Watershed Management: Managing Gully Erosion
One of the most important areas of watershed management is the prevention of gully erosion. Usually, gullies are made when an increase in surface run-off happens. This means that minimizing surface run-off is important in gully erosion control. There are many reasons why watersheds deteriorate. Among them include man's misuse of the land, short intensive rainstorms, prolonged rains of moderate intensity, and rapid snow melts. Furthermore, some precipitation factors can transform into high run-off which may lead to flooding and the formation of gullies.
To be able to control the gully the following techniques can be applied in the order given. First, there must be an improvement of the gully catchments to reduce and regulate the run-off rates. Then, there must be a diversion made for the of surface water above the gully area. Finally, there must be stabilization for the gullies by structural measures and accompanying revegetation.
In countries with temperate climates, the first and/or the second methods will lead to the stabilization of these small or incipient gullies without having to use the third method. Conversely, in tropical and subtropical countries where there is heavy rains, the three aforementioned methods must be carried out for successful gully control.
In areas where many rills join and gain more than 30 cm depth, that is where the gullies are formed. The rate of gully erosion is dependent on the run-off-producing characteristics of the watershed. These characteristics include the drainage area; soil characteristics; the alignment, size and shape of the gully; and the gradient of the gully channel. These gullies are pretty destructive and could not be eradicated by simply tilling or ploughing the soil because of their depth.
There are three stages of development in the formation of a gully. These three distinct stages are waterfall erosion; channel erosion along the gully bed; and landslide erosion on gully banks. To be able to prevent these, correct gully control measures have to be done according to these development stages. Waterfall erosion happens in three steps. In the first stage, sheet erosion develops into rills, then the rills gain depth and reach the B-horizon of the soil. In the 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. Finally, in the 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. Eventually, the excavation has become too deep which leads to the collapse of the steep gully-head wall. The process is repeated and the gully head progresses backwards to the upper end of the watershed. The process is known as gully-head advancement. Whenever the gully head advances backwards and crosses lateral drainage ways caused by waterfall erosion, new gully branches are developed. Branching of the gully may continue until a gully network or multiple-gully systems cover the entire watershed.
The second stage development pertains to the channel erosion along gully beds. This means that the soil from the bottom and the sides of the gully are being scoured by flowing water. As waterfall erosion causes the gully head to advance backwards, the length of the gully channel increases. Simultaneously, the gully continues to become deeper and wider because of the channel erosion.
The final stage is channel erosion along the gully beds and these are caused by landslides on the gully banks. In the rainy season, as 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. This type of erosion on the gully banks also happens in regions with temperatures that alternate between freezing and thawing. As the temperature goes below zero degrees Celsius, the wet gully banks freeze. Soon as the temperature rises above zero, the banks thaw, loosening the soil, and the loose gully banks easily slide during the first rainy season. The landslides in the gully banks lead to the development of more new branch gullies in the disturbed banks. In this stage of the gully development, the gullies become deeper and longer as well as wider. There is no end to the three stages of gully development - waterfall erosion, channel erosion along the gully bed, and landslides on gully banks - unless the gully is stabilized by structural control measures and revegetation.
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