MIKE by DHI UK and Ireland Blog

Coupling groundwater and ecological models (MIKE-SHE & ECO Lab) - Roberta-Serena Blasone (DHI)



Delivered at: -

Groundwater Modelling Workshop 2014:
Pushing the Boundaries – New Issues and Applications in Groundwater Modelling
13 May 2014 - Birmingham, UK

Example MIKE 21 Coupled Model to investigate swell wave propagation in the English Channel

Introduction

In February this year (2011) a large storm in the North Atlantic caused swell waves with periods of 20-25 seconds to propagate into the English Channel. Whilst this is not unusual in itself (beaches along the south coast have historically been subject to similar storms), the combination with high water of these small wave height, long period waves led to a series of flooding “near misses” along the South Coast as swell waves led to damage and overtopping of some defences including natural barrier beaches.

It is important to note that many of the beaches along the south coast are gravel/shingle dominated barriers which are typically considered reflective in morphological classifications (Short 1979). Greenwood (1984) noted that reflective beaches, which are already prone to sub-harmonic edge waves, can interact with longer period swell waves leading to an amplification of wave run up, a mechanism which leads to higher than anticipated overtopping and overwashing of the crest with consequent implications for barrier stability and hinterland flooding. With increasing emphasis from operating authorities being placed on beach management of such natural defences, it is important to understand the driving conditions in an event such as this to plan effective management.

With this in mind, DHI have developed a demonstration model of this event to show how the Flexible Mesh series of models allows you to take large scale North East Atlantic basin driving forces to understand the impact at a local scale. The coupled model developed makes use of the Spectral Wave and Hydrodynamic modules of the MIKE by DHI software and uses freely available data sources as inputs.

For this regionally focused modeling study, bathymetry data has been sourced from the GEBCO 08 Grid bathymetry data set which provides a 30 arc-second grid of the entire globe. Tidal data was obtained from the MIKE 21 toolbox to allow the flow model to be adequately resolved.

In addition the driving forces of the storm in the North East Atlantic needed to be represented. For this the NOAA/NCEP reanalysis data was used. This data set can provide driving wind fields and pressures for the entire globe and a temporal and spatial subset was utilized for this demonstration. The NOAA/NCEP reanalysis data is 6 hourly at a spatial resolution of 2.5 degrees. Available pressure charts from the UK Met Office provide a validation of the data set used as seen in Figure 1 below.


Figure 1 – Met office and NOAA/NCEP reanalysis data used as forcing conditions for the model.

The mesh used for the modelling was optimized to provide a suitable representation of the features of interest in the storm and whilst starting out as a mesh of the entire North Atlantic for the entire duration of the storm system, this was reduced to a smaller North-East Atlantic model for a period including 3 days prior to the storm striking the South coast of the UK. This was to enable the wave model to be run in fully spectral mode, the most suitable for allowing the wind/wave generation and transport process to take place and at a computational speed that would be achievable for operational forecasting models (~1-2 hour model run times for combined wave and flow conditions).