Contribution to the morphodynamic chronology of Beskydian rivers (Morávka River )

Transcription

1 Contribution to the morphodynamic chronology of Beskydian rivers (Morávka River ) Mgr. Jan Hradecký Katedra fyzické geografie a geoekologie, Přírodovědecká fakulta Ostravské univerzity, Chittussiho 10, Ostrava - Slezská Ostrava, , Česká republika Study of temporal changes of geomorphologic regime of water streams represents one of significant spheres of fluvial geomorphology, respectively of dynamic geomorphology. Collecting data on the development of fluvial systems and processing them bring valuable information not only for solving theoretical aspects of the development of the georelief but these can be also used in applied research and practice. The study of time changes can be framed in various chronological dimensions with respect to specific interactions of a water stream as a morphodynamic individual and surrounding relief. Chronological dimensions can create four basic categories: long timescale (>10 5 years), medium timescale ( years), short timescale ( years), instantaneous time (<10-1 years) (SCHUMM AND LICHTY, 1965, modified by KNIGHTON, 1998). Fig. 1. The location of the area under study within the territory of the vicinity Frýdek-Místek town. Explanations: 1 area of analyzed aerial photographs and historic maps, 2 areas urbanized, 3 area of intensive changes of geomorphic river regime, 4 weir. In the case of Beskydian water streams we are going to concentrate on the morphodynamics in the historic period ( ). We have chosen the Morávka River as an example locality. It represents ideal object of study from the point of geomorphologic regime and types 50

2 of changes influenced by a man. The studied locality itself is the length between the weir in Vyšní Lhoty and the confluence of the Morávka and the Ostravice Rivers in Frýdek-Místek (see Fig. 1). The Morávka River has preserved specific features of a mountain stream despite a wide range of human-induced interference with the stream channel. Basic aspect of the geomorphologic regime of the stream is preserved braided channel pattern that has had more intensive and more extensive character recently. The stream was and still is influenced by a wide range of human impacts that represented the interference with the recent development of the fluvial system. The impacts caused transformation of the dynamics mechanisms of geomorphologic processes that can be explained through theory of stability, theory of dynamic equilibrium and theory of geomorphic thresholds. To understand the mechanisms it is necessary to view the water stream as a time continuum whose individual time periods involve sways in character and intensity of morphogenetic processes as influenced by various factors. However, viewed from the longer period of time, the sways stand for the dynamics proper to the system while the trend of the development is not influenced (robust behaviour). Preservation of the current development line is possible provided that the threshold limits preserving the current geomorphologic regime have not been exceeded. Otherwise, the water stream enters a new development trajectory (responsive behaviour). These two aspects of dynamic geomorphology themselves can be easily identified within the studied locality. The term geomorphic thresholds was introduced into literature by SCHUMM (1979) and he defines it as: a threshold of landform stability that is exceeded either by intrinsic change of the landscape itself, or by progressive change of and external variable. From this point of view there is an interesting relationship between two types of thresholds and development and dynamics (see Fig. 2). There are two basic groups of thresholds (according to SHUMM, 1980): extrinsic threshold one that is exceeded by the application of a force or process external to the system fundamental influence on the stream development - natural changes and changes induced by human activity (e.g. climatic fluctuation, land-use change, base level change), intrinsic threshold one in which change occurs without a change in an external variable natural changes (e.g. changes of position gravel bars, lateral bank cutting leading to slope failures, etc.). From the point of human-induced pressure the fluvial system of the Morávka River can be understood as a perfect laboratory where it is possible to study consequences of these influences and carry out following prognoses of the development. If need be, its geomorphologic history could be used as a warning example of the behaviour of a man in the landscape. The overview of human-induced changes of the fluvial system of the Morávka River is summed in Tab. 1. Water streams with geomorphologic braided channel pattern are marked with frequent anabranching in thickly gravelled beds as a consequence of unbalanced flows (pre-glacier zone) or as a consequence of periodic (or, as the case may be, episodic) flooding. The last example is typical for the Beskydian region. Intensive disturbing processes lead to destruction of bank growth and vegetation covering ephemeral or permanent bars and islands in the stream channel. Repeated activity of the stressing factor of floods keeps the braided stream channel out of thicker covering by vegetation. Therefore, aerial photographs can be used during the spatial analysis of the stream channel. 51

3 Fig. 2. Relation between robust (I.+III., upper diagram) and responsive behaviour (II., upper diagram), geomorphic thresholds and three states of dynamic equilibrium observed in Morávka R. channel (modified after Werritty In: Thorne et al.(eds.) and Selby, 1974). 52

4 Tab.1. Types of human-induced changes in the studied area ( modified after KNIGHTON, 1998) River regulations Channel modifications Direct changes Water storage by reservoirs Bank stabilizations or channel-phase changes Diversion of water Channel straightening Stream gravel extraction Land-use changes Land drainage Removal of vegetation, Agricultural drains especially deforestation Indirect changes Afforestation Storm-water sewerage or systems land-phase changes Changes in agricultural practices Building construction Urbanization Mining activity To study the time changes we made use of two historic maps (1780 and ) and a set of aerial photographs of the same area taken in different periods and. With help of the Military Topographic Institution in Dobruška we have acquired chronological series of photographs of the Morávka River of the period of Argus, a.s., a private company offered us aerial photographs of the period before floods in March On the base of their analysis it is possible to make a time series of the Morávka River channel pattern. Thus, we can analyze potential changes in the intensity of morphodynamic processes or if need be the change of their substance (i.e. the change of geomorphologic regime of the water stream). The morphodynamic changes of the area of interest were analyzed in following periods: Data of the length of the oldest research within the studied period can be interpreted from the map of the First Military Mapping (1780). Despite the fact that the map did not rise on the base of accurate geodetic studies it brings valuable information on the condition of the stream channel. The water stream branched out throughout the whole studied length; the most intensively in the vicinity of an undercut slope of the Skalická Strážnice Mt (438 m) and to the north of the line Staré Město (Altstat) Nošovice (Noschowitz). Before the proper mouth of the Morávka River to the Ostravice River there is a very interesting length where the two main stream channels branch out into a few parallel branches. The pattern of the stream channel resembles inner delta. The map of the Third Military Mapping ( , issued in 1910) corresponds with the previous length to a great extent. The part of the water stream before the mouth does not show branching of the previous period. On the map there are objects in the stream channel unidentified by that time (given mark does not appear in the legend of the map). The same marks can be found in the stream channel to the north of the elevation point of Przewrat czernazem (408,3; now Vrchy). They probably mark forlorn, unflowed anabranches of the stream. This supports the fact that the intensity of braiding was considerably lower in the period of the Third Mapping This period is marked with typical geomorphologic braided pattern throughout the whole part of the studied river length. However, detailed morphology of the stream channel in intensively gravelled parts cannot be identified, i.e. smaller ephemeral channels (channels flowed through in the course of higher water level) are not palpable from the aerial 53

5 photographs. By 1955 lateral bars rose in the vicinity of Frýdek-Místek. Their extent was considerably bigger in comparison with the previous period. Changes within bigger stream channels can be quite easily interpreted. In course of time branching (central and eastern part of the area) spread. This is supported by the photograph of However, its quality is not high enough. Thus, it is possible that even in 1937 there was a number of branches that were recorded in 1955 or vice versa, some branches were interpreted in a wrong way. Important index of changes is represented by the existence of unstable large bars and islands. The period documented in the photograph of 1937 is marked with fords that to a certain extent diversify morphology of the gravel-bed channel. The year of 1955 was very significant for the following morphological development of the water stream with intensive human-induced interference with the geomorphologic regime. In the western part of the area we can identify transverse constructions on the riverbanks This period is marked with a considerate tendency to the straightening of the stream. A lot of anabranches disappear. One of potential reasons for the lay-out is represented by river regulations and channel modifications on the upper Morávka River. Construction of the weir in Vyšní Lhoty that is used for water diversion to the Lučina water basin took place from From Morávka water reservoir was constructed. Considerable changes in morphology of the stream channel can be observed in the vicinity of Frýdek-Místek where the stream deposited a great amount of gravel sediments. The photograph of 1955 shows still evident increasing of the number of anabranches while the year 1966 shows stream stability The same tendency that was described in the previous stage of the stream development is typical for this period. Branching eliminated to the minimum. Still, it remained preserved in the furthest eastern part of the profile (to the southwest of Dobrá). In comparison with the previous period the extent of gravel accumulation lowered on the eastern margin of Frýdek- Místek where stream branching can be observed in a low degree. Large islands and bars in the central part of the area ceased to exist. In 1979 a construction of waste dump was palpable on the left bank of the Morávka River (central part of the photograph). As a consequence of lateral erosion of the stream the part of the bank containing the dump became undercut. Terrain observations during made it obvious that the Morávka River has a tendency to the lateral attacking of the bank zone which is immediately connected to the dump (northern marginal part of the dump lies on the contact with the stream). The photographs clearly show unsuitable localization of the dump on the undercut bank Channel anabranching to the east of Frýdek-Místek definitively stopped; the intensity of the process damped. Construction in the vicinity of the stream was probably connected with removal of gravel accumulation (extractional gravel accumulation?). In 1992 only not big accumulations of the type of lateral bars could be observed. Conspicuous bars and ephemeral islands disappeared in other parts. The most intensive river regulations and channel modifications within the stream took place in the studied area (making banks firmer with stone straightening, building transverse constructions stone chutes in the eastern part of the profile). Detailed interpretation of the photograph of 1992 shows signs of deep erosion of the stream (central part of the area). 54

6 The last photograph of the chronology originated in March 1997, i.e. just before a considerable hydrological-geomorphologic event floods in July There were changes in the geomorphologic regime of the stream in the central part of the profile with conspicuous displays of deep erosion. These changes are not so intensive on the previous photograph. Bank zone connected with deep incision of the stream and landslide activity on destabilized banks (zigzag course of the bank line) was extended in New gravel accumulations appear in the lower parts along the stream flow. Described changes of the geomorphologic regime in the length of Dobrá Frýdek-Místek lead to the rebuilding of channel pattern. Original braided channel is being changed into siltclay channel with combination of bed rock channel (localities of Tcheschinite intrusions). It is necessary to compare the geomorphologic changes with hydrological data and data of human impact in the studied area. Present and future research will concentrate on contemporary changes of channel landforms to quantify the intensity of the changes. Methods of digital terrestrial photogrammetry will be used. This paper was supported by grant project No. IZEP References GREGORY, K. J., WALLING, D.E. (1973): Drainage Basin Form and Process. Edward Arnold, London, 456 pp. HRADECKÝ, J. (2000): Současná morfodynamika koryt beskydských toků fenomén řeky Morávky. In: Lacika, J.(ed.): Zborník referátov. 1. konferencia ASG při SAV, , Liptovký Ján, Bratislava, 2000, s KILLIANOVÁ, H. (2000): Řeka Morava na mapách III. vojenského mapování z let : příspěvek k fluviální dynamice. Geol. výzk. Mor. Slez. v roce 1999, VII, Brno, s KIRCHNER, K., IVAN, A. (1999): Anastomózní systémy v CHKO Litovelské Pomoraví. Geol. výzk. Mor. Slez. v roce 1998, VI, Brno, s KNIGHTON, D. (1998). Fluvial forms & processes. Arnold, London New York Sydney Auckland, p SCHUMM, S.A. (1979): Geomorphic thresholds: the concept and its applications. Transactions of the Institute of British Geographers (NS), 4, p SCHUMM, S.A. (1980): Some applications of the concept of geomorphic thresholds. In: Coates, D.R., Vitek, J. (Eds.): Thresholds in Geomorphology. George Allen & Unwin, London, p SCHUMM, S.A., LICHTY, R.A. (1965): Time, space, and causality in geomorphology. American Journal of Science, 263, p SELBY, M.J. (1974): Dominant geomorphic events in landform evolution. Bulletin of the International Association of Engineering Geology, 9, p THORNE, C.R., HEY, R.D., NEWSON, M.D. (1997). Applied Fluvial Geomorphology for River Engineering and Management. Wiley, Chichester New York Weinheim Brisbane Singapore Toronto, p WILLIAMS, G.P., COSTA, J.E. (1988). Geomorphic measurements after a flood. In: Baker, V.R., Kochel, R.C., Patton, P.C., (Eds.): Flood geomorphology. Wiley, New York Chichester Brisbane Toronto Singapore, p

7 Souhrn Příspěvek k morfodynamické chronologii beskydských toků (Morávka ) Studium časových změn geomorfologického režimu vodních toků představuje jednu z významných oblastí fluviální geomorfologie, potažmo její dynamické části. V případě beskydských vodních toků studie řeší otázky morfodynamiky v historické době ( ). Jako příklad byl zvolen vodní tok Morávka, který z hlediska geomorfologického režimu a typů změn ovlivněných člověkem představuje ideální objekt studia. Lokalitou vlastního výzkumu je úsek toku mezi jezem ve Vyšních Lhotách a soutokem Morávky s Ostravicí ve Frýdku-Místku (viz Fig. 1). Vodní tok prošel celou řadou antropogenních změn (Tab. 1), které představovaly zásah do vývoje fluviálního systému v historické době. Zásahy transformovaly mechanismy dynamiky geomorfologického režimu, které lze vysvětlit prostřednictvím teorie dynamické rovnováhy a teorie prahů (viz Fig. 2). Pro studium časových změn byla k dispozici série leteckých snímků s časovým posunem ( ) a dvě historické mapy (1780 a ). Na základě jejich analýzy lze sestavit časovou řadu prostorového uspořádání koryta Morávky a na tomto základě analyzovat případné změny v intenzitě morfogenetických procesů, resp. změnu jejich podstaty (tzn. změnu geomorfologického režimu toku). Základním aspektem změny geomorfologického režimu toku Morávky je transformace původního divočícího toku na tok s intenzivními projevy hloubkové a boční eroze. 56