Incorporating river bed level changes into flood risk modelling

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Douglas Pender, Sandhya Patidar, Gareth Pender, Heather Haynes

Thursday 2 july 2015

15:20 - 15:35h at Europe 2 (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Flood risk assessment

Parallel session: 12K. Floodrisk - Assessment

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The ever changing nature of river channels and catchments and the high level of uncertainty in future conditions and practices; makes incorporating these into risk studies problematic. Typically, the design and analysis of flood defence schemes is based on a single 1:N year and a single survey of the channel and flood plain geometry. Although there is the possibility of altering the flood hydrograph to account for future flows, overall this approach is flawed as it does not account for any factor within the channel or catchment that may change during the design life of the scheme. One such factor that can have considerable influence on future flood levels is the sediment transport and morphological regime of the river (Stover and Montgomery, 2001; Lane et al., 2009). In its simplest form, flood risk is defined as the changes in river stage, which are controlled by the channel characteristics (i.e. roughness and conveyance) and flow magnitude. Including climate change and its associated morphological response can add significant uncertainties to the prediction of future flood risk, by reducing the conveyance capacity of the channel. Therefore, the overall aim of this paper is to present a modelling methodology that can be implemented in order to assess future flood risk, as a result of changes in the river channel morphology. The methodology combines the use of: a novel stochastic model, for generating streamflow sequences; a 1D hydraulic and sediment transport model (HEC-RAS) to estimate 50 years of morphological change; and a 1D/2D hydraulic model (TUFLOW) to estimate the influence of this change on future flood levels. The River Caldew, UK is used as a case study. The methodology is demonstrated through evaluation of the change in inundation for eight flood events with Return Periods (RPs) of 1, 2, 5, 10, 25, 50, 100 and 200 years. Results show that, whilst all events exhibit an increase in flooded area and volume, these changes are more pronounced at the lower, more frequent, RPs (160% compared to 9% increase in flood extent for 1 and 200 year RPs respectively). This has the potential to increase the frequency of flooding by turning previous in-channel high flow events into out-of-bank flood events.