Plenary Session 3

Speaker: Professor Mike Hartnett (NUI Galway)

Date & Time: 13 April 2017, 9:00 am

Title of the plenary lecture: Modelling complex flows in coastal urban floodplains using multi-scale spatial nesting

The complexity of hydraulic models to simulate surface flows varies greatly from simple 1D hydraulic models through, reduced complexity 2D models  to 1D-2D linked models.  Application of these models to complex flows in coastal urban floodplains has shown that it is difficult to achieve high accuracy while maintaining reasonable computational costs.  It has been shown that high spatial resolution is required to accurately reproduce hydrodynamics in urban floodplains; spatial scales of 2-3m are required to simulate ‘street flows’.

A flood model must simulate both rivers flowing downstream into cities simultaneously with tide and storm surges propagating upstream.  For computational efficiency, it is important that the model uses appropriate spatial and time scales for different regions of the overall model domain.  In the downstream estuary relatively large spatial scales may be used, whereas in the urban floodplains and in the upstream river reaches finer scales may apply.  MSN_Flood model was developed to enable multi-scale nesting of highly complex flows.  The model uses a novel nesting routine treating boundaries between coarse and fine resolution as internal boundaries through the development of ‘ghost cells’; this treatment conserves continuity of mass and momentum.  One-way and two-way versions of the model have been developed; further, the model can include sub-domains that are not orthogonal to the original parent grid.

The width of upstream river floodplains may significantly broaden and narrow during flood propagation; MSN_Flood is numerically robust and, importantly, facilitates the implementation of moving boundaries and recirculating boundaries between adjacent coarse and fine mode sub-domains.  Through this development computational efficiency is obtained.  The model has been applied with success to a major urban flood event which resulted in €100M worth of damage.