Two-Dimensional Flood Plain Flow. II: Model Validation

Information from two floods on the Waihao River rural flood plain in New Zealand was used to validate a two-dimensional flood plain flow model, Hydro2de. An aerial photogrammetric survey digitally described the terrain. Measurements during and after the floods, and a global positioning system survey of positions from photographic records and other information recalled years later by flood plain residents, provided flood level, depth, and extent data to test the model. Uncalibrated, with inputs from the river and levee overtopping or breaches, the model underestimated levels, depths, and the area covered by floodwaters. The estimates were sufficiently close to be useful for flood plains without previous flood measurements. Calibrated to reproduce the measured flood extents, the model gave levels and depths closer to reality that were good at the edges of the flood plain subareas, but underestimated depths in the center of the flow. The underestimates occurred because the model did not include details of houses, hedges, and fences, nor any wave action of the water flow. A comparison is made to one-dimensional modeling using the MIKE11 model. Two-dimensional flood plain flow modeling promises benefits over one-dimensional modeling, in particular because the former does not require operator choice of a network of "channels" to represent the flood plain. The two-dimensional modeling reported here shows that improving the accuracy of the digital terrain model would provide the most improvement to accuracy of the results. Model functionality could also usefully be improved.