A field study was conducted on three selected grasses, which are Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) in order to determine the hydraulic roughness coefficient (Manning’s n), the effect of bed slope (0.2%, 0.3% and 0.4%) on the n-value and to select from the results obtained the most suitable vegetation that can be used to control erosion. The set-up of this experiment consists of 12 trapezoidal open channels with a length of 5m, breadth (top width) of 0.12m and maximum flow depth of 0.03m. The experiment was carried out at different flow depths of 0.001m, 0.002m, 0.003m, 0.004m and 0.005m. The vegetation height varied for different slope and all the grasses were tested under unsubmerged condition. Discharge was determined using timed gravimetric method, hence, the hydraulic roughness coefficient (n) can be obtained using Manning’s equation. At the three slopes, the range of n-value for Spear grass was 0.020-0.046, 0.031-0.111 for Guinea grass and 0.034-0.198 for Bahama grass. Bahama grass has the highest Manning’s n, this can be attributed to its deep root system and creeping nature. Linear relationships were developed between Manning’s n and Degree of submergence (Y/H), Reynolds number Re and VR, Vegetation density, Flow depth and Drag coefficient Cd. It was found at that at each slope, the degree of submergence increases as the Manning’s n decreases, Reynolds number and VR increases as the Manning’s n decreases, and as flow depth increases, the Manning’s n decreases. At a constant flow depth, the vegetation density increases as the Manning’s n increases, except for Guinea grass and Bahama grass. It is also noticed that Reynolds number increases and drag coefficient Cd decreases, as the flow depth increases for all the vegetation.
1.3 Aim and Objectives of the Study. 3
2.1.1 Classification of Open Channel Flow.. 5
2.2 Methods for Determination of Hydraulic Roughness Coefficient. 8
2.2.2 Storage methods (SCS method): 9
2.4 Hydraulic Roughness Coefficient 13
2.4.1 Manning’s n variations with Degree of Submergence. 14
2.4.2 Manning’s n variations with Reynolds number, Re. 15
2.4.3 Manning’s n variations with Vegetation Density. 16
2.4.4 Manning’s n variations with Flow Depth. 16
2.4.5 Manning’s n variations with Drag Coefficient 17
2.4.6 Manning’s n variation with the Vegetation Arrangement or Distribution 19
2.5 Factors Influencing Manning’s Roughness Coefficient 20
2.5.1 Size and Shape of the Grains in the Channel 20
2.5.3 Size and Shape of the Channel 21
2.5.4 Irregularities in the Channel 21
2.5.8 Silting and scouring of the channel 23
2.6.1 Spear Grass (Imperata cylindrica) 24
2.6.2 Guinea Grass (Panicum maximum) 25
2.6.3 Bahama grass (Cynodon dactylon) 27
2.7 Design of Channel for Uniform Flow.. 28
2.7.1 Geometric Shapes of Channel 28
2.7.2 Geometric Elements of the Channel 29
3.4 Field Clearing, Marking and Construction of the Channel 42
3.5 Experimental Procedures. 43
3.6 Determination of Channel Slope. 44
3.7 Determination of Velocity of flow.. 44
3.8 Determination of the Hydraulic Roughness Coefficient (n) of the selected Vegetated species. 45
3.9 Determination of Drag Coefficient of the Vegetation. 45
3.10 Determination of Reynolds Number of the selected Vegetation. 45
4.1 Manning’s Roughness Coefficient (n) of the Vegetation. 46
4.2 Relationship between Manning’s n and Degree of submergence (Y/H) 51
4.3 Relationship between Manning’s n and Reynolds Number Re 56
4.4 Relationship between Manning’s n and Vegetation Density. 62
4.5 Relationship between Manning’s n and Flow Depth (d) 66
4.6 Relationship between Drag Coefficient (Cd) and Reynolds number (Re) 70
CONCLUSION AND RECOMMENDATIONS. 75
SPEAR GRASS (Imperata cylindrica) 81
GUINEA GRASS (Panicum maximum) 82
BAHAMA GRASS (Cynodon dactylon) 83
CONTROL (WITHOUT VEGETATION) 84
Plate 1: Construction of trapezoidal channels in Agricultural and Bio-resources Engineering Experimental site ………………………………….……43
Plate 2: Transplanting Bahama grass (Cynodon dactylon) in 0.2% sloped channel …………………………………………86
Plate 3: Collecting the flow at the outlet of the channel at a fixed length of time (30 seconds) – Timed gravimetric method …………………………………87
Table 2.1: Recommended sideslopes for open channels …………………………33
Table 4.1: Manning’s roughness coefficient (n) of Spear grass (Imperata cylindrica) at stem heights of 0.060m, 0.054m and 0.054m in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% respectively …………………………46
Table 4.2: Manning’s roughness coefficient (n) of Guinea grass (Panicum maximum) at stem heights of 0.094m, 0.085m and 0.102m in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% respectively …………………………47
Table 4.3: Manning’s roughness coefficient (n) of Bahama grass (Cynodon dactylon) at stem heights of 0.029m, 0.026m and 0.032m in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% respectively ………………………….48
Table 4.4:Manning’s roughness coefficient (n) of Control (without grasses) in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% ………………..49
Table 4.5: Values of Manning’s n and Degree of submergence (Y/H) of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) at different flow depths in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% with varying stem heights respectively ………………………………………………………………………..51
Table 4.6: Values of Manning’s n and Reynolds Number (Re) and VR of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) and Control (without vegetation) at different flow depths in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% with varying stem heights respectively ……………………….58
Table 4.7: Values of Manning’s n and Vegetation Density (De) at constant flow depth of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% ……………………………62
Table 4.8: Values of flow depth and the Manning’s n of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), Bahama grass (Cynodon dactylon) and the Control (without vegetation) in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% ………………….…………………66
Table 4.9: Values of Reynolds number (Re) and Drag coefficient (Cd) of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% at an increasing flow depth ………………………..…….70
Figure 2.1: Spear grass (Imperata cylindrica) ……………………………….24
Figure 2.2: Guinea grass (Panicum maximum) ………………………………26
Figure 2.3: Bahama grass (Cynodon dactylon) ………………………………27
Figure 2.4: Channel cross-section notation and formulas ……………………31
Figure 2.5: Channel cross-sections showing the bottom width (b), top width (T) and depth of flow (y) ………………………………………………33
Figure 3.1:Satellite view of the Agricultural and Bio-resources Engineering Experimental site, located in Nnamdi Azikiwe University, Awka..37
Figure 3.2: Experimental Layout …………………………………………..….38
Figure 3.3: Sectional view of the channel ………………………………..…….40
Figure 3.4: Front view of the trapezoidal channel ……………………..………41
Figure 3.5: Isometric view of the trapezoidal channel ………………..………..42
Figure 4.1: Graph of Manning n versus Degree of submergence (Y/H) for Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) in 0.2%, 0.3% and 0.4% sloped trapezoidal channels ………………………………….……………56
Figure 4.2: Graph of Manning n versus Reynolds number and Manning n versus VR for Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) and Control (without vegetation) in 0.2%, 0.3% and 0.4% sloped trapezoidal channels…62
Figure 4.3: Graph of Manning’s n versus Vegetation density at constant flow depth of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% ……………………………………………..66
Figure 4.4: Graph of Manning’s n versus Flow depth of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), Bahama grass (Cynodon dactylon) and Control (without vegetation) in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% ……………………………………71
Figure 4.5: Graph of Drag coefficient (Cd) versus Reynolds number (Re) of Spear grass (Imperata cylindrica), Guinea grass (Panicum maximum), and Bahama grass (Cynodon dactylon) in trapezoidal channels sloped at 0.2%, 0.3% and 0.4% at an increasing flow depth ………………..75
The following symbols are used in this study:
= Frontal area of the vegetation (m2) the unit term
= Drag coefficient (m-1) = Projected Area (m2)
= 1.0 for units in meters and seconds
= Friction slope (L/L)
A= Cross sectional area (m2)
b = Bottom width (m) d = Depth of flow (m) = Side slope angle (o)
D = Depth of Channel (m) T or H = Height of vegetation (m)
d = design depth (L) z = side slope ratio (L/L) (horizontal/vertical expressed as z:1).
n1 = a correction factor for the effect of surface irregularities n2 = a value for variations in shape and size of the channel cross section, n3 = a value for obstructions, n4 = a value for vegetation and flow conditions, and m = a correction factor for meandering of the channel.
Q = Discharge of flow or flow rate (
Re = Reynolds number = fluid density (M/L3) S = Slope of energy line (ft./ft.) n = Coefficient of roughness (Manning’s n)
v = Kinematic viscosity of water (m2/s) R = hydraulic radius (L) µ = dynamic viscosity (kgm-1sec-1)
V = Mean velocity of flow (m/sec) g = Acceleration due to gravity (m2/sec) D = Hydraulic Depth
Y = Height of the inundated part of the vegetation
INTRODUCTION
1.1 Background of Study
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.
1.2 Statement of Problem
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.
1.3 Aim and Objectives of the Study
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.
1.4 Justification of Study
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.
1.5 Scope of the Study
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.
1.6 Significance of Study
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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