. But can these be extended to the sea? Can we study the climate of the Sea? The answer is yes. The climate of the sea can also be described using the height, period, and direction of its waves.
The main aim of this study was to come up with a simple and easy-to-use model that calculates wave heights and periods. The models tested in the study were relatively simple and use wind speed, wind directions, the depth of the sea and the fetch length (the maximum distance over which a wind blows)... (More)
It is common to hear weather forecasts and talk of the climate these days everywhere. Most of these forecasts, especially the ones made available to the public are rainfall and temperature measurements. And they most of the time concern the atmosphere. But can these be extended to the sea? Can we study the climate of the Sea? The answer is yes. The climate of the sea can also be described using the height, period, and direction of its waves.
The main aim of this study was to come up with a simple and easy-to-use model that calculates wave heights and periods. The models tested in the study were relatively simple and use wind speed, wind directions, the depth of the sea and the fetch length (the maximum distance over which a wind blows) as inputs.
The necessary data used for testing the models were collected from Swedish meteorological and hydrologic institute (SMHI) wave and wind stations over the Baltic Sea. The data contained several years of wind speed and wind direction measurements. Moreover wave heights, periods and wave direction were all obtained from these stations.
One of the models tested was the Hanson-Larson model, named after the researchers who came up with it. The model assumes exponential growth or decay of wave heights to a particular equilibrium condition, which is determined by the existing wind conditions. The model performed satisfactorily as to accurately predict the wave conditions over the sea. Even if there were some instances where there were disagreements between model-calculated and observed wave heights and periods, for most part, the agreements were fair.
There were also other variations to the Hanson-Larson model, which were tested for their prediction of the wave climate of the sea. These variations, instead of the wave heights, assume the growth and decay of wave energy. The model results were in agreement with observed values, but only with a lesser degree than the Hanson-Larson model.
The Hanson-Larson model can be used in conjunction with other sophisticated models to simulate complex conditions. However the advantage of this simplified model will be in calculating wave heights and wave periods over an extended time period within a short time. ,which would have taken a long time using other models, (Less)
Please use this url to cite or link to this publication:
author
Shanko, Alemayehu Dula LU
supervisor
organization
course
VVRM01 20161
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Model, Hanson Larson evolution model, growth and decay, wave height, wave period
report number
TVVR-16/5008
ISSN
1101-9824
language
English
id
8886354
date added to LUP
2016-07-12 09:37:33
date last changed
2016-07-12 09:37:33
@misc{8886354,
  abstract     = {The Hanson Larson evolution model assumes that wave height grows or decays from an initial condition towards an equilibrium wave height through a simple exponential growth. The model uses wind input data, fetch length and depth measurements. Using this input parameters, it both determines the wave height and period values, thus predicts wave climate. The model is developed using deep water conditions where group velocity is assumed be constant. 

The model is validated using measured wave data from the Baltic Sea. The wave and input wind data are retrieved from the Swedish meteorological and hydraulic institute’s (SMHI) data base. The wind stations used in the analysis have a correlation coefficient of 0.72. The wind inputs were also checked for their reliability to represent the wind condition at the wave buoy. This was done by comparing the wind speed and wind direction measured at the wind stations. Whenever the difference between speeds was less than 5 m/s and wind direction difference less than 45 degrees, the wind input was taken as reliable. 

The wave heights calculated were in a fair agreement with observed values for most of the trial runs, except when wind direction was abruptly changing. The error observed was more for wave periods as compared to wave height, but the growth and decay pattern calculated by the model was similar as the observed pattern. 
Other two more models based on wave energy analysis are also tested as to predict wave height and wave period, from wind input data. In comparison to the Hanson Larson evolution model, these models resulted in more disagreement between calculated and observed values, rendering them less useful.},
  author       = {Shanko, Alemayehu Dula},
  issn         = {1101-9824},
  keyword      = {Model,Hanson Larson evolution model,growth and decay,wave height,wave period},
  language     = {eng},
  note         = {Student Paper},
  title        = {A simple model for wave generation under varying wind condition},
  year         = {2016},
}