Wykorzystanie numerycznych modeli terenu do generowania systemu drenażu powierzchniowego, funkcjonującego podczas opadów nawalnych. Podstawy metodyczne na podstawie studium przypadku zlewni Zalasówki (Pogórze Ciężkowickie) = Use of digital terrain models to generate the surface drainage network functioning during heavy rainfall. Methodological aspects based on the Zalasówka catchment (Ciężkowickie foothills)

Abstract

In considering the process by which flash floods form, core information concerns the parameters of an area’s surface drainage system. That system is composed of elements of natural origin (rivers and valleys), as well as those of an anthropogenic nature (roads, ditches and rills), which together operate as a single drainage system at times of heavy rainfall. In line with this understanding, the work underpinning this article has focused on: 1) a characterisation of different types of DTM in the context of their application to detailed surface drainage system generation in small Carpathian catchments, 2) methodological aspects of DTM modification allowing elements of anthropogenic origin, such as roads, ditches and rills to be included within the surface drainage system, 3) a characterisation of the differences between the river system operating year-round and the surface drainage system functioning at times of heavy rainfall. The results reveal that the most popular DTMs, such as the SRTM, ASTER, TBD and SMOK, do not allow detailed surface drainage systems (including anthropogenic origin elements such as roads, ditches, etc.) to be generated. Such a goal may be achieved by analysis of a DTM generated on the basis of LiDAR (Light Detection And Ranging) data. However, such a DTM includes certain “obstacles” (bridges, culverts, etc.) that modify real concentrated flow paths. A methodology for LIDAR-type DTM modification was therefore proposed, with this including: 1) selection and digitisation (as line-type vector data) of the said “obstacles” (on the basis of field data and analyses of aerial photographs), 2) characterisation of the vectors (“obstacles”) by reference to the four attributes of buffer, incision, channel and resolution) – Fig. 2, and 3) modification of the DTM through burning of the “obstacles” using the attributes mentioned above. Such an approach allows for the generation of a surface drainage system similar to that observed in the terrain. The surface drainage system in question was generated using the D8 algorithm, with the threshold values required for first-order stream generation being calculated on the basis of field studies following on from a rainfall event (26/27-06-2009). The methodology proposed in this study seems to be correct. The surface drainage system generated on the basis of the DTM in the Zalasowka catchment was composed of elements of anthropogenic and natural origin, and was comparable with the system operating at the time of the rainfall event examined. The results for the period of heavy rainfall revealed a surface drainage system 9 times more developed (at 13.7 km·km–2) than the river system (1.5 km·km–2). There were significant changes in the stream pattern reflected in the Horton and Schumm ratios. There was also increases in the maximum stream order, the bifurcation ratio RB, the length ratio RL and the area ratio RA; as well as a decrease in the mean length and mean area of the first-order stream.