The development of a coupled surface water inundation modelling system

The capability to provide a coupled land-surface, surface-water inundation modelling system in the Unified Model is currently being developed within the Met Office. The main aim is to investigate whether using a coupled inundation and atmospheric model provides a skilful way of forecasting surface water flooding, given the uncertainty of the rainfall and the limitations imposed by the available computer power.

The community land surface scheme JULES (Joint UK Land Environment Simulator), which forms the land component of the Met Office Unified Model, has been coupled to LISFLOOD-SGC, the University of Bristol's inundation model. LISFLOOD-FP is a high resolution two-dimensional hydrodynamic model specifically designed to simulate floodplain inundation, whilst LISFLOOD-SGC is a revised version that includes a representation of sub-grid channels. The model predicts water depths in a grid box at each time step, and hence simulates the propagation of flood waves through a river catchment and over fluvial, coastal and estuarine floodplains.

The two models have been coupled together by spatially downscaling the JULES surface water runoff fluxes onto a higher resolution LISFLOOD-FP grid, and using variable sub-timesteps to ensure stability. At present the system has not been coupled to the full Unified Model, but is driven off-line given atmospheric data. The longer term aim will be to fully integrate the system and consider the possibility for feedbacks from inundated areas onto the surface fluxes of heat and moisture. To assess the performance of the system, data from the Met Office operational high resolution 1.5 km. atmospheric model over the UK (UKV) has been used to provide the atmospheric forcing for a surface water flooding event in Wales during June 2012. Results from this simulation will be presented and the capability of the system will be discussed. Current limitations will be highlighted and future developments be identified.