Reynolds number, one of the similar criteria for flows of viscous liquids and gases, characterizing the relationship between inertial forces and viscous forces: Re=r vl/m, where r is density, m is the dynamic viscosity coefficient of a liquid or gas, v- characteristic flow speed, l- characteristic linear size. Thus, for flow in round cylindrical pipes it is usually taken l=d, Where d- pipe diameter, and v=v cp, where v cp - average current speed; when flowing around bodies, / is the length or transverse size of the body, and v = v¥ , where v ¥ - the speed of the undisturbed flow impinging on the body. Named after O. Reynolds.

The flow regime of the liquid, characterized by the critical radio frequency, also depends on the RH. Re kr . At R<Re kр only laminar fluid flow is possible, and at Re>Re kр the flow can become turbulent. Meaning Re kр depends on the type of flow. For example, for the flow of a viscous fluid in a round cylindrical tube Re kр = 2300.

Distribution of current speeds in a river flow.

One of the features of the movement of water particles in rivers is irregular random changes in speed. Continuous changes in the direction and magnitude of velocities at each point in a turbulent flow are called pulsations. The higher the speed, the greater the turbulent pulsation. Then at each point of the flow and at each moment of time, the instantaneous flow velocity is a vector. It can be decomposed into components in a rectangular coordinate system (υ x, υ y, υ z), they will also be pulsating. Most hydrometric instruments measure the longitudinal component of velocity (υ x), averaged over a certain time interval (in practice, 1-1.5 minutes).

Velocities vary along the depth and width of the river's living cross-section. At each individual vertical, the lowest speed is observed at the bottom, which depends on the roughness of the riverbed. Towards the surface, the velocity increases to the vertical average value at a depth of 0.6h, and the maximum is noted on the surface or at a distance of 0.2h from the surface, in the open channel. The graph of velocity changes over depth is called a hodograph (velocity diagram).

The velocity distribution in depth depends on the bottom topography, the presence of ice cover, wind and aquatic vegetation. The presence of boulders, large stones and aquatic vegetation at the bottom leads to sharp decrease speed in the bottom layer. Ice cover and slush also reduce speed, but in the layer of water under the ice. The average vertical speed is determined by dividing the area of ​​the diagram by the vertical depth.

Along the width of the flow, the speed basically follows the change in depth - from the banks the speed increases towards the middle. The line connecting the points with the highest speeds along the length of the river is called the core (line of greatest depths).

The distribution of velocities in plan can be reflected by isotachs - lines connecting points with equal velocities in a live section.

The line connecting the points of individual live sections along the river with maximum speeds is called the dynamic axis of the flow.

Hydrology 2012

LECTURE 8. SPECIAL ISSUES IN THE HYDROLOGY OF RIVERS AND RESERVOIRS

Questions:

Movement of water in rivers

Sediment movement in rivers

Channel processes

Thermal and ice regime of rivers and reservoirs

Lakes and their morphometric characteristics

1. Movement of water in rivers.

The movement of water in rivers occurs under the influence of gravity in the presence of a longitudinal slope or pressure. The speed of the flow depends on the ratio of the horizontal component of gravity, determined by the slope and pressure difference, and the frictional force, determined by the interaction between the particles within the flow and the particles and the bottom.

Rivers are characterized by a turbulent regime of water movement, the distinctive feature of which is the pulsation of speed or its change over time at each point in value and direction relative to the average value.

Due to the unevenness of losses across the width of the channel, current velocities are distributed unevenly in the river flow: the highest velocities are observed on the surface of the flow above the deepest part of the channel, the lowest - near the bottom and banks. In the most common conditions of a regular distribution of flow velocities, the diagram (distribution graph) of average velocities along the depth of a river flow has a maximum (u max) near the surface, a velocity close to the average on the vertical - at a depth of 0.6h from the bottom (h - total depth ) and the minimum (u min), not equal to zero, is at the bottom (Fig. 8.1, a ).

Rice. 8.1. Vertical distribution of current speeds in a river flow:

A - typical; 6-under ice cover; V - under a layer of inland ice (sludge); d - with tailwind and headwind; d- under the influence of vegetation; e - under the influence of bottom irregularities; 1 - ice cover; 2-layer sludge; V-wind direction; u max - maximum flow speed; -And - reverse current

However, under the influence of ice cover, wind, vegetation, and uneven bottom and shore topography, this distribution of velocities is disrupted (Fig. 8.1, b -e).

The average flow velocity in the cross section v is calculated from the known water flow rate - Q and cross-sectional area -  using the formula: v=Q/.

The simplest patterns are observed when the fluid moves uniformly in a channel close to rectilinear. In this case, the average flow velocity in the channel can be described by the Chezy formula.

, (8.1)

where C is the Chezy coefficient;

h av – average depth in the channel, m;

I – slope of the water surface.

When the ratio of the width of the channel (B) and the average depth (h av) ​​is less than 10, instead of h av, use the hydraulic radius R = / ( is the living section area,  is the wetted perimeter).

The Chezy coefficient is calculated using empirical formulas, among which the most common are

Manning's formula (for rivers):

C=h avg 1/6/n. (8.2)

Pavlovsky formula (for artificial watercourses - canals, ditches):

C=(1/n) R y /n (8.3)

y = 0.37+2.5
- 0,75(
-0,1) 
,

where n is the roughness coefficient, which is found using special tables (in Russia - according to the Sribny, Karasev tables, in the USA - Bradley tables).

For smooth, unovergrown channels with a sandy bottom n = 0.020 - 0.023; for winding channels with an uneven bottom n= 0.023-0.033; for floodplains overgrown with bushes, n = 0.033 - 0.045.

Chezy's formula shows that the greater the depth of the channel and the slope of the water surface and the less rough the channel, the greater the flow speed in a river flow.

By multiplying both sides of the Chezy formula by the cross-sectional area , taking into account formula (8.1), one can obtain a formula for determining water flow:

. (8.4)

If the morphometric characteristics of a river flow change along the length of the river, then the movement of the river flow will be uneven and the flow speed will change along the river. In a small section of the river, where the flow rate does not change, from the law of conservation of mass of matter, we can write the continuity equation

 1 v 1 =  2 v 2 = Q= const. (8.5)

It follows that an increase in the cross-sectional area along the river (from section 1 to section 2) will entail a decrease in the current speed in this section, as, for example, during low water on a reach, while a decrease in the cross-sectional area along the river will lead to an increase in this section current speed, as, for example, during low water on a riffle.

In the case of uneven movement, the slope of the water surface will no longer be equal to the bottom slope, therefore, backwater phenomena (increasing water depth with increasing distance) or recession phenomena (decreasing depth with increasing distance) can be observed along the river. The cause of uneven movement can be various structures erected in the riverbed - dams, dikes, bridge crossings, straightening and clearing of river beds.

More complex cases of movement arise at the turn of the channel, where, along with the force of gravity, centrifugal force influences the flow speed. This leads to a deviation of the flow in the surface layers towards the concave bank, which creates a transverse distortion of the water level. As a result of excess hydrostatic pressure near the concave coast, a current appears in the bottom layers directed towards the convex coast. Combined with the main longitudinal transport of water in the river, multidirectional currents on the surface and at the bottom create a spiral movement of water at the bend of the river channel - transverse circulation (Fig. 8.2).

Fig.8.2. Scheme of transverse circulation at a bend in a river flow in plan (a) and cross section (b) and diagram of acting forces (c):

1 – surface jets; 2) bottom jets.

Cross slope ( I pop = sin ), which occurs at a turn in the channel, can be determined by the formula

. (8.6)

Where v-average current speed;

g – free fall acceleration, m/s2;

r - radius of bend of the channel.

The amount of level distortion between both banks (  H pop) is equal to

H pop = Ipop IN, (8.7)

Where IN- channel width.

Example. At speed v=1 m/s, r=100 m, B=50 m, the value Ipop=0,001,  H pop = 0.05 m.

Along with the force of gravity, friction and centripetal force, the deflecting force of the Earth's rotation acts on the fluid particles.

Due to the daily rotation of the Earth with an angular velocity =2/86400 = 0.0000729 rad/s, any material point moving relative to the Earth with a speed v experiences additional acceleration (). The force corresponding to this acceleration is called the Coriolis force (F Coriol), and is equal to

F coriol =m g =2 mvsin. (8.8)

The Coriolis force is directed in the northern hemisphere at a right angle to the right to the direction of motion of the particle, in the southern hemisphere - to the left.

The transverse slope caused by the Coriolis force is equal to

I coriol = v sin/67200, (8.9)

For northern latitude =45 sin=0.707 I coriol= v/95000, at v=1 m/s I coriol =1.0510 -5. With a river width B=50 m, the level difference is H=0.00052 m (0.05 cm), which is 100 times less than the slope due to centrifugal force. The influence of the Coriolis force is most pronounced for large rivers (Volga, Dnieper, Yenisei, Ob, etc.), which was once discovered by the Russian academician and naturalist K. Baer. However, due to its smallness, the Corriolis force is not taken into account in hydraulic calculations.

Sediment movement in rivers

Along with water, sediments and soluble impurities move in rivers. The main sources of sediment entering rivers are the surface of catchment areas, which are subject to erosion or the process of destruction of soils and soils. flowing water and the wind during the period of rains and snowmelt, and the river beds themselves, washed away by the river flow.

Erosion of the surface of a watershed is a complex process, depending both on the eroding ability of rain and melt water flowing over its surface, and on the anti-erosion resistance of the soils of the watershed. The erosion of the surface of watersheds (and the flow of its products into rivers) is usually greater, the heavier the rains and the more intense the snowmelt, the more uneven the relief, the looser the soils (loess soils are most easily eroded), the less developed the vegetation cover, and the more plowed slopes. Erosion of river channels is the stronger, the higher the flow speed in the rivers and the less stable the soils that make up the bottom and banks. Part of the sediment enters river beds during abrasion (wave destruction) of the banks of reservoirs and river banks on wide reaches. The sediment that forms the bottom of rivers is called bottom sediments, or alluvium.

The most important sediment characteristics are:

geometric size, expressed in terms of the diameter of sediment particles (D mm);

hydraulic fineness, i.e., the rate of sedimentation of sediment particles in still water (w, mm/s, mm/min);

particle density(rn, kg/m 3), equal for the most common quartz sands to 2650 kg/m 3;

sediment density(soil density) (p ex, kg/m 3), depending on the particle density and porosity of the soil according to the formula (the density of silt deposits at the bottom of rivers usually averages 700-1000 kg/m 3, sand 1500-1700, cm ­ shanny 1000-1500 kg/m 3);

concentration(content) of sediment in the stream, which can be presented both in relative values ​​(the ratio of the mass or volume of sediment to the mass or volume of water) and in absolute values; in the latter case, the concept of water turbidity (s, g/m3, kg/m3) is used, which is calculated using the formula

where m is the mass of sediment in the water sample; V is the volume of water sample. Turbidity is determined by filtering water samples taken using pitometers and weighing the filters.

The highest concentration of sediment (water turbidity) is found in rivers with a flood regime and flowing in conditions of an arid climate and easily eroded soils. The muddiest rivers on Earth are Terek, Sulak, Kura, Amu Darya, Ganges, Yellow River. The average annual turbidity of the Terek, Amudarya and Yellow Rivers under natural conditions was, for example, 1.7; 2.9 and 25.8 kg/m3, respectively. During the flood, the turbidity of the Yellow River water reached 250 kg/m 3! Currently, the turbidity of the listed rivers has become noticeably less. For comparison, we present data on the average annual turbidity of water in the Volga in its lower reaches: before the regulation of the river it was about 60 g/m 3 , and after regulation it decreased to 25-30 g/m 3 .

Based on the nature of movement in rivers, sediment is divided into two main types - weighted And attracted. The intermediate type is saltating sediments, moving spasmodically in the bottom layer; sediments of this intermediate group are conventionally combined with bedload sediments.

Dragged sediment - This is sediment transported by river flow in the bottom layer and moves by sliding, rolling or saltation. The largest particles of sediment (sand, gravel, pebbles, boulders) move along the bottom by gravity.

Thus, the criterion for the beginning of the movement of traction sediments in rivers is the condition

(8.11)

where u bottom is the actual bottom current velocity.

Between the "initial velocity" and the volume or weight of the moving particles:

F g ~D"~u 6 bottom0 . (8.12)

This formula is called Airy's law, which states that the weight of transported sediment is proportional to the sixth power of the current speed. From Airy's formula it follows that an increase in flow speed, for example by 2, 3, 4 times, leads to an increase in the weight of sediment particles moving along the bottom by 64, 729, 4096 times, respectively. This explains why on lowland rivers with low flow speeds the flow can carry only sand along the bottom, and on mountain rivers at high speeds - pebbles and even huge boulders. To move along the bottom of sand, bottom current speeds of at least 0.10-0.15 m/s are required, gravel - at least 0.15-0.5, pebbles - 0.5-1.6, boulders - 1.6- 5 m/s. The average flow rate should be even higher.

Dragged sediment can move along the bottom of rivers either in a continuous layer or in the form of accumulations, i.e. discretely. The second type of movement for rivers is the most typical. Accumulations of traction sediment are represented by bottom ridges of various sizes (Fig. 8.3). Sediment moves in a layer along the upper slope of the ridge and rolls down the lower slope (its slope is close to the angle of repose) into the basement of the ridge. Here nanoson particles can be “buried” by the advancing ridge and will begin to move again only after the ridge has been displaced along its entire full length.

Fig.8.3. Bottom ridges on the river bottom at two consecutive points in time (1 and 2).

Suspended sediments are transported in the thickness of the river flow. The condition for such movement is the relation

u + z  w, (8.13)

where u + z is the upward-directed vertical component of the current velocity vector at a given point in the flow; w is the hydraulic size of the sediment particle.

The most important characteristics during the movement of suspended sediment in rivers are the water turbidity s, determined by formula (8.10), and the flow rate of suspended sediment:

R=10 -3 sQ, (8.14)

where R is in kg/s, s is in g/m3, Q is in m3/s.

Suspended sediments are distributed unevenly in the river flow: in the bottom layers, turbidity is maximum and decreases towards the surface, and for suspended sediments of larger fractions it is faster, for sediments of small fractions it is slower.

Along with water runoff, sediment runoff is determined in hydrology. The sediment runoff of a river includes the runoff of suspended and tractional sediment runoff, and the main role usually belongs to suspended sediment. It is believed that the share of transported sediment accounts for on average only 5-10% of the suspended sediment flow of rivers, and this share, as a rule, decreases as the size of the river increases.

The maximum total flow rate of both suspended and transported sediment that a river can carry under given conditions is called the transport capacity of the flow R tr. According to theoretical and experimental research Rtr depends primarily on flow rates and water consumption:

(8.15)

Where str- water turbidity corresponding to the transporting capacity of the flow;

v-average flow speed;

hcp - average depth;

w- average hydraulic size of sediment particles.

Many different formulas of the form (8.15) have been proposed in our country and abroad. In this case, the water turbidity s tr, corresponding to the transporting capacity of the flow (i.e., the maximum possible turbidity under given hydraulic conditions), is often expressed as a function of the average flow speed: s rp = av n, Where A And n - parameters, and n varies from 2 to 4.

In real conditions, the actual flow of sediment in the river and the transporting capacity of the stream may not coincide, which becomes the cause of channel deformations.

The sediment flow of a river (primarily suspended sediment) is usually calculated using the relationships between water flow and suspended sediment flow R=f(Q) constructed on the basis of measurements. This relationship has two important features: it is nonlinear, and R grows faster than Q; This dependence can sometimes be written very approximately in the form of a power equation:

R = kQ m , (8.15)

where, according to N.I. Makkaveev, n = 2  3 .

Very often the relationship between R and Q turns out to be ambiguous (loop-shaped). This is explained by the discrepancy between changes in river water flow and sediment flow over time (Fig. 6.18). The maximum turbidity of water in rivers (and the maximum sediment discharge too) usually exceeds the maximum water discharge, since the most active washout of soil from the surface of the catchment occurs during the period of rising floods or floods.

Rice. 8.4. Typical graphs of changes in water flow and suspended sediment (a) and connections between them ( b): 1 - rise of flood; 2 -recession of high water

Using a communication graph R= f(Q) From the known average daily values ​​of Q, it is easy to determine the corresponding values ​​of R.

The average sediment discharge for any period R is determined in exactly the same way as the average water discharge. Sediment runoff is calculated using the formula:

W n = RT, (8.16)

where sediment runoff W n, kg; average consumption sediment R, kg/s; time interval T, s.

It is often more convenient to present sediment flow not in kilograms, but in tons or even millions of tons. In these cases, the formulas are used

W n (t)= RT 10 -3, (8.17)

If we are talking about annual values, then write

W n (million tons) = R 31.510 -3. (8.18)

The sediment runoff module is the sediment runoff in tons per 1 km 2 of catchment area (A):

M H =Wн/A. (8.19)

For annual values ​​of sediment runoff we obtain M n, t/km 2:

M n = R31.510 3 /F. (8.20)

The sediment runoff modulus characterizes the erosive activity of river flows (recall, however, that the actual denudation in river basins is many times greater than the sediment runoff modulus calculated by the methods just described, since a huge amount of sediment washed away from the slopes does not fall into the rivers, but is deposited at the foot slopes, at the mouths of ravines, ravines, small tributaries, on floodplains.

The module of suspended sediment runoff and the average turbidity of river water, as well as the module of water runoff, are unevenly distributed over the territory. Thus, in the north of the European territory of Russia (tundra, forest zone) it often does not exceed 1-2 t/km 2 per year, in the northern and western parts of the European Plain it rises to 10-20 t/km 2. In the south of the European territory of the former USSR it reaches 50-100 t/km 2, and in a number of regions of the Caucasus - even 500 t/km 2 per year. For the basins of some rivers in the world, the suspended sediment runoff module under natural flow conditions was: for the Volga - 10.3 t/km 2, for the Danube - 63.6, Terek - 350, Yellow River - 1590 t/km 2 per year. Turbidity rivers fairly regularly distributed throughout the territory. For example, the average annual turbidity of rivers in the north of the European part of Russia is very low - 10-50 g/m3, in the Oka, Dnieper, Don basins it increases to 150-500 g/m3, in the North Caucasus it sometimes exceeds 1000 g/m 3.

From the total annual sediment runoff of all rivers in the world (15,700 million tons) the largest share in natural conditions falls on the Amazon (1200 million tons), Yellow River (1185 million tons), Ganges with Brahmaputra (1060 million tons), Yangtze (471 million tons), Mississippi (400 million tons) (see. table 6.1). Among the most turbid rivers on the planet are the Yellow River (average annual water turbidity is more than 25 kg/m3, and the maximum is 10 times more), Indus, Ganges, Yangtze, Amu Darya, Terek.

In the previous paragraphs, the laws of equilibrium of liquids and gases were discussed. Now let's look at some phenomena associated with their movement.

The movement of fluid is called with the current, and a collection of particles of a moving fluid is a stream. When describing the movement of a fluid, the speeds at which fluid particles pass through a given point in space are determined.

If at each point in space filled with a moving fluid, the speed does not change with time, then such motion is called steady, or stationary. In a stationary flow, any fluid particle passes through a given point in space with the same speed value. We will consider only the steady flow of an ideal incompressible fluid. Ideal called a liquid in which there are no frictional forces.

As is known, a stationary liquid in a vessel, according to Pascal’s law, transmits external pressure to all points of the liquid without change. But when a fluid flows without friction through a pipe of variable cross-section, the pressure in different places pipes are not the same. The pressure distribution in a pipe through which liquid flows can be assessed using an installation schematically shown in Figure 1. Vertical open pressure gauge tubes are soldered along the pipe. If the liquid in the pipe is under pressure, then in the manometric tube the liquid rises to a certain height, depending on the pressure in a given place in the pipe. Experience shows that in narrow areas of the pipe the height of the liquid column is less than in wide areas. This means that there is less pressure in these tight spots. What explains this?

Let us assume that an incompressible fluid flows through a horizontal pipe with a variable cross-section (Fig. 1). Let us mentally select several sections in the pipe, the areas of which we denote by and . In a steady flow, equal volumes of liquid are transferred through any cross section of a pipe over equal periods of time.

Let be the velocity of the fluid through the section, and let be the velocity of the fluid through the section. Over time, the volumes of liquids flowing through these sections will be equal to:

Since the fluid is incompressible, then . Therefore, for an incompressible fluid. This relationship is called the continuity equation.

From this equation, i.e. fluid velocities in any two sections are inversely proportional to the cross-sectional areas. This means that liquid particles accelerate when moving from the wide part of the pipe to the narrow part. Consequently, a certain force acts on the liquid entering the narrower part of the pipe from the liquid still in the wide part of the pipe. Such a force can only arise due to the pressure difference in various parts liquids. Since the force is directed towards the narrow part of the pipe, the pressure in the wide section of the pipe should be greater than in the narrow section. Taking into account the continuity equation, we can conclude: during stationary fluid flow, the pressure is less in those places where the flow speed is higher, and, conversely, it is greater in those places where the flow speed is lower.

D. Bernoulli was the first to come to this conclusion, therefore this law called Bernoulli's law.

Application of the law of conservation of energy to a flow of moving fluid allows us to obtain an equation expressing Bernoulli’s law (we present it without derivation)

- Bernoulli's equation for a horizontal tube.

Here and are the static pressures and the density of the liquid. Static pressure is equal to the ratio of the force of pressure of one part of the liquid on another to the area of ​​​​contact when the speed of their relative motion is zero. This pressure would be measured by a pressure gauge moving with the flow. A stationary monometric tube with an opening facing the flow will measure pressure

Slope of the riverbed. The most characteristic feature of any river is the continuous movement of water from source to mouth, which is called with the current. The reason for the flow is the inclination of the channel, along which, obeying the force of gravity, the water moves at a greater or lesser speed. As for the speed, it is directly dependent on the slope of the riverbed. The slope of the channel is determined by the ratio of the difference in heights of two points to the length of the section located between these points. So, for example, if from the source of the Volga to Kalinin 448 km, and the difference in height between the source of the Volga and Kalin and the nom is 74.6 m, That average slope Volga in this section is 74.6 m, divided by 448 km, i.e. 0.00017. This means that for every kilometer of the length of the Volga in this section the fall is 17 cm.

Longitudinal profile of the river. Let us plot the length of different sections of the river along a horizontal line, and the heights of these sections along vertical lines. By connecting the ends of the verticals with a line, we get a drawing of the longitudinal profile of the river (Fig. 112). If you do not pay special attention to the details, the longitudinal profile of most rivers can be simplistically represented as a descending, slightly concave curve, the slope of which progressively decreases from source to mouth.

The slope of the longitudinal profile of the river is not the same for different sections of the river. So, for example, for the upper section of the Volga, as we have already seen, it is equal to 0.00017, for the section located between Gorky and the mouth of the Kama it is 0.00005, and for the section from Stalingrad to Astrakhan it is 0.00002.

The Dnieper is approximately the same, where in the upper section (from Smolensk to Orsha) the slope is 0.00011, and in the lower section (from Kakhovka to Kherson) 0.00001. In the area where the rapids are located (from Lotsmanskaya Kamenka to Nikopol), the average slope of the longitudinal profile of the river is 0.00042, i.e. almost four times greater than between Smolensk and Orsha.

The examples given show that the longitudinal profile of different rivers is far from the same. The latter is understandable: the longitudinal profile of the river reflects the relief, geological structure and many other geographical features of the area.

For example, consider the “steps” on the longitudinal profile of the river. Yenisei. Here we see sections of large slopes in the area of ​​​​the intersection of the Western Sayan, then the Eastern Sayan and, finally, at the northern end of the Yenisei Ridge (Fig. 112). The stepped nature of the river's longitudinal profile. The Yenisei indicates that uplifts in the areas of these mountains occurred (geologically) relatively recently, and the river has not yet had time to level out the longitudinal curve of its bed. The same can be said about the Bureinsky Mountains, cut through by the river. Cupid.

So far we have talked about the longitudinal profile of the entire river. But when studying rivers, it is sometimes necessary to determine the slope of the river at a given small area. This slope is determined directly by leveling.

Cross profile of the river. In the transverse profile of a river we distinguish two parts: the transverse profile of the river valley and the transverse profile of the river itself. We already have an idea of ​​the transverse profile of the river valley. It is obtained as a result of ordinary terrain surveying. To get an idea of ​​the profile of the river itself, or, more precisely, the river bed, it is necessary to measure the depths of the river.

Measurements are made either manually or mechanically. For manual measurements, a mark or hand lot is used. The basting is a pole made of flexible and durable wood(spruce, ash, hazel) round section diameter 4-5 cm, length from 4 to 7 m.

The lower end of the basting is finished with iron (iron protects against splitting and helps with its weight). The basting is painted white and marked in tenths of a meter. The zero division corresponds to the lower end of the basting. Despite the simplicity of the device, basting gives accurate results.

Depth measurements are also carried out using a hand survey. The flow of the river causes the lot to deviate from the vertical by a certain angle, which forces an appropriate correction to be made.

Measurements on small rivers are usually made from bridges. On rivers reaching 200-300 m width, with a current speed of no more than 1.5 m per second, measurements can be made from a boat along a cable stretched from one bank of the river to the other. The cable should be taut. When the river width is more than 100 m it is necessary to anchor a boat in the middle of the river to support the cable.

On rivers whose width is more than 500 m, the measurement line is determined by the channel signs placed on both banks, and measurement points are determined by goniometric instruments from the shore. The number of measurements along the target depends on the nature of the bottom. If the bottom topography changes quickly, there should be more measurements; if the bottom is uniform, there should be fewer. It is clear that the more measurements are taken, the more accurate the river profile is obtained.

To draw a river profile, a horizontal line is drawn, on which measurement points are plotted to scale. A perpendicular line is drawn down from each estrus, on which the depths obtained from measurements are also plotted in scale. By connecting the lower ends of the verticals, we get a profile. Due to the fact that the depth of rivers is very small compared to the width, when drawing a profile, the vertical scale is taken larger than the horizontal one. Therefore, the profile is distorted (exaggerated), but more visual.

Having a river bed profile, we can calculate the cross-sectional area (or water cross-sectional area) of the river (Fm 2 ), width of the river (B), length of the wetted perimeter of the river ( Rm), greatest depth (hmaxm ), average river depth ( hcpm) and the hydraulic radius of the river.

Live cross section of the river called the cross section of a river filled with water. The channel profile obtained as a result of measurements gives an idea of ​​the living cross-section of the river. The living cross-sectional area of ​​a river is for the most part calculated analytically (less often determined from a drawing using a planimeter). To calculate the living cross-sectional area ( Fm 2) take a drawing of the transverse profile of the river, on which the verticals divide the area of ​​​​the living cross-section into a series of trapezoids, and the coastal sections have the form of triangles. The area of ​​each individual figure is determined using formulas known to us from geometry, and then the sum of all these areas is taken.

The width of the river is simply determined by the length of the upper horizontal line representing the surface of the river.

Wetted perimeter - this is the length of the river bottom line on the profile from one edge of the river bank to the other. It is calculated by adding the length of all segments of the bottom line on the drawing of the living cross-section of the river.

Hydraulic radius is the quotient of dividing the open cross-sectional area by the length of the wetted perimeter ( R= F/R m).

Average depth - this is the quotient of dividing the living cross-sectional area

rivers by river width ( h Wed = F/ Bm).

For lowland rivers, the value of the hydraulic radius is usually very close to the value of the average depth ( R≈ hcp).

Greatest depth is restored based on measurement data.

River level. The width and depth of the river, the open cross-sectional area and other values ​​we give can remain unchanged only if the river level remains unchanged. In reality, this never happens, because the river level changes all the time. From this it is quite clear that when studying a river, measuring the fluctuations in river level is the most important task.

For the water-measuring station, an appropriate section of the river with a straight channel is selected, the cross-section of which is not complicated by shoals or islands. Observation of river level fluctuations is usually carried out using foot rod. A footpole is a pole or rail, divided into meters and centimeters, installed near the shore. The zero of the foot rod is taken (if possible) to be the lowest level of the river in a given place. The zero selected once remains constant for all subsequent observations. The zero of the foot rod is associated with a constant rapper .

Observation of level fluctuations is usually carried out twice a day (at 8 and 20 hours). At some posts, self-recording limnigraphs are installed, which provide a continuous record in the form of a curve.

Based on the data obtained from observations of the foot rod, a graph of level fluctuations is drawn for one period or another: for a season, for a year, for a number of years.

River flow speed. We have already said that the speed of the river flow is directly dependent on the slope of the riverbed. However, this dependence is not as simple as it might seem at first glance.

Anyone who is at least a little familiar with the river knows that the speed of the current near the banks is much less than in the middle. Boatmen know this especially well. Whenever a boatman has to go up a river, he sticks to the bank; when he needs to quickly go down, he stays in the middle of the river.

More accurate observations made in rivers and artificial streams (having a regular trough-shaped channel) showed that the layer of water immediately adjacent to the channel, as a result of friction against the bottom and walls of the channel, moves at the lowest speed. The next layer has a higher speed, because it does not come into contact with the riverbed (which is motionless), but with the slowly moving first layer. The third layer has an even greater speed, etc. Finally, the highest speed is found in the part of the flow furthest from the bottom and walls of the channel. If we take a cross section of the flow and connect places with the same flow speed with lines (isotachs), then we will get a diagram that clearly depicts the location of layers of different speeds (Fig. 113). This peculiar layered flow movement, in which the speed successively increases from the bottom and walls of the channel to the middle part, is called laminar. Typical features of laminar flow can be briefly characterized as follows:

1) the speed of all particles in the flow has one constant direction;

2) the speed near the wall (at the bottom) is always zero, and with distance from the walls it gradually increases towards the middle of the flow.

However, we must say that in rivers where the shape, direction and character of the channel are very different from the regular trough-shaped bed of an artificial stream, regular laminar movement is almost never observed. Already with just one bend of the channel, as a result of the action of centrifugal forces, the entire system of layers moves sharply towards the concave bank, which in turn causes a number of other

movements. If there are protrusions at the bottom and along the edges of the channel, vortex movements, countercurrents and other very strong deviations arise, further complicating the picture. Particularly strong changes in the movement of water occur in shallow places of the river, where the current is divided into jets arranged in a fan-shape.

In addition to the shape and direction of the channel, an increase in flow speed has a great influence. Laminar movement, even in artificial flows (with a regular bed), changes sharply with increasing flow speed. In fast-moving flows, longitudinal helical jets appear, accompanied by small vortex movements and a kind of pulsation. All this greatly complicates the nature of the movement. Thus, in rivers, instead of laminar movement, a more complex movement is most often observed, called turbulent. (We will dwell in more detail on the nature of turbulent movements later when considering the conditions for the formation of a flow channel.)

From all that has been said, it is clear that studying the speed of river flow is a complex matter. Therefore, instead of theoretical calculations, one often has to resort to direct measurements.

Measuring current speed. The simplest and most in an accessible way flow velocity measurement is a measurement using floats. By observing (with a clock) the time a float passes by two points located along the river at a certain distance opposite each other, we can always calculate the required speed. This speed is usually expressed in meters per second.

The method we have indicated makes it possible to determine the speed of only the uppermost layer of water. To determine the speed of deeper layers of water, two bottles are used (Fig. 114). In this case, the top bottle gives an average speed between both bottles. Knowing the average speed of water flow on the surface (the first method), we can easily calculate the speed at the desired depth. If V 1 will be the speed on the surface, V 2 - average speed, A V - the required speed, then V 2 =( V 1 + V)/2 , where the required speed comes from v = 2 v 2 - v 1 .

Incomparably more accurate results are obtained when measuring with a special device called turntables. There are many types of turntables, but the principle of their design is the same and is as follows. The horizontal axis with a bladed propeller at the end is movably mounted in a frame that has a steering feather at the rear end (Fig. 115). The device, lowered into the water, obeys the rudder, stands just against the current,

and the bladed propeller begins to rotate along with the horizontal axis. There is an endless screw on the axis that can be connected to the counter. Looking at the watch, the observer turns on the counter, which begins to count the number of revolutions. After a certain period of time, the counter turns off, and the observer determines the flow speed by the number of revolutions.

In addition to these methods, measurements are also used with special bottlemeters, dynamometers and, finally, by chemical means, known to us from studying the speed of groundwater flow. An example of a bathometer is the bathometer of Prof. V. G. Glushkova, which is a rubber cylinder, the hole of which faces the flow. The amount of water that manages to get into the cylinder per unit time makes it possible to determine the flow speed. Dynamometers measure the force of pressure. The pressure force allows you to calculate the speed.

When it is necessary to obtain a detailed understanding of the distribution of velocities in the cross section (live section) of the river, proceed as follows:

1. The transverse profile of the river is drawn, and for convenience, the vertical scale is taken 10 times larger than the horizontal one.

2. Vertical lines are drawn along those points where current velocities were measured at different depths.

3. On each vertical, the corresponding depth in scale is marked and the corresponding speed is indicated.

By connecting points with the same speeds, we obtain a system of curves (isotaches), which gives a visual representation of the distribution of speeds in a given living section of the river.

Average speed. For many hydrological calculations, it is necessary to have data on the average speed of water flow in the living section of the river. But determining the average speed of water is a rather difficult task.

We have already said that the movement of water in a stream is not only complex, but also uneven over time (pulsation). However, based on a number of observations, we always have the opportunity to calculate the average flow speed for any point in the living cross-section of the river. Having the value of the average speed at a point, we can plot the distribution of speeds along the vertical we have taken. To do this, the depth of each point is plotted vertically (from top to bottom), and the flow speed horizontally (from left to right). We do the same with other points of the vertical we took. By connecting the ends of the horizontal lines (depicting velocities), we get a drawing that gives a clear idea of ​​the velocities of currents at various depths of the vertical we have taken. This drawing is called a velocity graph or velocity hodograph.

According to numerous observations, it was revealed that to obtain a complete picture of the vertical distribution of current velocities, it is enough to determine the velocities at the following five points: 1) on the surface, 2) at 0.2h, 3) by 0.6h, 4) by 0.8hand 5) at the bottom, counting h - vertical depth from surface to bottom.

The velocity hodograph gives a clear idea of ​​the change in velocities from the surface to the bottom of the flow along a given vertical. The lowest velocity at the bottom of the flow is mainly due to friction. The greater the roughness of the bottom, the sharper the current speeds decrease. IN winter time When the surface of the river is covered with ice, friction also occurs on the surface of the ice, which is also reflected in the speed of the flow.

The velocity hodograph allows us to calculate the average speed of the river flow along a given vertical.

The average vertical flow velocity of the free cross-section of the flow can most easily be determined using the formula:

where ώ is the area of ​​the velocity hodograph, and H is the height of this area. In other words, to determine the average vertical velocity of the flow across the live cross section of the flow, you need to divide the area of ​​the velocity hodograph by its height.

The area of ​​the velocity hodograph is determined either using a planimeter or analytically (i.e., by dividing it into simple figures- triangles and trapezoids).

The average flow rate is determined in various ways. The simplest way is to multiply the maximum speed (Vmax) by roughness coefficient (P). The roughness coefficient for mountain rivers can be approximately considered 0.55, for rivers with a bed lined with gravel, 0.65, for rivers with an uneven sandy or clay bed, 0.85.

To accurately determine the average flow velocity of the live cross-section of the flow, various formulas are used. The most commonly used is the Chezy formula.

Where v - average speed of the live flow section, R - hydraulic radius, J- surface flow slope and WITH- speed coefficient. But here, determining the speed coefficient presents significant difficulties.

The speed coefficient is determined using various empirical formulas (i.e., obtained based on study and analysis large quantity observations). The simplest formula is:

Where P- roughness coefficient, a R - the hydraulic radius already familiar to us.

Consumption. Amount of water in m, flowing through a given living section of a river per second is called river flow(for this item). Theoretically, consumption (A) easy to calculate: he equal to area live section of the river ( F), multiplied by the average current speed ( v), i.e. A= Fv. So, for example, if the cross-sectional area of ​​a river is 150 m 2, and speed 3 m/sec, then consumption will be 450 m 3 per second. When calculating the flow rate, a cubic meter is taken as a unit of water quantity, and a second is taken as a unit of time.

We have already said that theoretically the river flow for one point or another is not difficult to calculate. Fulfilling this task in practice is much more difficult. Let us dwell on the simplest theoretical and practical ways, most often used in the study of rivers.

There are many in various ways determining water flow in rivers. But all of them can be divided into four groups: volumetric method, mixing method, hydraulic and hydrometric.

Volumetric method successfully used to determine the flow of the smallest rivers (springs and streams) with a flow rate of 5 to 10 l (0,005- 0,01 m 3) per second. Its essence is that the stream is dammed and the water flows down the gutter. A bucket or tank is placed under the gutter (depending on the size of the stream). The volume of the vessel must be accurately measured. The filling time of the vessel is measured in seconds. The quotient of dividing the volume of the vessel (in meters) by the time of filling the vessel (in seconds) as. times and gives the desired value. The volumetric method gives the most accurate results.

Mixing method is based on the fact that at a certain point in the river a solution of some salt or paint is introduced into the stream. By determining the salt or paint content at another, lower flow point, the water flow rate is calculated (the simplest formula

Where q - flow rate of brine solution, k 1 - concentration of salt solution at release, to 2- concentration of the salt solution at the underlying point). This method is one of the best for stormy mountain rivers.

Hydraulic method based on the use of various types hydraulic formulas when water flows through both natural channels and artificial spillways.

Let us give a simple example of a spillway method. A dam is built, the top of which has a thin wall (made of wood, concrete). A rectangular spillway with precisely defined dimensions of the base is cut into the wall. Water flows over the spillway, and the flow rate is calculated using the formula

(T - weir coefficient, b - width of the spillway threshold, H- pressure above the edge of the weir, g -gravity acceleration), With the help of a weir it is possible to measure flow rates from 0.0005 to 10 with great accuracy m 3 /sec. It is especially widely used in hydraulic laboratories.

Hydrometric method is based on measuring the living cross-sectional area and flow velocity. It is the most common. The calculation is carried out according to the formula, as we have already discussed.

Stock. The amount of water flowing through a given living section of a river per second is called flow. The amount of water flowing through a given living section of a river over a longer period is called drain. The amount of runoff can be calculated per day, per month, per season, per year and even over a number of years. Most often, runoff is calculated over seasons, because seasonal changes for most rivers are especially strong and characteristic. Of great importance in geography are the values ​​of annual runoff and, in particular, the value of the average annual runoff (runoff calculated from long-term data). The average annual flow makes it possible to calculate the average river flow. If the consumption is expressed in cubic meters per second, then the annual flow (to avoid very large numbers) is expressed in cubic kilometers.

Having information about the flow rate, we can obtain data about the flow for a given period of time (by multiplying the flow rate by the number of seconds of the given time period). The amount of flow in in this case expressed volumetrically. The flow of large rivers is usually expressed in cubic kilometers.

For example, the average annual flow of the Volga is 270 km 3, Dnepra 52 km 3, Obi 400 km 3, Yeniseya 548 km 3, Amazon 3787 km, 3 etc.

When characterizing rivers, the ratio of the amount of runoff to the amount of precipitation falling on the area of ​​the basin of the river we have taken is very important. The amount of precipitation, as we know, is expressed by the thickness of the water layer in millimeters. Consequently, to compare the amount of runoff with the amount of precipitation, it is necessary to express the amount of runoff also by the thickness of the water layer in millimeters. To do this, the amount of runoff for a given period, expressed in volumetric measures, is distributed evenly over the entire area of ​​the river basin lying above the observation point. This value, called the runoff height (A), is calculated by the formula:

A is the height of the drain, expressed in millimeters, Q - consumption, T- time period, 10 3 serves to convert meters to millimeters and 10 6 to convert square kilometers to square meters.

The ratio of the amount of runoff to the amount of precipitation is called runoff coefficient. If the runoff coefficient is denoted by the letter A, and the amount of precipitation expressed in millimeters is h, That

The runoff coefficient, like any ratio, is an abstract quantity. It can be expressed as a percentage. So, for example, for r. Neva A=374 mm, h= 532 mm; hence, A= 0.7, or 70%. In this case, the river runoff coefficient. The Neva allows us to say that of the total amount of precipitation falling in the river basin. Neva, 70% flows into the sea, and 30% evaporates. We see a completely different picture on the river. Nile. Here A=35 mm, h =826 mm; therefore a=4%. This means that 96% of all precipitation in the Nile basin evaporates and only 4% reaches the sea. Already from the given Examples it is clear how important the runoff coefficient is for geographers.

Let us give as an example the average value of precipitation and runoff for some rivers in the European part of the USSR.

In the examples we have given, the amount of precipitation, the amount of runoff, and, consequently, the runoff coefficients are calculated as annual averages based on long-term data. It goes without saying that runoff coefficients can be derived for any period of time: day, month, season, etc.

In some cases, flow is expressed as liters per second per 1 km 2 pool area. This flow value is called drain module.

The value of the average long-term runoff can be plotted on a map using isolines. On such a map, runoff is expressed in runoff modules. It gives an idea that the average annual runoff on the flat parts of the territory of our Union has a zonal character, and the amount of runoff decreases to the north. From such a map you can see how important relief is for runoff.

River feeding There are three main types of river feeding: feeding by surface waters, feeding by groundwater and mixed feeding.

Recharge by surface waters can be divided into rain, snow and glacial. Rain-fed rivers are common in tropical regions, most monsoon regions, and many areas Western Europe characterized by a mild climate. Snow feeding is typical for countries where a lot of snow accumulates during the cold period. This includes most of the rivers of the USSR territory. IN spring time They are characterized by powerful floods. It is especially necessary to highlight the snows of high mountainous countries, which provide the greatest amount of water at the end of spring and in summer time. This nutrition, called mountain snow nutrition, is close to glacial nutrition. Glaciers, like mountain snow, provide water mainly in the summer.

Groundwater recharge occurs in two ways. The first way is to feed the rivers with deeper aquifers that emerge (or, as they say, wedge out) into the river bed. This is a fairly sustainable food for all seasons. The second way is the supply of groundwater to alluvial strata directly connected to the river. During periods of high water standing, the alluvium is saturated with water, and after the water declines, it slowly returns its reserves to the river. This diet is less sustainable.

Rivers that receive their nutrition from surface water alone or groundwater alone are rare. Rivers with mixed feeding are much more common. In some periods of the year (spring, summer, early autumn) surface waters are of predominant importance for them, in other periods (winter or during periods of drought) ground water becomes the only source of nutrition.

We can also mention rivers fed by condensation waters, which can be both surface and underground. Such rivers are more common in mountainous areas, where accumulations of blocks and stones on the peaks and slopes condense moisture in noticeable quantities. These waters can influence the increase in runoff.

River feeding conditions at different times of the year. Pain in winterMost of our rivers are fed exclusively by groundwater. This feeding is quite uniform, so the winter flow for most of our rivers can be characterized as the most uniform, decreasing very slightly from the beginning of winter to spring.

In spring, the nature of the flow and, in general, the entire regime of rivers changes dramatically. Precipitation accumulated over the winter in the form of snow quickly melts, and huge quantities of meltwater flow into rivers. The result is a spring flood, which, depending on the geographical conditions of the river basin, lasts for a more or less long time. We will talk about the nature of spring floods a little later. In this case, we note only one fact: in the spring, a huge amount of spring melted snow water is added to the ground supply, which increases the runoff many times. For example, for the Kama, the average flow rate in spring exceeds winter consumption 12 and even 15 times, for Oka 15-20 times; The flow of the Dnieper near Dnepropetrovsk in the spring in some years exceeds the winter flow by 50 times; in small rivers the difference is even more significant.

In the summer, rivers (in our latitudes) are fed, on the one hand, by groundwater, and on the other, by direct runoff of rainwater. According to the observations of academician Oppokova in the upper Dnieper basin, this direct runoff of rainwater during the summer months reaches 10%. In mountainous areas, where flow conditions are more favorable, this percentage increases significantly. But it reaches a particularly large magnitude in those areas that are characterized by widespread permafrost. Here, after each rain, the river level rises quickly.

IN autumn time As temperatures fall, evaporation and transpiration gradually decrease and surface runoff (rainwater runoff) increases. As a result, in the fall, the runoff, generally speaking, increases until the moment when liquid precipitation (rain) is replaced by solid precipitation (snow). Thus, in the fall, like

we have ground plus rain feeding, and rain feeding gradually decreases and by the beginning of winter it stops altogether.

This is the course of feeding of ordinary rivers in our latitudes. In high mountainous countries, melt water from mountain snows and glaciers is added in the summer.

In desert and dry-steppe areas, melt water from mountain snow and ice plays a dominant role (Amu Darya, Syr Darya, etc.).

Fluctuations in water levels in rivers. We have just talked about the feeding conditions of rivers at different times of the year and, in connection with this, noted how the flow changes at different times of the year. These changes are most clearly shown by the curve of fluctuations in water levels in rivers. Here we have three graphs. The first graph gives an idea of ​​the fluctuations in river levels in the forest zone of the European part of the USSR (Fig. 116). The first graph (Volga river) is characterized by

rapid and high rise with a duration of about 1/2 month.

Now pay attention to the second graph (Fig. 117), typical for rivers in the taiga zone of Eastern Siberia. There is a sharp rise in the spring and a series of rises in the summer due to rain and the presence of permafrost, which increases the speed of runoff. The presence of the same permafrost, which reduces winter ground nutrition, leads to especially low water levels in winter.

The third graph (Fig. 118) shows the fluctuation curve of river levels in the taiga zone of the Far East. Here, due to the permafrost, there is the same very low level during the cold period and continuous sharp fluctuations in the level during warm periods. They are caused by snowmelt in spring and early summer, and later by rain. The presence of mountains and permafrost accelerates runoff, which has a particularly dramatic effect on level fluctuations.

The nature of fluctuations in the levels of the same river in different years is not the same. Here is a graph of fluctuations in p levels. Kama for different years (Fig. 119). As you can see, the river has very different patterns of fluctuations in different years. True, the years of the most dramatic deviations from the norm are selected here. But here we have a second graph of fluctuations in p levels. Volga (Fig. 116). Here all the fluctuations are of the same type, but the range of fluctuations and the duration of the spill are very different.

In conclusion, it must be said that the study of fluctuations in river levels, in addition to scientific significance, also has enormous practical significance. Demolished bridges, destroyed dams and coastal structures, flooded, and sometimes completely destroyed and washed away villages have long forced people to pay close attention to these phenomena and begin to study them. It is no wonder that observations of fluctuations in river levels have been carried out since ancient times (Egypt, Mesopotamia, India, China, etc.). River navigation, the construction of roads, and especially railways, required more accurate observations.

Observation of fluctuations in river levels in Russia began, apparently, a very long time ago. In the chronicles, starting with XV c., we often find indications of the height of the river floods. Moscow and Oka. Observations of fluctuations in the level of the Moscow River were made daily. At first XIX V. daily observations were already carried out at all major piers of all navigable rivers. From year to year the number of hydrometric stations has continuously increased. In pre-revolutionary times, we had more than a thousand water-measuring stations in Russia. But these stations achieved special development during Soviet times, which is easy to see from the table below.

Spring flood. During the spring melting of snow, the water level in the rivers rises sharply, and the water, usually overflowing the channel, overflows its banks and often floods the floodplain. This phenomenon, characteristic of most of our rivers, is called spring flood.

The timing of the flood depends on climatic conditions terrain, and the duration of the flood period, in addition, depends on the size of the basin, individual parts of which may be under different climatic conditions. So, for example, for r. On the Dnieper (according to observations near the city of Kyiv), the duration of the flood is from 2.5 to 3 months, while for the tributaries of the Dnieper - Sula and Psyol - the duration of the flood is only about 1.5-2 months.

The height of the spring flood depends on many reasons, but the most important of them are: 1) the amount of snow in the river basin at the beginning of melting and 2) the intensity of spring melting.

The degree of water saturation of the soil in the river basin, permafrost or soil thaw, spring precipitation, etc. are also of some importance.

Most large rivers in the European part of the USSR are characterized by a spring rise in water up to 4 m. However, in different years the height of the spring flood is subject to very strong fluctuations. So, for example, for the Volga near the city of Gorky, water rises reach 10-12 m, near Ulyanovsk until 14 m; for r. Dnieper for 86 years of observations (from 1845 to 1931) from 2.1 m up to 6-7 and even 8.53 m(1931).

The highest rises in water lead to floods, which cause great damage to the population. An example is the flood in Moscow in 1908, when a significant part of the city and the Moscow-Kursk railway were under water for tens of kilometers. A number of Volga cities (Rybinsk, Yaroslavl, Astrakhan, etc.) experienced very severe flooding as a result of an unusually high rise in the river water. Volga in the spring of 1926

On large Siberian rivers, due to congestion, the water rise reaches 15-20 meters or more. So, on the river Yenisei up to 16 m, and on the river Lena (near Bulun) up to 24 m.

Floods. In addition to periodically recurring spring floods, sudden rises in water are also observed, caused either by heavy rains or some other reasons. These sudden rises of water in rivers, in contrast to periodically recurring spring floods, are called floods. Floods, unlike floods, can occur at any time of the year. In the conditions of flat areas, where the slope of the rivers is very small, these floods can cause sharp increases in levels 1 mainly in non- big rivers. In mountainous conditions, floods also occur on larger rivers. Particularly severe floods are observed in our Far East, where, in addition to mountainous conditions, we have sudden prolonged downpours, giving more than 100 mm precipitation. Here, summer floods often take on the character of strong, sometimes destructive floods.

It is known that forests have a huge influence on the height of floods and the nature of runoff in general. First of all, they ensure the slow melting of snow, which lengthens the duration of the flood and reduces the height of the flood. In addition, forest litter (fallen leaves, pine needles, mosses, etc.) retains moisture from evaporation. As a result, the surface runoff coefficient in the forest is three to four times less than in the arable land. Hence, the height of the flood decreases to 50%.

In order to reduce spills and generally regulate flow in our USSR, the government has addressed Special attention to preserve forests in river feeding areas. Resolution (from 2/VII1936) provides for the conservation of forests on both banks of the rivers. At the same time, in the upper reaches of the rivers, forest strips of 25 km width, and in the lower reaches 6 km.

The possibilities for further combating spills and developing measures to regulate surface runoff in our country are, one might say, unlimited. The creation of forest shelterbelts and reservoirs regulates flow over vast areas. The creation of a huge network of canals and colossal reservoirs further subordinates the flow to the will and greatest benefit of the individual in a socialist society.

Low water. During the period when the river lives almost exclusively from groundwater in the absence of rainwater, the river level is at its lowest. This period of the lowest water level in the river is called low water. The beginning of low water is considered to be the end of the decline in the spring flood, and the end of low water is the beginning of the autumn rise in level. This means that the low-water period or low-water period for most of our rivers corresponds to the summer period.

Freezing of rivers. Rivers in cold and temperate countries are covered with ice during the cold season. Freezing of rivers usually begins near the coast, where the current is weakest. Subsequently, crystals and ice needles appear on the surface of the water, which, collecting in large quantities, form the so-called “fat”. As the water cools further, ice floes appear in the river, the number of which gradually increases. Sometimes continuous autumn ice drift lasts for several days, and in calm frosty weather the river “rises” quite quickly, especially at bends where a large number of ice floes accumulate. After the river is covered with ice, it switches to groundwater, and the water level often drops and the ice on the river sag.

The ice gradually thickens by growing from below. The thickness of the ice cover, depending on climate conditions, can be very different: from several centimeters to 0.5-1 m, and in some cases (in Siberia) up to 1.5- 2 m. From the melting and freezing of fallen snow, the ice can thicken on top.

Outputs from a large number of sources bringing more warm water, in some cases lead to the formation of a “hole”, i.e. an unfreezing area.

The freezing process of a river begins with the cooling of the upper layer of water and the formation of thin films of ice known as lard As a result of the turbulent nature of the flow, water is mixed, which leads to cooling of the entire mass of water. In this case, the water temperature can be slightly below 0° (on the Neva River up to - 0°.04, on the Yenisei River -0°.1): Supercooled water creates favorable conditions for the formation of ice crystals, resulting in the so-called deep ice. Deep ice formed at the bottom is called bottom ice. Deep ice in suspension is called Suga. Suga can be suspended or float to the surface.

Bottom ice, gradually growing, breaks away from the bottom and, due to its lower density, floats to the surface. At the same time, bottom ice, breaking away from the bottom, takes with it part of the soil (sand, pebbles and even stones). Bottom ice that floats to the surface is also called slush.

The latent heat of ice formation is quickly consumed, and the river water remains supercooled all the time, right up to the formation of the ice cover. But once the ice cover forms, heat loss to the air largely stops and the water is no longer supercooled. It is clear that the formation of ice crystals (and therefore deep ice) stops.

At significant current speeds, the formation of ice cover slows down greatly, which in turn leads to the formation of deep ice in huge quantities. As an example, we can point to p. Hangar. There's sludge here. And. bottom ice, clogging the channel, forms gluttons. Blockage of the riverbed leads to a high rise in water levels. After the formation of ice cover, the process of formation of deep ice is sharply reduced, and the river level quickly decreases.

The formation of ice cover begins from the coast. Here, with a lower current speed, ice is more likely to form (zaberegi). But this ice is often carried away by the current and, together with the mass of slush, causes the so-called autumn ice drift. Autumn ice drift is sometimes accompanied by congestion, i.e., the formation of ice dams. Jams (like ice jams) can cause significant water rises. Congestion usually occurs in narrowed sections of the river, at sharp turns, on riffles, and also near artificial structures.

On large rivers flowing north (Ob, Yenisei, Lena), the lower reaches of the rivers freeze earlier, which contributes to the formation of especially powerful jams. The rising water level in some cases can create conditions for the occurrence of reverse flows in the lower sections of the tributaries.

From the moment the ice cover forms, the river enters a period of freeze-up. From this point on, the ice slowly grows from below. In addition to temperatures, the thickness of the ice cover is greatly influenced by the snow cover, which protects the river surface from cooling. On average, the ice thickness on the territory of the USSR reaches:

Polynyas. It is not uncommon for some sections of the river to not freeze in winter. These areas are called polynyas. The reasons for their formation are different. Most often they are observed in areas of fast flow, at the outlet of a large number of sources, at the site of factory water discharge, etc. In some cases, such areas are also observed when a river exits a deep lake. So, for example, R. Angara at the exit from the lake. Baikal for 15 kilometers, and in some years even 30, does not freeze at all (the Angara “sucks in” the warmer water of Lake Baikal, which does not soon cool down to the freezing point).

Opening up of rivers. Under the influence of spring sun rays The snow on the ice begins to melt, causing lens-shaped accumulations of water to form on the surface of the ice. Streams of water flowing from the shores increase the melting of ice, especially near the coast, which leads to the formation of edges.

Usually, before the autopsy begins, there is ice movement. At the same time, the ice begins to move and then stops. The moment of movement is the most dangerous for structures (dams, dikes, bridge abutments). Therefore, ice near structures breaks off in advance. The beginning of a rise in water breaks up the ice, which ultimately leads to ice drift.

Spring ice drift is usually much stronger than autumn, which is caused by a significantly larger amount of water and ice. Ice jams in spring are also greater than in autumn. Especially large sizes they reach the northern rivers, where the opening of the rivers begins from above. The ice brought by the river lingers in the lower areas, where the ice is still strong. As a result, powerful ice dams are formed, which in 2-3 hours raise the water level by a few meters. The subsequent dam failure causes very severe destruction. Let's give an example. The Ob River opens near Barnaul at the end of April, and near Salekhard at the beginning of June. The ice thickness near Barnaul is about 70 cm, and in the lower reaches of the Ob there are about 150 cm. Therefore, congestion is quite common here. When jams form (or, as they call it here, “jags”), the water level rises by 4-5 in 1 hour m and decreases just as quickly after the ice dams break through. Enormous flows of water and ice can destroy forests over large areas, destroy banks, and create new channels. Congestion can easily destroy even the strongest structures. Therefore, when planning structures, it is necessary to take into account the locations of structures, especially since traffic jams usually occur in the same areas. To protect structures or winter anchorages of the river fleet, the ice in these areas is usually blasted.

The rise of water during congestion on the Ob reaches 8-10 m, and in the lower reaches of the river. Lena (near the town of Bulun) - 20-24 m.

Hydrological year. Stock and others character traits The lives of rivers, as we have already seen, are different at different times of the year. However, the seasons in the life of the river do not coincide with the usual calendar seasons. So, for example, the winter season for a river begins from the moment when rain feeding stops and the river switches to winter ground feeding. Within the territory of the USSR, this moment occurs in the northern regions in October, and in the southern regions in December. Thus, there is no one precisely established moment suitable for all rivers of the USSR. The same must be said regarding other seasons. It goes without saying that the beginning of the year in the life of a river, or, as they say, the beginning of the hydrological year cannot coincide with the beginning of the calendar year (January 1). The beginning of the hydrological year is considered to be the moment when the river transitions to exclusively groundwater feeding. For different places in the territory of even one of our states, the beginning of the hydrological year cannot be the same. For most rivers in the USSR, the beginning of the hydrological year falls on the period from 15/XIup to 15/XII.

Climatic classification of rivers. Already from what has been said O rivers at different times of the year, it is clear that climate has a huge impact on rivers. It is enough, for example, to compare the rivers of Eastern Europe with the rivers Western and Southern Europe to notice the difference. Our rivers freeze in the winter, open in the spring and give an exceptionally high rise in water during the spring flood. The rivers of Western Europe very rarely freeze and give almost no spring floods. As for the rivers of Southern Europe, they do not freeze at all, and the most high level have water in winter. We find an even sharper difference between the rivers of other countries lying in other climatic regions. Suffice it to recall the rivers of the monsoon regions of Asia, the rivers of the northern, central and South Africa, rivers South America, Australia, etc. All this taken together gave our climatologist Voeikov the basis to classify rivers depending on the climatic conditions in which they are located. According to this classification (slightly modified later), all rivers on Earth are divided into three types: 1) rivers fed almost exclusively by meltwater from snow and ice, 2) rivers fed only by rainwater, and 3) rivers fed by both methods indicated above .

Rivers of the first type include:

a) rivers of deserts bordered by high mountains with snowy peaks. Examples include: Syr-Darya, Amu-Darya, Tarim, etc.;

b) rivers of the polar regions (northern Siberia and North America), located mainly on the islands.

Rivers of the second type include:

a) rivers of Western Europe with more or less uniform rainfall: Seine, Main, Moselle, etc.;

b) rivers of Mediterranean countries with winter floods: rivers of Italy, Spain, etc.;

c) rivers of tropical countries and monsoon regions with summer floods: Ganges, Indus, Nile, Congo, etc.

Rivers of the third type, fed by both melt and rainwater, include:

a) rivers of the East European, or Russian, plain, Western Siberia, North America and others with spring floods;

b) rivers receiving food from high mountains, with spring and summer floods.

There are other newer classifications. Among them, it is worth noting the classification M. I. Lvovich, who took as a basis the same classification of Voeikov, but for the purpose of clarification took into account not only qualitative, but also quantitative indicators of river feeding sources and the seasonal distribution of flow. So, for example, it takes the annual runoff and determines what percentage of the runoff is due to one or another power source. If the runoff value of any source is more than 80%, then this source is given exceptional importance; if the flow rate is from 50 to 80%, then it is preferential; less than 50% - predominant. As a result, he gets 38 groups water regime rivers, which are combined into 12 types. These types are as follows:

1. Amazonian type - almost exclusively rain fed and the predominance of autumn runoff, i.e. in those months that are considered autumn in the temperate zone (Amazon, Rio Negro, Blue Nile, Congo, etc.).

2. Nigerian type - predominantly rain fed with a predominance of autumn runoff (Niger, Lualaba, Nile, etc.).

3. Mekong type - almost exclusively rain-fed with a predominance of summer runoff (Mekong, upper reaches of Madeira, Marañon, Paraguay, Parana, etc.).

4. Amur - predominantly rain fed with a predominance of summer runoff (Amur, Vitim, upper reaches of the Olekma, Yana, etc.).

5. Mediterranean - exclusively or predominantly rain fed and the dominance of winter runoff (Moselle, Ruhr, Thames, Agri in Italy, Alma in Crimea, etc.).

6. Oderian - predominance of rain nutrition and spring runoff (Po, Tissa, Oder, Morava, Ebro, Ohio, etc.).

7. Volzhsky - mainly snow-fed with a predominance of spring runoff (Volga; Mississippi, Moscow, Don, Ural, Tobol, Kama, etc.).

8. Yukon - predominant snow supply and dominance of summer runoff (Yukon, Kola, Athabasca, Colorado, Vilyui, Pyasina, etc.).

9. Nura - predominance of snow supply and almost exclusively spring runoff (Nura, Eruslan, Buzuluk, B. Uzen, Ingulets, etc.).

10. Greenland - exclusively glacial feeding and short-term runoff in summer.

11. Caucasian - predominant or predominantly glacial feeding and dominance of summer runoff (Kuban, Terek, Rhone, Inn, Aare, etc.).

12. Loansky - exclusive or predominant nutrition from groundwater and uniform distribution of flow throughout the year (Loa River in northern Chile).

Many rivers, especially those that are long and large area nutrition may turn out to be their separate parts in different groups. For example, the Katun and Biya rivers (from the confluence of which the Ob is formed) are fed mainly by meltwater from mountain snow and glaciers with rising water in the summer. In the taiga zone, the tributaries of the Ob are fed by melted snow and rainwater with overflows in the spring. In the lower reaches of the Ob, tributaries belong to the rivers of the cold zone. The Irtysh River itself has a complex character. All this, of course, must be taken into account.

- Source-

Polovinkin, A.A. Fundamentals of general geoscience/ A.A. Polovinkin. - M.: State educational and pedagogical publishing house of the Ministry of Education of the RSFSR, 1958. - 482 p.

Post Views: 55

The average depth velocity is the ratio of the hodograph area to the maximum river depth. The area of ​​the hodograph can be calculated either from the palette, or by calculating the area of ​​the living cross-section of the river (see task 2).

Task 2

Determine the open cross-sectional area of ​​the river using the data in Table 8:

Table 8

Cross-sectional depth of the river

The living cross-sectional area of ​​a river is calculated as the sum of a number of elementary geometric figures (Fig. 9).

The figures A 1 A 2 B 1 and A 5 B 4 A 6 are triangles, the area of ​​each of them is equal to half the product of the base and the height. The remaining figures are trapezoids. The area of ​​each trapezoid is equal to the product of half the sum of the bases and the height.

Rice. 9. Cross section rivers

Points A 1, A 2, A 3, etc., at which depth measurements were carried out, are called measuring points. The starting point from which measurements A 1 are made is called the permanent beginning of the alignment.

Task 3

Calculate the water flow in the river if it is known that the open cross-sectional area is 42.2 m2, the maximum water speed in the river is 0.5 m/s, and the average depth of the river is 4.5 m.

Calculation of the average river speed based on the maximum surface speed is carried out using the formula:

,

where, V av - average speed; V max - maximum speed, K - coefficient of transition of maximum speed to average. The coefficient K is presented in table. 9.

Table 9

Values ​​of the coefficient of transition from maximum speed to average

Task 4

Determine using the Chezy formula (
, Where WITH speed coefficient, R– hydraulic radius, i– the average slope of the river), the average speed of the river, if it is known that in a given section the bottom of the channel is composed of sandy material, there are islands and shoals. The average slope of the river is 0.000056, hydraulic radius is 1.8 m.

The speed coefficient C in the Chezy formula is determined by the Bazin formula
.

The roughness coefficient y is determined according to table 10.