Current speed at depth. Determining river flow speed

River flow velocities (or flow kinematics) are studied in detail in a hydraulics course. Here we will pay attention only to those features of flow kinematics that are necessary to know to understand the main branches of hydrology.

Water in rivers moves under the influence of gravity. The flow speed depends on the relationship between the magnitude of the gravity component, parallel line the longitudinal slope of the flow and the resistance force that arises in the flow as a result of friction of the moving mass of water between the bottom and the shore. The magnitude of the longitudinal component of gravity depends on the slope of the channel, and the resistance force depends on the degree of roughness of the channel. If the resistance is equal to the driving force, then the movement of water becomes uniform. If the driving force exceeds the resistance force, the movement acquires acceleration; when the ratio of these forces is reversed, the movement slows down. There are two categories of water movement - laminar and turbulent.

Laminar flow is a parallel flow movement. Laminar movement is distinguished by the following features: 1) All particles of the flow move in one general direction, without experiencing transverse deviations; 2) the speed of water flow smoothly increases from zero at the channel wall to a maximum on the free surface; 3) the flow speed is directly proportional to the slope of the free surface and depends on the viscosity of the liquid.

Turbulent motion has the following features: 1) flow velocities pulsate, that is, the direction and magnitude of the velocity at each point fluctuates all the time; 2) The current speed from zero on the wall quickly increases within the thin bottom layer; further, towards the water surface, the speed increases slowly; 3) the speed of water flow does not depend or almost does not depend on the viscosity of the liquid and, in the absence of the influence of viscosity, is proportional to the square root of the slope; 4) water particles move not only along the flow, but also vertically and transversely, i.e. the entire flowing mass of water moves.

Thus, in turbulent motion it has been established that in open flows the amplitude of pulsations increases from the surface to the bottom. In the cross section of the flow, the amplitude of pulsations increases from the flow axis to the banks.

Due to the tortuosity and various forms of channels, the flow of water in rivers is almost never parallel to the banks, and the water flow is divided into separate so-called internal currents. These currents erode the channel, transport erosion products (sediments) and deposit them in the channel, resulting in spits, meanders, riffles, passes and other underwater obstacles.

The following internal currents exist in a river flow: 1) current caused by the curvature of the channel; 2) flow that occurs when the earth rotates around its axis; 3) rotational (vortex) movement of water, caused by insufficient streamlining of channel forms.

A distinction is made between instantaneous velocity and local velocity at a point of flow. Instant speed (U) (see Fig. 1) is the speed at a given point in the flow at a given instant. In a rectangular coordinate system instantaneous speed has a longitudinal component directed horizontally along the longitudinal axis of the flow and a vertical component directed along the vertical axis of the flow.

In practical calculations, as a rule, one has to deal with flow velocities averaged over time. The flow velocity at a point of flow, averaged over a sufficiently long period of time, is called local velocity and is given by the expression

(1)

where is the area of ​​the speed pulsation graph within a time period T(Fig. 1).

Rice. 1. Graph of pulsations of the longitudinal component of water flow velocity.

Velocity distribution in a river flow.

The distribution of water flow velocities in a river flow is varied and depends on the type of river (plain, mountain, etc.), morphometric features, channel roughness, and the slope of the water surface. Despite all the diversity, there are some general patterns in the distribution of velocities along the depth and width of the river.

Let us consider the distribution of longitudinal velocities at various vertical depths. If we plot the velocity values ​​from the vertical direction and connect their ends with a smooth line, then this line will represent a velocity profile. A figure limited by the velocity profile, vertical direction, and lines of the water surface and bottom is called a velocity diagram (Fig. 2). As can be seen from Figure 2, the highest speed (in an open flow) is usually observed on the surface (U surface). The speed at the bottom of the flow is called bottom speed (U d).

If we measure the area of ​​the velocity diagram and divide it by the vertical depth, we obtain a value called average vertical speed and is expressed by the formula

(2)

average speed on the vertical of the open flow is located at a depth from the surface equal to approximately 0.6h.

The normal view of the velocity profile shown in Fig. 2, in the conditions of natural watercourses it can be distorted by the impact various factors: bottom irregularities, aquatic vegetation, wind, ice formations, etc.

With significant bottom unevenness, the speed at the bottom can sharply decrease, approximately as shown in Fig. 3.

With wind downstream, surface velocities may increase and water levels may decrease slightly; with wind against the current, the opposite picture is observed (Fig. 4).

Similar to velocity diagrams on verticals, it is possible to construct a velocity diagram along the width of the river (Fig. 5), for example, surface or average velocities on verticals; the contours of the diagram usually follow the contours of the bottom; the location of the highest speed approximately coincides with the position of the greatest depth.

In the presence of ice cover, the influence of the roughness of the lower surface of the ice causes a shift in the maximum speed to a certain depth from the surface, usually by (0.3-0.4)h (Fig. 6a). If there is subglacial slush, then the downward shift of the maximum velocity can be even more significant, up to (0.6-0.7)h (Fig. 6b).

For determining river water flow still needs to be determined average river flow speed. This can be done in various ways:

1. surface floats;
2. by maximum speed;
3. using hydrometric poles or poles;
4. using deep floats;
5. hydrometric turntables.

Determination of river flow speed using surface floats.

Having chosen a straight section of the river,

• We install 8 slats (milestones) on both banks in pairs, one behind the other;
• each pair of slats should be placed perpendicular to the direction of the river flow;
• the distance between the slats that make up a pair should be the same for all pairs (for example, 5 m each).

Thus, we install four sections: I - starting, II - upper, III - main, IV - downstream of the river.

These sites are located at the same distance from each other, the value of which depends on the size of the river, for example, at a distance of 15 m from each other.

Before throwing floats, you need to record the start time of work, and after finishing - the end time of work; then note the work environment:

1. the state of the river at the gauging station (clean, in some places covered with vegetation);
2. weather conditions (clear, cloudy, fog, rain);
3. wind characteristics (calm, weak, medium, strong; downstream, against the current; from the left or right bank);
4. characteristics of the flow surface (calm, rippled, rough).

The wind has a particularly great influence on the speed of the river flow: it increases (tail wind) or reduces (head wind) the flow speed, therefore, for greater accuracy in determining the flow speed, corrections are made. There are special tables for introducing amendments.

Next, having placed observers on target, you can begin throwing floats. Floats are usually used in the form of circles, sawed off from dry logs with a diameter of 10-25 cm and a thickness of 5-6 cm. To make the float better visible on the river, it is painted with white paint, and sometimes bright red. If the river is small, then you can limit yourself to three to five floats.

At the launch site, the floats are thrown sequentially: first closer to the right bank, then to the middle of the river, then closer to the left bank.

A signal is given at the top gate. When the float is on target, the observer standing at the main target marks the time, that is, starts a stopwatch or simply notes the time on a watch with a second hand. The observer standing at the lower gate, when the float passes through the gate, gives a signal to the observer at the main gate, and he stops the stopwatch or notes the time on the clock. To determine the speed of movement of the floats, it is more convenient to make observations using the table below.

If the distance between the gates is 15 m, then the distance between the upper and lower gates will be equal to 30 m. We throw four floats in turn from the launch gate in different places of the river (i.e., first the first float; when it goes all the way, then we throw the second and etc.) and get the data that is written in the table below.

Float no.

Float path (m)

Float stroke duration (sec)

Current speed (m/sec)

Average surface current speed (m/sec)

We divide the path of the float by the time of its movement and find out the speed of the float, and to determine the average speed of the current, we add up the speeds of all the floats and divide by their number.

Determination of average speed for small rivers based on maximum surface speed.

We multiply the highest speed Vmax by the correction factor K, which depends on the degree of roughness of the riverbed. As a result, we obtain the average speed of the river. For mountain rivers with a boulder bottom, K = 0.55, for rivers with a gravelly bottom, K = 0.65, for rivers with an uneven sandy and clayey bed, K = 0.85. For example, if K = 0.75, then the average speed in our example

Vav = 0.75-0.65 - 0.49 m/sec.

Determining the average flow rate using hydrometric poles or stakes.

To do this, take a hydrometric pole with a length less than the minimum depth for a given section of the river, otherwise the pole will get stuck in shallow water. A stone of such size is tied to the pole so that the hydrometric pole protrudes slightly above the water, and its speed is determined in different points rivers in the same way as they do with surface floats; find out the average speed. In this case, the average speed will not be surface, but along the live section, but for greater accuracy, a correction should be introduced using the formula:

where H is the average depth of the river from the water surface to the bottom, h is the immersion depth of the hydrometric pole, v is a certain speed.

Determination of average speed using deep floats.

To determine the speed using this method, you need to take two bottles. The bottles are tied to each other with a cord, the length of which will depend on the depth of the river being studied. One bottle (bottom) is filled with water and sealed with a cork; sand is poured into the second bottle (top) in such an amount that only part of its neck is above the water, and also sealed.

By observing the top bottle, determine the average speed of both bottles. Using two bottles, you can also determine the speed at a certain depth equal to the depth of the bottom bottle. For example, we want to determine the speed at a certain depth of a river in a given section. Then, having tied the bottom bottle to a depth of 0.2 h (where h is the depth of the river), we first determine the average speed of two bottles - the top and the bottom, i.e. vcp, and using surface floats we determine the average surface velocity Vav.pov

and find the required speed using the formula:

V 0.2 h = 2 Vav - Vav.pov

This method can also determine the speed at depths: 0.4 h; 0.6 h; 0.8 h; thus, we can find out the average speed over the live section. To do this, you need to add all five speeds and divide by 5:

Observations show that current velocities are distributed unevenly across the cross-section of rivers. They reach their maximum value either on the freest surface or at a slight depth from it. As you approach the bottom, the speed decreases. The speed distribution picture can be depicted on a graph. To do this, the depth of each point is plotted vertically (from top to bottom), and the flow speed horizontally (from left to right). By connecting the ends of horizontal lines depicting flow velocities, we obtain a curve called speed hodograph.

Knowing the average flow velocity and the open cross-sectional area, we can determine water flow in the river according to the formula:

For example, above we determined that F = 7.08 m2, and the average speed Vav = 0.27 m/sec; therefore Q = 7.08-0.27 = 1.91 m3/sec. In round numbers we can assume Q==2 m3/sec.

Now we determine the water flow in our second example using the formula: Q = F - Vav, where F = 7.4 m2, and Vav = 0.4 m/sec; Q - 7.4 * 0.4 = 2.96 m3/sec. In round figures we can assume Q = 3 m3/sec.

Determination of fluctuations in water level in the river.

If possible, you should use a water gauge to monitor fluctuating water levels in the river within a few days. Typically, the water level is measured once a day, at 8 a.m.; if this is difficult, then observations can be made once every 10 days, and daily observations can be carried out only during high water or floods. Difference between high level water (h max) and low (h min) is called vibration amplitude(A) water level.

The amplitude value has great importance when designing various hydraulic structures.

To monitor the fluctuation of water level in the river, you can take the conditional water level at the depth of the pile, which never leaves the water during the year. From this conditional level, called the zero of the graph, a graph of water fluctuations in the river is drawn.

In general, the zero of the graph is taken to be a water horizon deeper than the minimum water level, which can be found at the nearest gauging station. When constructing a graph, the time of determining the depth is plotted on the abscissa axis, and the elevations above the zero of the graph or the water level mark for each day are plotted on the ordinate axis.

1. From the proposed list, select a cold current:
A) Gulf Stream
B) Kuroshio
C) Peruvian
D) Guinean

2. Name the “extra” mountains by their location:
A) Himalayas
B) Andes
C) Tibet
D) Alps

3. Which climate zone of Eurasia occupies the largest territory?
A) subarctic
B) subtropical
C) subequatorial
D) moderate

4. Which of the following mountains are the lowest?
A) Himalayas
B) Cordillera
C) Ural
D) Andes

5. A continent that does not belong to any state:
A) Antarctica
B) Africa
C) Eurasia
D) Australia

6. The following sea does not belong to the Arctic Ocean basin:
A) Chukotka
B) Barentsevo
C) Baltic
D) Laptevs

7. The saltiest ocean:
A) Quiet
B) Arctic
C) Atlantic
D) Indian

8. At meteorological stations, atmospheric pressure is determined using:
A) thermometer
B) barometer
C) weather vane
D) precipitation gauge

9. What winds are seasonal?
A) trade winds
B) westerly winds
C) monsoons
D) breezes

10. What type of air mass is characterized by low humidity and high summer temperatures?
A) tropical
B) moderate
C) arctic
D) equatorial

11. Lions, hippos, giraffes, antelopes live in which natural area?
A) equatorial forests
B) deserts
C) hard-leaved forests
D) savannas

12. The sea is the outskirts:
A) Black
B) White
C) Barentsevo
D) Baltic

13. What determines the strength of the wind?
A) on the speed of rotation of the Earth
B) from the proximity of the oceans
C) from the difference atmospheric pressure
D) depending on the time of year

14. An excessively moist area of ​​land with moisture-loving vegetation is...
A) reservoir
B) swamp
C) river
D) lake

15. What is the height above sea level called?
A) relative
B) horizontal
C) vertical
D) absolute

help with questions 1. what determines the formation of tides in the oceans and seas 2. what conditions affect the mixing of ocean waters 3. how can

decide ecological problems oceans and seas, confirm with specific examples 4. using a physical map, give a description (optional) of one of the seas according to the following plan; a) which ocean basin does the sea belong to; b) is it an internal or external sea; c) in what direction does it stretch; d) at what distance from your area is it located; does the water freeze? g) what rivers flow into it. 5. what is the importance of lakes in the economy 6. how do people use groundwater 7. why the protection of groundwater is one of the main problems of our time. 8. How do cover glaciers differ from mountain glaciers? 9. write a geographical essay on the topic “my project for the protection of the ocean (sea, lake, river)” thank you very much in advance for helping, I give a lot of pcs thank you

Where are the boundaries between the plates of the lithosphere? a) along ravines; b) along plains and rivers; c) along mid-ocean ridges and deep-sea trenches; d) along

continental coastlines. What are the ancient stable areas of lithospheric plates called? a) folded areas; b) platforms; c) plains; d) ocean bed. What is the name of the long-term weather pattern that repeats in a given area from year to year? a) climate; b) weather; c) isotherm; d) greenhouse effect. The closer to the equator, the: a) the greater the angle of incidence of the sun's rays and the less heat earth's surface b) the angle of incidence of the sun's rays is smaller and the air temperature in the troposphere is higher.) the angle of incidence of the sun's rays is greater and the earth's surface is heated more strongly, which means the air temperature in the surface layer of the atmosphere is higher d) the angle of incidence of the sun's rays is less and the earth's surface is heated less. Which winds predominate in tropical latitudes? a) trade winds; b) Western; c) northern; d) monsoons. Where on Earth are areas of low pressure? a) near the equator and in temperate latitudes; b) in temperate and tropical latitudes c) at the poles; d) only over continents. At what latitudes is upward air movement observed? a) in tropical; b) in the equatorial; c) in Antarctica; d) in the Arctic. In which climatic zone During the year, 2 air masses dominate: temperate and tropical?a) in temperate; b) in the tropical; c) in the subtropical; d) in the subequatorial. For what climate. zones are characterized by the dominance of westerly winds and distinct seasons? a) for the tropical; b) for equatorial; c) for moderate; d) for the Arctic. What determines the salinity of ocean waters? a) on the amount of precipitation; b) from evaporation; c) from the influx of river waters; d) from all of the above reasons. The temperature of surface ocean waters: a) is the same everywhere; b) varies and depends on latitude; c) changes only with depth; d) changes with depth and latitude. What causes the alternation natural areas on land?a) the amount of moisture; b) amount of heat; c) vegetation; d) the ratio of heat and moisture. Part B: What are the three layers that make up the continental crust? What is the importance of the atmosphere for living organisms? (at least 3 factors) Indicate why all components geographic envelope connected into a single whole? Define the concept of race, and indicate the main human races. Part C. What force moves the lithosphere plates? Why do air masses move during the year, either north or south? What is altitudinal zonation? And its main pattern.

There are several ways to measure river speed. This can be done by solving mathematical problems, when there is some data, and this can be done by applying practical actions.

River flow speed

The speed of the current depends directly on the slope of the riverbed. The slope of the channel is the ratio of the difference in heights of two sections, points to the length of the section. The greater the slope, the greater the speed of the river flow.

You can find out what the speed of a river's current is by sailing a boat upstream and then downstream. The speed of the boat with the current is V1, the speed of the boat against the current is V2. To calculate the river flow speed you need (V1 - V2): 2.

To measure the speed of water flow, a special lag device is used, a pinwheel, consisting of a blade, body, tail section, and rotor.

There is one more the simplest way how to find the speed of a river. You can measure 10 meters upstream, in steps. Your height will be more accurate. Then make a mark on the bank with a stone or branch and throw a sliver of wood into the river above the mark. After the sliver reaches the mark on the shore, you need to start counting the seconds. Then divide the measured distance of 10 meters by the number of seconds over this distance. For example, a sliver traveled 10 meters in 8.5 seconds. The river flow speed will be 1.18 meters per second.