Factors, Impacts and Prevention of Gully Erosion

Factors, Impacts and Prevention of Gully Erosion

Submitted by:

Ramirez, Eunice G.

2011-65121

A term paper submitted

In partial fulfillment of the requirements in

ABE 73

Soil and Water Conservation Engineering

Date Submitted: April 13, 2016

GULLY EROSION

Soil erosion is one of the critical phenomenon that is studied in the field of soil and water conservation engineering. Specifically, runoff erosion that has sub-processes which are detachment and transport. Accounting the temporal aspect of erosion, this runoff erosion will develop into sheet erosion, then rill erosion and would finally lead to gully erosion (Gonzales, 2016). Gully erosion is considered highly visible form of soil erosion since it causes deterioration of the land in terms of productivity and usage. As stated by Ziebell and Richards, 1999, gully erosion is the removal of soil through surface runoff water. By definition, gullies are moderately steep-sided channel where water flows during intense rainfall (Carey, 2006).

Gullies can be classified based on size; channel cross-section, geometry and morphologic. Based on size, gullies can be small, medium and large. Meanwhile, based on channel cross-section, it can be U-shaped, V-shaped and trapezoidal depending with the resistance of the layers (subsoil and topsoil). When the subsoil and topsoil resistance are equal, U-shaped gully is formed. V-shaped gullies are the most common kind of gully since it is produced when subsoil resistance is greater than the topsoil resistance. During the existence of a resistant gully bottom than the topsoil and subsoil. Another classification of gullies is based on its continuation. Gullies can be continuous or discontinuous. Lastly, morphologically, gully classification would be divided as linear, bulbous, dentrite, parallel and compound (FAO and Gonzales, 2016).

FACTORS

According to Food and Agriculture Organization (FAO), factors that affect gully formation is divided into two: man-made factors and physical factors. Man-made factors include improper land use, forest fires, grass fires, overgrazing, mining, road construction, livestock trails, vehicle trails, and destructive logging. As the name implies, man-made factors are controlled by humans. Improper land use includes the excessive cutting of trees and burning of fields like what is done in kaingin system. Meanwhile, physical factors include precipitation, topography, soil properties, and vegetative cover. Under precipitation, distribution of rainfall, intensity and run-off are the prevailing factors that affect gully erosion. In addition to, shape & size of catchment area affects gully erosion through the intensity and frequency of flash floods. Lastly under topography, length and gradient of slope in which steeper slope induces more erosion to the land.

Formation of gullies is caused by different phenomena which help in development of gully erosion. Some of these said phenomena are cultivation or over-grazing on soil. Another cause of gully erosion is increased in run-off and the land cover of the area. It was also observed that the higher productive area, the higher the chance of formation of gully. In line with this, run-off concentration from furrows and waterways also cause gully erosion. Moreover, inadequate design, construction or maintenance of the irrigation lines of the given catchment area causes formation of gullies and low seepage flow in a given long period of time (Carey, 2006; Ziebells & Richards).

Development of gullies have three distinct stages as mentioned by FAO. These are waterfall erosion, channel erosion, and landslide erosion. Waterfall erosion is further divided into three stage. The first stage is sheet erosion becomes rill and would reach the B-horizon. Next, water will attain the C-horizon and eventually will weaken the parent material. Lastly, the third stage involves gull-head advancement, which involve repeated splashing at the bottom of gully. Next, channel erosion occurs when the removal of the bottom and sides of the gully bed took place. Land-slide erosion on gully banks was the scenario when sliding and rolling of big soil blocks occur at the gully channel.

IMPACTS

Gully erosion affects in different fields of agriculture and different kinds of area. As stated by Valentin, C. et. al, 2005, some of the areas of impacts of gully erosion are farming systems, hydrological functions, and sediment production. Regarding the farming systems, the impact of gully erosion is reduced soil fertility. Based on studies, sudden erosion removes more topsoil compared to the gradual erosion. Specifically, gully erosion produces fast and severe removal of the soil. Moreover, the said loss of topsoil and higher labor cost would be another impact for the farming system. In some areas like in Southeast Asia that has gentler slopes, water erosion rates accelerate from steep slope towards the gentler slopes. Regarding the hydrological functions, gullies are affected by the topography of the area. In general, the impacts of the gullies are increased drainage and strengthened aridification processes. Specifically, in areas with narrow valleys, gullies prevent the flow of water to be irrigated the whole area of the valley since gullies are formed in narrow channels. On the other hand, extremely crusted areas are influenced by gully bottoms that become water transmission sources. With regards to sediment production, studies have proven that about 80% of sediment within the reservoir originated from the nearby gully and channel erosion (Wasson et. al, 2002, cited by Valentin, C., et. al, 2005)