Assessing sediment connectivity in dendritic and parallel calanchi systems

Calanchi, a type of Italian badlands created by a combination of water erosion processes and environmental constraints controlling their development, is a striking example of long-term landscape evolution. Sediment connectivity can be defined as the degree to which a system facilitates the fluxes of sediments through itself. The goal of this research is testing the use of simple morphometric variables for assessing sediment connectivity of calanchi landforms distinguishing between dendritic and parallel systems. For detecting the morphological characteristics controlling the sediment connectivity of calanchi basins, literature data (146 calanchi basins) and measurements carried out in two areas located in Sicily (Italy) (119 calanchi basins) were examined. Application of Hack's law to the investigated calanchi basins, carried out distinguishing between landforms with dendritic and parallel systems, demonstrated that a calanco basin with a dendritic system has an exponent (k = 0.49) which is close to that (k = 0.55-0.60) typical of a river basin. The analysis also showed that Melton number assumes only for calanchi basins with a dendritic system a value (0.648) which is similar to that of the river case (0.694). For establishing calanchi hillslope connectivity, the hillslope sediment transport efficiency was also measured by the travel time, which is the ratio between the length and the square root of the hillslope steepness of each cell of the calanchi DEM, and the corresponding sediment delivery ratio. The concept of sediment delivery ratio of each hillslope cell SDRi was used as a measure of sediment connectivity since low values of this ratio imply a lack of connectivity (structural and/or hydrological). For each calanco the cumulative frequency distribution of the travel time allowed both to test the applicability of the exponential relationship used in SEdiment Delivery Distributed (SEDD) model to calculate SDRi and to estimate the coefficient beta appearing in this relationship. Finally, the median travel time and the sediment delivery ratio SDRM of each calanco basin were calculated to evaluate the structural connectivity at calanchi basin scale. The analysis demonstrated that both a dendritic system has median travel time values less than those of a parallel system and that SDRM values of a dendritic system are higher than those of a parallel one. In conclusion, sediment connectivity of a dendritic system is higher than that of a parallel one.