The physical model of a terraced plot: first results

Bench terraces reducing the terrain slope and the length of the overland flow, provide critical hydrological system such as quantitatively controlling the runoff flow velocity, facilitating the drainage and thus leading to a reduction of soil erosion. The study of the hydrologic-hydraulic function in the terraced slopes is essential in order to evaluate the possible use of terraces as a structural measure to reduce flood risk, maintaining a low environmental impacts. The goal of this study is highlighting the analysis of the response in time of the water, which is provided by terraced slope with the dry-stone walls, considering the surface and groundwater component. In this contest a physical model, characterized by a realistic scale, has been installed at the University of Padua (TESAF Department) to reproduce the hydrological behavior of a 3% terraces slope characterized by bare soil. The model is represented by a metal box able to contain a representation of a terrace with the consequent measure: 1 m large, 3,3 m length and 2 m height. It is completed by the installation of two piezometers, 9 TDR sensors for the analysis volumetric water content, a scale for the measure of the discharge at the outlet and a spillway located upslope for the release of the water. Thanks to the model and the devices installed, different hydrological analyses have been carried out; in particular, the outcomes obtained are related with three different analysis described as: i)Volumetric water content analysis: study of the variation of the humidity during different stages of the experiment; ii) Hydrological analysis: study of the response in time of the water considering the surface and groundwater component; iii)Computation a specific curve number for a terraced area, relative comparison with a normal hillslope and values from literature. The performance of the test has been carried out by the release of a laminar sheet of water for a selected period of time. The tests are divided according to the initial humidity condition of the terrain, non-saturated and saturated conditions, and three different decided discharges; maximum equal to 19,50 l/min, the medium of about 12 l/min and the minimum equal to 5 l/min. A total of 12 experiments have been performed since for every condition has been carried out a first test and a replication. The volumetric water content analysis produced by the 9 TDR sensors installed was able to provide a great overview of the trend of humidity in the soil during the experiments. The hydrological analysis gives back great results in terms of time of response of the different component of output discharge. It was possible to identify the different times needed for the surface runoff as the groundwater to reach the outlet. Moreover, even the time of depletion and the level of the piezometers has been considered. The computation of the specific curve numbers for the terraced area show values rather elevated in respect of what is reported by the literature. This phenomena is probably related with two factors which can positive influence the production of runoff: 1.The decided input discharges are non-representative of real precipitation events but they are more intense, considering their duration, if compared to real rain events; 2.The soil reaches a low permeability due to the increasing compactness of the surface layer caused by the succession of experiments. The pioneering experiences with the model have produced some remarkable and useful outcomes as many ideas on improving the system to better interpret the hydrological behavior of the hillslope-terrace model.