The article presents results of experiment and its theoretical justification aimed to study the mechanism and intensity of the early stage of gully head formation. The experiment was carried out using the jet installation that allows to change the angle of the water stream from 0.5 to 90° with the flow velocity of up to 6 m/s. The flow angle was changed with a 10° step, while the flow velocity was maintained in the range of 1.01–1.04 m/s. The intensity of soil erosion positively correlates with the flow angles (angles of attack) in the range from zero up to 40°. When the angle was increased to 50–90°, the intensity continuously declined. The lowest absolute values of erosion intensity were observed when the water flow was normal to the soil surface. There are two main reasons why the intensity of soil erosion changes with changing angle of stream flow; the first is hydraulic, the second is hydro-mechanical. The former determines weakening and destruction of inter-aggregate bonds by the water penetrating into the soil; the latter – by the combined vectors of forces of hydrodynamic head and forces keeping the particle (aggregate) in place. The experiment showed that the maximum impact of water jet on a separate soil particle occur at angle of 41°. The flow in a gully happens occasionally and the results obtained should be attributed to the early stage of erosion: the stage before formation of a so called gully headcut.

The article presents results of experiment and its theoretical justification aimed to study the mechanism and intensity of the early stage of gully head formation. The experiment was carried out using the jet installation that allows to change the angle of the water stream from 0.5 to 90° with the flow velocity of up to 6 m/s. The flow angle was changed with a 10° step, while the flow velocity was maintained in the range of 1.01–1.04 m/s. The intensity of soil erosion positively correlates with the flow angles (angles of attack) in the range from zero up to 40°. When the angle was increased to 50–90°, the intensity continuously declined. The lowest absolute values of erosion intensity were observed when the water flow was normal to the soil surface. There are two main reasons why the intensity of soil erosion changes with changing angle of stream flow; the first is hydraulic, the second is hydro-mechanical. The former determines weakening and destruction of inter-aggregate bonds by the water penetrating into the soil; the latter – by the combined vectors of forces of hydrodynamic head and forces keeping the particle (aggregate) in place. The experiment showed that the maximum impact of water jet on a separate soil particle occur at angle of 41°. The flow in a gully happens occasionally and the results obtained should be attributed to the early stage of erosion: the stage before formation of a so called gully headcut.

The erosive capacity of slope water flows, a key parameter in the quantitative assessment of soil erosion, is defined as the difference between the transporting capacity of the flow and the total content of its suspended load and bedload. Therefore, it is necessary to assess the factors and intensity of soil aggregates attrition in the water flow that determine the shares of suspended and dragged particles in the sediment load. The earlier simulation of the attrition of river sediments (H. Sternberg) and soil aggregates (G.I. Shvebs) fail to fully reflect the condition of interaction between soils and slope flows. The further attempts to describe the attrition process using empirical dependences have not given any significant improvements. A fundamentally different model of particle attrition based on the laws of mechanics allows us to describe the attrition of soil aggregates broken away by water flow differentiating the total load between the bedload and suspended load. The experimental verification of the model calculations appears to be satisfactory.

It has been experimentally shown that there are two ranges of water flow velocities, at which the erodibility of a monofractional soil (of aggregates 1–2 mm) sharply differs. In the low-velocity range, the erodibility varies from 171.53 to 3.17 m –2 s 2 at an increase in the soil density from 1.2 to 1.5 g/cm 3 . In the range of high velocities, it varies from 36.88 to 0.88 m –2 s 2 . The simultaneous solution of equations for the two velocity ranges enables us to obtain the boundary values of the flow velocity. Above them, other erodibility values should be taken into account at calculations. The boundary velocities for the model soil are within 1.6–1.7 m/s. This is explained by the fact that at a slow flow, water removes aggregates, which have lost the contact with the main soil as a result of its peptization by water. At high-velocity water flow, aggregates are detached under the effect of hydrodynamic forces.

The article is dedicated to the 50th anniversary since the N. I. Makkaveev’s Scientific Research Laboratory for soil erosion and channel processes was organized at the Moscow University. The history of scientific school, created by N. I. Makkaveev, establishment is considered, as well as the discipline on integral erosion-depositional process and its evolution to the theory of catchment erosion-fluvial systems. The analysis of the fundamental researches performed by the Laboratory is submitted, and the application tasks in the area of soil erosion, gully erosion, channel and estuarine processes are outlined. Main directions of research are formulated, the prospects for their development are evaluated.

The effect of different factors and preparation conditions of monofraction samples from the arable horizon of leached chernozem on soil erodibility and its relationship with soil tensile strength (STS) has been studied. The exposure of samples at 38°C reduces their erodibility by two orders of magnitude. The drying of samples, on the contrary, increases their erodibility. It has been shown that erodibility decreases during the experiment. It has been found that the inoculation of soil with yeast cultures ( Naganishia albida , Lipomyces tetrasporus ) reliably increases the STS value in 1.5–1.9 times. The sterile soil is eroded more intensively than the unsterile soil: at 4.9 and 0.3 g/(m 2 s), respectively. The drying of soil followed by wetting to the initial water content (30%) has no significant effect on the STS value in almost all experimental treatments.

It has been shown in experiments in a hydraulic flume with a knee-shaped bend that the rate of soil erosion more than doubles at the flow impact angles to the channel side from 0° to 50°. At higher channel bends, the experiment could not be performed because of backwater. Results of erosion by water stream approaching the sample surface at angles between 2° and 90° are reported. It has been found that the maximum erosion rate is observed at flow impact angles of about 45°, and the minimum rate at 90°. The minimum soil erosion rate is five times lower than the maximum erosion rate. This is due to the difference in the rate of free water penetration into the upper soil layer, and the impact of the hydrodynamic pressure, which is maximum at the impact angle of 90°. The penetration of water into the interaggregate space results in the breaking of bonds between aggregates, which is the main condition for the capture of particles by the flow.

The effect of density of monofraction samples from the plow horizon of leached chernozem on the rupture rate of interaggregate bonds in water has been studied. The rupture rate of bonds has been determined in a hydraulic flume by alternating passive phases of 1–5 min in duration, during which the sample occurs under a nonmoving water layer, with short (15-s long) active phases with a water flow in the flume. Samples have also been tested for tensile strength and water infiltration rate. It has been shown that the rupture rate of interaggregate bonds is related by a hyperbolic law to the soil density and by an exponential law to the rate of water infiltration to the soil. The latter relationship varies within a year and, hence, can be used as reliable parameter for predicting the seasonal dynamics of soil erodibility.

It has been experimentally shown with monoaggregate model samples of chernozemic soil as an example that the weakening of cohesion between soil particles is due to the molecular interaction of soil particles with water as a dipole substance rather than to the hydraulic forces of the flow. Therefore, soil erosion should be considered as a two-stage process. First, the bonds between particles are weakened due to the interaction of soil particles with water; then, the particles that lost bonds with neighboring ones are entrapped by the flow. Thus, the erosion rate of a consolidated soil is determined by the destruction of bonds between particles during their interaction with water, rather than the flow velocity, although this factor also affects the erosion rate.

Vertical and inclined shores washout processes were simulated in the small-scaled water tunnel with curved bed and in the rectangular-sectioned hydraulic flume. Intensity of the washout of lithologically homogeneous material of vertical shore is in direct correlation with the attack angle of flow on the coast: Wa = 349.8(1 + + 3.21sin3 a), where Wa is the intensity of washout at the attack angle a. The power-low relation (Wb = 82 + 1.72b0.71) of underwater river bank washout intensity (Wb) from its incline (b) to the flow surface is fixed. Mathematic simulation results show that the ratio of side erosion to bottom erosion shifts to bottom erosion if flow velocity was increased. Due to their morphometric, timing and hydraulic parameters these models are closest to the processes parameters and morphology of the beds of slope drainage network of shallow waterflows. Sharp river bed curves, high flow velocities and flow turbulence on the top of high water are typical for them. All these features occur in the river drainage networks as well, especially on the mountainous rivers. On the plain rivers the most adequate are the high water processes analogues as long as high water passes in the berm edges of flood-plain coasts.

It is known that tensile strength of soil samples is by three orders of magnitude greater than the shear stresses on the bottom of slope streams responsible for the detachment and transport of soil particles by water current. C.E. Mirtskhulava believed that detachment of soil particles by water current occurs due to the fatigue destruction of bonds between soil particles.Taking this fact into account, tensile strength is lower by two orders of magnitude. M.A. Nearing had an opinion that detachment of soil particles occurs in the points of separation of vortices from the bottom of the stream, where the shear stress is by two orders of magnitude higher than the average. These approaches did not explain overcoming by slope streams of the cohesion forces between soil particles. Studies of the influence of water temperature on the washout rate of model samples have shown that the soil erosion is highly dependent on the water temperature, which is close to the Van’t Hoff’s rule. This means that destruction of bonds between soil particles is probably the result of interaction between the soil solid phase and water molecules. Experiments have also shown that destruction of bonds between soil particles in the sample of chernozem monoaggregate soil occurs under a layer of still water. Upon the start of the water flow, particles that lost bond with the rest of the soil body immediately break away. The number of particles with disrupted bonds grows with the duration of the sample exposure to still water, although with some flattening. Experiments confirm the validity of the hypothesis of non-hydraulic nature of forces that disrupt inter-aggregate bonds during water erosion.

The article is devoted to the theoretical analysis and experimental investigation of the bottom and lateral erosion in shallow flows on slopes. The analysis of the ratio between the forces detaching and retaining a soil particle has showed that the erosion of the bed sidewall exceeds manifold the erosion of the bottom at the flow velocity close to the threshold value. When the flow velocity increases, the differences in the rate of erosion between the bottom and the sidewalls of the rill are leveled. The rate of sidewall erosion strongly depends on the slope of rill sides. The experimental studies of the effect of the sample surface inclination have completely confirmed the theoretical conclusions. It should be kept in mind that the lateral erosion under natural conditions is also limited by the laws of hydraulics. When the rill bed is widened due to the lateral erosion, the flow width increases and, hence, its velocity decreases to below the threshold value, which stops the erosion of the bed.