Bogdana Izmaiłow, Maria Kamykowska, Kazimierz Krzemień 1. Introduction 2. Research Method the GEOMORPHOLOGICAL EFFECTS of FLAS
Total Page:16
File Type:pdf, Size:1020Kb
PRACE GEOGRAFICZNE, zeszyt 116 Instytut Geografii i Gospodarki Przestrzennej UJ Kraków 2006 Bogdana Izmaiłow, Maria Kamykowska, Kazimierz Krzemień THE GEOMORPHOLOGICAL EFFECTS OF FLASH FLOODS IN MOUNTAIN RIVER CHANNELS. THE CASE OF THE RIVER WILSZNIA (WESTERN CARPATHIAN MOUNTAINS) Abstract: The objective of the River Wilsznia channel study was to identify patterns of river channel development during extreme flood events in an area with diverse geological resistance and varied land use. A flash flood event produced local erosional downcutting and lateral erosion along bedrock reaches, while channel migration and avulsion prevailed along alluvial reaches. Key words: Carpathian Mts., mountain river channels, flash floods. 1. Introduction The contemporary diversity in river channels has developed in a long evolutionary process. In mountainous areas this process has primarily been driven by erosion. Mountain river channels tend to be shaped by eddy erosion, resulting in local deepening, rather than by progressive erosion, which would even-out the channel longitudinal profile. A crucial role in their transformation is normally played by major river-flood events, such as flash floods, and the identification of their geomorphological impacts remains an important topic of mountain geomorphology research. The objective of the research project in the River Wilsznia channel was to identify patterns of river channel development during catastrophic river flood events in an area with diverse geological resistance and varied land use. 2. Research method The fieldwork in theWilsznia river network was carried out in two phases. In the summer of 2001, as phase one, the Wilsznia river and its tributary channels were mapped (Kałuża 90 PRACE GEOGRAFICZNE, ZESZYT 116 2001). The field mapping method applied to the structural research employed a specially designed blank form with a protocol on filling it in (Kamykowska et al., 1999). Develo- ped in various mountain areas, but especially in the Carpathian Mts., the method ensured uniform mapping of the entire Wilsznia river channel system. The outline of the mapped channel and its specific set of relief features helped the identification of individual channel reaches, which were then characterised in detail. This involved the identification of erosion and accumulation-driven terrain features, their qualitative and quantitative characteristics, rubble granularity and a scale of river management. All of the measurements were performed at low water levels. Altogether 41 reaches of the Wilsznia river system channels, with the combined length of 26.3 km, were mapped and characterised (Figure 1). Finally, using this material, local river channel types were identified. The second fieldwork phase was carried out after a flash flood on the Wilsznia river and its tributaries caused by a local torrential rainfall on the night of 18 July 2003 (Figure 2). Figure 2. Pluviogram from weather station in Krosno of 18 July 2003 The same channel reaches were mapped and the same methodology was applied, as in 2001. Since the greatest transformation occurred in the Wilsznia river channel below Olchowiec the second phase of fieldwork focused on reaches 9-17 (Figure 3), over a com- bined length of 6.8 kilometres. A vast photographic documentation was also collected. 3. Research area The Wilsznia river (11 km) is a right-bank tributary of the Upper Wisłoka river. Run- ning through the Beskid Niski range of the Western Carpathian Mts., it has a drainage basin of 66.31 km2. The geology is dominated by resistant Flysch series of the Magurska Nappe. Only in the northern and north-eastern parts of the drainage basin are there the less resistant Dukielska Nappe series (Ślączka 2003). At 754-342 meters asl, the research area has a trellis relief with parallel ridges running along a NW-SE line, consistent with the rock layers. The mountain ridges, reaching on average 600-750 m, are built of resistant Magura sandstones. The river network forms a grid pattern with alternating valley reaches parallel and perpendicular to the mountain ridges and to rock fold structures. The main valley has a broad (100-200 m) flat bed lined with gravel alluvia. The Wilsznia is a order-5 waterco- urse according to Horton-Strahler, with Ropianka and Roztoka being its largest right-bank tributaries and Hucianka as the main left-bank tributary. THE GEOMORPHOLOGICAL EFFECTS OF FLASH FLOODS IN MOUNTAIN RIVER CHANNELS... 91 Figure 3. Characteristics of the Wilsznia river channel in 2001 Explanations: A – channel reach numbers, B – channel cross section, C – channel long profil, D – geology: a – shales and thin-bedded sandstones, b – thick bedded sandstones and shales, E – number of bars per km, F – bar area [m2] per km, G – number of undercuts per km, H – number of thresholds per km, I – number of marmites per km, J – number of pools and riffles per km, K – largest size rubble, L – gradient in ‰, Ł – channel form factor, M – braiding coefficient, O – number of boundaries, P – channel types and subtypes: A – bedrock channels, a1 – bedrock straight-line channels with intensive down-cutting, B – bedrock-alluvial channels, b1 – bedrock/alluvial sinuous channels with down-cutting and local accumulation, b2 – bedrock/alluvial with lateral erosion and local intensi- ve accumulation, C – alluvial channels, c1 – alluvial with strong accumulation, c2 – alluvial with low gradient with strong accumulation and local lateral erosion. 92 PRACE GEOGRAFICZNE, ZESZYT 116 In the research area the rivers and streams feature a typically mountain-type hydro- logical regimen; uneven, rain/ground/snow-supplied, with a domination of thaw swelling in springtime (March-April) and secondary high-water periods caused by prolonged rain- fall in summertime and early wintertime (Dynowska 1971). The maximum precipitation of 600-650 mm (70% of the annual total) is recorded between April and September with a peak in June. A sizable proportion of this figure is accounted for by torrential rains and thunderstorms linked with the development of either convection currents or with the pas- sage of atmospheric fronts (Obrębska-Starklowa 2003). In the Beskid Niski range such events are typically of limited geographical spread. The maximum daily precipitation tops 100 mm. There are 24 days with precipitation greater than 10 mm daily, mostly between June and August (Obrębska-Starklowa 2003). Sheet flow prevails over infiltration due to the low ground retention capacity in a geology dominated by shale formations, with relatively thin layers of fissured sandstones and the low permeability of the waste-mantle. On small watercourses, water level ampli- tudes reach up to 2 m (Soja 2003). The Wilsznia network rivers swell up to 2 m above the channel bed on average every five years. Forests occupy 74.3% of the Wilsznia catchment. They cover most of the hilltops and slopes, while most of the non-wooded areas are concentrated along the valley floors. This land use pattern is a result of afforestation and the conversion of arable land into meadows after second World War. 4. Characteristics of the Wilsznia channel system and its tributaries The streams of the Wilsznia river system are typical of the Beskid Niski range. They run in alternating narrow and broad valley reaches linking to geologies of varying resistance. Their patterns are sinuous, running at an angle or parallel to geological strata, with only short perpendicular reaches. The channel long profiles are uneven. The mostly natural channel banks vary in height from 0.5 to 1.5 m with some higher banks, 3-5 m and up to 20 m in places, only in bank undercut sections. Where there is local bank reinforce- ment, the banks tend to be steep and the channel cross-sections are normally symmetrical, mostly rectangular or taper-shaped. The top-view pattern of the Wilsznia channel varies along its way. The first two reaches are straight. Further on, the sinuosity coefficient increases from 1.03 to 1.16. The one exception is the river’s most sinuous reach 9 with a coefficient of 1.33. The floor of the Upper Wilsznia river valley is narrow, thus limiting the width of the mostly flat floodplain to a maximum of 10 m. Downstream it broadens to 50 m at reach 10, which is accompanied by a more diverse micro-relief with a system of steps and remains of old channels. At reach 15 the floodplain spreads further to 500 m featuring wet oxbow lakes and dense red osier or grass vegetation. According to a 2001 study (Kałuża 2001), there are five river channel types and subtypes along the Wilsznia river, which were joined into three types (Figures 1). These include: bedrock straight-line channels with intensive down-cutting (reaches 1-2); bedrock/alluvial sinuous channels with down-cutting and local accumulation (reaches 3-9); bedrock/alluvial with lateral erosion and local intensive accumulation (reaches 10-14); THE GEOMORPHOLOGICAL EFFECTS OF FLASH FLOODS IN MOUNTAIN RIVER CHANNELS... 93 alluvial with low gradient with strong accumulation and local lateral erosion (reaches 15 and 17); and alluvial with strong accumulation (reach 16). The Wilsznia channel long-profile is uneven with clearly seen thresholds in reaches 2, 11 and 17 (Figure 3). The initial reaches (1-2) are straight and steep at 52-74%. The only features include steps and potholes mostly cutting into solid rock, only sometimes into thick layers of rubble and organic material. Further downstream the bedrock/accumulation channel is alternately cutting into the bedrock and alluvia and is more gradual (3-33%), but also sinuous. From reach 4 down rocky steps cut into the thick-bedded Magura sand- stone formations. Chutes and rocky steps have formed in the less resistant thinner bedded shale. Alongside the erosion features, bank undercuts are also found from reach 3 onwards. They are formed in waste mantle (reaches 10-13) and alluvial (14-17) material. From reach 10 downstream the number of downcutting-related features dropped, while the number of undercuts bank increased (Figure 3).