Hydroecological safety of Russia and its relationship with sediment yield and river channel deformations

N. I. Alekseevskiy, R. S. Chalov

Department of Hydrology, Faculty of Geography, Moscow State University
Vorobievy Gory, Moscow, Russia, 119992, phone 7 (495) 9391001 n_alex@hydro.geogr.msu.su

Hydroecological safety depends on water sources availability, water quality and dangerous hydrological processes development. Theirs changes occur under natural and anthropogenic factors and lead to the risk of social, economic and ecological damages. Channel deformations are the reason of risk occurrence [Berkovitch etc., 2000]. Their dangerous expressions develop under changes of hydrological regime, streams hydraulic characteristics, water and sediment runoff because instability of channel deposits, channel forms and channels themselves is caused by natural or anthropological shifts of stream velocities, water and sediment discharges.

The relationship between sediment yield and channel deformations is explained by the equation of sediment mass balance for the channel reach

W₂ – W₁ = ±DW, (1)

where W₂ and W₁ are the sediment loads on the downstream and upstream reach borders, DW is the balance resultant. If the lack of sediment supply by tributaries and from slides exists, DW depends on the volume of river (floodplain and channel) deposits changes DW₀:

. (2)

The eq. (2) shows that the resultant of sediment mass balance and river deposits are equals with contrary sign [Alekseevskiy, 1998]. Under the longitudinal sediment load increase the value of DW > 0. This process is determined by erosion of river sediments (channel banks and/or beds) and accompanied by theirs volume decrease DW₀. Sediment volume decrease is expresses by vertical deformations and bottom level drop or horizontal deformations and bank erosion [Chalov, 1979]. Opposite processes occur under the longitudinal sediment load decrease (DW < 0). The increase of river sediments exists (DW₀ > 0). In this case the vertical deformations correspond to bottom level increase, horizontal – to the deposition on floodplain banks. When the sediment particles are transported along the channel reach (at the equality of deposited and entrained sediments, when DW = 0) the volume of river sediments is constant (DW₀ = 0) and stable channels exist [Karaushev, 1977]:

W_sl = W, (3)

where W_sl is transporting capacity. It equals to the rate of sediment yield, possible to transport in each time interval i of the representative period T under certain annual water regime and stream hydraulic characteristics. Thus, W_sl depends on flow rate and is characterized by suspended sediment concentration s_sl corresponded to the maximum sediment load under certain hydrological and hydraulic characteristics.

In stable channels bed and bank deformations have lack of directed development (fig. 1a) and are expressed only in geological time scale. For the unstable channels the eq. (3) is disturbed because of natural and anthropogenic transformations of sediment yield W and transporting capacity W_sl. In each case there are 2 possible variants of deformations development. If the increase of W_sl and decrease of W induce the existing of W_sl >W, the could be removed by the sediment yield increase and transporting capacity decrease. It occurs by the channel deformations process. Because of bed and bank erosion the balance resulting DW>0 and decrease of channel deposits volume appears. The maximal intensity of channel deposits diminution occurs near the upper reach border. As the result the channel slope decreases, what is the factor of transporting capacity reduction. The equality (3) is achieved.

The opposite processes develop at the reaches with W_sl <W. This inequality reflects the sediment load yield increase or transporting capacity reduction. Accumulation processes occur and W_sl grows and W decreases. The maximal sediment accumulation is observed near the upper reach border where the volume of sediment deposits increases (DW₀>0). Bed level grows with higher intensity. Sediment load discharge reduces, channel slope and transporting capacity decrease and finally the new state of the “channel-water stream” system leads to the eq. (3).

Figure 1. The response stable stream-channel system to sediment load yield increase: reduction (b) or growth (c) of transporting capacity

Channel processes influence on the social and economic safety and nature management is controlled by sediment yield and transporting capacity factors shift. River sediment yield includes suspended load yield (W_R) and bed load yield (W_G). Sediment yield components depend on river size and geographical conditions of river basin determining erosive processes intensity [Sediment yield, 1979; Dedkov, Mozherin, 1984]. When other factors are similar the stream order increase is accompanied by suspended and bed load yield growth.

Channel deformations intensity controls risk of engineering objects damage, water intake shoaling, the riparian lands losses and depends on W_G/W_{G+R ratio}. It varies in wide ranges in according to changes of sediment yield components [Chalov, Lui Shuguang, 2005]. On the North Russian rivers the relatively small sediment yield is combined with W_G/W_G+R = 40–50% and more. It is the consequence of forest cover and poor erosive processes. Only incised channels with limited channel deformations are stable and channel deformations are safe for most human activities. Channels with wide floodplains are unstable. On small rivers the annual erosion velocities are about 1-3 m, on large rivers – up to 10 meters and more. Side-bars, central bars and riffles movement on middle and large rivers equals 50–500 m/year. It makes difficult the use of water ways, installation of pipelines passages across rivers, stable functioning of water intake. The estimation of channel processes at different phases of human activities is necessary.

Plateaus composed by loess, abundant soil and gully erosion are the sources of the southern rivers sediments where W_G/W_R+G < 10%. The total sediment yield is 2–3 times more as northern rivers. Bed load materials are the only channel forming sediments in the North. In the South the main part of the suspended load materials is also channel forming sediments. It determines the instability of southern rivers, intensity of horizontal deformations (10–100 m/year), bed level shifts because of large riffles movement up to 100–1000 m/year and as a result the high risk of water resources use (Terek, Kuban and Ob rivers in Russia). Sediment yield and transporting capacity are transformed because of evolution of river systems and changes of basin natural conditions and also under nature management in river basins and channels. Deforestation, marshes drainage, mining, riverbed excavation are the main human activities affecting the suspended sediment load variations. Transporting capacity shifts mainly due to water runoff (flow) regulation, riverbed excavation and changes of channel width, depth and length (tab. 1). The impact of hydraulic project and quarries digging differs between upstream and downstream reaches of works facilities. Inside-basin and inter-basin water redistribution are also an important factors. In the rivers – receiving waters transporting capacity increases, in the rivers – water sources transporting capacity decreases. W_sl and/or W changes are accompanied by the channel deformation, thus natural management is usually the reason of damage expressions.

The spatial-temporal scales of the equation (3) transgression are much different. Global environmental changes, large hydraulic projects, inter-basin water redistribution, deforestation are spread over vast territories and river basins in historical and geological duration. The risk level of main directed channel deformations is determined by velocity and rate of these changes. Also during natural evolution of channel forms the eq. (3) may be broken. As a result the deepening or shoaling of braided rivers branches, the channel banks erosion or accumulation occur. The same processes exist under human impact on channels, water runoff and sediment yield. Under the installation of water reservoirs and mass quarries digging the disturbance of the equality W_sl =W involve large stream reaches including theirs tributaries; the influence of the single quarry is exerted over a short reach and leads to the local deformations. In one case the disturbance of hydroecological safety is characterized by general or regional and permanent scale, in another – it is local and brief.

Table 1. Anthropogenic impact on river sediment movement (Alekseevkiy, 1998)

Anthropogenic impact	Sediment yield change factors			Suspended load DR and bed load DG yield change
	Water discharge change DQ	Suspended sediment concentration change Ds	s sтр	во времени Dt	по длине (Dx) участка реки
Hydraulic projects: upstream dams downstream dams	D Q 0 DQ ¹ 0	Ds < 0 Ds < 0	s > sтр s< sтр	DR/Dt<0 DG/Dt <0 DR/Dt<0 DG/Dt<0	DR/Dx<0 DG/Dx<0 DR/Dx>0 DG/Dx>0
Channel quarries: upstream downstream	DQ 0	Ds ≥ 0 Ds < 0	s £ sтр s < sтр	DR/Dt0 DG/Dt0 DR/Dt<0 DG/Dt<0	DR/Dx0 DG/Dx>0 DR/Dx>0 DG/Dx>0
Channel cut-off	DQ 0	Ds > 0	s < sтр	DR/Dt>0 DG/Dt>0	DR/Dx>0 DG/Dx > 0
Water redistribution: downstream water intake Inter-basin redistribution	DQ<0 DQ>0	Ds≥0 Ds>0	s>sтр s<sтр	DR/Dt0 DG/Dt<0 DR/Dt>0 DG/Dt>0	DR/Dx<0 DG/Dx<0 DR/Dx>0 DG/Dx>0

The level of channel processes risk depends on the intensity of channel deformations, their directed or recurrent development, frequency of occurrence on each river – on the channel stability as a whole. That’s why the estimation of channel deformations risk is produced according to the channel stability characteristics (tab. 2) – Lohtin number and Makkaveev stability coefficient:

, , (4)

where d is size of channel deposits (mm), I – slope (‰), b – channel width, m.

The impact of channel processes risk on social and industrial units is assesses by the risk scale from 0 till 5 points. The upper border (point 5) is absent for the rivers of Russia. On rivers of other regions more intensive deformations are possible. The absence of the risk (0 point) is rather conditional because channel processes always are able to have a dangerous expression (there is no rivers without any dangerous expression of channel processes). On stable rivers in rocks their banks almost are not eroded, but water intake working is disturbed. Large rocks are dangerous for navigation.

The level of channel processes risk depends on channel pattern type. Meandering, braided or relatively straight channels are characterized by the special rules of location of accumulation and erosion areas, during different phases of channel forms development. Because of their evolution the intensity of deformations increases or decreases. Conditions of channel deformations development (geological and geomorphologic) are also an important factor. Incised channels or channels with wide floodplain occur.

In natural conditions the directed vertical deformations are distinguished by small intensity (mm/year), their results are expressed during thousands of years. Thus they may be disregarded at hydroeconomic projects. Vertical deformations lead to bed level shifts according to development of channel forms. Conditions of intensive vertical deformations are infrequent. In Russia they exist on the lower Terek, lower Amur and its tributaries. As a result transformations of river system and floods risk appear. Examples of the intensive incision limited by underlying layer of solid rocks are less known.

Table 2. Types of channel processes risk [Berkovitch etc., 2000]

Risk of channel processes (channel stability)	Points	Stability characteristics		Channel deformations characteristics
		L	ks	Cг	Uср	Umax