Factors Affecting Gully Formation | Soil Management

Gullies are normally formed by the increased surface runoff. The gully formation depends on all those factors, which directly or indirectly affects the watershed’s deterioration. The main factors may be the man’s misuse of land, intense rainstorm, moderate intensity rainstorm for longer duration, and rapid snow meltings.

The factors associated to rainstorm affect the quantum and rate of runoff, which starts flow following depressed parts of the land; and forms big size path if turbulent runoff is not checked. Apart from runoff affecting the gully formation, there are also a host of other factors, listed in table 4.1.

All above listed factors are described as under:

1. Man Made Factors:

i. Improper Land Use:

Due to rapidly increasing population, the people usually migrate to uplands in the forests or rangelands for cultivation. There people cut the trees, burn the litters and grasses, and cultivate the hill lands without using conservation measures. They cultivate the land for the duration, until the land returns some yield. On continuous unscientific cultivation, the soil ultimately becomes infertile. In other words, the soil erodibility gets increase. At the same time, the soil remains under disturbed condition.

In such condition the rainfall erodes the soil very easily. In Result, these is formation of rills due to uncontrolled overland flow. Ultimately, the rills, which have been formed in the area, if not properly removed, they get enlarge by next rainfall events. The enlarged rill that cannot be removed by tillage operations is called gully. In worse situation due to lack of control measures several gullies are linked together, and thus the extent of gullied area gets enlarge. After gully formation, they extend ahead by their head advancement.

ii. Forest and Grass Fires:

The forest fire is mainly clue to uncontrolled burning used under shifting cultivation by the migrated people. The fire gets easily spread in the forest area; and destroys the vegetative growth and litters from there. In shifting cultivation the people burn the vegetation near to the end of dry season to obtain young shoots for their livestock or cleaned land for cultivation. In brief, the forest fire or grass fire makes the land unclothed (without vegetative cover) as result the exposed soil becomes very much susceptible to get affect by the rainfall; and finally there is formation of rills/gully in the area.

iii. Overgrazing:

Due to overgrazing the grasses/vegetations are-removed from the area, which results into loss of protective cover from the soil. In this condition the soil gets compact, apart from development of other bad effects. All these effects reduce the infiltration capacity of the soil; and thus there is significant increase in runoff. The increased runoff ultimately causes the formation of gully, and also enhances the size of existing gullies.

iv. Mining:

Mining is either for petroleum or any other purposes, is also a causative factor affecting the gully formation in the area. The possibility of mining is predicted on the basis of symptoms, like appearance of cracks in the ground, soil creep etc. In mining operation, the entire area gets disturbed, as result in rainy season when intense rainfall takes place the surface runoff gets concentrate through them, which ultimately takes the shape of gully.

v. Road Construction:

In the area where roads are under construction, there is formation of rill or gully due to rainfall, if proper care is not taken. In such area, immediately, after construction of roads, the road cut and filled slopes should be vegetated to check the runoff flow. The formation of gully in road area takes place from both the sides. In addition to vegetation there should also be made the provision of adequate drainage system such as culverts, ditches etc., to drain the rainwater.

vi. Livestock and Vehicle Trails:

In hilly areas, the animals normally graze the grasses. Which results into development of trails on the ground surface, apart from removal of vegetations from there. In rainy season, the rainwater gets concentrate to flow from the formed trails. If flow of rainwater is continued from the trails, then the size of trails gets increase due to forthcoming rainfalls.

Due to this reason, this factor is also considered to be as one of the causative factors affecting the gully formation. Trailing action also makes the soil surface compact, which causes greater amount of runoff generation from the area. The increased runoff is very effective to develop the gully in the area.

vii. Destructive Logging:

Although, the forests have very good vegetative cover to check the formation of runoff; and thereby to check the soil erosion or soil loss from there. But that forest, which has been deteriorated due to destructive logging, becomes very prone to water erosion, and thus to develop the gully. In such forest regions, the logging with tractors down the slope leads to gully formation, because of concentration of runoff along the skid trails.

2. Physical Factors:

The physical factors affecting the gully formation refer to those factors, which affect the rate and amount of surface runoff, which directly in turn to cause the gully formation. The precipitation, land topography, soil properties and vegetative cover are mainly considered as the physical factors affecting gully formation.

These factors are described as under:

i. Precipitation:

The precipitation as one of the important physical factors affecting the gully formation depends on:

a. Monthly distribution of rainfall

b. Rainfall intensity, and

c. Snowmelt with rapid rate.

The rainfall distribution affects the gully formation by varying the quantum of runoff likely to be developed from given rainstorm. The rainfall distribution also affects the amount of available moisture content in the soil media. If monthly rainfall is very good, then certainly there is sufficient moisture content in the soil. This causes into less loss of rainwater through infiltration action, as result there is good amount and rate of surface runoff formation from the area.

This phenomenon ultimately supports the formation of gully, provided that the soil condition is favorable. But in other hand, in such area since moisture content is in good status, therefore, the growth of vegetation is being very good, which causes into reduction of surface runoff, significantly. As result the formation of gully gets affected, significantly. Because of this particular reason, in humid regions, which receives better rainfall, the gully formation may not be a serious problem.

In contrast, in the areas with not good rainfall distribution, the vegetations are not in good form due to very poor moisture content in the soil. This effect causes very effective soil erosion on occurrence of rainfall/runoff, which may cause gully formation if soil condition favours.

The rainfall intensity, runoff, density of vegetative cover and catchments size are interrelated to each other. These have well-defined relationship. If the amount of rainfall is more than the water holding capacity of soil, then there is increase in surface runoff, followed by soil erosion and gully formation, thereby. In temperate regions, a prolong rainfall of moderate intensity and an intense rain storm of 15 to 90 minutes duration can cause landslides, gully formation and flood because of the increased runoff in u/s area.

The torrential flood, which generally takes place after short and intense rainstorm can damage the agricultural lands, residential areas, roads etc. The rainfall alters the status of moisture content in the soil, which affects the growth of vegetation. At prolong dry condition the vegetative growth is about to nil, which makes the soil uncovered, as result there is the soil erosion or gully formation on occurrence of intense rainfall.

Snowmelt at faster rate results very high runoff rate. The increased runoff is always a very causative factor to enhance the rate of gully formation.

ii. Topography:

The topographical feature of watershed includes the characteristics such as size & shape and the length & gradient of land surface. These topographical features affect the runoff, soil erosion and soil loss, significantly.

These are described as under:

a. Shape:

The shape of catchment has dominating effect on soil erosion due to variations in runoff likely to be generated from the area. In Fig. 4.1, two shapes of catchment with same area have been shown, in which the catchment with horizontally expanded [Fig. 4.1 (a)] involves very less time of concentration or greater rainfall intensity, as result the resulting runoff will be at high rate. In this condition, the soil erosion or soil loss will be at very high rate because of increased runoff rate.

Also, because of catchment’s shape, there is possibility of formation of peak runoff very frequently due to lesser time of concentration. The occurrence of peak runoff from the catchment is always destructive to cause very severe soil erosion. In case of other shape of catch­ment [Fig. 4.1 (b)], the time of concen­tration is more, which results into less rainfall intensity (rainfall depth/time of concentration).

At lesser rainfall intensity the runoff gets reduce; and thereby the soil erosion or soil loss also gets reduce, accordingly. Also, there is less possibility of peak runoff formation in the catchment, because of greater time of concentration. As conclusion, if all other factors are being same, then a long and narrow catch­ment has fewer runoffs than the square or round shaped catchments.

b. Size:

The catchment size affects the runoff. In general, if area is more than there would be more runoff. But in very large size catchment, the distribution of rainfall (intensity) is not being uniform throughout the catchment. In this condition, there is variation in runoff yield from the area. And accordingly, there is variation in soil erosion/soil loss or gully formation, thereby.

c. Length and Gradient:

A long slope length causes deposition of eroded mass on the soil surface. At higher slope gradient the soil erosion is more, and transportation of eroded soil mass is for greater distance. Due to greater slope steepness the flow velocity of runoff water gets increase, which causes scouring of soil mass from the flow path. If soil scouring is not checked then there is formation of rills, and thereby the gullies.

iii. Soil Properties:

The soil classes based on the soil texture are the sand, loamy sand, sandy loam, loam, silt, loam, clay loam and clay. The trend of infiltration rate of these soil classes is different. Infiltration rate gets increase from clay to sand (for loamy sand 2.5-5 cm/h), but resistance against erosion gets decrease.

Vegetative Cover:

The vegetative covers play very significant and multilevel roles on runoff formation and soil erosion/soil loss from the catchment; and accordingly on the gully formation.

Few of them are described as under:

a. Vegetative cover intercepts the rainwater, and reduces the amount of surplus water for creating runoff; and soil erosion/loss, accordingly.

b. Maintains the soil structure and pore spaces in proper forms, as result there is reduction in level of runoff formation.

c. Directly dissipates the KE of falling raindrops and sheet flow, both; which in turn to reduce the level of soil erosion.

d. The decomposed vegetative materials in the soil enrich the level of organic matters, as result the soil structure gets improve. And accordingly, the runoff, soil erosion and soil loss also get change.

Factors Related to Flow Velocity:

The factor that can reduce or increase the kinetic energy of runoff is its velocity. The runoff velocity depends on the depth of runoff and the slope and roughness of channel. The slope increases the flow velocity; and hence there is enhancement in gully formation, although the gullying can easily get start on less than 1% slope.

The topographical position of the area can also be a major factor – The ground water can get drain away from the soil until it reaches the valley bottom, but gullying can develop where the groundwater emerges.

Difference in elevation (ground surface) also influence the gully formation – The height from which water falls into the gully causes a considerable vortex energy, which accelerates the soil erosion rate; and also causes advancement of gully head.

There is relationship for predicting the gully erosion based on the following para­meters:

i. Amount of rainfall (r in mm).

ii. Surface area of catchment in km².

iii. Height of fall at the head of the gully (h).

The relationship is given as below:

Gully erosion = 6.87 x 10^–3 r^1.34 x S¹ x h^0.52

Plant cover has a complex influence on linear erosion.

It is as per below:

i. Plant cover protects the soil against the impact of rain drops which causes maintenance of soil permeability; and thus reducing the volume of runoff.

ii. Its litter attracts meso-fauna, which dig the macro-pores and develops greater potential for absorbing runoff water.

iii. The plant cover develops soil roughness, which depends on the number of stalks per unit area. The plant cover of grasses with many stalks is more effective than the few trees for protecting the soil against runoff.

The soil itself affects the runoff and erosion in various ways – The roughness of soil surface slows down the runoff and also affects its volume. The stability of soil structure affects the rain splash. If the soil surface contains gravel or large size stones, then there can be two effects, in which one, if the stones are on the soil surface, then they protect the soil against splash and also protect the underlying macro-pores, and hence there is positive effect on infiltration rate.

On the other hand, if the stones are in the sediments or slaking crusts, then runoff will be less. If the soils have been compacted, then soil becomes impervious in nature, causing into greater runoff yield. If soil profile is homogeneous, erosion will produce a V-shaped gully, whereas if soil resistance is irregular, then a U-shaped gully with vertical sides is formed because erosion rate depends on how well the materials resists the soil cutting.

In the year 1935, Hjulstrom studied the soil credibility as the function of soil texture and flow velocity; and mentioned that the soil erosion starts at a minimum flow velocity of 25 cm/s with the soil texture of 100 microns size fine sands.