Storage-based approaches to build floodplain inundation modelling capability in river system models for water resources planning and accounting

• We develop two innovate approaches for floodplain modelling in river system models.
• The two approaches can estimate floodplain fluxes and stores model at river reach.
• Performance of the second approach is equivalent to a hydrodynamic model.
• The second approach is suitable for rapid inundation estimate at high spatial scale.
• New developments enable river models to improve environmental flow modelling.

SummaryTwo conceptual storage-based approaches have been developed for incorporating floodplain inundation modelling capability in two water resources planning models. Approach one is a simple method suitable for data limited environment, in which, flow in a river reach within a floodplain is partitioned into two components, in-stream and overbank flow, based on the in-stream capacity. A flood volume–area relationship derived from the flood inundation time series, which is generated by analysing daily MODIS imagery, is used to estimate flooded area for the overbank flow. The losses due to evaporation and groundwater seepage from floodplain are calculated using the estimated flooded area. This approach was implemented in the floodplain area of the Murray–Darling Basin in Australia. The simulated flow was compared with gauged data in 216 stations. The model has produced daily time series of floodplain stores and fluxes. The mass-balance analysis shows that the long term mass balance error was negligible for all floodplain reaches.Approach two is more comprehensive and suitable for areas with high resolution topography data. LiDAR data is used to divide a floodplain into multiple storages based on pre-defined threshold of floodplain inundation heights. The storage characteristics including disconnected storage volume (or, dead storage) and hydraulic connectivity between floodplain storages and a river reach are derived from the LiDAR data using a spatial data processing technique. This information is used to estimate floodplain inundation area for overbank flow. Approach 2 successfully simulated inundation depths, duration and extents in two reaches of Murrumbidgee floodplain. The performance of Approach 2 is highly satisfactory in terms of flood extent, depth and duration at a very high spatial resolution.The key limitations of Approach one are that this approach is unable to produce the spatial variability of floodplain inundation depth and extent and it does not incorporate dead storage. These limitations can be overcome by integrating it with Approach two. A key advantage of Approach two is the quick run time compared to a two dimensional hydrodynamic model. This makes the approach highly suitable to be incorporated in river system models for various scenario analyses for planning and management, which requires the model to run for over 100 years at a daily time step. The limitations of this approach are the requirement of very high resolution topographic data and the inability to produce flood velocity.