Generally, gullies are formed by an increase in surface run-off. Therefore, minimizing surface run-off is essential in gully control. Watersheds deteriorate because of man's misuse of the land, short intensive rainstorms, prolonged rains of moderate intensity, and rapid snow melts. These precipitation factors also turn into high run-off which causes flooding and forms gullies.
In gully control, the following three methods must be applied according to the order given:
(1) Improvement of gully catchments to reduce and regulate the run-off rates (peak flows);
(2) Diversion of surface water above the gully area;
(3) Stabilization of gullies by structural measures and accompanying revegetation.
When the first and/or second methods are applied in some regions of countries with temperate climates, small or incipient gullies may be stabilized without having to use the third method. On the other hand, in tropical and subtropical countries which have heavy rains (monsoons, typhoons, tropical cyclones, etc.), all three methods must be carried out for successful gully control.
The factors affecting gully formation can be categorized into two groups, man-made factors, and physical factors.
In developing countries, rapidly-increasing populations usually migrate upland to occupy forests or rangeland. Most migrants cut trees, burn litter and grasses, and cultivate hillsides without using conservation measures. After a few years, the productivity of the soil is lost because of sheet, rill and gully erosion, and the land is abandoned. This kind of cultivation, (slash and burn or shifting cultivation) is repeated by farmers on other hillsides until the land loses its productivity there as well. Thus, the whole of an area may be completely destroyed by gullying as the gully heads advance to the upper ends of the watershed.
Many forest fires are caused by the uncontrolled burning used in shifting cultivation. These fires can easily spread into the forest and destroy the undergrowth and litter. Grass fires are usually ignited by farmers near the end of the dry season in order to obtain young shoots for their livestock or new land for cultivation. On slopes, the soil that is exposed after forest and grass fires is usually, gull ied during the first rainy season.
Overgrazing removes too much of the soil's protective vegetal cover and trampling compacts the soil; thus the infiltration capacity of the land is reduced. The increased run-off, caused by the insufficient water holding capacity of the soil, produces new gullies or enlarges old ones.
Underground (block cave) mining is another factor that can cause gullying. Initially, cracks in the ground and soil creep (a kind of gravity erosion) are observed in the mining areas. Then, during rainy seasons, gullies are formed. Gullying in open-pit mining areas is also a big problem in many countries.
If road cuts and fill slopes are not revegetated during or immediately following road construction, gullies may form on both sides of the road. Inadequate drainage systems for roads (small number of culverts, insufficient capacity of road ditches, etc.) are a major cause of gullying. Widening operations along roadsides do not often follow road construction but, where widening is practiced, the operation usually causes landslide erosion and then gullying during the first rainy season.
Gullies are also formed on livestock and vehicle trails that run along hillsides. This is because the traffic on them compacts the soil and reduces the water holding capacity.
In forest regions, logging with tractors down slopes can lead to gully erosion, because the run-off becomes concentrated along the skid trails. Highland logging with slack cables also causes gullying on forest land.
As mentioned before, gullies are formed by increased surface run-off which acts as a cutting agent. The main physical factors effecting the rate and amount of surface run-off are precipitation, topography, soil properties and vegetative cover.
(a) Monthly distribution of rainfall
The duration of wet and dry seasons cannot be deduced from total annual rainfall. The monthly distribution of rainfall is more significant than total annual rainfall because of its effects on the growth of vegetation, as well as the fact that it gives some indications about rainfall intensity.
In humid regions with uniform distribution of rainfall, surface erosion, including gully formation, may not be a serious problem because vegetation grows throughout the year.
However, in areas that do not have uniform rainfall, the vegetation (especially grass) dries up during the prolonged dry season (3 to 5 months or more). If the land is not properly used, or if forest or grass fires occur during the dry period, it cannot sufficiently hold rainwater and so the increased surface run-off in the rainy season produces large scale landslides and gullies.
(b) Rainfall intensity and run-off
There is a relationship between rainfall intensity, rate of run-off, density of vegetative cover, and the size of a catchment area. This relationship is generally expressed in equations. The Rational Formula which is used in engineering designs for gully and torrent control is a good way to demonstrate this relationship.
If the amount of rainfall is more than the holding capacity of the soil, there will be an increase in surface run-off, followed by surface erosion and gullying. In some tropical and subtropical countries, after the soil is completely saturated, almost all of the rainfall turns into run-off during the wettest months, which include the monsoon season, tropical cyclones and especially typhoons. It rains intensively for two or three days without stopping during each typhoon period and the increased run-off causes landslides, huge gullies and devastating floods.
In continental and temperate-climate countries, prolonged rains of moderate intensity (duration several days). or short, intensive rain storms lasting from 15 to 90 minutes (maximum rainfall intensity about 3mm/minute), cause landslides, gullies and floods because of the increased run-off in the watersheds. Torrential floods, which generally occur after the short, intensive rain storms, destroy agricultural lands, residential areas, roads, irrigation ditches and canals at the base of the valley below a deteriorated watershed.
Rainfall intensity and run-off rates (peak flows) are expressed in milliliters per hour or minute and cubic meters per second, respectively. In designing engineering measures such as check dams or diversions in gully and torrent control, the rate of run-off is more important than the amount of run-off.
(c) Rapid snowmelts
Rapid snowmelts turn into high run-off. This increased surface run-off acts as a cutting agent and produces gullies. Like prolonged rains of moderate intensity and short intensive rain storms, rapid snowmelts cause destructive floods.
The size and shape of a drainage area, as well as the length and gradient oŁ its slopes have an effect on the run-off rate and amount of surface water. Therefore, all topographic characteristics should be studied in detail before gully control work begins.
(a) Shape of catchment
In Fig. 1, the two catchments have the same area, but have different shapes. Both have symmetrical drainage patterns, but the distance to the outlet in the long catchment is greater than in the short one. Therefore, the long catchment's gathering time (time of concentration) will be longer, its corresponding intensity lower, and its maximum run-off rate (Q max, cubic m/second) less. This explains why, if all other factors are equal, long narrow catchments have fewer flash floods than square or round catchments.
(b) Size of catchment
The larger the catchment, the greater the amount of run-off. The catchment area of a gully can be measured easily and accurately by using a 1/10 000 scaled map. If it is not available, a map can be prepared after surveying the catchment area of the gullies and torrents with a theodolite or transit. Mapping continuous gullies or gully networks can also be undertaken by surveying the closed traverses with a clinometer (0-90 degrees), handcompass (0-360 degrees) and 50 m measuring tape.
(c) Length and gradient of the slope
On long slopes, there is generally an accumulation of water towards the base. To prevent the gully formation, this water (run-off) should be conducted safely downhill over a long distance to stable, natural water courses or vegetated outlets. Otherwise, the water should be infiltrated into the ground by land treatment measures such as contour ditches (infiltration trenches), level terraces (gradoni), wattling, staking, etc.
The steeper the slope, the higher the velocity and erosive power of the run-off. Watershed land treatment measures not only reduce the amount of surface water, but they also decrease its velocity, and so its erosive power.
The following seven soil classes are based on soil texture: sand, loamy sand, sandy loam, loam, silt, loam, clay loam and clay. The infiltration rate increases from clay to sand (for loamy sand 2.5-5 cm/hour), but resistance against erosion decreases.
The role of vegetative cover is to intercept rainfall, to keep the soil covered with litter, to maintain soil structure and pore space, and to create openings and cavities by root penetration. This is best achieved by an undisturbed multistory forest cover. Under special conditions, however, a well-protected, dense grass cover may also provide the necessary protection.
In general, it is management and protection rather than the type of the vegetative cover which determines its effectiveness in gully control. Any vegetation which is well-adapted to local conditions and which shows vigorous growth may be used. In some cases, these may be broadleaf species, in others conifers, tall grasses, etc. In critical areas, it may be necessary to exclude any use of the protecting vegetation. Whenever possible, however, it is desirable to establish a vegetative cover which serves a dual purpose, for example, provision of fodder, fuelwood, fruit, etc.
Sheet erosion, which is a uniform removal of soil in thin layers from sloping land, occurs where the velocity of surface run-off is about 0.3 to 0.6 meters per second. More commonly, however, the direct impact of raindrops on soil particles causes their detachment and gradual downhill movement - splash erosion. Sheet erosion is barely detectable in the short term because it is a gradual process. However, over a long period, the consequent exposure of roots and subsoil can be easily observed.
Rill erosion is the removal of soil by surface flows that either form small, shallow channels or streamlets - neither is deeper than 30 cm. Because of its higher surface-flow velocities, rill erosion has a greater capacity than sheet erosion to remove and transport soil.
Still, because they are small, rills can easily be eliminated by normal tilling or ploughing.
Gullies are formed where many rills join and gain more than 30 cm depth. The rate of gully erosion depends on the run-off-producing characteristics of the watershed: the drainage area; soil characteristics; the alignment, size and shape of the gully; and the gradient of the gully channel. Gullies are very destructive and cannot be eliminated by tilling or ploughing because of their depth.
A gully develops in three distinct stages; waterfall erosion; channel erosion along the gully bed; and landslide erosion on gully banks. Correct gully control measures must be determined according to these development stages.
Waterfall erosion can also be broken down into three steps:
(a) First Stage
First, sheet erosion develops into rills, then the rills gain depth and reach the B-horizon of the soil.
(b) Second Stage
The gully reaches the C-horizon and the weak parent material is removed. A gully head often develops where flowing water plunges from the upstream segment to the bottom of the gully.
(c) Third stage
The falling water from the gully head starts carving a hollow at the bottom of the gully by direct action as well as by splashing. When the excavation has become too deep, the steep gully-head wall collapses. This process is repeated again and again, so that the gully head progresses backwards to the upper end of the watershed. This process is called gully-head advancement (Fig. 2).
As the gully head advances backwards and crosses lateral drainage ways caused by waterfall erosion, new gully branches develop. Branching of the gully may continue until a gully network or multiple-gully systems cover the entire watershed.
Channel erosion along a gully bed is a scouring away of the soil from the bottom and sides of the gully by flowing water. The length of the gully channel increases as waterfall erosion causes the gully head to advance backwards. At the same time, the gully becomes deeper and wider because of channel erosion. In some cases, a main gully channel may become as long as one kilometer.
Channel erosion along gully beds is the main cause of land slides on gully banks. During the rainy season, when the soil becomes saturated, and the gully banks are undermined and scoured by channel erosion, big soil blocks start sliding down the banks and are washed away through the gully channel.
Land-slide erosion on gully banks also occurs in regions with temperatures that alternate between freezing and thawing. When the temperature drops below zero (Celsius), wet gully banks freeze. After the temperature rises above zero, the banks thaw, the soil loosens, and the loose gully banks easily slide during the first rainy season. After land slides have occurred on all gully banks, a considerable number of new branch gullies may begin along the disturbed banks. During the third stage of gully development, gullies become deeper and longer as well as wider.
The three stages of gully development (waterfall erosion, channel erosion along the gully bed, and landslides on gully banks) will continue unless the gully is stabilized by structural control measures and revegetation.
Gullies are classified under several systems based on their different characteristics.
One gully classification system is based on size - depth and drainage area. Table 1 describes small, medium and large gullies and is commonly used in manuals on erosion.
Table 1
Gully classes based on size
|
Gully classes |
Gully depth
|
Gully drainage area
|
|
m |
ha
|
|
|
(a) Small gully |
less than 1 | less than 2 |
|
(b) Medium Gully |
1 to 5 | 2 to 20 |
|
(c) Large gully |
more than 5 | more than 20 |
Source: Frevert et., al., 1955).
This system classifies gullies according to the shape of their cross-sections (Fig. 3).
(a) U-Shaped gullies are formed where both the topsoil and subsoil have the same resistance against erosion. Because the subsoil is eroded as easily as the topsoil nearly vertical walls are developed on each side of the gully.
(b) V-Shaped gullies develop where the subsoil has more resistance than topsoil against erosion. This is the most common gully form.
(c) Trapezoidal gullies can be formed where the gully bottom is made of more resistant material than the topsoil and
(a) Continuous gullies consist of many branch gullies. A continuous gully has a main gully channel and many mature or immature branch gullies. A gully network (gully system) is made up of many continuous gullies. A multiple-gully system may be composed of several gully networks.
(b) Discontinuous gullies may develop on hillsides after landslides. They are also called independent gullies. At the beginning of its development, a discontinuous gully does not have a distinct junction with the main gully or stream channel. Flowing water in a discontinuous gully spreads over a nearly flat area. After some time, it reaches the main gully channel or stream. Independent gullies may be scattered between the branches of a continuous gully, or they may occupy a whole area without there being any continuous gullies.
In deteriorated mountain watersheds, each continuous gully in a gully network usually has a distinct catchment and a main gully channel, but it may or may not have a fan (Fig. 4). The main gully channel of each continuous gully is about one kilometer long and its catchment area is not usually more than 20 hectares. In general, there is not any vegetative cover in a gully system.
A torrent catchment usually comprises several gully systems; forest and rangelands destroyed or in good condition; hillside farming areas; low croplands; and urban areas (villages, towns). Therefore, the catchment area of a torrent may spread over more than 1 000 hectares and be 2 kilometers long. In order to control a torrent and avoid upstream floods, it is essential to stabilize all the gullies throughout the entire catchment area.
Gully control is one of the most important restoration methods used in watershed management, and timing is an essential element. The field work in all structural and vegetative control measures selected should be completed during the dry and early rainy season. This is the most important aspect of gully control - especially in tropical and subtropical countries. Otherwise, the incomplete structural work can easily be destroyed during the first rainy season. In addition, vegetative measures such as the planting of tree seedlings, and shrub and grass cuttings cannot begin until structural work is complete. Each continuous gully in a gully system should be regarded as a basic treatment unit, and all the control measures in that unit should be finished before the rainy season.
For a continuous gully, the main criteria for selecting structural control measures are based on the size of the gully catchment area, the gradient and the length of the gully channel. The various portions of the main gully channel and branch gullies are stabilized by brush fills; earth plugs; and brushwood, log, and loose-stone check dams. The lower parts are treated with loose-stone or boulder check dams. At a stable point in the lowest section of the main gully channel, for example, on a rock outcrop, a gabion check dam or cement masonry check dam should be constructed. If there is not a stable point, a counter-dam (gabion or cement masonry) must be constructed in front of the first check dam. The points where the other check dams will be constructed are determined according to the compensation gradient of the gully channel and the effective height of the check dams. General standards for selecting control measures for each portion of a continuous gully are given in Table 2.
The required structural measures for each portion of a main gully channel are shown in the last column of the table. The criteria used for selecting gully control measures in Table 2 are to be used for continuous gullies, gully networks, or multiple-gully systems located in deteriorated mountain watersheds. In upstream watersheds with very steep slopes, the gradient of the main gully channel can easily reach 100 percent or even 125 percent.
In using these standards, there is not much difference between the gullies located in mountain watersheds and those in rolling lands. In rolling lands, the highest gradient of a main gully channel may be 30-40 percent. In nearly-flat areas (agricultural and rangelands on foothills), this gradient is much less than 30-40 percent. The remaining criteria are the length of the main gully channel's portions (100 m or less and 900m) and the catchment area of the gully portions (two ha or less and 2-20 ha). They are the same for gullies located on rolling land and in nearly-flat areas.
