CHANNEL DEFORMATIONS OF THE MOUNTAIN RIVER UNDER INFLUENCE
A MASS OF SEDIMENTS ARRIVAL FROM DEBRIS FLOWS TRIBUTARIES.
N. N. Vinogradova*, I. V. Krylenko*, V. V. Surkov*
*Laboratory of soil erosion and channel processes, Faculty of Geography, Moscow State University
Vorobievy Gory, 119899 Moscow, Russia, phone 7 (495) 939 56 97
ABSTRACT
The results of field studies on the Baksan (Northern Caucasia) serve as an example of bed deformations at three
sections of this mountain river, which are influenced by debris flow. There are big sections on the valley floor’s
terrain which are formed by silts of debris flow. The period between debris flow is characterized by processing
of sedimentation, bed incision and, with that, growth of flooded marks on the valley bottom. These processes
pass at the speed of 0.2-1.5m per year. The growth of marks on the valley bottom is followed by their weediness
and formation of bottom land.
Keywords: channel processes, mountain rivers, debris flow
INTRODUCTION
In 2001-2006 the study of channel processes passing at the Baksan river’s upper course,
region of Elborus, Northern Caucasia, started. The river Baksan is an interesting – and unique
in many aspects – natural object. Its bed is being formed under intensive debris flow activity
conditions. Debris flow, avalanches, glaciers, landfalls and screes let solid materials comes
into the river, form relief of the valley bottom and influence on sediment transportation. The
investigation included field studies of the dynamics of channel on key sections, repeated
geodesic surveying, longitudinal and transversal profile leveling. Such aspects as sediment
granulometric and lithologic сomposition, roundness and morphology of separate boulder and
gravel material were studied.
OBJECT OF INVESTIGATION
The Baksan valley passes along a big linear fracture which divides the northern flank of the
Big Caucasian ridge. The headstream (the Bolshoi Azau river) starts from a glacier on the SE
Elbrus flank. Length of the river is 173 km. Its average weighted inclination is 12‰; and at
the upper course this value varies from 50 to 200‰. The basin area is 6,800km², from which
7% belong to glaciers. The flow is formed, to a great extent, of glacier- and snow- waters.
This fact makes spring and summer flood periods last long, from April/May to
August/October. The highest water level is usually registered in July and August. High water
peaks coincide with periods of debris flow descents from tributaries. At the studied sector the
average long-term discharge changes from 10 m³/s (Tegenekly, 12 km from source) to 23,7
m³/s (Tyrnauz town, 45 km from source). and the average maximum discharge changes from
51,9 m³/s to 134 m³/s. The annual level amplitude is not very big – it seldom exceeds 2m.
The detailed research concerning channel dynamics was made at three key sectors. The upper
one, 300 m-long, is situated upper the Azau tourist centre, 2 km from the source. The
medium, 400 m-long, passes near Neitrino settlement (18,5 km from the source); and the
lower situated in Tyrnyauz town (45 km from the source). All the sectors have similar
geomorphologic conditions for channel forming. These sectors are broadenings in valley bed
situated over debris flows cones. The lower sector sit in head of water zone upper the
Gerkhozhan’s debris flows cone – this river is the major debris flows tributary of the Baksan.
A significant part of sediment at this sector is arrived by the Bolshoy Mukulan and the Maly
Mukulan, very active debris flows tributaries distanced 5 km upstream. The medium sector is
also situated between cones of two debris flows tributaries (the Yzuk and the Sagaevsky); the
upper lies 200 m upper the avalanche channel. Every year descending avalanches dam the
Baksan’s bed, and there is always a ‘snow bridge’ over the river.
INVESTIGATIONS
Debris flows passing along the upper course of the Baksan create channel forming belt on the
valley bottom: the river flow migrates within this belt’s limits and forms main elements of
channel relief (water-falls, rifts, point bars, braid bars, etc.) and original terrace surfaces. The
latter are not mainly formed by the activity of water flow but in a greater degree by debris
flows floods. These surfaces are relatively flat, with chaotically formed masses of boulders,
pebble and crushed ballast. They are covered with water only during very high floods, and
existing in highland conditions has got minimum vegetation. They are maximum 2-2.5 m
high. With time zones of these surfaces which are more distanced from the channel get
covered with bushes and grass, and get transformed into flood-plane or debris flow terraces.
At the upper sector the width of channel forming belt (50 m) practically coincides with the
width of valley bed. The level amplitude at this sector is 15 m, and average slope, 75‰. The
width of low-water channel is 3-8 m. During the investigation period no big debris flow were
registered. Gradual erosion of bed is taking place. In 2004-2005 the bottom degradation was
0.4-1 m (Fig.1), and the volume of erosion, 500 m³. In the above period the marks of ridges
on channel forming belt insignificantly increased (0.2-0.5 m). The growth is partially caused
by sand-gravel and small-sized pebble coming from the channel, and partially by crushed
ballast and debris which avalanches bring to the valley bed.
Figure 1. Longitudinal section of water surface of Baksan river on upper section (water
discharge 10 m³/s)
The valley width at the medium sector reaches 700 m, and the one of channel forming belt,
150 m. The low-water bed’s width is 10-15m. Directional accumulation were of sediment the
reason for forming here a multiple channel (simple linked forks, Fig.2) and periodical changes
in the leading branches position. The upper island is 50 m wide and up to 200 m long. The
second one, situated 150 m downstream, has smaller dimension – 35x120 m. The altitude
amplitude at the sector is 3 m, and average slope, 9‰.
Figure 2. Medium section: 1 – direction of debris flow (2003), 2 – position of main stream
directly after debris flow, 3 – position of main stream in period between debris flow.
The channel dynamics at the sector can be characterized as periodical changes in the leading
position of main branch. The defining role is played by debris flow material which enters to
the river from the upper-flowing right tributary (Sagaevsky). The debris flow dams –
completely, in fact – the right branch of the upper fork. As result, the main part of the flow
gets directed to the left branch. It is here that the main volume of debris flow deposition enters
in the first 1-2 years. This process leads to bottom marks’ increase in the left branch and to
appearance, a bit upper, of a transverse slope to right bank. In post-debris flow periods –
when floods are relatively low – the flow’s energy is not enough to take away sediment from
the left branch; thus the main stream finds itself in the right branch again. In the downstream
forking, according to the ‘figure-of-eight rule’, the position of leading branch change. In the
first 1-2 years the main stream passes right from the island. While the right branch in the
upper forking develops, the main stream moves to the left branch. With that, the right branch
near the low island can be completely covered with sediment and get dry. In the first 2-5 years
after debris flow at the sector accumulation of sediment are registered, and bottom marks’
general increase. The main relief forms (challows, rifts, pools) move 10-30 m downstream.
High floods which come afterwards influence on bottom erosion in the main branches (right
branch in upper and left in lower forking) and make the main branches’ position fixed. River
challows and water-falls move upper along the bed. The marks on point bars and braid bars
increase due to accumulation of sand on their surface. Sedimentation of fine materials makes
point bars get covered with vegetation, forms flood-plain and narrows the channel forming
belt. This process lasts until a new big debris flow comes from a tributary, and the whole
cycle repeats.
The above process was registered at the sector in 2002-2006. After a debris flow came from
the Sagaevsky tributary in 2003, the main stream was passing from the left to the right branch
in the upper forking. In the lower fork, in 2004, the right branch was still under development
(bottom erosion surpassed sedimentation by 600 m³); but during the flood occurred in 2005 it
got completely dry. By that time the section received the major volume of re-making debris
flow materials. The increase of valley bottom marks during the flood-2005 was from 0.5 to
1.5 m, and the positive balance of sediments, about 3000 m³. In 2006 slow erosion of main
branches began.
At the lower sector the bed meets artificial bank-defense walls’ resistance; hence the channel
forming belt’s width is 50-70 m, and channel deformations take place with the boulder-pebble
rifts moving. The height amplitude at the sector is 3.5 m, and the average slope, 10.5‰. In
2000 a catastrophic debris flow, which dammed the Baksan, formed a sub-pond lake above
the river Gerkhozhan’s outfall. The sector found itself in the upper part of the Baksan’s ‘inner
estuary’ which appeared above the sub-pond lake. This fact led to simultaneous increase of
bottom marks by 0.5-1 m. Afterwards, when the lake came down, river incision and bottom
marks’ decrease occurred. As a result, the sector got a meandering bed with long-lasting
chequer-wise point bars, covered with water during flood peaks. The point bars’ width was
30-50 m, and the length, up to 100 m. Point bars have fixed position – they do not move
downstream. In the years of low floods (2001-2004) there was registered at the sector weak
bed erosion and accumulation of fine materials on point bars. Thus, during the flood-2004 the
main stream erosion was 0.2-3.1 m taking account wash-out of dams (Fig. 3). Accumulation
of the sediment on point bars was 0.3-1.5m. The erosion volume (1150 m³) was three times
more than the accumulation (380 m³). These processes are the same as the ones which pass at
the upper sector during periods between debris flow. In 2005 significant changes happened at
the lower sector. They were caused by accumulation of sediment which entered the river
Рис. 3. Деформации дна р. Баксан на нижнем участке в 2004 – 2005 гг.
during the high flood, after erosion of debris flow terraces in the river Bolshoi Mukulan’s
outfall. The channel’s morphodynamic type remained untouched, but the features of point
bars essential changed. Theirs relative width increase from 20 m to 30-50 m, height rose 0.5-1
m. The bed itself was still under erosion, and the erosion value was from 0.3 to 0.8 m. In this
period the sedimentation (4500 m³) surpassed the erosion (3700 m³), and the general balance
of sediment became positive.
CONCLUSIONS
Debris flows play a decisive role in the channel-forming activity of the Baksan upper reaches.
They supply to the channel debris material, and, as result, change bed position, form valley
bottom relief. In the periods between debris flows erosion of low-water bed takes place and
simultaneously increasing marks of point bars and riffs due to sedimentation of fine materials
on them. This fact leads to their overgrown and flood-land forming. In 2002-2006, when there
were no big debris flows, the area of point bars with bushes and grass in the upper Baksan
valley increased 25-30%.
ACKNOWLEDGEMENTS
This study was supported by RFFI grant (project 04-05-65030)