Compound flooding from tropical cyclone-induced sea surge and precipitation in Sri Lanka (C-FLOOD)

Institute

  • University of Plymouth

Background

Coastal communities in the north and east of Sri Lanka face significantly greater risk of coastal flooding from storm surges associated with seasonal cyclones than those in the rest of the country. These storm surges are essentially local elevations in sea level caused by the weather system, which subsequently inundate the land.

Storm surges are caused by a combination of:

  • low atmospheric pressure 'lifting' the sea surface (barometric tide)
  • frictional drag of the wind blowing over the sea causing a slope in the water surface (wind stress)
  • breaking waves transferring their momentum into the water column (wave setup)

Hazard maps to indicate predicted storm surge inundations around the Sri Lankan coastline were produced by Prof J Wijetunge of the University of Peradeniya, in association with the Disaster Management Centre. The computer models on which these hazard maps were based were limited to describing only the barometric tide and the wind stress. Subsequent advancements in understanding mean that wave setup can now be included. It is critical to do so, because the wave setup effect may contribute 40 per cent of the surge in some locations; in other words, some communities may face a more grave risk than hitherto realised.

The situation is potentially much worse than this, however, as scientists are beginning to understand the interaction of storm surges with severe rainfall events, which almost always accompany the cyclones in the Indian Ocean region. The mechanism for this so-called 'compound flooding' is that rivers swollen from heavy rainfall are prevented from effectively discharging to the sea due to storm surges coming inland. To protect against flooding events in the west we are familiar with flood defence structures; Sri Lanka has no such hard-engineered structures. However, there are natural protective features such as mangrove forests and salt marshes. The potential benefits of mangroves in particular have received some attention since the devastating Boxing Day tsunami of 2004, though the intentional implementation in formal coastal schemes is still in its infancy.

Aims

Prof W Taylor of the University of Western Australia recently received funding from the Global Challenge Research Fund to investigate how design codes might incorporate their effects. Against this backdrop, the C-FLOOD project will produce a new generation of compound flood-hazard maps, based upon state-of-the-art computer modelling that will consider all the storm-surge components and the rainfall effect. It will also consider a variety of climate change scenarios that will influence flooding due to predicted rising sea levels.

The final outcome will be improved predictions of flooding inundation, with engagement of the selected communities, leading to improved resilience to compound flooding. The hazard map-production techniques and flood-impact mitigation methods could then be implemented across other vulnerable communities.

Approach

This will be done by Prof J Wijetunge at the University of Peradeniya in Sri Lanka, and Dr M Jayaratne at the University of East London, with their related expertise. Furthermore, the protective effects of the natural vegetation will be included in the modelling and maps, by conducting experiments at the University of Plymouth's COAST Laboratory. This will be undertaken by Dr A Raby (Plymouth) and Prof W Taylor (University of Western Australia). The C-FLOOD project will focus on three communities that are deemed most vulnerable due to their geography and levels of poverty associated with the past military conflict.

The project team will work with community members in addition to local and regional leaders and administrators to maximise the benefits and uptake of the new hazard maps. Individual, localised hazards will also be captured in comprehensive multihazard maps for the communities. Dr K Kitagawa from the University of East London and Mr R Ranawaka of the Coast Conservation and Coastal Resource Management Department have past experience of such activities and will be overseeing these critical aspects.