Instytut Geografii i Gospodarki Przestrzennej UJ Kraków 2006
Bogdana Izmaiłow, Maria Kamykowska, Kazimierz Krzemień
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
THE GEOMORPHOLOGICAL EFFECTS OF FLASH FLOODS IN MOUNTAIN RIVER CHANNELS.
THE CASE OF THE RIVER WILSZNIA
(WESTERN CARPATHIAN MOUNTAINS)
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.
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.
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);
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). Along the initial reaches of the upper river course the bars were linked as a result of the blocking vegetation. Downstream the number of bars diminished while their size increased. The mouth section of the Wilsznia channel (reaches 16-17) was cutting into alluvia. The reach had a very gradual slope with numerous bars and islets forming a typical braided channel. In the summer of 2001, during the mapping, no downcutting features were recorded and only a small number of bank undercuts.
The tributary channels were similar to the Wilsznia channel, even if the lengths and sequences of the bedrock, bedrock/alluvial and alluvial reaches differed. The 2001 maps revealed that the Wilsznia system channels were mostly cutting into the bedrock or alluvia and in the latter case there were local sections cutting through to the bedrock as well. Only the mouth section of the Wilsznia river featured a typical braided alluvial channel.
5. Catastrophic flooding
On 18 July 2003, a portion of the Wilsznia river network experienced a catastrophic flooding event. It was caused by a local torrential rainstorm over the Wilsznia and Ropianka valleys and the Wisłok river network further to the east. The local pluviogramme from the town of Krosno shows the main rainfall to have taken place between 20:00 and 21:00 (Figure 2). During that single hour, 38.3 mm of rain fell in Krosno, while the total rainfall amounted to 52 mm. Similar proportions were recorded by other precipitation stations, but there were no precipitation or water level stations in the study area.
Based on the nearest precipitation readouts the most intense daily rain fell around Olechowiec towards Wisłok Wielki and Orzechówka to the east. The actual metered rainfall values were as follows: 55.4 mm at Krempna; 38.4 at Dukla; 63.6 at Wyszowatka; 48.6 at Barwinek; 88.0 at Wisłok Wielki; 98.7 at Orzechówka; 49.8 at Komańcza; and 52.0 mm at Krosno. Interviews with the local population appear to indicate that in the Wilsznia valley the rain started around 19:00, peaked between 20:00 and 21:00 and continued until 22:30. According to a certain assessment in excess of 100 mm water may have fallen in the area of Olechowiec during 1.5 hours. Marks on the river and stream channels show that the highest water levels reached 3.5-4.0 m along the Wilsznia river from reach 9 to the river mouth at its confluence with the Wisłoka river. Water levels were increasing very rapidly until 22:00. While on the Upper Wisłoka upstream of the Wilsznia mouth, near Krempna,
the levels did not show a significant increase and grew up to 1.1 m in the Hucianka river basin, the Wilsznia river valley itself experienced a catastrophic flash flood. At Polany a bridge was destroyed and there were casualties (Photo 1). Returning to the main channel near the church at Polany, a side stream of the Wilsznia swept a car from the road killing all five inside. The car and its passengers were dragged for more than 1.7 km downstream (between reaches 12 and 16). Local interviews confirmed that this was the greatest flood in the valley in living memory.
6. Wilsznia channel transformation during the catastrophic flood
The fact that the Wilsznia and its tributaries had been mapped before the flood better enables one to understand the role of catastrophic floods in mountain valley floors.
The greatest transformation was experienced in the Wilsznia channel, especially along reaches 9-11 (Photo 2). This was caused by a number of factors. The most intensive rain fell near Olchowiec, a junction of three streams, the Wilsznia, Ropianka and Olchowczyk, which carried most logs and branches blocking channels and causing their avulsion into the valley floor. Similar cases were also recorded during catastrophic floods in other Carpathian river and stream valleys (Kaczka 1999, Wyżga et al. 2002-2003). Downstream of the Roztoka mouth both the valley floor and the Wilsznia channel broaden considerably causing the river energy to be distributed over a larger area and eliminating the clogging effect of the logs and branches carried by the stream, thus reducing the geomorphological impact of the flood along that reach (Photo 3).
However, despite the great transformation of the Wilsznia river channel in the 2003 flood, the only change was in the number of relief features.In the upstream reaches that number diminished. In the downstream reaches the number of features either dimini- shed or slightly increased (Figures 4 and 5). In the narrow-channel reaches the number of features mostly dropped, but their area increased. The overall numberof bank undercuts and channel bars increased. There was a general increase of downcutting, especially along reaches 11-12 and 15-17, as evidenced by the number of rock steps (Figure 4). New rock thresholds emerged in reaches 15 and 17. This means that the river had effectively cut into
Figure 4. Number of rocky thresholds along the Wilsznia river
Figure 5. Overall area of bars along the Wilsznia river before and after the flood
the Flysch bedrock through the alluvial cover that was, apparently, relatively thin even in the braiding reaches, similar to other Carpathian river and stream channels (Krzemień 1976, 2003, Wyżga 1992).
Compared to the status of 2001, the proportion of the largest-sized rubble accumulated along the entire length of the Wilsznia channel increased. This was caused by the greater river energy during the flash flood capable of tearing rocks from the sub-base (Photo 4).
Such rocks, up to 1 m in diameter, were deposited separately or in piles with clearly mar- ked imbricate structures (Photo 5). The increased number of features linked with erosion along certain reaches and the marked increase in the quantity of the largest-sized rubble along the entire length of the channel clearly point to erosion as the driver of the Wilsznia channel transformation during the catastrophic flood.
Where the channel was blocked by tree logs and branches, rocks up to 0.5 m in diameter were thrown out into the floodplain (ca. 1.6 m above the channel bed; reach 10).
In certain reaches such organic matter accumulation zones contributed to the development of bars up to several hundred square meters in area on the floodplain (Photo 6). However, where the valley floor was wide the accumulated organic material did not cause any significant geomorphological modifications of the floodplain (Photo 7). The deep cuts, down to 1.5 m in solid rock or 2.2 m in alluvia/solid rock, found in both the channel and on the floodplain are evidence of a great role played by organic material, such as logs and branches, in di- verting the huge water energy to the deepening of the channel through erosion. This type of downcutting is common in mountain stream channels during large floods. Elsewhere in the Beskidy Mountains similar intensive erosion led to the deepening of channels by 0.8-1.0 m (Krzemień 1976, 2003, Froehlich 1998). The downcutting of the Wilsz- nia channel was particularly strong downstream from the organic and inorganic matter accumulation zones, where the river’s energy was clearly greater. The channel banks and bed were also eroded where, as a result of organic and inorganic matter accumulation, downcutting, lateral erosion or channel avulsion into the valley floor occurred. This was
particularly visible along reaches 10-11 or 16-17, where the channel rubble accumulated against piles of tree branches and logs either as large separate bars or a broad aggradation zone across the channel width (Photo 8).
The fluvial investigation of the Wilsznia river system demonstrates that local flash floods can lead to large-scale transformations of stream channels. Additionally, large quantities of rubble tends to be displaced and delivered into the main river channels.
Taking into account studies of areas of the Beskidy Mts., it can be inferred that events such as that in the Wilsznia channel constitute a pattern in the functioning of small mountain streams during flash floods.
7. Conclusions
The transformation of a mountain channel system is restricted to a deep chute cut- ting into a terraced valley floor. During catastrophic floods, the bedrock channel reaches experience local downcutting and lateral erosion. In certain places the process leads to the destruction of rock threshold systems developed on thin or medium-bedded sand- stone formations. Downcutting and lateral erosion provide fresh rubble material into the channel, a fact confirmed by a marked increase in the quantity of the largest-sized bedlo- ad. The alluvial reaches, on the other hand, feature a strong increase in channel migration.
Where a channel is blocked with logs and branches or where hydro-engineering or training structures have been destroyed this process may lead to avulsion. As a result of floods, however, it is not the number of relief features that increases, but their individual size.
The square area of such features is strictly related to the local valley morphology and to the size of the high-water channel. The greatest transformation occurs in the main chan- nels, especially in the zones clogged by tree logs. The best evidence of the transporting force of a river or stream is the development of imbricate structures. On the floodplain the transformation tends to be only at a local scale. This may be either rubble build-up or erosion cutting, principally where the main stream is diverted onto the floodplain from a blocked channel. On the way back to the main channel the eroding or accumulating capacity of such streams tends to be minor and does not normally have any morphological impact, except for some accumulated organic material traces.
The patterns identified in the research area are primarily applicable to mountain channels in wooded areas ensuring ample supply of organic material, such as tree logs and branches.
The paper has been prepared under a project No PBZ-KBN-086/P04/2003, Extreme meteorological events in Poland. (Evaluation of events and forecasting their effect on the human environment).
References
Dynowska I., 1971, Typy reżimów rzecznych w Polsce, Zeszyty Nauk. UJ, Prace Geogr., 28.
Froehlich W., 1998, Transport rumowiska i erozja koryt potoków beskidzkich podczas powodzi w lipcu 1997 roku, [in:] Powódź w dorzeczu górnej Wisły w lipcu 1997 roku, PAN, Kraków.
Kaczka R., 1999, The role of coarse woody debris in fluvial processes during the flood of the July 1997, Kamienica Łącka valley, Beskidy Mountains, Poland, Studia Geomorph. Carp.-Balcan., 33.
Kałuża S., 2001, Typy koryt rzecznych w dorzeczu Wilszni (Beskid Niski), MSc Thesis, IGiGP UJ.
Kamykowska M., Kaszowski L., Krzemień K., 1999, River channel mapping instruction, Key to the river bed description, [in:] K. Krzemień (ed.), River channels, Pattern, structure and dynamics, Prace Geogr. IG UJ, 104.
Krzemień K., 1976, Współczesna dynamika potoku Konina w Gorcach, Folia Geogr., ser.
Geogr.-Phys., 10.
Krzemień K., 2003, The Czarny Dunajec River, Poland, as example of human-induced development tendencies in a mountain river channel, Landform Analysis, 4.
Obrębska-Starklowa B., 2003, Warunki klimatyczne, [in:] A. Górecki, K. Krzemień, S. Skiba, B. Zemanek (ed.), Przyroda Magurskiego Parku Narodowego, MPN, UJ, Krempna-Kraków.
Soja R., 2003, Wody, [in:] A. Górecki, K. Krzemień, S. Skiba, B. Zemanek (ed.), Przyroda Magur- skiego Parku Narodowego, MPN, UJ, Krempna-Kraków.
Ślączka A., 2003, Budowa geologiczna, [in:] A. Górecki, K. Krzemień, S. Skiba, B. Zemanek (ed.), Przyroda Magurskiego Parku Narodowego, MPN, UJ, Krempna-Kraków.
Wyżga B., 1992, Zmiany w geometrii koryta i układzie frakcji jako odzwierciedlenie transformacji reżimu hydrologicznego Raby w czasie ostatnich dwustu lat, Czasopismo Geogr., 43.
Wyżga B., Kaczka R., Zawiejska J., 2002-2003, Gruby rumosz drzewny w ciekach górskich – formy występowania, warunki depozycji i znaczenie środowiskowe, Folia Geogr., ser. Geogr.-Phys., 33-34.
Bogdana Izmaiłow Maria Kamykowska Kazimierz Krzemień
Institute of Geography and Spatial Management Jagiellonian University
Gronostajowa 7 30-387 Cracow Poland
Figure 1. W Explanations
Photo 2. The River Wilsznia channel dissected in a zone of piled branches and tree logs (reach 10), during the flood of July 2003
Photo by K. Krzemień
Photo by K. Krzemień
Photo 4. The rocky Wilsznia channel newly dissected during the flood of July 2003 (reach 10)
Photo 6. The rocky Wilsznia channel newly dissected with rubble accumulated within the flood terrace during the flood of July 2003 (reach 13)
Photo by K. Krzemień
Photo by K. Krzemień
Photo 8. A new rubble bar within the River Wilsznia flood plain (reach 9)
