Nearshore wave transformation table (Dutch coast)
Project Phase: Planning and Design
Purpose: Quick assessment of wave parameters for projects affected by wave dynamics
Requirements: basic knowledge on wave modelling/ monitoring
Relevant Software: OpenEarth Viewer (web application) or Matlab (stand-alone tool)
The Wave Transformation Table is a look-up table to quickly obtain elementary wave parameters (i.e. significant wave height, peak period and direction) nearshore. This information can be relevant to any type of coastal and offshore engineering or marine ecology study in which waves are important, especially if nearby wave measurements are missing, too expensive or too time-consuming. The tool is available both as a web application and as a Matlab application. The look-up table only covers the Holland Coast, but in principle the technique can be applied to nearshore areas anywhere in the world (provided that sufficiently long offshore wave time series are available).
Building with Nature interest
The tool is especially applicable in the Planning and Design Phase of a coastal engineering project as part of the data analysis or as input for numerical models. The wave time series can be used both to study situations in the past and to derive wave boundary conditions (wave climates) for predictive modelling efforts. The Wave Transformation Table was initially meant to quickly obtain nearshore wave characteristics for research and monitoring of the Sand nourishment - Sand Engine Delfland, North Sea, NL, as there were initially no wave buoys installed to obtain the nearshore conditions from. The Wave Table can be of use to engineers, ecologists and other scientists active on the beach or the foreshore. The tool allows users to obtain time series of basic spectral wave parameters (i.e. significant wave height, peak wave period and wave direction) on any location near a coast (presently only the Holland Coast). This can be useful information when nearshore measurements are lacking, too expensive or too time-consuming. In principle, the technique underlying the tool can be applied to any coast, provided that sufficiently long offshore wave time series are available.
How to Use
The intended users of the Wave Transformation Table are engineers and scientists dealing with nearshore processes. The tool is available as a web application through the OpenEarth Viewer. Users need an internet connection and a Google Chrome browser and can obtain time series of spectral wave parameters with a few button clicks. For more advanced users, the tool is also available as a Matlab application. For proper interpretation of the results it is recommended to have some experience/expertise in the field of wave modeling and/or monitoring.
The Wave Transformation Table is available in two versions:
- As an online application accessible through the OpenEarth Viewer.
This version is meant for end-users, i.e. users who want to use the application as is and are not interested in modifying the underlying code. The only requirements for using this application are an internet connection and installation of the Google Earth plug-in (indicated automatically when needed).
Note: This application is known to encounter problems when accessed with Internet Explorer and Firefox. We advise to use Google Chrome instead.
- As a Matlab application in OpenEarth.
Obtaining wave time series with the Wave Transformation Table involves the following steps:
- Browse to viewer.openearth.nl, open the project Building with Nature, select the case Holland Coast and navigate to the tool Wave Transformation Table. A window should pop-up as indicated in the figure below.
- Using the Ctrl-button and the left mouse button the yellow pushpin can be dragged to the place of interest. Please make sure that the pushpin remains within the area indicated by the green line, as this is the model extent. In the pop-up window the user can select the start and stop dates and times for the wave time series. When pressing transform, the tool will start to interpolate the wave timeseries to the point of interest. Depending on the length of the time period this may take a little while. Eventually, the results should pop-up as indicated in the figure.
- To open the individual results in a new window, please follow the link "Open file in new window". Here, the user can also save the file.
- Scrolling to the bottom of the balloon with the results, the user will find background information on the set-up of the tool and the working principles. This document also contains validation information of the tool.
The Wave Transformation Table is based on wave time series measured at the stations IJmuiden, Eierlandse Gat and Europlatform 3 in the period January 1979 to December 2000. From the time series a set of wave conditions is derived that force a SWAN wave model in stationary mode. The results of the SWAN simulations are used to derive the Wave Transformation matrix. This matrix consists of factors that specify the relation between the offshore (model forcing) and nearshore (model results) wave parameters. With this matrix a wave condition offshore can be translated to any nearshore location. The offshore wave condition is obtained from one of the offshore wave measurements stations (depending on the pinpointed location relative to the offshore stations and the wave direction) based on the dates and times entered by the user. The Wave Transformation Table has been validated against measurements for the stations Noordwijk Meetpost (1979-2001), Lichteiland Goeree (1979-2001) and two stations near Petten (1995-2001), see the correlation diagrams for the significant wave height in the figures attached. The validation results typically show correlation coefficients in the order of 0.9 or higher for significant wave height, wave direction, peak period and surge, except for the stations near Petten where the peak wave period has a correlation in the order of 0.6. The lower correlation coefficient for Petten may be related to the relatively large number of extreme values in the time series, which may be outliers that have not been removed from the dataset prior to the validation.
To apply the technique underlying the tool to other coasts the following steps need to be taken:
- obtain (sufficiently long, i.e. typically more than 20 years) measured offshore wave time series in/near the coast of interest
- set up a wave model (for example SWAN) to transform offshore wave conditions to the nearshore
- run the model for a matrix of wave conditions and derive transformation factors
- write/ adjust the Matlab code (based on the locations of the measurements) to transform time series of offshore spectral wave parameters to nearshore based on the matrix
- link to an online database that regularly updates with recent time series (of spectral wave parameters)
The Wave Transformation Table can be used to obtain nearshore wave characteristics for coastal and offshore engineering and marine ecology studies. It requires sufficiently long time series (typically 20 years or longer) of offshore wave conditions to obtain reasonable wave climates in the nearshore.
- Sand Motor Delfland
- Interactive Design Tool - Holland Coast
Within the Building with Nature program, the Sand nourishment - Sand Engine Delfland, North Sea, NL is a practical example of a project in which nearshore wave characteristics were relevant to model the physical processes, while nearshore wave buoys were not yet installed. The Wave Transformation Table also proved to be useful to derive wave climates for the Coastline intervention tool - Holland Coast - ITHC. In its current state the tool is focusing on the Holland Coast, but technically the principles can be applied to coastal zones anywhere in the world.
Limitations and Lessons Learned
- The Wave Transformation Tool proved to be a useful alternative for wave measurements to obtain elementary nearshore wave parameters. The tool can be practical when wave measurements are not available or too expensive in the area of interest.
- Since the Wave Transformation Table depends on bathymetry, wave climates and the SWAN version used, it is recommended to update the transformation matrix whenever these factors change. This will improve the predictive capabilities of the matrix.
- Currently, the influences of (tidal) currents and water levels on the waves are not accounted for in the model. Incorporating these effects may improve the Wave Transformation Table.
- Since the transformation matrix is designed for average or frequently occurring wave events, it may be less suitable to predict the nearshore wave characteristics for extreme wave events.
- Setting up a similar tool for other coastal areas in the world requires offshore wave time series of reasonable length (typically over 20 years) to obtain quality results.
- SWAN user manual
- Weerts and Diermanse (2004). Golfstatistiek op relatief diep water 1979-2002. WL|Delft Hydraulics report: Q3770.
Access to the tool
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