Fig. 1.
A – Location of the study area in south-eastern Poland, as shown in Figure 1B. B – Location of the detailed study area shown in Figure 1C, overlaid on the landforms and river network of the middle San River basin (Przemyśl Foothills). The map indicates the location of fan delta No. 4. C – Geomorphological map of the studied cones and incised V-shaped valleys in the Stupnica River valley. Debris-flow cones numbering (1–3) corresponds to descriptions in the text. 1 – ridges, 2 – landslide tongues, 3 – distinct landslide niches, 4 – indistinct landslide niches, 5 – incised V-shaped valleys, 6 – trough-shaped (dellen type) valleys, 7 – foot of the escarpment slope, 8 – debris (torrent) cones, 9 – floodplain, 10 – streams and riverbeds, 11 – point bars, 12 – road cuts. Lithological complexes: Shs – shales with sandstone intercalations; SaS – medium- to thin-bedded sandstones with shale intercalations. Lithological boundaries after SMGP 1:50 000 (Rauch et al. 2009) and field observations.
Fig. 2.
Effects of the flash flood in the Stupnica and Jawornik catchments (the San tributaries) in the Przemyśl Foothills. 1 – channel erosion, 2 – deposition in the channel and on the floodplain, 3 – debris-flow (torrents) cones and alluvial cones, 4 – landslides and earth flows, 5 – tree jams, 6 – destroyed bridges, 7 – flooded houses, 8 – settlements.
Table 1.
Morphometric characteristics of cones and the type and fraction of material deposited at the mouths of V-shaped valleys during the torrential rainfall on 26 June 2020, in the Stupnica Valley (Przemyśl Foothills).
| No | Type of cone | Length | Base width | Height | Slope | Type of sediments | Thickness | Maximum particle size |
|---|---|---|---|---|---|---|---|---|
| [m] | [%] | [cm] | ||||||
| 1. | Debris | 20 | 50 | 2.0 | 5.5 | Boulders, gravels, tree logs, cobbles | 20–40 | 75 × 40, 70 × 45, 70 × 20, 57 × 39, 49 × 12, 40 × 15, 30 × 15, 30 × 17, 20 × 11, 16 × 7 |
| 2. | Debris | 96 | 90 | 4.7 | 5.8 | Boulders, cobbles, soil aggregates, sand | 10–60 | 92–50 |
| 3. | Muddebris | 47 | 55 | 4.5 | 6.4 | Silt, cobbles, soil aggregates, boulders, tree logs, branches | 30–40 | Boulders (50–40) cobbles (30–20) |
| 4. | Silty-sandy | 30 | 16 | 1.0 | 1.0 | Silt, sand, plant detritus, small wood, tree trunks | 35–107 | 0.2–0.05 |
Table 2.
Morphometry of incised V-shaped valleys and cone surface area in the Stupnica catchment (Przemyśl Foothills) formed as a result of the extreme rainfall on 26 June 2020.
| No. | Type of valley | Height max | Height min | Length | Slope | Catchment area | Cone surface area |
|---|---|---|---|---|---|---|---|
| [m a.s.l.] | [m] | [%] | [km2] | [ha] | |||
| 1. | V-shaped | 373 | 252 | 900 | 13.4 | 0.12 | 0.26 |
| 2. | V-shaped | 378 | 252 | 1100 | 11.5 | 0.53 | 0.55 |
| 3. | Trough-V-shaped | 312 | 257 | 400 | 13.7 | 0.03 | 0.22 |
| 4. | V-shaped | 445 | 283 | 3500 | 4.6 | 2.88 | 0.05 (0.09*) |
| 5. | V-shaped | 392 | 248 | 1650 | 8.7 | 1.40 | 1.00 |
* value in parentheses indicate the cone surface area, including the built-up area (inaccessible for research), where accumulation was also observed. The numbering of the selected valleys and cone surface areas (Nos. 1–4) in the table corresponds to the sequence in which they are discussed in the text.
Fig. 3.
A – Daily precipitation totals in June 2020 at the Bircza weather station (Archival data, IMGW-PIB). B – Ten-min precipitation sums at the Bircza station on 26 June 2020 (Archival data, IMGW-PIB).
Fig.4.
Radar image of storm cells showing the spatial distribution of the maximum rainfall intensity index in the Stupnica and Jawornik catchments on 26 June 2020 (source: Archival data, IMGW-PIB). 1 – precipitation monitoring stations, 2 – other localities mentioned in the article, 3 – detailed study area (see Fig. 1C), No. 4 marks the fan delta at the outlet of the Korzonka stream tributary.
Fig. 5.
Erosion in the headwater section of a dry small valley (pol. debrza) caused by an earth-organic debris flow following a downpour on 22 June 2020.
Fig. 6.
A – Debris-flow (torrent) cone No. 1. The convex lobate landform of mid-fan with coarse deposition of boulders, pebbles, and wood debris. In the foreground, a clear boundary is visible between proximal-fan gravel deposition and distal-fan sand and silt deposition. B – The bedrock channel of the V-shaped valley was deepened and widened as a result of a debris–mud flow event on 26 June 2020.
Fig. 7.
Longitudinal profile and cross-sections of the lower segment of the source-incised valley for debris-flow cone No. 1. 1 – valley cut into bedrock, 2 – bedrock steps, 3 – colluvial debris-clay cover, 4 – incision depth of the valley floor (in meters).
Fig. 8.
A – Overview of debris-flow cone No. 2. The central (mid fan) sector of the cone shows an accumulation of coarse debris lobe, whereas the marginal zones are composed of gravelly sandy sediments. B – The apex part of debris cone No. 2 with incised channel. Sandstone blocks and boulders, up to 90 cm in diameter, accumulated at the mouth of the forested V-shaped valley.
Fig. 9.
Delta fan deposits, up to 1 m thick, accumulated in a small valley (a tributary of the Korzonka stream) during the peak of a flood wave backed up by a road embankment and a tree jam blocking the road culvert.