A Review of Landscape Design as a Means of Controlling Gully Erosion

Gully erosion is the most visible and devastating form of soil erosion known to be one of the leading causes of land degradation worldwide. Landscape design is one of the techniques used in combating this problem. The objective of this paper is to review the use of landscape design in controlling gully erosion. The method used is a review of academic articles, conference papers, internet materials, textbooks and publicly available materials on landscape design and gully erosion. The results of this study indicated that previous authors whose works were reviewed have a convergent view that the use of vegetative approach, an aspect to landscape design, must be practiced for effective control of gullies. On the other hand, however, previous authors have divergent views on the use of structural approach, another aspect to landscape design, for control of gully erosion. Recommendations of this study include: (1) planting of a combination of woody trees and grasses; (2) use of some locally accessible structures for example, sieve structures; and (3) practise of agroforestry. Keywords— Landscape Design, Gully Erosion, Review, Sustainable Development, Structure,


INTRODUCTION
Gully erosion has been recognized as the major cause of land degradation worldwide (Musa, Ahmed, Muhammed and Abdul, 2016). According to them, it is fast becoming the most reoccurring disaster in many countries of the world. Gullies are steep sided watercourses, marked by stepped longitudinal profile and commonly an abrupt channel head, subject to intermittent flow of water (Khairulmaini and Fatemeh, 2011). Poesen (2011) observed that gullies are among the morphological indication of long periods of soil erosion revealing the effect of atmospheric adjustment such as heavy rainfall and land use practices in the landscape. Gully erosion has attracted a growing interest as reflected by two recent international conferences: one in Leuven, Belgium  and one in Chengdu, China (Valentin, Poesen and Li, 2005).
By removing vegetation cover, the erosion-resisting capacity of the soilbecomes disturbed (Rickson, 2001). He also opined that when the kinetic energy of rainfall splash increases, it results in increases in soilseparation. He further stated that hydraulic surface flow mostly increases with lack of vegetation cover, which inevitablyincreases soil susceptibility to erosion mostly gully, by reducing cohesion and shear strength. Ehiorobo and Audu(2012) reported that gully erosion occurs due to extreme overflow of fluid with a very high speed and energy to remove and transmit soil particles down-hill slope.In most instances, the development of gullies is caused byovergrazing, road construction and urbanization, log haulage, improper farming andirrigation practices (Valentin et al.,2005). The need to understand how to monitor and prevent gulling is particularly acute (Poesen, Nachtergaele Verstraten and  and the search forinexpensive, durable, low maintenance techniques to control gully erosion has proven elusive (Norton, Bowannie, Peynetsa, Quandelacy and Siebert, 2002). It is important therefore, that a holistic understanding of gully erosion should involve collaborative inputs from experts in diverse fields like chemists, geologists, biologists and others (Brevik, Cerda, Mataix-Solera, Pereg, Quinton, Six and Van, 2015). Landscape as an essential part of the environment which includes topography, vegetation and associated plants and soil, water bodies, is one of the most visual needs of people (Zheng, Zhang and Chen, 2011). Williams and Tilt (2006) opined that an effective landscape design can become an integral part of a good community environment. A well defined landscape space can enhance the quality of living areas which meets people`s preferences (VanDerZanden and Rodie, 2008). A multifunctional landscape design solution must embrace the various ecosystem services that have already been bequeathed to a land area. These services include: (1) supporting and biophysical services (e.g., protecting and enhancing biodiversity and water quantity and quality); (2) provisioning services (e.g., production of energy and other utilitarian resources); (3) regulating services (e.g., waste reduction and reuse); and (4) cultural and social services (e.g., visual quality, beauty, human health, and recreational opportunity) (Lundy and Wade, 2011). Landscape design is thus defined as the art of modifying an area for aesthetic or practical reasons (David, 2017). Amangabara(2012) stated that erosion control is an important factor in landscape design and the prevention of erosion must be top priority during the life cycle of planning for soil conservation in any landscape design. The application of landscape design for controlling gullies requires a good knowledge of hydrometeorology and surface hydrology (Professional Landscape Design, 2007).Controlling gully erosion is a multi-approach goal in that there are three main areas to review in any landscape design project involving conservation and they are design consideration, vegetation selection and soil treatments (Blair, 2014).This paper is thus focused on reviewing the use of landscape designs whether vegetation or artificial installations in controlling gully erosion.

Statement of the Problem
Gully erosion usually has unpredictable impacts that are often serious and flashy (Nyssen, Poesen, Moeyersons, Deckers, Miiku and Lang, 2004).They also asserted that in the Ethiopian highlands, the development ofgullies has led to an enlarged drainage of the inter gully areas, resulting in soil moisturedecrease and a corresponding crop yield reduction on plots located near the gully walls. In tropicalnorth-western Australia, about 80% of the sediment in the Lake Argyle reservoir hascome from gully and channel erosion and less than 10% from the catchment in the area of highly erodible soils formed on sedimentary rocks (Wasson,Caitcheon, Murray, McCullochand Quade, 2002). Gully erosion has also caused loss of farmland leading to drastic decrease in soil productivity, loss of property and threat to life resulting from food shortage and famine (Abdulfatai, Okunlola, Akande, Momohand Ibrahim, 2014). According to Blair (2014), artificial installations used forerosion control in landscape design are effective to certain extents but may however besubject to decayand become less effective overtime. He was also of the opinion that some of these installations are limited in capacity and that it has been common practice to implant concrete or rock barriers intoerosion gullies in the hope that a solid barrier will counteract erosiveactivity and capture sediment.He further stated that these efforts to control gully erosionwith a solid barrier though helpful at the onset of installation usually fail.An example is the sieve structure which can only slow down gully water flows but not stop them (Layne, BreinDemisachew, Jaldesa, Badasa and Dereje, 2015). Hydrological knowledge needed for effective implementation of landscape designs is most times relegated to the background especially in developing countries,hence, the problem of gully erosion and its negative impacts continue(Amangabara, 2012).

Objective
The objective of this study is to review the use of landscape design in controlling gully erosion.

II.
CONCEPTUAL FRAMEWORK: SUSTAINABLE DEVELOPMENT This research is based on the concept of sustainable development. Morelli (2010) saw sustainable development as meeting the resources and services needs for current and future generations without compromising the health of the ecosystems that provide and more specifically as a condition of balance, residence and interconnection that allows human society to satisfy its needs while neither exceeding the capacity of its ecosystems to continue to regenerate the services necessary to meet those needs nor by our actions diminishing biological diversity.World Conference on Environment and Development (WCED) (1987) opined that sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. This research sets to review landscape design as a means for gully erosion control so as to build in sustainability into the management of the phenomenon.

III.
METHOD The researchers gathered a total of 34 materials for this research, but were able to summarize the characteristics of 10 that were deemed more relevant tolandscape design as means of controlling gully erosion and sustainable development. This research made use of academic articles, conference papers, internet materials, textbooks and publicly available materials on landscape design and gully erosion.

IV.
LITERATURE REVIEW Markus, Miloš, Jozef, Štefan and Pavol(2013) stated that erosion as one of the major and most widespread forms of land degradation, poses severe limitations to sustainable agricultural land use, reduces on-farm soil productivity and contributes to water-quality problems from the accumulation of sediments and agro-chemicals in waterways. They also claimed that gullies are formed mostly after deforestation,the beginnings of agricultural utilization and that they are often controlledby access roads or by other linear artificial landscape elements. They reported further that accelerated water erosion in general includinggulling was most effective when human interference wascombined with colder and wetter climatic fluctuations. Prolonged erosion causes irreversible soil loss over time, reducing the ecological functions of soil: mainly biomass production, crop yields due to removal of nutrients for plant growth, and reduction in soil-filtering capacity due to disturbance of the hydrological cycle; from precipitation to runoff (Víctor, Dur´an, Carmen and Rodr´iguez ,2008). They further stated that runoff is a fundamental process in land degradation, causing soil erosion and influencing the soil water balance and hydrology of the catchments. They also noted that in a wide range of environments, both runoff and sediment loss will decrease exponentially as the percentage of vegetation cover increases and that soilerosion resistance increases exponentially with greater root density. McGarigal (2006) defined landscape as an area that isspatially heterogeneous in at least one factor of interest. He pointed out that the concept of landscape differs from thetraditional ecosystem concept in focusing on groups of ecosystems and the interactions amongthem and that the focus is on spatial heterogeneity and its impact on process.According to Williams and Tilt (2006), landscape design is the art of developing property for its greatest use and enjoyment and canbecome an integral part of a good community environment.They reported that increasing evidence has shown that designcan achieve multifunctional benefits if the rolethat natureplays is taken into consideration. They further opined that landscape design has multiple benefits such as decentralized and naturalized ways of managing stormwater, runoff deduction and water quality enhancement. Musacchio (2009) was of the opinion that there is the need to be an intricate balance between the environmental and socio-economic aspects of a design to achieve sustainability goals. He further claimed that every design has its main focus and project success should be measured based on its main project goal, rather than by a rigid set of metrics. Katherine, Valerie, Carissa and Eric (2002) stated that understanding gully erosion mechanism is very important to design the gully erosion measurement system and develop its control. They observed that the use of vegetation to control erosion has been practised in many countries for centuries. They enumerated some factors to consider when using vegetation to control erosion which include texture and layering of materials, existing vegetation and surface and groundwater movement from upslope and so on. In the view of Yifan, Yongqiu and Wen (2011), the control of gully erosion can be divided into three approaches. They stated that the firstis to try to stabilize the gully using the vegetation cover method, the second is to control the runoff flow from upstream of the gully and the third is to build some soil conservation works inside the gully to restore the hydraulic balance of the gully. There are two essential components to managing the erosion problem: rehabilitating the landscape to control the source of soil loss, and reducing sediment flow through the gully system (Layne, Brien, Demisachew, Jaldesa, Bedasa, and Dereje, 2015). They argued that gully erosion cannot be stopped completely, especially when gullies receive rushing floods of water from heavy rainfall but that gullies can be treated to achieve longterm suppression of sediment transport, and when combined with better landscape management the erosion can be substantially reduced. In the opinion of Valentin, Poesen and Li (2005), many techniques have proved to be effective for gully prevention and control, including vegetation cover, zero or reduced tillage, stone bunds, exclosures, terracing and check dams. They also opined that aboveground vegetation is known to favour water infiltration and to protect soil from erosion. They further claimed that gully erosion is reduced when soil physical properties such as structural stability and infiltrability is improved by the inherent strength of the tree root mat that binds the surface soils. Izinyon, Ehiorobo and Adedeji (2013) asserted that stabilization of gullies involves the use of appropriate vegetative measures in the head, floor and sides of the gully. They further stated that once gullies begin to form, they must be treated as soon as possible, to minimize further damage and restore stability and that there are a multitude of physical and biological techniques which can be applied for effective gully treatment. In their opinion, the combination of the two measures (biophysical approach) is the best solution for effective gully control and for productive use of the gully area andthe construction of gully physical structures will be followed by the establishment of biological measures. They also reported that natural regeneration which comes after the gullies are protected and enclosed should also be considered in the overall rehabilitation scheme and attention must always be given to keeping the gully catchment well vegetated. According to Mwango, Msanya, Mtakwa, Kimaro, Deckers, Poesen, Massawe and Bethuel (2014), roots bind particles in the topsoil, which offer protection to soilthat is under pressure of detachment by sheet flow or concentrated flow. They claimed that the presenceof roots also increases the soil's roughness, thereby providing a greater capacity forinfiltration and for reducing surface runoff velocity. Ken and Wallie (2011) suggested that various artificial installations are useful in controlling gully erosion. They were also of the opinion that plants reduce soil erosion by intercepting raindrops, enhancing infiltration, transpiring soil water and by providing additional surface roughness by adding organic substances to the soil. They further stated that plant roots have a mechanical effect on soil strength and that by penetrating the soil mass, roots reinforce the soil and increase the soil shear strength. They also reported that an important aspect of rehabilitation work, which is most often neglected, is the follow-up maintenance of rehabilitation efforts and after installation, the erosion control structures need constant attention (particularly after rainfall) to ensure that they are still effective and that they will continue to contribute to improvement. Yang, Ming-Han and Shujuan (2013) asserted that increasing tree canopy coverage is not only beneficial in the control of gullies but also mitigates Urban Heat Island (UHI) effect and may reduce the incidence of heat-related diseases. Existing rooted vegetation should not be disturbed in any landscape as this encourages the occurrence of soil degradation.
Using a combination of methods outlined will be more effective for rehabilitation and also be more cost-effective. 3 Yang, Ming-Han and Shujuan(2013). It is important to begin gully remediation at the gully head otherwise erosion will continue to cut back upslope.
It is necessary to adopt a landscape approach that tackles the entire gully network, beginning where the gully erosion starts.
Source: Researchers' design, 2018 V. RESULTS AND DISCUSSION Landscape designis a highly effective means of controlling gully erosion. Table 1

VI. RECOMMENDATIONS
The specific recommendations for the use of landscape design as a means for controlling gully erosion emanating from this paper are: 1. Introduction of vegetation in affected communities should not just be limited to planting of woody trees whose canopies intercept raindrops andform larger ones that have more kinetic energy to detach soil particles, but should include planting of grasses that would absorb the kinetic energy of the larger raindrops formed by the canopies. 2. Use of structures such as terraces which alter existing environmental conditions of the soil in any community should be discouraged. This is because, in the case of terraces, the ground is cemented causing absorptive capacity of the soil to be compromised hence, increasing runoff that detaches and transports soil particles. 3. The use of structures such as sieves which do not degrade the soil and can be easily accessed locally should be introduced to compliment vegetative control of gullies. 4. The practice of agroforestry which is a combination of planting of economictrees and cropping activities should be encouraged. This will encourage control of gullies and generation of income for the affected community. 5. Environmental legislations/regulations which discourage cementing of residences of individuals within gully erosion prone communities should be established and fines levied to offenders.

VII.
CONCLUSION This paper discussed landscape design as a means of controlling gully erosion through a review of works done by previous authors. It reviewed two major aspects of landscape design: use of vegetation and the use of structures or installations in controlling gullies. The authors generally agreed that vegetation helps to curb gully erosion however; some of them were against the use of only structures in controlling gully erosion while others encouraged it. Landscape design is an effective tool in controlling gully erosion even though it may be subject to decay especially where the use of structures is concerned as illustrated by some of the authors' works reviewed.
This study therefore concludes that landscape design should be adopted to prevent and control gully erosion giving special attention to the use of vegetation as this not only helps to control gullies but also has other environmental benefits (e.g. improvement of aesthetic quality of the environment).