Page - 1139 - in Book of Full Papers - Symposium Hydro Engineering

The preparations for the HPP chain construction took a long time, while over the recent few years intensive analysis concerning the design and implementation of the selected solutions took place. In the individual phases of construction, which took several years, the runoff conditions gradually changed, therefore the previous runoff conditions in the HPP impact area were analysed. Good cooperation with the HPP design engineer and competent authorities was essential to identify relevant or unfavourable scenarios of flood (and other) events to be investigated hydraulically, and to review and evaluate the interim calculation results. 2. HYDRAULIC MODELLING – BREŽICE HPP The advantage of numerical modelling over physical models is that it is easier to change the geometry and boundary and operational conditions, but unfortunately in the most complex cases it still does not achieve the necessary robustness of results compared with physical models. Therefore, hybrid hydraulic modelling (HHM) was used, which applies the manner of modelling (physical/numerical) that is the most relevant in a particular case. In this case, we applied conventional and distorted physical models for flood areas and various structures, while for the numerical analyses of runoff conditions in the entire area the combined one- and two-dimensional (1D-2D) software MIKE FLOOD was used. The appropriate numerical simulation of the faster 1D flow in the Sava River channel and the slower 2D flow in floodplains was achieved by an extensive calibration in relation to the previous flood events and simulations of conditions in the physical model. The 1D module applied allowed for simulation of the impact of various structures and operational dynamics according to the temporal dynamics of the flow from upstream areas. This enabled a hydraulic analysis of the situation in the wider areas (the entire HPP impact zone) as well as modelling of immediate areas (e.g. the construction pit area) and of individual structures (e.g. spillways). To analyse the flow conditions in the HPP’s immediate areas during the individual construction phases fully-2D modelling was applied, e.g. using the hydraulic software tool CCHE2D [5], which allowed for detailed analyses of velocity fields in critical areas. This tool is well calibrated, too; the same boundary conditions were applied for the HPP’s immediate area as well, i.e. the flow conditions from the wider site analysis using HHM. Such case in shown in Figure 3 where left, based on an orthophoto image, the immediate zone of the HPP Brežice construction pit is shown, while on the right the numerical mesh of the same area for 2D analyses is shown. 1139