Optimal geometric description in one-dimensional hydraulic models

It is well known that choosing a suitable set of cross-sections for the representation of the natural geometry of a river is critical to the efficiency of one-dimensional (1D) hydraulic models. The model implementation is generally driven by two conflicting goals. On the one hand, a model that utilizes a higher number of cross-sections is expected to provide a better description of the hydraulic behavior of the river system. On the other hand, the topographic survey of an excessive number of cross sections may unduly increase the overall costs for model implementation. The scientific literature provides a few indications of the optimal distance between cross-sections (see e.g., Cunge et al., 1980; Samuels, 1990).These indications result from approximated theoretical analyses or from the application of common sense. Yet the identification of the optimal geometric description is still an open problem. This study aims at investigating this issue through an innovative numerical procedure. The procedure is applied to a 55km reach of the River Po (Italy) and a 16km reach of the River Severn (United Kingdom), for which high quality laser scanning altimetry are available. The high resolution Digital Terrain Models (DTMs) of the two river reaches enabled the construction of a series of hypothetical topographical ground surveys by retrieving cross-sections directly from the DTM within a GIS (Geographical Information System) environment. The hypothetical surveys, characterized by different resolutions (i.e. different spacing between cross sections), were used to build different 1D hydraulic models of each reach. The models were applied to simulate historical and synthetic flood events for the two river reaches. The results of the simulations were then analyzed to quantify the efficiency of each model, and to assess how the decrease of survey resolution impacts the performance of the models themselves. The study results integrate with the indications reported in the scientific literature and provide practical and useful guidelines for 1D hydrodynamic modeling.