Finite element modelling of floodplain inundation
Flood inundation phenomena typically occur over reach lengths of 5- 30 km and
incorporate a number of complex flow mechanisms. These include a momentum
transfer between the main channel and floodplain and turbulent mixing caused by
the delivery of water to the floodplain from the channela nd its subsequenrte turn.
However, currently available one dimensional schemes applicable at scales
appropriate to floodplain inundation processes cannot effectively simulate such
processes. This is due to both an incomplete description of the flow physics and a
failure to treat floodplain areas in realistic fashion. More complex two and three
dimensional models, which have these capabilities, have only been applied over
very short reach lengths (c. 0.5 -2 km) and rarely to compound meandering
This thesis reports on the further development of a generalized two dimensional,
finite element code (RMA-2) to meet this research need. This is achieved via a
series of modifications to the numerical model and to the physical representation by
finite elements that enable river channel/floodplain flow at the long reach scale to be
Evaluationo f the enhancedR MA-2 schemef ollows a three stages trategy. Firstly,
the assumptions underlying the scheme are examined to identify possible
inconsistencies. Secondly, tests are undertaken to assess whether the specified
physical model has been correctly transferred into computer code. This is achieved
via sensitivity analysis, examination of numerical stability issues and investigation
of model response to abnormal parameterization. Thirdly, model predictions of
flow field information are compared to observed field data in the context of an
application of the enhanced model to an 11 km reach of the River Culm, Devon,
Results from this evaluation process indicate that the enhanced RMA-2 model is
capable of simulating main channel/floodplain momentum transfer and the two
dimensionale ffects associatedw ith compoundm eanderingc hannelsa t this scale.
Model simulations compare favourably to field data, both for specific cross sections
and over the entire mesh.
Finally, extension of this core modelling capability is begun via the development of
two model application scenarios. These demonstrate the likely utility of the
enhanceds chemef or the assessmenotf flood risk and the investigationo f sediment
depositionp rocessesin floodplain systems.