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Essay on Gully Erosion
- Essay on the Meaning of Gully Erosion
- Essay on the Causes of Gully Formation
- Essay on Triggers for Gully Development
- Essay on Gully Development
- Essay on the Process of Gullying
- Essay on the Intensity of Gullying
- Essay on Preventive Measures of Gully
- Essay on Gully Control Measures
Essay # 1. Meaning of Gully Erosion:
Gully erosion is the advance stage of channel or rill erosion in which the size of rills is so enlarged that they cannot be smoothened by ordinary tillage implements. Process of gully formation follows sheet and rill erosions. It also occurs, when runoff volume from a sloppy land gets increase sufficiently or increase in flow velocity to cut deep inclusions or when concentration of runoff water is long enough through the same channel.
Gullies may also be developed from those rills which are unchecked. Often, gullies are also developed from natural depressions on the land surface in which runoff water accumulates, and in extreme situation they join together and formation of water flow through them. Development of gullies also takes place through the tracks formed by the movement of machineries, down the slope.
The rate and extent of gully development are closely related to the amount and velocity of runoff water. Because large amount of flowing water tends to detach and transport the soil relatively in large amount. The amount of runoff water available is dependent on the size and runoff producing characteristics of the drainage area, involved. The rate of gully erosion is further conditioned by the soil characteristics, size & shape of the gully and slope of the gully bed.
Overall, the followings are main factors:
1. Resistance offered by the top soil and underlying hard layer.
2. Rainfall characteristics, which favour to increase the volume of runoff over the land surface.
3. Vegetative cover on the soil surface.
4. Topography of the area including land slope.
Essay # 2. Causes of Gully Formation:
Various causes are as under:
1. Increase of Surface or Subsurface Water Flow:
The formation of gully in any area depends very much on increase in the magnitude of surface or subsurface water flow. The increase in water flow can be due to several reasons such as improvement in drainage channels and removal of water holding areas at upstream part, which may be the swamp or bush; enlargement of catchment contributing the flow or change in tillage practices and cropping system in the watershed.
2. Decrease in Soil Resistance to Erosion:
When resistance of soil to erosion gets decrease then there is the possibility of gully formation. There are several reasons by virtue of which the soils resistance to erosion gets reduce, in which the elimination of vegetative cover, breakdown of soil structure due to poor tillage practices and cropping practices, constant saturation by trickling tiles or compaction by tractor wheels, are the main.
3. Sloughing and Undermining:
The sloughing occurs at the base of gully due to following reasons:
i. Strong spring.
ii. Constant saturation of the bank area by water ponding or due to waterfall from a higher elevation to lower elevation, e.g., from cultivated field into channel.
The undermining process causes extension of gully head towards upstream; and enlargement of gully width.
The undermining may be because of flowing reasons:
i. Wave action developed in the ponded water in gully.
ii. Piping action.
iii. Rodent’s hole.
In brief the above points are summarized as under:
i. Making the land surface without vegetation.
ii. Adoption of faulty tillage practices.
iii. Overgrazing and other forms of biotic pressure on the vegetative cover, existing on the land surface.
iv. Absence of vegetative cover.
v. Not smoothening of rills, channels or depressions present on the ground surface.
vi. Improper construction of water ways, roads, rail lines, cattle trails etc.
Essay # 3. Triggers for Gully Development:
The gully development may be triggered by the following points:
i. Cultivation or grazing of vegetations on the soils susceptible to gully erosion.
ii. Increased runoff due to change in land use, such as tree cutting for construction of new residential areas, etc.
iii. Runoff concentration in furrows, contour banks, waterways etc.
iv. Improper design, construction and maintenance of waterways in the area.
v. Poor vegetative cover may be due to overgrazing, fires or salinity problems.
vi. Poor flow or prolong seepage flow.
vii. Down-cutting in creek.
viii. Diversion of drainage ways to the area of high erosion risk such as steep creek bank or highly erodible soils.
Essay # 4. Gully Development:
Gullies are developed by several processes which may be either singly or in combination at the same time.
These processes are given as:
1. Scouring of the soil particles from the bottom and sides of the gully by flowing water mixed with abrasive materials such as hard soil particles or debris.
2. Water fall erosion at the gully head which results the cutting of gully bank, and thereby extension of gully into non-gullied lands.
3. Sliding or mass movement of the soil from gully banks due to seepage, alternate freezing and thawing or undercutting by flow.
Gully development is accomplished under various stages.
The four recognized stages of gully development are given as under:
Stage 1 – During this stage, the channel erosion and deepening of the gully bed take place. This stage normally proceeds with slow speed where the top soil is fairly resistant to erosion. Stage 1 is basically the initiation stage of gully development.
Stage 2 – This stage is known as development stage, in which due to runoff flow from the up-stream portion of the gully head, the size of gully i.e., width and depth is enlarged. The gully depth reaches up to the ‘C’ horizon and the weak parent materials are rapidly washed away. A waterfall often starts, when flow plunges from upstream segment to the eroded channel, below.
Stage 3 – This is the healing stage, in which vegetations are started to grow in the gully. During this stage, there is no appreciable erosion in any form, from the gully section.
Stage 4 – This is the last stage of gully development, in which the gully has been fully stabilized. There is no further change to develop the gully, unless healing process is disturbed. The gully attains a stable gradient and its walls gain a stable slope. The vegetation begins to grow in abundance to cover the entire gully section, and also development of a new top soil. The healing stage is necessary to stabilize and to grade from one stage to another.
During the latter two stages (i.e., 1 and 2) the gully head is progressed towards the upper end of the watershed and resultant runoff entering into the gully becomes less, as drainage area is reduced. The runoff enters at different points along the length of the gully.
The evaluation or prediction of gully development is very difficult, because the factors which are involved, are not so easy to determine them. However, Beer and Johnson (1962) have developed a prediction model on the basis of aerial photograph and field survey for the deep loess regions of Western Iowa.
The equation is based on 20-years return period, given as under:
A = 0.01 Q0.10.At–0.044.L0.80.Ld–0.25.e–0.36I … (4.1)
A = change in gully surface area
Q = surface runoff expressed in terms of depth
At = level terraced area of the watershed
L = length of gully at the beginning period,
Ld = gully length measured from upper end to the watershed divide line
I = deviation from normal precipitation, expressed in terms of depth.
The kinetic energy of raindrops on a slope is more or less constant depending on the wind-speed. Runoff, on the other hand tends to accumulate and run together on a longer slope. If the peak flow gets increase, the soil surface will be cut and a rill will start to form.
The formed rills will then get deepen as the water-borne sediment abrades the bottom of the rill, the edges cave in, and the materials thus dislodged are carried away. In gully formation process, the initial step is the rill formation, thereafter the formation of V or U shaped gullies depending on the nature of sub-surface soil formation and lastly the process of tunneling.
These processes are described as under:
1. V-Shaped Gullies are the First Form of Gullying:
V-shaped gullies are the first form of gullying. These are very common in the formation of relatively loose, homogeneous materials such as sandy-clay and clay soils. The slopes of these gullies get alter as the rock weathers. In cold seasons through the alternation of freezing weather and in hot seasons the weathering may be due to alternation of dry and stormy effects. In Mediterranean areas, weathering can reach 4 to 10 mm per year. Subsidence takes place during exceptional rainstorms.
One or two rainfall events per year are enough to carry away all particles accumulated in the gully bottom during the year, and for the solid load in the runoff to abrade the valley bottom. During intermediate seasons, the fine matters accumulated on the slopes as the rock weathers gets slide down to the bottom of the gully due to raindrop impact, through formation, of small secondary rills and also due to mass sloughing of water-saturated soil mass. In case of V-shaped gullies therefore erosion control must be concentrated to prevent further excavation of gully bottom and re-establishing the equilibrium.
2. U-Shaped Gullies:
U-shaped gullies are second form of gullying. These are found to get develop in heterogeneous soil formations. The base of this gully contains very resistant materials; due to this reason the depth does not increase further, but broaden. Because of this reason the u-shaped gullies are broader than their depth. Also due to undermining action the bank soil becomes unstable; in this situation again width of u-gully gets enlarge.
Tunneling is the third form of gullying. They get develop on gentle slopes in the soil formations with surface cracks; in the soil rich in swelling clay; in the marl rich in gypsum or other soluble minerals etc. During end of dry season rainstorms, the water penetrates in these cracked soils, and moves down to the weathered rock, which percolates through the cracks to the bottom of the slope, where headwater cutting can form gullies. Also, when hypodermic runoff enters the cracks, then cracks get gradually modified into tunnels. The tillage is only the way of blocking these cracks and checking the gully formation by this method.
As soon as the velocity of surface runoff exceeds the critical limit at which external force of the flow in-terms of energy becomes greater than the internal force expressed by the soil cohesion, the washing of soil particles by the flow begins. The critical velocity of surface flow is the highest velocity at which washing of soil particles does not yet take place. Mirtskhulava (1970) developed following equation to describe the intensity of gullying –
Ig = intensity of gullying
d = mean diameter of soil particles
w = area of transverse section of water flow
VΔi = initial water velocity
VΔk = extreme velocity of water flow.
The flow velocity in the gully depends on the catchment area above; and length of gullying as l = (L – x), where L is the distance of the gully bottom and x is the point of beginning of gullying. The shorter the value of x, smaller will be the area and growth intensity of the gully. Depth of gullying in time (t) can be predicted by using the following equation–
where, ht is the depth of gullying in time t; h is the expected depth of gully stabilization; t is the time period of gullying and T is the time period of gully stabilization, which can be computed by using the following expression–
The value of Vk is considered at critical flow velocity to cause erosion from upper soil horizon, for which the formula is given by Velikanow (1948), as–
The critical water velocity is dependent on the resistance of the earth and rocks to gullying. The critical water flow velocities for various types of soil and rock formations are given in Table 4.2. When water accumulates in the rills, depressions or artificial furrows, channels etc. the frictional resistance gets decrease as result the velocity of water flow increases and consequently the erosive force and carrying capacity of surface water, both get rapidly increased. Gullying activity also gets accelerate, when overflow takes place from gullies.
In general, the gully density is usually not greater than 10 km/km watershed area; and area covered by the gullies does not exceed 15 percent of the total area of the watershed. When these values get exceed then linear erosion changes into polymorphous erosion, in which area is substantially more divided and dissected; also the other destructive phenomena such as intra soil erosion, landslides, solifluction etc. occur at the same time.
Regular monitoring is essential to detect early stages of gully formation.
There are host of measures to prevent the gully; the important measures amongst them are described as under:
1. Properly Development:
Few important points are mentioned below:
i. Management of catchments to ensure that the run-off is not increased.
ii. Assessment of the land’s capability to ensure whether it is suitable for the proposed use or not.
iii. Construction of roads, fences and laneways so that they cause minimum concentration and diversion of run-off.
2. Grazing Management:
Under this the followings are the main points:
i. Control of erosion on slopes and cultivated lands by stubble retention and construction and maintenance of contour banks and waterways.
ii. Construction of waterways of appropriate specifications – their stabilization and maintenance.
iii. Ensuring that the contour bank discharge is into waterways at safe location.
iv. Spreading the flood flows on cultivated floodplains; and avoiding those practices that concentrate flood flows.
i. Avoiding development of bare and compact areas, that may be in the school grounds or other heavily trafficked areas.
ii. Avoiding development of steep sites and drainage lines.
iii. Minimizing soil disturbance, stockpiling and re-spreading the topsoil and re-vegetating the affected areas.
iv. Constructing flood detention systems below high run-off areas.
The question that, why use of partial control measure is not successful for gully control. The answer is, it does not follow the first principle of gully control i.e., determining the cause of gully development and implementing necessary measures, accordingly. If the stability of the gully has been lost and its extension is still continued because of increment of flood volume then minor patching up of the damage will not solve the problem but needs to use some other measures.
The second principle states that, either to restore the original hydraulic balance or to create new conditions, which was not fulfilled during application of partial control measures.
The gully control measures include the following methods:
A long experienced soil conservation engineer stated that “in gully control a bag of fertilizer is more important than a bag of cement”. The purpose of growing vegetation is twofold. First, it provides soil cover and second protects the gully against scouring. It also reduces the flow velocity by increasing the hydraulic resistance of the channel section, thereby the scouring and abrading ability of runoff gets reduce to a great extent.
In addition, if the flow velocity is sufficiently reduced then some of the sediment loads carried by runoff is deposited over the gully bed. This leads to the formation of desirable situation for growth of vegetation in the deposited soil, which further makes a favourable situation to control the gully. The vegetation gets denser after few times and traps more and more silts, until the whole gully area is filled.
In this method, structures are used to control the flow velocity and thereby the gully erosion. The structures which are constructed to check the velocity of flowing water in the gully are known as check dams.
The working principles of check dams are given as under:
1. They reduce the degree of slope steepness of the gully bed by constructing a series of checks at regular intervals, across the width in the gully. These check dams divide the longitudinal slope of gully bed into a series of steps with low risers and long flat treads.
2. They reduce the velocity of running water by creating obstructions in the flow path; and thus making the silt deposition over the gully bed carried by the running water.
3. The series of such breaks increase the time of opportunity of flow in the gully, by which the scope for percolating of water into the soil gets increased.