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Experience in monitoring channel deformations on rivers with broad flood plains (as exemplified by the Kerzhenets river)
Geography and Natural Resources 29 (2008) 195–200
Experience in monitoring channel deformations on rivers with broad flood plains (as exemplified by the Kerzhenets river) O. V. Korableva * and A. V. Chernov Moscow Pedagogical State University, Moscow Received 9 January 2007
Abstract Presented are the results of a five-year (2001-2005) monitoring of deformations of the Kerzhenets river channel in its lower part. The concave-bank caving observations were made for three free bights compose d of sand and loamy sand: the steep segmental, segmental developed and segmental gently sloping blights. Keywords: monitoring, channel deformation, intense bank caving.
Formulation of the problem The ever increasing requirements imposed on information regarding the dynamics and anticipated development of natural objects that have been or are being involved in the human economic activities dictate a need to constantly perfect the methodological background to the study of natural geographical objects. To accomplish this implies not only devising new investigation methods but also improving the previously well-known research techniques. Among them, of course, is continuous monitoring of the functioning and development of an object under investigation, based on using an invariable or slightly differing technique at prescribed time intervals. Monitoring is particularly important for investigating the development of dynamical natural objects which are distinguished by drastic changes over the course of one generation. One such object is represented by rivers in general and their channels and floodplains in particular. On the other hand, while a hydrological monitoring of the water and sediment runoff has been carried out for a long time (for more than 150 years), a geomorphological monitoring the river channels and floodplains is confined to a highly limited scale. Intermittent observations of poorly navigable shoals have been carried out on may navigable rivers of Russia for more than 100 years. The frequency of observation varies from a single to several observations per navigation, and a monitoring of deformations of the channels is carried out only on
* Corresponding author.
large navigable rivers when comparing survey data on the channels obtained at intervals of at least 10 years. A combined analysis of such surveys (pilot charts) can only provide highly averaged data on bank caving and aggradation, and on places of their manifestation, which can result in serious errors and irreparable consequences in the case of site selection to accommodate the particular economic projects. In an earlier publication [1], we pointed to the errors in determining horizontal channel deformations arising in the case of large intervals between measurements: the mean rate of floodplain bank caving of a bend of the Kerzhenets river (the left tributary to the Volga in the Nizhni Novgorod TransVolga region) for a 13-year period was 1 m/year, and the maximum rate was 1.6 m/year. In the year 2001 when the Volga had high-water level, the mean rate of caving of the same bank was 2 m/year, and the maximum rate was 5.8 m/year in the lower wing of the bend, i.e. by a factor of 3.6 larger than the mean rate for many years. By contrast, in the year 2002 when the Volga had low-water level, the retreat of the entire front of the concave bank was below average, or only 0.6 m/year, but the maximum rate was 3.3 m/year, i.e. above average. Hence it was concluded that the horizontal channel deformations behaved unevenly in the long-term section and that they were closely associated with water abundance in a particular year in general, and with high-water levels in particular. To obtain more objective information on the character of horizontal channel deformations and determine the actual rates of bank caving, and aggradation and the periods of intensification and decay of this process requires continuous observations of the channel deformations in key stretches, i.e. a monitoring of channel processes.
Copyright © 2008 IG SB, Siberian Branch of RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.gnr.2008.06.003
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O. V. Korableva and A. V. Chernov / Geography and Natural Resources 29 (2008) 195–200
tive study of them. Specifically, B. F. Snishchenko [2] treats a monitoring of channel processes as the most important task of obtaining qualitative information on real channel processes and includes it as the basic component in the research program implemented by the State Hydrological Institute of Rosgidromet. The monitoring technique for river channels to be sued to determine the degree of their resistance and, hence, the potential hazard was developed by R. S. Chalov and A. V. Chernov [3]. Observations of the dynamics of the channel and shoals of the incised pebble Tarusa river, the left tributary to the Upper Oka, have made every year since 1995 [4]. 2001 saw the start of a monitoring of the behavior of the Kerzhenets river channel within the “Kerzhensky” state nature reserve (in the lower part of the river), initially on one plot, and since 2003 on a further two plots arranged on the bends. The results from the first year of observation on the former plot were published in [1]. In this paper, we discuss the monitoring data for three bends of the Kerzhenets river spanning the time interval 2001-2005. Description of the object under study The Kerzhenets river, the left tributary to the Volga, crosses the Nizhni Novgorod Trans-Volga region, a vast alluvial plain composed largely by alluvial sands of the ancient Volga that were reworked by the Kerzhenets and by the other rivers of the Dnieper fluvioglacial sediments [5]. Lithological structure of this kind predetermines the free conditions of development of channel deformations, and the broad-floodplain pattern of the Kerzhenets channel, and of its tributaries. The river has a curved channel, with steep and gently sloping segments predominating, and with more rarely occurring sinusoidal and loop-shaped free branches surrounded by the floodplain banks. The floodplain includes segments of ridges and is composed by easily scoured sands and loamy sands, and in places by peatbed lenses. The Kerzhenets river chan-
straightening of very steep meanders, and the development of new meanders in the place of straightening branches. Thus, for the period since 1964 along the 33-kilometer stretch of the Kerzhenets river within the nature reserve, of more than 60 meanders, three straitened in a natural way, and a further four meanders underwent a human-caused straightening. The mean caving of the concave banks of the meanders for that period was 1.5 m/year. According to the hydrological regime, the Kerzhenets is categorized as the Eastern-European type, characterized by high spring floods (they occur on the Kerzhenets in the second half of May), and by a normal (high or low) water level in the summertime with the possible rain-induced floods, not high autumn floods, and a low persistent wintertime water level [6]. The western boundary of the Kerzhenets state nature reserve runs along the axis of the Kerzhenets river channel; deformations of the river channel are constantly changing the territory and the area of the nature reserve – caving of the left bank, and aggradation of the right bank lead to a reduction in its area, and the inverse processes the situation is the reverse. Especially significant are the increases or decreases in the area in the case of a straightening of the meanders, where the valley-side spurs of the meanders suddenly find themselves on the other river bank – either within the nature reserve or outside of it. The influence of annual water levels on bank caving intensity For the purpose of making a forecast of deformations of the Kerzhenets channel in the protection area, which is only possible through detailed observations of horizontal channel deformations of the river, continuous observations of the caving of the concave banks of three meanders differing in shape and curvature were organized (Fig. 1). The bank caving of the meanders at different points along the concave bank is determined at the time of an intermittent change in the
Fig. 1. Schematic maps of the areas of the bends of the Kerzhenets river in which the channel processes are being monitored. Numerals – bench mark numbers; a – overgrowing shoal stretches; b – oxbow lake, and Kerzhenets tributary, the Vishnya river.
O. V. Korableva and A. V. Chernov / Geography and Natural Resources 29 (2008) 195–200
distance between the bank line and the recorded benchmarks which are represented by specially marked trees. Some of these benchmark trees can find themselves in the hazardous zone of caving and fall into the river with the passage of time; in this case, extra benchmarks are assigned in advance to be used in measuring the bank caving. Typically the measurements are made twice a year: in June after a flood rise, and in September-October at the transition of the summertime to wintertime low-water period. In years with rain-caused high-water levels, additional measurements can be made thereafter. Such a periodicity helps determine the season (the phase of the hydrological regime) when channel deformations are taking place with the highest intensity. Plot No. 1 (PP1), the measurements on which have been carried on since 2001, is a segment-shaped steep meander with its high concave floodplain bank undergoing the caving process: the coefficient setting the scale of meandering (Cm) of the meander is determined as l/L, where l is the length along the meander channel, and L is the length of its step equal to 1.82 (see Fig. 1, pl. 1). The floodplain is composed by sand-loamy sand alluvium, and the scarp is 3.3 m in
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height. The floodplain surface is covered with spruce forest and is complicated by hollows and ridges. Also, some of the hollows is open to the channel as a result of the bank caving process, and they show outcrops of loamy sands and peat. Table 1 presents the results from measuring the cavings of the concave bank of the meander for the period 2001-2005 during the different phases of the regime – at a high- and low-water period. In this case the local non-uniformity of bank caving at neighboring points is accounted for by the different influence of the trees at the edge of the bank undergoing the caving process. They retard the scour by heir root system over a long period of time, and lastly as soon as they collapse, however, they carry along massive blocks of the bank. Subsequently, the scouring intensity in this place is increased. 1. Almost all channel deformations occur at high-water period. Separate bank scours were recorded at the time of a rain-caused stream rise in 2004. No bank caving occurs at a low-water period – it was observed to retreat at separate points at the low-water periods of 2001, 2002, and 2003, due to the slickensliding of local blocks of the bank scarp whose stability was upset at the high-water period.
Table 1 Caving intensity (m) of the concave bank of the bend in trial area No. 1 at high-water, freshet and low-water periods (2001-2005 ) Bench Elemark ment of numbend ber 1 2 3 Upper wing 4 5 6 7 8 9 10 Head 11 12 13 14 15 16 17 18 19 Lower 20 wing 21 22 23 24 25 26 Mean caving
2001 high water
0.8 0.6 1.7 0.0 1.1 1.1 0.0 4.1 0.7 1.65 2.25 3.0 1.0 0.7 2.9 2.8 4.65 3.4 3.2 1.98
2002
2003
2004
2005
low water
high water
low water
high water
low water
high water
freshet
low water
high water
low water
0.35 0.0 0.0 0.3 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0
0.2 0.0 0.53 0.4 0.0 1.0 0.25 0.5 0.0 0.3 0.3 0.2 0.0 1.3 0.5 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 1.1 3.2 1.0 1.2 0.0 0.0 0.8 0.15
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0
0.0 1.6 0.0 0.1 0.0 0.0 0.2 0.0 0.8 1.3 1.7 1.4 0.1 0.8 1.6 0.0 0.0 0.2
0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.7 0.0 0.2
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.8 0.0 0.6 0.1 0.8 0.7 2.5 3.4 2.0 1.5 1.5 1.3
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.08 0.0 0.0 0.0 0.0 0.2
0.0 0.0 0.1 0.0 0.0 0.0
1.1 3.3 0.0 0.5 0.05 0.0
0.0 0.1 0.1 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.8 0.0
0.0 0.0 0.0 0.0 0.0 0.0
0.6 0.2 0.6 1.0 0.2 1.2
0.3 0.1 0.0
0.0 0.0 0.0
3.2 0.8 2.0
0.8 0.0 0.0
0.0
0.0
0.2
0.0
0.1
0.6
0.0
0.3
0.0
0.5
0.1
0.0
1.3
0.0
Total caving for points 0.55 1.6 0.53 0.8 0.0 1.3 2.25 1.3 3.1 1.7 5.0 6.6 3.6 6.5 8.9 3.0 4.15 4.1 3.0 7.0 5.2 5.7 4.3 5.9 4.6 3.2 3.9
Note. Blank cells – no data, as the bench marks are wholly caved, the mean caving is determined by dividing the total sum of all cavings by the number of years of observation.
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Table 2 Caving intensity (m) o the concave bank of the bend in trial area No. 2 at high-and low-water periods (2003-2005)
Table 3 Caving intensity (m) pf the concave bank of the bend in trial area No. 3 at high- and low-water periods (2003-2005)
0.6 1.0 1.3 4.2 4.0 3.2 2.1 4.0 3.0 4.6 7.3 9.8 2.1 3.6
1 2 3 4 5 6 7 8 9 10
Upper wing Head
Lower wing
Mean caving
low water
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
high water
0.2 0.0 0.7 4.2 2.2 1.9 1.5 4.0 2.8 4.6 7.1 7.9 2.0 3.0
low water
Total caving for points
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
high water
low water
.0 0.0 0.3 0.0 0.6 1.3 0.0 0.0 0.0 0.0 0.2 1.6 0.1 0.3
2005
low water
high water
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2004
high water
low water
0.0 1.0 0.3 0.0 1.2 0.0 0.6 0.0 0.2 0.0 0.0 0.3 0.0 0.28
2003 Element of bend
high water
1 2 Upper 3 wing 4 5 6 7 Head 8 9 10 Lower 11 wing 12 13 Mean caving
Bench mark number
low water
2005
high water
2004
Elements of bend
2003
Total caving for points
high-water periods; 2) the maximum bank caving (4-8 m/ year) more likely occurs in the lower part of the meander head (points 11 and 12), although at the very head of the meander (points 4-10) the maximum scouring rate is rather high, or 1-4 m/year; 3) the mean scouring rate of the concave bank at the high-water period of 2005 (3 m/year) exceeded the mean scouring rate of the same bank in the two preceding years, which is in some agreement with the measurements from plot No. 1, where maximum scours during 2003-2005 were also recorded in 2005, and 4) the mean scouring rate of the entire bank for three years was 1.2 m/year, whereas in 2005 the bank in the lower wing of the meander retreated at the place of maximum scouring by 7.9 m. Comparison of the scouring intensity on the first and second plots reveals that on the steeper meander (PP2) the fastest maximum scouring was 7.9 m (2005), whereas on the least steep meander (PP1) it was only 6.6 m. Plot No. 3 (PP3) is located on a very gently sloping evolving meander, the upper wing of which is still represented by an overgrowing tail of the above-lying side water surface. Scouring affects the weakly meandering bank of the high floodplain (3.5 m above a normal water level) covered with spruce and pine (in the lower wing) forest and composed by sand-loamy sand alluvium (see Fig. 1, pl. 3). The observations of the scouring of this bank have been made since 2003. The measurements (Table 3) confirm the conclusions drawn for the two previous plots: 1) no scouring occurs at a low-water period; 2) scouring affects largely the lower wing of the meander, which is generally typical of a gently sloping meander, and 3) the most intense horizontal channel deformations were observed during the flood of 2005 when, as was the case in the two previous years, their intensity was an order of magnitude lower. The maximum scouring rate, recorded in the lower wing of the meander, was 4.9 m (in
Bench mark number
2. In spite of the scatter of points with maximum scouring from year to year (sequentially from 2001 to 2005, points 15, 12, 11, and 15), they are all concentrated at the beginning of the lower wing of the meander. Analysis of the total scour of the bank for the last five years using the individual points shows that it exceeds 5 m at points 11, 12, 14, 15, 18, 20, 21, 22, 24, and 25 in the lower wing of the meander. By contrast, the total scour at the points of the upper wing (1, 2, 3, 4, and 5) is minimal. These data intimate that the scour of the lower wing of the steep meander is dominant and that there is a longitudinal component n its displacement. 3. The most intense scouring was recorded in 2001 and 2005, and it was much weaker in between. Thus, the scour of the bank (with due regard for the points at which no scouring was observed) in 2001 and 2005, respectively, was 1.97, and 1.28 m/year, respectively, whereas in 2002, 2003 and 2004 it was 0.6, 0.33, and 0.5 m/year, respectively. 4. The mean scouring rate of the banks of the meander for five years was about 1 m/year. The actual scouring rates of the banks on some stretches of the meander varied from 0 to 6.6 m/year. This lends support to the earlier conclusion [1] that the scouring rates of the banks, averaged over long periods, have low representativity. Plot No. 2 (PP2) is a steep, nearly loop-shaped, meander of the Kerzhenets river (see Fig. 1, pl. 2. Its concave bank where the scouring measurements are made, belongs to an elevated floodplain 3 m in height above the low-water edge. The surface of the floodplain is covered with pine forest with the inclusion of birch and lime, its relief near the rim is relatively subdued, and the floodplain scarp is composed by sand-loamy sand alluvium. The measurements on the plot have been made using an identical technique since 2003. The measurement results for the period 2003-2005 are presented in Table 2. It is also evident from the table that: 1) all bank scours occur only at
0.0 0.0 4.6 0.0 0.0 0.7 1.6 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 1.6 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.4 0.4 0.0 0.0 1.5 3.6 1.1 3.4 3.0 4.9
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.4 0.4 4.6 0.0 1.5 4.3 2.8 3.4 4.6 4.9
0.7
0.0
0.17
0.0
1.8
0.0
2.7
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Fig. 2. Plots of water level variation for the Kerzhenets river in the area of the settlement of Rustai for 2001-2005.
2005), which is lower than on the steep segment-like and the loop-shaped meander. The observations of the scouring of the concave banks of the three meanders with a different meandering call for explanation as regards their non-uniformity in separate years. In the course of the variation of the water levels of the Kerzhenets river for the period 2001-2005 (Fig. 2), three groups of years can be identified, with a relatively high flood (more than 0.4 above an arbitrary “0” on the diagram) (2001, and 2005); with a relatively low flood, and with rain-caused floods (2003-2004), and with a relatively low flood, and with no rain-caused floods (2002). Since bank scouring occurs mostly at a high water-period, within the context of studying channel deformations the second and third groups can be combined, and only groups with high and low water levels are left. Empirically, for the Kerzhenets river we can delineate the boundary between them from the flood height of 0.4 m. Such are the years 2001 and 2005 for the preceding time interval, i.e. the years during which the most intense scours of the concave banks of the meanders occurred (Fig. 3). In the other years, the floods were considerably lower; therefore, the intensity of channel deformations of the Kerzhenets river was also considerably lower, even in spite of the raincaused floods about 100 cm in height that occurred in the summer of 2003 and 2004. It should be noted that the mean scouring rates for the concave bank of the steep segment-shaped meander on plot N. 1 in 2001 and 2005 are uncorrelated with the flood height in the same years – the rise of the water level in 2001 reached 410 cm, and in 2005 it was 438 m, or nearly 30 cm as high, whereas the mean bank scour rate in 2001 and 2005 was 1.97
and 1.28 m, respectively. This difference is attributable to a change in the shape of the meander: in the scouring process of the concave bank it becomes steep, the coefficient of its meandering increases, and the advantage (in terms of energy) of the meandering shape of the channel decreases. According to N. I. Makkaveyev [7], it is maximal when Cm = 1.57; on the meander, however, it reaches 1.82 in 2001. The eroding capacity of the flow decreases at the concave bank. This fact could also be explained differently: in 2001 the high flood level persisted for about 10 days, whereas it stayed only three days in 005.
Fig. 3. Maximum water levels during floods (a), and mean bank caving (b) for three bends of the Kerzhenets river from 2001 to 2005 in the area of the settlement of Rustai.
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Conclusions A five-year-long monitoring of the channel deformations on three free meanders (with a different degree of meandering) of the Kerzhenets river suggests the following conclusions. 1. Bank scours occur almost always during floods. Neither rain-caused floods (of course, if their height does not exceed the flood height) nor a long-lasting high standard water level does not cause bank scour; instead, it is caused by the slight deformations which occur at the time of a flood and have a residual character after floods. 2. The lower wing is mostly affected by scouring on the three meanders; hence the longitudinal component in the displacement of the meanders is present in the dynamics not only of gently sloping meanders but also of steep meanders. In the latter case, it does not exceed the transverse component which is ensured by the scouring of the heads of the meanders. 3. The intensity of horizontal channel deformations is closely associated with the flood height – it is maximal if the flood height corresponds to or slightly exceeds the height of the floodplain. Bank scours are proceeding markedly more slowly in the case of lower flood levels. The other characteristics of a year’s water abundance do not influence markedly the scouring intensity. 4. The values of the bank scour rates, averaged over a long-term period of time, are considerably lower than the actual rates in years with high floods. Thus, the mean scouring rate of the concave banks on the three meanders of the Kerzhenets river for the last 3-5 years is 1 m/year, whereas it was (in the case of a high flood) 6.6 and 7.9 m in 2005, in some places on the steep meanders. By continuing a monitoring of the deformation of the free meanders of the broad floodplain Kerzhenets river, it will be possible to extend the as yet small series of observations of these processes, validate or invalidate some conclusions drawn on the basis of a 5-year series of observations and, possibly, to reveal new, hitherto unknown, details in the development of river channels. They could serve as a basis for a monitoring of the floodplains nearby the meanders under investigation and, in particular, determine the formation patterns of young floodplain massifs due to aggradation of the concave banks of the meanders, the character of the
floodplain succession on the convex banks of the meanders, and the dynamics of the relief and landscapes on the undercut banks as the leading edge of the caving is displaced deep into the floodplain. Furthermore, a monitoring of the most typical meanders of the Kerzhenets river would help make forecasts of the channel deformations along the entire length of the channel of this river within the nature reserve. This is of important significance for delineating the boundaries of the protection territory which tend to change constantly on the channel stretch. This work was done with financial support from the RF President’s Grant for financial backing of scientific schools (NSh-4884.2006.5), and from the Russian Foundation for Basic Research (07-05-004210). References 1. Chernov A. V. and Korableva O. V. On the results of bank caving observations on the rivers of the Forest Trans-Volga Region and their analysis. Proc. Acad. of Problems of Water Management Sciences. Moscow: Izd-vo Mosk. un-ta, 2003, issue 9: The Problems of Channel Studies, pp. 206-214. 2. Snishchenko B. F., Debolsky V. K., Chalov R. S. et al. The problems of studying and monitoring the channel process, erosion and sediment runoff to meet the current requirements of the economy. 6th All-Russian Hydrol. Congress: Abstracts of Papers. SPb: Gidrometeoizdat, 2004, pp. 42-47. 3. Chalov R. S. and Chernov A. V. The ecological consequences of channel processes and their monitoring. The Sergeyev Lectures. Moscow: GEOS, 2000, issue 2, pp. 115-119. 4. Chernov A. V. Characteristics of channel deformations on small rivers in conditions of limited development of channel deformations. 18th Plenary Inter-University Coordination Meeting on the Problems of Erosion, Channel and Estuary Processes. Kursk: Izd-vo Kursk. un-ta , 2003, pp. 213-214. 5. Fridman B. I. and Korableva O. V. Geology and relief of the Kerzhensky Nature Reserve. Proc. “Kerzhensky” State Nature Reserve. Nizhni Novgorod, 2001, v. 1, pp. 7-70. 6. Mankish V. D. and Bayanov N. G. The hydrological and hydrochemical regime of the Kerzhenets river and its tributaries in its middle and lower part. Proc. “Kerzhensky” State Nature Reserve. Nizhni Novgorod, 2001, v. 1, pp. 79-108. 7. Makkaveyev N. I. The River Channel and Erosion Within Its Basin. Moscow: Izd-vo AN SSSR, 1955, 354 p.
Experience in monitoring channel deformations on rivers with broad flood plains (as exemplified by the Kerzhenets river) O. V. Korableva * and A. V. Chernov Moscow Pedagogical State University, Moscow Received 9 January 2007
Abstract Presented are the results of a five-year (2001-2005) monitoring of deformations of the Kerzhenets river channel in its lower part. The concave-bank caving observations were made for three free bights compose d of sand and loamy sand: the steep segmental, segmental developed and segmental gently sloping blights. Keywords: monitoring, channel deformation, intense bank caving.
Formulation of the problem The ever increasing requirements imposed on information regarding the dynamics and anticipated development of natural objects that have been or are being involved in the human economic activities dictate a need to constantly perfect the methodological background to the study of natural geographical objects. To accomplish this implies not only devising new investigation methods but also improving the previously well-known research techniques. Among them, of course, is continuous monitoring of the functioning and development of an object under investigation, based on using an invariable or slightly differing technique at prescribed time intervals. Monitoring is particularly important for investigating the development of dynamical natural objects which are distinguished by drastic changes over the course of one generation. One such object is represented by rivers in general and their channels and floodplains in particular. On the other hand, while a hydrological monitoring of the water and sediment runoff has been carried out for a long time (for more than 150 years), a geomorphological monitoring the river channels and floodplains is confined to a highly limited scale. Intermittent observations of poorly navigable shoals have been carried out on may navigable rivers of Russia for more than 100 years. The frequency of observation varies from a single to several observations per navigation, and a monitoring of deformations of the channels is carried out only on
* Corresponding author.
large navigable rivers when comparing survey data on the channels obtained at intervals of at least 10 years. A combined analysis of such surveys (pilot charts) can only provide highly averaged data on bank caving and aggradation, and on places of their manifestation, which can result in serious errors and irreparable consequences in the case of site selection to accommodate the particular economic projects. In an earlier publication [1], we pointed to the errors in determining horizontal channel deformations arising in the case of large intervals between measurements: the mean rate of floodplain bank caving of a bend of the Kerzhenets river (the left tributary to the Volga in the Nizhni Novgorod TransVolga region) for a 13-year period was 1 m/year, and the maximum rate was 1.6 m/year. In the year 2001 when the Volga had high-water level, the mean rate of caving of the same bank was 2 m/year, and the maximum rate was 5.8 m/year in the lower wing of the bend, i.e. by a factor of 3.6 larger than the mean rate for many years. By contrast, in the year 2002 when the Volga had low-water level, the retreat of the entire front of the concave bank was below average, or only 0.6 m/year, but the maximum rate was 3.3 m/year, i.e. above average. Hence it was concluded that the horizontal channel deformations behaved unevenly in the long-term section and that they were closely associated with water abundance in a particular year in general, and with high-water levels in particular. To obtain more objective information on the character of horizontal channel deformations and determine the actual rates of bank caving, and aggradation and the periods of intensification and decay of this process requires continuous observations of the channel deformations in key stretches, i.e. a monitoring of channel processes.
Copyright © 2008 IG SB, Siberian Branch of RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.gnr.2008.06.003
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tive study of them. Specifically, B. F. Snishchenko [2] treats a monitoring of channel processes as the most important task of obtaining qualitative information on real channel processes and includes it as the basic component in the research program implemented by the State Hydrological Institute of Rosgidromet. The monitoring technique for river channels to be sued to determine the degree of their resistance and, hence, the potential hazard was developed by R. S. Chalov and A. V. Chernov [3]. Observations of the dynamics of the channel and shoals of the incised pebble Tarusa river, the left tributary to the Upper Oka, have made every year since 1995 [4]. 2001 saw the start of a monitoring of the behavior of the Kerzhenets river channel within the “Kerzhensky” state nature reserve (in the lower part of the river), initially on one plot, and since 2003 on a further two plots arranged on the bends. The results from the first year of observation on the former plot were published in [1]. In this paper, we discuss the monitoring data for three bends of the Kerzhenets river spanning the time interval 2001-2005. Description of the object under study The Kerzhenets river, the left tributary to the Volga, crosses the Nizhni Novgorod Trans-Volga region, a vast alluvial plain composed largely by alluvial sands of the ancient Volga that were reworked by the Kerzhenets and by the other rivers of the Dnieper fluvioglacial sediments [5]. Lithological structure of this kind predetermines the free conditions of development of channel deformations, and the broad-floodplain pattern of the Kerzhenets channel, and of its tributaries. The river has a curved channel, with steep and gently sloping segments predominating, and with more rarely occurring sinusoidal and loop-shaped free branches surrounded by the floodplain banks. The floodplain includes segments of ridges and is composed by easily scoured sands and loamy sands, and in places by peatbed lenses. The Kerzhenets river chan-
straightening of very steep meanders, and the development of new meanders in the place of straightening branches. Thus, for the period since 1964 along the 33-kilometer stretch of the Kerzhenets river within the nature reserve, of more than 60 meanders, three straitened in a natural way, and a further four meanders underwent a human-caused straightening. The mean caving of the concave banks of the meanders for that period was 1.5 m/year. According to the hydrological regime, the Kerzhenets is categorized as the Eastern-European type, characterized by high spring floods (they occur on the Kerzhenets in the second half of May), and by a normal (high or low) water level in the summertime with the possible rain-induced floods, not high autumn floods, and a low persistent wintertime water level [6]. The western boundary of the Kerzhenets state nature reserve runs along the axis of the Kerzhenets river channel; deformations of the river channel are constantly changing the territory and the area of the nature reserve – caving of the left bank, and aggradation of the right bank lead to a reduction in its area, and the inverse processes the situation is the reverse. Especially significant are the increases or decreases in the area in the case of a straightening of the meanders, where the valley-side spurs of the meanders suddenly find themselves on the other river bank – either within the nature reserve or outside of it. The influence of annual water levels on bank caving intensity For the purpose of making a forecast of deformations of the Kerzhenets channel in the protection area, which is only possible through detailed observations of horizontal channel deformations of the river, continuous observations of the caving of the concave banks of three meanders differing in shape and curvature were organized (Fig. 1). The bank caving of the meanders at different points along the concave bank is determined at the time of an intermittent change in the
Fig. 1. Schematic maps of the areas of the bends of the Kerzhenets river in which the channel processes are being monitored. Numerals – bench mark numbers; a – overgrowing shoal stretches; b – oxbow lake, and Kerzhenets tributary, the Vishnya river.
O. V. Korableva and A. V. Chernov / Geography and Natural Resources 29 (2008) 195–200
distance between the bank line and the recorded benchmarks which are represented by specially marked trees. Some of these benchmark trees can find themselves in the hazardous zone of caving and fall into the river with the passage of time; in this case, extra benchmarks are assigned in advance to be used in measuring the bank caving. Typically the measurements are made twice a year: in June after a flood rise, and in September-October at the transition of the summertime to wintertime low-water period. In years with rain-caused high-water levels, additional measurements can be made thereafter. Such a periodicity helps determine the season (the phase of the hydrological regime) when channel deformations are taking place with the highest intensity. Plot No. 1 (PP1), the measurements on which have been carried on since 2001, is a segment-shaped steep meander with its high concave floodplain bank undergoing the caving process: the coefficient setting the scale of meandering (Cm) of the meander is determined as l/L, where l is the length along the meander channel, and L is the length of its step equal to 1.82 (see Fig. 1, pl. 1). The floodplain is composed by sand-loamy sand alluvium, and the scarp is 3.3 m in
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height. The floodplain surface is covered with spruce forest and is complicated by hollows and ridges. Also, some of the hollows is open to the channel as a result of the bank caving process, and they show outcrops of loamy sands and peat. Table 1 presents the results from measuring the cavings of the concave bank of the meander for the period 2001-2005 during the different phases of the regime – at a high- and low-water period. In this case the local non-uniformity of bank caving at neighboring points is accounted for by the different influence of the trees at the edge of the bank undergoing the caving process. They retard the scour by heir root system over a long period of time, and lastly as soon as they collapse, however, they carry along massive blocks of the bank. Subsequently, the scouring intensity in this place is increased. 1. Almost all channel deformations occur at high-water period. Separate bank scours were recorded at the time of a rain-caused stream rise in 2004. No bank caving occurs at a low-water period – it was observed to retreat at separate points at the low-water periods of 2001, 2002, and 2003, due to the slickensliding of local blocks of the bank scarp whose stability was upset at the high-water period.
Table 1 Caving intensity (m) of the concave bank of the bend in trial area No. 1 at high-water, freshet and low-water periods (2001-2005 ) Bench Elemark ment of numbend ber 1 2 3 Upper wing 4 5 6 7 8 9 10 Head 11 12 13 14 15 16 17 18 19 Lower 20 wing 21 22 23 24 25 26 Mean caving
2001 high water
0.8 0.6 1.7 0.0 1.1 1.1 0.0 4.1 0.7 1.65 2.25 3.0 1.0 0.7 2.9 2.8 4.65 3.4 3.2 1.98
2002
2003
2004
2005
low water
high water
low water
high water
low water
high water
freshet
low water
high water
low water
0.35 0.0 0.0 0.3 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0
0.2 0.0 0.53 0.4 0.0 1.0 0.25 0.5 0.0 0.3 0.3 0.2 0.0 1.3 0.5 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 1.1 3.2 1.0 1.2 0.0 0.0 0.8 0.15
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0
0.0 1.6 0.0 0.1 0.0 0.0 0.2 0.0 0.8 1.3 1.7 1.4 0.1 0.8 1.6 0.0 0.0 0.2
0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.7 0.0 0.2
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.8 0.0 0.6 0.1 0.8 0.7 2.5 3.4 2.0 1.5 1.5 1.3
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.08 0.0 0.0 0.0 0.0 0.2
0.0 0.0 0.1 0.0 0.0 0.0
1.1 3.3 0.0 0.5 0.05 0.0
0.0 0.1 0.1 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.8 0.0
0.0 0.0 0.0 0.0 0.0 0.0
0.6 0.2 0.6 1.0 0.2 1.2
0.3 0.1 0.0
0.0 0.0 0.0
3.2 0.8 2.0
0.8 0.0 0.0
0.0
0.0
0.2
0.0
0.1
0.6
0.0
0.3
0.0
0.5
0.1
0.0
1.3
0.0
Total caving for points 0.55 1.6 0.53 0.8 0.0 1.3 2.25 1.3 3.1 1.7 5.0 6.6 3.6 6.5 8.9 3.0 4.15 4.1 3.0 7.0 5.2 5.7 4.3 5.9 4.6 3.2 3.9
Note. Blank cells – no data, as the bench marks are wholly caved, the mean caving is determined by dividing the total sum of all cavings by the number of years of observation.
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Table 2 Caving intensity (m) o the concave bank of the bend in trial area No. 2 at high-and low-water periods (2003-2005)
Table 3 Caving intensity (m) pf the concave bank of the bend in trial area No. 3 at high- and low-water periods (2003-2005)
0.6 1.0 1.3 4.2 4.0 3.2 2.1 4.0 3.0 4.6 7.3 9.8 2.1 3.6
1 2 3 4 5 6 7 8 9 10
Upper wing Head
Lower wing
Mean caving
low water
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
high water
0.2 0.0 0.7 4.2 2.2 1.9 1.5 4.0 2.8 4.6 7.1 7.9 2.0 3.0
low water
Total caving for points
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
high water
low water
.0 0.0 0.3 0.0 0.6 1.3 0.0 0.0 0.0 0.0 0.2 1.6 0.1 0.3
2005
low water
high water
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2004
high water
low water
0.0 1.0 0.3 0.0 1.2 0.0 0.6 0.0 0.2 0.0 0.0 0.3 0.0 0.28
2003 Element of bend
high water
1 2 Upper 3 wing 4 5 6 7 Head 8 9 10 Lower 11 wing 12 13 Mean caving
Bench mark number
low water
2005
high water
2004
Elements of bend
2003
Total caving for points
high-water periods; 2) the maximum bank caving (4-8 m/ year) more likely occurs in the lower part of the meander head (points 11 and 12), although at the very head of the meander (points 4-10) the maximum scouring rate is rather high, or 1-4 m/year; 3) the mean scouring rate of the concave bank at the high-water period of 2005 (3 m/year) exceeded the mean scouring rate of the same bank in the two preceding years, which is in some agreement with the measurements from plot No. 1, where maximum scours during 2003-2005 were also recorded in 2005, and 4) the mean scouring rate of the entire bank for three years was 1.2 m/year, whereas in 2005 the bank in the lower wing of the meander retreated at the place of maximum scouring by 7.9 m. Comparison of the scouring intensity on the first and second plots reveals that on the steeper meander (PP2) the fastest maximum scouring was 7.9 m (2005), whereas on the least steep meander (PP1) it was only 6.6 m. Plot No. 3 (PP3) is located on a very gently sloping evolving meander, the upper wing of which is still represented by an overgrowing tail of the above-lying side water surface. Scouring affects the weakly meandering bank of the high floodplain (3.5 m above a normal water level) covered with spruce and pine (in the lower wing) forest and composed by sand-loamy sand alluvium (see Fig. 1, pl. 3). The observations of the scouring of this bank have been made since 2003. The measurements (Table 3) confirm the conclusions drawn for the two previous plots: 1) no scouring occurs at a low-water period; 2) scouring affects largely the lower wing of the meander, which is generally typical of a gently sloping meander, and 3) the most intense horizontal channel deformations were observed during the flood of 2005 when, as was the case in the two previous years, their intensity was an order of magnitude lower. The maximum scouring rate, recorded in the lower wing of the meander, was 4.9 m (in
Bench mark number
2. In spite of the scatter of points with maximum scouring from year to year (sequentially from 2001 to 2005, points 15, 12, 11, and 15), they are all concentrated at the beginning of the lower wing of the meander. Analysis of the total scour of the bank for the last five years using the individual points shows that it exceeds 5 m at points 11, 12, 14, 15, 18, 20, 21, 22, 24, and 25 in the lower wing of the meander. By contrast, the total scour at the points of the upper wing (1, 2, 3, 4, and 5) is minimal. These data intimate that the scour of the lower wing of the steep meander is dominant and that there is a longitudinal component n its displacement. 3. The most intense scouring was recorded in 2001 and 2005, and it was much weaker in between. Thus, the scour of the bank (with due regard for the points at which no scouring was observed) in 2001 and 2005, respectively, was 1.97, and 1.28 m/year, respectively, whereas in 2002, 2003 and 2004 it was 0.6, 0.33, and 0.5 m/year, respectively. 4. The mean scouring rate of the banks of the meander for five years was about 1 m/year. The actual scouring rates of the banks on some stretches of the meander varied from 0 to 6.6 m/year. This lends support to the earlier conclusion [1] that the scouring rates of the banks, averaged over long periods, have low representativity. Plot No. 2 (PP2) is a steep, nearly loop-shaped, meander of the Kerzhenets river (see Fig. 1, pl. 2. Its concave bank where the scouring measurements are made, belongs to an elevated floodplain 3 m in height above the low-water edge. The surface of the floodplain is covered with pine forest with the inclusion of birch and lime, its relief near the rim is relatively subdued, and the floodplain scarp is composed by sand-loamy sand alluvium. The measurements on the plot have been made using an identical technique since 2003. The measurement results for the period 2003-2005 are presented in Table 2. It is also evident from the table that: 1) all bank scours occur only at
0.0 0.0 4.6 0.0 0.0 0.7 1.6 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 1.6 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.4 0.4 0.0 0.0 1.5 3.6 1.1 3.4 3.0 4.9
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.4 0.4 4.6 0.0 1.5 4.3 2.8 3.4 4.6 4.9
0.7
0.0
0.17
0.0
1.8
0.0
2.7
O. V. Korableva and A. V. Chernov / Geography and Natural Resources 29 (2008) 195–200
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Fig. 2. Plots of water level variation for the Kerzhenets river in the area of the settlement of Rustai for 2001-2005.
2005), which is lower than on the steep segment-like and the loop-shaped meander. The observations of the scouring of the concave banks of the three meanders with a different meandering call for explanation as regards their non-uniformity in separate years. In the course of the variation of the water levels of the Kerzhenets river for the period 2001-2005 (Fig. 2), three groups of years can be identified, with a relatively high flood (more than 0.4 above an arbitrary “0” on the diagram) (2001, and 2005); with a relatively low flood, and with rain-caused floods (2003-2004), and with a relatively low flood, and with no rain-caused floods (2002). Since bank scouring occurs mostly at a high water-period, within the context of studying channel deformations the second and third groups can be combined, and only groups with high and low water levels are left. Empirically, for the Kerzhenets river we can delineate the boundary between them from the flood height of 0.4 m. Such are the years 2001 and 2005 for the preceding time interval, i.e. the years during which the most intense scours of the concave banks of the meanders occurred (Fig. 3). In the other years, the floods were considerably lower; therefore, the intensity of channel deformations of the Kerzhenets river was also considerably lower, even in spite of the raincaused floods about 100 cm in height that occurred in the summer of 2003 and 2004. It should be noted that the mean scouring rates for the concave bank of the steep segment-shaped meander on plot N. 1 in 2001 and 2005 are uncorrelated with the flood height in the same years – the rise of the water level in 2001 reached 410 cm, and in 2005 it was 438 m, or nearly 30 cm as high, whereas the mean bank scour rate in 2001 and 2005 was 1.97
and 1.28 m, respectively. This difference is attributable to a change in the shape of the meander: in the scouring process of the concave bank it becomes steep, the coefficient of its meandering increases, and the advantage (in terms of energy) of the meandering shape of the channel decreases. According to N. I. Makkaveyev [7], it is maximal when Cm = 1.57; on the meander, however, it reaches 1.82 in 2001. The eroding capacity of the flow decreases at the concave bank. This fact could also be explained differently: in 2001 the high flood level persisted for about 10 days, whereas it stayed only three days in 005.
Fig. 3. Maximum water levels during floods (a), and mean bank caving (b) for three bends of the Kerzhenets river from 2001 to 2005 in the area of the settlement of Rustai.
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Conclusions A five-year-long monitoring of the channel deformations on three free meanders (with a different degree of meandering) of the Kerzhenets river suggests the following conclusions. 1. Bank scours occur almost always during floods. Neither rain-caused floods (of course, if their height does not exceed the flood height) nor a long-lasting high standard water level does not cause bank scour; instead, it is caused by the slight deformations which occur at the time of a flood and have a residual character after floods. 2. The lower wing is mostly affected by scouring on the three meanders; hence the longitudinal component in the displacement of the meanders is present in the dynamics not only of gently sloping meanders but also of steep meanders. In the latter case, it does not exceed the transverse component which is ensured by the scouring of the heads of the meanders. 3. The intensity of horizontal channel deformations is closely associated with the flood height – it is maximal if the flood height corresponds to or slightly exceeds the height of the floodplain. Bank scours are proceeding markedly more slowly in the case of lower flood levels. The other characteristics of a year’s water abundance do not influence markedly the scouring intensity. 4. The values of the bank scour rates, averaged over a long-term period of time, are considerably lower than the actual rates in years with high floods. Thus, the mean scouring rate of the concave banks on the three meanders of the Kerzhenets river for the last 3-5 years is 1 m/year, whereas it was (in the case of a high flood) 6.6 and 7.9 m in 2005, in some places on the steep meanders. By continuing a monitoring of the deformation of the free meanders of the broad floodplain Kerzhenets river, it will be possible to extend the as yet small series of observations of these processes, validate or invalidate some conclusions drawn on the basis of a 5-year series of observations and, possibly, to reveal new, hitherto unknown, details in the development of river channels. They could serve as a basis for a monitoring of the floodplains nearby the meanders under investigation and, in particular, determine the formation patterns of young floodplain massifs due to aggradation of the concave banks of the meanders, the character of the
floodplain succession on the convex banks of the meanders, and the dynamics of the relief and landscapes on the undercut banks as the leading edge of the caving is displaced deep into the floodplain. Furthermore, a monitoring of the most typical meanders of the Kerzhenets river would help make forecasts of the channel deformations along the entire length of the channel of this river within the nature reserve. This is of important significance for delineating the boundaries of the protection territory which tend to change constantly on the channel stretch. This work was done with financial support from the RF President’s Grant for financial backing of scientific schools (NSh-4884.2006.5), and from the Russian Foundation for Basic Research (07-05-004210). References 1. Chernov A. V. and Korableva O. V. On the results of bank caving observations on the rivers of the Forest Trans-Volga Region and their analysis. Proc. Acad. of Problems of Water Management Sciences. Moscow: Izd-vo Mosk. un-ta, 2003, issue 9: The Problems of Channel Studies, pp. 206-214. 2. Snishchenko B. F., Debolsky V. K., Chalov R. S. et al. The problems of studying and monitoring the channel process, erosion and sediment runoff to meet the current requirements of the economy. 6th All-Russian Hydrol. Congress: Abstracts of Papers. SPb: Gidrometeoizdat, 2004, pp. 42-47. 3. Chalov R. S. and Chernov A. V. The ecological consequences of channel processes and their monitoring. The Sergeyev Lectures. Moscow: GEOS, 2000, issue 2, pp. 115-119. 4. Chernov A. V. Characteristics of channel deformations on small rivers in conditions of limited development of channel deformations. 18th Plenary Inter-University Coordination Meeting on the Problems of Erosion, Channel and Estuary Processes. Kursk: Izd-vo Kursk. un-ta , 2003, pp. 213-214. 5. Fridman B. I. and Korableva O. V. Geology and relief of the Kerzhensky Nature Reserve. Proc. “Kerzhensky” State Nature Reserve. Nizhni Novgorod, 2001, v. 1, pp. 7-70. 6. Mankish V. D. and Bayanov N. G. The hydrological and hydrochemical regime of the Kerzhenets river and its tributaries in its middle and lower part. Proc. “Kerzhensky” State Nature Reserve. Nizhni Novgorod, 2001, v. 1, pp. 79-108. 7. Makkaveyev N. I. The River Channel and Erosion Within Its Basin. Moscow: Izd-vo AN SSSR, 1955, 354 p.