Fluvial processes on the wet Miszkowice Fan. Part I. Erosion and erosional landforms

Abstract

Abstract The Miszkowice Fan is a small wet alluvial fan located at the mouth of the River Złotna valley, Western Sudetes. The Złotna is a small mountain stream (Qmean 0.45 m3 s-1) characterized by a gravelly bed, steep slope, and large fluctuations in discharge (0.05-40 m3 s-1 in the last eight years). The lowest stretch of the Złotna running through the Miszkowice Fan was investigated since 1971. Field observations and measurements of the mode and rate of fluvial erosion were made systematically in two years (1977-1978). At that time three floods have been noted, namely in May (Q ca. 12-15 m3 s-1), and August 1977 (Q ca. 35-40 m3 s-1) and in February/March 1978 (Q ca. 9-11 m3 s-1). Field investigations have demonstrated that the location within the channel of zones of strong lateral and downward erosion depends on the position of the streamtube of maximum velocity, which changes with the discharge and Froude number. The higher the Froude number the better the conformability between theoretical and observed positions of the streamtube of maximum velocity. With high Froude numbers the stream-tube of maximum velocity lies characteristically at the inner bank in the entrance to the bends and approaches the outer cut bank at the bend axis or some point downchannel from it. The structure of turbulence during a flood discharge is very complex and variable. Nevertheless it has been found that the greatest rate of erosion is commonly attributable to stationary, horizontal, spiral vortexes. Their structure may be recorded both in bank morphology and bed configuration. The distribution within the channel of zones of erosion and deposition depends on the water and sediment discharge and the condition of bed material. With mean discharge and stable gravelly bed erosion takes place along riffle slopes as well as in upper parts of pools that follow them (backward erosion on riffles). At the same time deposition frequently occurs within middle and lower portions of the pools (backward deposition in pools). During a flood discharge and live-bed conditions deposition takes place on the riffles (vertical and downchannel accretion), while erosion is operating within middle and lower portions of the pools (downward and lateral erosion). The best explanation of the observed differentiation in location of erosion and deposition zones is the phenomenon of velocity reversal (Keller 1971, 1977). During mean stage the flow undergoes contraction over the riffle slopes (high velocity) and expands in the pools (low velocity), while in flood it expands over the riffles (low velocity) and becomes contracted in bends or straight pools (high velocity). During a flood discharge riffles and pools may also develop within unstable straight reaches. These may be genetically connected with either symmetrical or asymmetrical erosion. Successive pools are generally 5-8 channel widths apart. Field measurements have also demonstrated that both a low discharge of megascopically clear water and a strongly overloaded one may result in erosion of alluvial channels. With a low discharge erosion takes place along the actual wetted perimeter provided that flow velocity exceeds that necessary to transport fine sand. An overloaded discharge, on the other hand, results in vertical accretion and a rise in elevation of the channel bed. It has been also noted that strong lateral erosion of alluvial channel banks may be brought about by such flows and that the erosion is apparently simultaneous with the deposition. Under conditions of stable gravelly bed the depth of scour is related to the grain size of the bed material, while with a live-bed it is controlled mostly by the intensity of turbulence and the transporting ability of the stream. Low floods, carrying low amounts of bed load, may produce deep scours and relatively small modifications of bank configuration rendered by both lateral erosion and deposition of coarse bed material. High floods, on the other hand, lead to substantial changes in bank morphology attributable to either lateral erosion and/or lateral accretion. During high floods strong downward erosion occurs rather locally and there is a general tendency for coarse bed-load material to accumulate in some reaches of lower gradient and, particularly, in channels crossing alluvial fans. Thus deposition within alluvial channels is commonly connected with high flow velocities and erosion with low flow velocities. During the August 1977 flood gravelly channel plugs have developed on a number of Sudetic alluvial fans. These are typical especially of central-fan segments, downfan from the points of intersection. Thus the gravelly plugs occur characteristically on those segments of the fans which are characterized by the steepest longitudinal and transverse slopes. There is increasing evidence that the following cycle of channel patterns is characteristic of the wet Sudetic fans under the present-day climatic conditions and the activity of man: plugging of an incised channel during a high flood → unstable braided pattern → straight or low-sinuosity incised channel originated from gully erosion → incised meandering stream → incised meander belt. The occurrence of meandering channels on the Sudetic fans is perhaps conditioned by climate from the one hand and the presence of thick alluvial loams covering fan gravels from the other. The whole cycle may last from a dozen or so till several tens of years. Alluvial fans, being places of deposition of the coarsest bed load available, control to a large degree the character and amount of load carried by mountain streams. Passing through the Miszkowice Fan the Złotna leaves behind up to 85% its bed-load and changes in character from a bed-load to a mixed-load stream. This is well manifested by the thickness and composition of alluvial loams, which are much thicker in the fanbase area as well as below the confluence of the Złotna and the Bóbr. The most regular meandering patterns not uncommonly coincide with those places. Meandering channels of pebbly mountain streams may exist under relatively a wide range of slope and water and sediment discharge. However, if a flood discharge is deficient in bed-load a meandering reach tends to degenerate for meander bends become too wide (erosion at the outer bank exceeds deposition at the inner bank). By the time channel sinuosity diminishes and there may occur a tendency to downward erosion. If a flood discharge is strongly overloaded the meandering channel becomes unstable and a complex meandering-braided or braided pattern may originate. Straight channels may be stable in the mountains provided that they attain in flood a normal slope and depth (uniform water and sediment discharge under conditions of quasi-steady flow) and that channel capacity is large enough to keep an undisturbed movement of all the bed material supplied to the reach.Procesy fluwialne na „mokrym” stożku Miszkowic. Część I. Erozja i formy erozyjneAbstract Wyjątkowo mokry rok hydrologiczny 1976/77 przyniósł w dorzeczu górnego Bobru dwie powodzie: w dniach 20-22 maja oraz w pierwszej dekadzie sierpnia 1977. Powódź majowa była następstwem deszczu nawalnego w dniu 20 maja, kiedy to notowano w oddalonej o 6 km od terenu badań Lubawce 40,2 mm opadu. Katastrofalna powódź sierpniowa wystąpiła na stożku Miszkowic w dniach 1 - 3 sierpnia po deszczu nawalno-rozlewnym w dniach 31 VII-2 VIII 1977. W Lubawce zanotowano wtedy 126,3 mm opadu, co stanowiło 12,7% opadu w roku 1977 (tabela 1). Lokalnie powodzie te spowodowały znaczne zmiany morfologiczne w strefie aktywnych koryt rzecznych, a zwłaszcza na stożkach napływowych. Praca niniejsza zawiera podsumowanie wyników badań terenowych prowadzonych w dolnym odcinku rzeki Złotnej (fig. 2), w dolinie Bobru powyżej zapory w Bukówce oraz w niektórych innych dolinach rzek sudeckich. Na dolnej Złotnej systematyczne obserwacje prowadzono od roku 1971, a ciągłe pomiary natężenia erozji wykonano w latach 1977-1978.