Designing Early Warning Systems (EWS) for Small Island Developing States (SIDS) can be a challenge due to a lack of data, capacity to maintain complex IT systems, and ability to communicate warnings with the public. Yet, SIDS are particularly susceptible to natural disasters, especially in the coastal zone. The Republic of Mauritius (Mauritius) is a group of islands in the southwestern Indian Ocean and was the first SIDS in the region to develop its own operational EWS. Mauritius experiences the effects of tropical cyclones almost yearly, and the associated storm surge and flooding can occasionally cause significant damage to coastal communities. The country is also regularly subject to high swell wave events generated by extra-tropical storms.

Mauritius Meteorological Services (MMS), the national meteorological institute, hosts, manages, maintains, and uses the national EWS. The EWS has been developed jointly by MMS and Deltares, in stages. The first version, which included tide and storm surge forecasts, was developed in 2015 and funded by the Adaptation Fund and UNDP. The second version, which expanded the system to include wave forecasts, the effect of waves on water levels, and inundation mapping, was completed in 2019 with support from the Adaptation Fund and WMO.

The main purpose of the EWS is to provide the information needed to inform coastal communities of a high-water event with enough time to evacuate safely. When an event is expected, MMS sends warnings to various government agencies, who disseminate the information further. 

 

The EWS provides real-time forecasts of tides, waves, storm surge, and inundation for the three main islands of Mauritius (Mauritius, Rodrigues, and Agalega). It relies on freely available modelling and forecasting software, developed by Deltares, and publicly-available global datasets. Five sets of tide, storm surge, and wave models were developed using coupled Delft3D-FLOW and -WAVE (SWAN) models. Meteorological forcing for two regional models is provided by the 0.25° NOAA Global Forecast System, while 2D spectral wave boundary conditions, including the swell component, are derived from 0.5° Global WAVEWATCH III Production Multigrid forecasts. Locally generated tropical cyclone winds based on cyclone track information, when available, are blended with meteorological forecasts.

 

Three detailed flow-wave models, one for each of the main islands, are nested in the two regional models. Meteorological and water level observations are imported every six hours. A GIS-based flood mapping algorithm converts coastal water levels to inundation depth maps.

 

The EWS produces 6-hourly HTML bulletins with water level forecasts and warning levels (Safe, Watch, Alert, or Warning) with a 3-day lead time. The bulletin was designed to be easy to interpret and disseminate, in collaboration with MMS and the National Disaster Risk Reduction and Management Center (NDRRMC). 

 

All import, data processing, modelling, and reporting workflows are managed automatically by Delft-FEWS. The EWS is a stand-alone Delft-FEWS application (rather than operator client) and runs on a dedicated PC at MMS. The stand-alone system is easy to maintain and debug and is connected to a back-up power supply. A standby version of the EWS also runs on a second PC at MMS. A third workstation at MMS has the EWS software installed in order to run offline scenarios.   

Using cyclone track forecasts to force hydrodynamic models

Another important characteristic of the EWS is the option to use cyclone track forecasts as forcing to the hydrodynamic and wave models. When available, cyclone bulletins from the Joint Typhoon Warning Center (JTWC) are imported into the EWS. These bulletins provide forecast information on position, radius of maximum wind, and pressure drop of the cyclone. The Wind Enhancement Scheme (WES) then uses these parameters to generate spatial wind and pressure fields. Forecasters have the option to modify track parameters based on expert judgement before running the WES module. Wind and pressure fields generated by WES are merged with NOAA GFS Numerical Weather Prediction (NWP) before running hydrodynamic and wave models. 

Generating spectral wave boundary conditions from global forecasts

Due to its unique location and geology, the Republic of Mauritius, especially the islands of Mauritius and Rodrigues, is subject to high water levels from cyclones, swell waves, and sometimes both at the same time.  

Until now, most operational coastal forecasting systems have relied on simple wave parameter timeseries (e.g. Hs, Tp, Dir) as boundary conditions for regional wave models. In many cases, this works well, when either swell or wind waves dominate a system. However, when both wind and swell waves are present, as can sometimes be the case in Mauritius, the wave energy spectrum cannot be derived accurately from a single wave height, wave period, and direction. 

The Mauritius EWS is the first Delft-FEWS which uses spectral wave boundary conditions derived from freely available and global WAVEWATCH III Production Multigrid forecasts. This data feed provides three wave time series. The first represents wind waves and the other two represent two swell components (“swell1” and “swell2”). These time series can each be converted to individual wave spectra and added together to produce a more complete wave spectrum (see Figure 3). The combined wave spectrum (forecast) can then be provided as a boundary condition to regional wave models.

A new Delft-FEWS transformation module was developed as part of the Mauritius EWS project to generate 1D and 2D wave spectra from a simple wave timeseries, using a JONSWAP spectrum. This functionality is available in Delft-FEWS versions 2017.02, 2019.01, and newer. Simple User transformations can then be used to add the separate 1D or 2D wave spectra together. 

 

Today the EWS is being used on a day-to-day basis for operational coastal forecasting and warning. Three wave buoys are currently being installed along the coast of Mauritius (2) and Rodrigues (1), which could provide valuable information about wave and water level forecast performance.

Would you like to know more? Please contact Tom Bogaard or Nena Vandebroek.