Performance Assessment of Bench-Terraces Through 2-D Modelling

The use of dry-stone walls to build bench-terraces has traditionally played a key role in the management of hilly and mountainous agricultural areas. They are typically built to increase suitable crop areas, provide safe working conditions and improve terrain stability during rainfall events. Because the mean gradient is greatly reduced by the presence of bench-terraces, the hydrological response of a terraced slope should result in a reduction of peak runoff at the toe of a hillslope and a delay in the passage of peak flows. This research develops and quantifies this hypothesis. The FLO-2D model is used to analyse the runoff propagation mechanism of a terraced slope (sequence of dry-stone walls) by varying number and spacing of terraces and assuming two hydrological soil setting scenarios in terms of antecedent moisture conditions within the Soil Conservation Service-Curve Number method. The model analysis shows that the majority of runoff modifications at the outlet of a terraced system result from topographical modifications rather than local hydrological variations at the dry-stone wall zone. Repeated modelling applications show that, given a quite-typical scenario of a 20°-sloped hillslope and a reference intense rainstorm, the peak discharge reduction at the hillslope outlet depends on the percentage of the area managed with terraces. The reduction can be calculated with a logarithmic-type function (for example, an increase of terraced area from 10% to 30% might bring to runoff peak reduction of almost 45%). This information can help determine where terrace additions are more effective in terms of hydrological benefit.