Soil erosion is known to be the major cause of environmental degradation in most developing countries. It appears to be the worst among the natural disasters especially in Nigeria (Onwuka et al, 2012). Soil erosion is simply the process of detachment, transportation and deposition of soil particles (sediments) by erosion agents such as water and wind. It can be caused by both natural factors (water and wind) and human factors (e.g. man’s removal of the protective cover of vegetation). Surface erosion includes processes of rain splash, sheet wash, rilling and gullying, these are for erosion caused by water. Erosion caused by water are affected by factors such as climate, vegetation, type of soil, topography and cultivation activities. Erosion is known to be a serious threat to human beings. In agriculture, it reduces productivity due to the removal or washing away of plant nutrients and organic matter.

According to Ogunlela and Makonjuola (2000), water erosion can be controlled using two major approaches: (1) reducing the erosive capacity of the flowing water through structural measures (e.g. check dams) and (2) increasing the resistance of the soil relative to the erosive capacity of the flowing water through vegetation lining. Vegetation can be used to control erosion due to its buttress and sprawling root systems that are responsible for increasing their resistance to erosion. Greenway (1987) stated that roots strengthen the soil thereby increasing the soil shear strength and are responsible for holding the soil particles at the ground surface thereby reducing its susceptibility to erosion. It is important to maintain vegetative cover in order to prevent erosion of the bare soil. Vegetation tends to reduce detachment of soil particles by intercepting the raindrop impact, and also reduce the transportation of these soil particles. It can be said that vegetation acts as a check in soil erosion. Chow (1959) noted that the presence of grasses/ vegetation causes loss of energy and retardance to flow. The flow across the channel is influenced by the vegetation in the channel and its extent of influence is based on the characteristics of the vegetation and the flow characteristics. The vegetation characteristics includes; the vegetation species, degree of submergence (submerged or unsubmerged), density, distribution and flexibility. The flow characteristics includes; flow area, depth and side walls of the channel. Velocity of flow is the main effect of vegetation in the channel, vegetation tends to increase the roughness or flow resistance or retardance (Fischenich, 2000).

This property that vegetation offers to resist flow is called Manning co-efficient of roughness, n also known as retardance co-efficient, the n-value depends on various factors such as the size and shape of the soil grains on the channel (soil type), type of vegetation, size and shape of channel, change of season, presence of obstruction etc. and all these factors are mutually dependent on one another (Chow, 1959). Ree and Palmer (1949) stated that the ability of vegetation to resist flow can be identified by the relative flow depth to the vegetation height. Rodney et al., (2011), concluded that the roughness coefficient varies for different vegetation season to season.

Many researchers have worked in this area of study obtaining varying results from different vegetation. Different models and formulae have been developed in order to predict the flow resistance of vegetation. But, this seems to be unreliable because different vegetation possess different characteristics (for example, the height of the vegetation affects the flow resistance, but it is decreased by the bending of the vegetation) which affects the hydraulic roughness coefficient, n (n-values) and these varies from place to place and time to time. Therefore, it is possible to say that these values vary constantly.

The aim of this research is to study on some Nigerian grasses (which are Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon)) in order to identify their suitability in controlling erosion by collecting a wide range of data showing the relationship between the flow characteristics and the vegetation characteristics in order to determine its hydraulic roughness coefficient, n (n-value). The specific objectives of this study are as follows:

• To determine the suitability of the selected vegetation in controlling erosion.
• To determine the extent of flow retardance of each of these vegetation cover or grass.
• To determine the effect of bed slope on the hydraulic roughness coefficient.

The study will attempt to solve soil erosion using vegetation for erosion control to retard the velocity of flow on the open channel, thereby solving one of the major engineering problems involved in soil and water conservation in order to maintain productivity of the crop. This is possible due to the effect of the vegetation on the soil which resist flow but varies for different vegetation. This study has been able to produce or develop results that can be used by engineers in the design of channels, highways and bridges and also important to farmers in order to select a vegetation with a higher hydraulic roughness coefficient for controlling erosion on their farmlands through the reduction of erosivity of flow.

This study is to determine the hydraulic roughness co-efficient, n, of some vegetation which is aimed at selecting vegetation that has the ability to control erosion.

The significance of this study is to obtain results on the hydraulic roughness coefficient, n of these grasses and to identify those that have the ability to resist flow considering its vegetal characteristics.

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