Controlling soil erosion through determination of hydraulic roughness coefficient of different type of vegetation
ABSTRACT: An experimental study was conducted to analyze the effects of different types of vegetation (Bahama grass, spear grass, guinea grass) on the hydraulic roughness coefficient as a means of soil erosion control, n in an open channel and to develop relationships between the characteristics of the vegetation (density and degree of submergence), Reynolds number, drag coefficient and Manning’s, n. The experimental set up consist of twelve (12) trapezoidal shaped channels with dimensions of top width= 0.12m, depth of flow = 0.03m, bottom width = 0.03m. The effect of bed slope and flow depth on roughness coefficient (n) values was tested. The impact of the three vegetation on erosion control was examined. The maximum flow depth that was used is 0.05m and the time gravimetric method was used to measure flow velocity. The channel bed slope that was used to do the analysis include 0.2%, 0.3% and 0.4%. The manning equation was adopted in getting the value of (n). The results show that Manning’s, n, for flows with Bahama grass, spear grass, guinea grass increased with the decrease in flow depth for unsubmerged conditions studied. The Manning’s, n was also found to increase as the value of degree of submergence (Y/T) decreases. Moreover, the values of Manning’s, n decreased with the increase of Reynolds number, Re, for unsubmerged condition studied. A linear relationship was found between Manning’s, n and vegetation density for unsubmerged flow conditions studied. The value of drag coefficient was also found to decrease as the values of Reynolds number increases for all the three grasses studied. Bahama grass was found to have higher value of manning n when the bed slope, flow depth and stem height was constant followed by guinea grass and spear grass respectively.
TABLE OF CONTENT
Title page ii
Declaration iii
Approval page iv
Dedication v
Acknowledgement vi
Abstract vii
Table of Content viii
List of Figures xiv
List of Tables xvi
List of Plates xvii
Notations xviii
CHAPTER ONE: INTRODUCTION
- Background of Study 1
- Statement of Problem 3
- Aim and Objectives 3
- Justification 4
- Scope of Study 4
- Significance of Study 5
CHAPTER TWO: LITERATURE REVIEW
2.0 Introduction 6
2.1 Open Channel Flow 7
2.1.1 Classification of Open Channel Flow 8
2.1.1.1 Laminar and Turbulent Flow 8
2.1.1.2 Uniform and Non-Uniform Flow 9
2.1.1.3 Steady Flow and Unsteady Flow 9
2.1.1.4 Subcritical Flow, Critical Flow and Supercritical Flow 10
2.1.2 Channel Design 10
2.1.3 Channel Cross Sections 10
2.1.3.1 Side Slopes 11
2.1.3.2 Bottom Width (b) 12
2.1.3.3 Depth of flow (y) 12
2.1.3.4 Top width (T) 12
2.1.3.5 Wetted perimeter (P) 12
2.1.3.6 Wetted Area (A) 13
2.1.3.7 Hydraulic radius (R) 13
2.1.3.8 Hydraulic depth (D) 13
2.1.4 Channel Velocity and Tractive Force 13
2.1.5 Measurement of Flow Velocity 14
2.1.5.1 Doppler meters 14
2.1.5.2 Impeller Meters 14
2.1.5.3 Slope Area 14
2.1.5.4 Float Method 14
2.1.5.5 Weirs and Flumes 15
2.1.5.6 Timed Gravimetric 15
2.1.5.7 Tracer Dilution 15
2.2 Hydraulic Equations 15
2.3 Hydraulic Roughness Coefficient. 17
2.3.1 Methods for Determination of Hydraulic Roughness Coefficient 18
2.3.1.1 Storage methods (SCS method) 18
2.3.1.2 Table method 18
2.3.1.3 Photographic method 19
2.3.1.4. Empirical method 19
2.3.1.4.1 Cowan equation 19
2.3.1.4.2. Manning’s equation 20
2.3.1.4.3. Darcy-Weisbach equations 20
2.3.1.4.4. Chezy equation 20
2.3.2 Factors Affecting Hydraulic Roughness Coefficient: 21
2.3.2.1 Surface Roughness 21
2.3.2.2. Vegetation 21
2.3.2.3 Channel Irregularity 21
2.3.2.4 Channel Alignment 22
2.3.2.5. Silting and Scouring 22
2.3.2.6. Obstruction 22
2.3.2.7. Stage and Discharge 22
2.4. Vegetative Roughness 22
2.4.1. Drag Coefficient for Unsubmerged Vegetation 23
2.4.2. Drag Coefficient for Submerged Vegetation 25
2.4.3. Relationship Between Drag Coefficient of Vegetation and Manning Roughness Coefficient 26
2.4.4. Relationship Between Hydraulic Roughness Coefficient (N) Of Vegetation
and Reynold Number (Re) 27
2.4.5. Relationship Between Hydraulic Roughness Coefficient (n) and Flow Depth (d)28
2.4.6 Relationship Between Manning’s n and Degree of Submergence 29
2.4.7 Relationship Between Manning’s n and Vegetation Density 30
2.5 Vegetative Selection 30
2.5.1 Factors Influencing the Selection of the Three Vegetation (Grasses) In This
Study 30
2.5.1.1 Availability 31
2.5.1.2 Ease of Establishment 31
2.5.1.3 Spreading Ability 31
2.5.1.4 Improvement of Soil Fertility 31
2.5.1.5 Ability to Control Soil Erosion 31
2.6 Bahama Grass (Cynodon Dactylon) 31
2.6.1 Description of Bahama Grass 31
2.6.2 Morphology of Bahama Grass 32
2.6.3 Utilization of Bahama Grass 32
2.6.4 Distribution of Bahama Grass 32
2.6.5 Environmental Impact of Bahama Grass: 33
2.6.5.1 Soil Erosion Control, Reclamation and Cover Crop 33
2.7 Guinea Grass (Panicum Maximum) 33
2.7.1 Description of Guinea Grass 33
2.7.2 Morphology of Guinea Grass 34
2.7.3 Utilization of Guinea Grass 34
2.7.4 Distribution of Guinea Grass 34
2.7.5 Environmental Impact of Guinea Grass 35
2.8 Spear Grass (Imperata Cylindrical) 35
2.8.1 Description of Spear Grass 35
2.8.2 Utilization of Spear Grass 36
2.8.3 Distribution of Spear Grass 36
2.8.4 Environmental Impact of Spear Grass 37
2.8.4.1 Carbon Reservoir and desertification controller 37
2.8.4.2 Erosion Control 37
CHAPTER THREE: MATERIALS AND METHODS
3.1 Study Area 38
3.2 Experimental Layout 39
3.3 Channel Design 40
3.4 Designed Drawing: 42
3.5 Field Clearing, Marking and Construction of the Channel 42
3.6 Experimental Procedures 43
3.7 Determination of Channel Slope 44
3.8 Determination of Velocity of flow 44
3.9 Determination of the Hydraulic Roughness Coefficient (n) of the Selected
Vegetated Species 45
3.10 Determination of Drag Coefficient of the Vegetation 45
3.11 Determination of Reynold Number of the selected Vegetation 45
3.12 Data Analysis 45
CHAPTER FOUR: RESULT AND DISCUSSION
4.1 Data Presentation 46
4.2 Data Analysis 51
4.2.1 Variation of Hydraulic roughness coefficient (Manning n) with Reynolds number (Re) 51
4.2.2 Variation of Hydraulic roughness coefficient (Manning n) with Flow Depth54
4.2.3 Variation of Hydraulic roughness coefficient (Manning n) with Degree of Submergence (Y/T) 57
4.2.4 Variation of Hydraulic roughness coefficient (Manning n) with vegetation density (Dv) 59
4.2.5 Variation of Drag coefficient with Reynold number 61
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.1 Conclusion 64
5.2 Recommendation 64
References 66
Appendix I: Bahama Grass (Cynodon dactylon) 71
Appendix II: Guinea Grass (Panicum maximum) 72
Appendix III: Spear Grass (Imperata cylindrica) 73
Appendix IV: Control (Without vegetation) 74
Appendix V 75
Appendix VI 76
Appendix VII 77
LIST OF FIGURE
Figure 2.1: Force balance of unsubmerged vegetation 24
Figure 2.2: Force balance of submerged vegetation 25
Figure 2.3: Bahama grass (Cynodon dactylon) 31
Figure 2.4: Guinea Grass (Imperata Cylindrical) 34
Figure 2.5: Spear grass (Imperata cylindrica) 36
Figure 3.1 Satellite view of the experimental site 38
Figure 3.2: Experimental layout. 40
Figure 3.3: Sectional view of the channel 40
Figure 3.4: Front View of the Trapezoidal Channel 42
Figure 3.5: Isometric View of the Trapezoidal Channel 42
Figure 4.1a: Variation of Hydraulic roughness coefficient (Manning n) with Reynolds number (Re) for Bahama grass 52
Figure 4.1b: Variation of Hydraulic roughness coefficient (Manning n) with Reynolds number (Re) for Guinea grass 53
Figure 4.1c: Variation of Hydraulic roughness coefficient (Manning n) with Reynolds number (Re) for Spear grass 53
Figure 4.1d: Variation of Hydraulic roughness coefficient (Manning n) with Reynolds number (Re) for control (no vegetation). 54
Figure 4.2a: Variation of Hydraulic roughness coefficient (Manning n) with Flow Depth for Bahama grass 55
Figure 4.2b: Variation of Hydraulic roughness coefficient (Manning n) with Flow Depth for Guinea grass 55
Figure 4.2c: Variation of Hydraulic roughness coefficient (Manning n) with Flow Depth for Spear grass 56
Figure 4.2d: Variation of Hydraulic roughness coefficient (Manning n) with Flow Depth for control (no vegetation). 56
Figure 4.3a: Variation of Hydraulic roughness coefficient (Manning n) with Degree of Submergence (Y/T) for Bahama grass 58
Figure 4.3b: Variation of Hydraulic roughness coefficient (Manning n) with Degree of Submergence (Y/T) for Guinea grass 58
Figure 4.3c: Variation of Hydraulic roughness coefficient (Manning n) with Degree of Submergence (Y/T) for Spear grass 59
Figure 4.4a: Variation of Hydraulic roughness coefficient (Manning n) with vegetation density (Dv) for Bahama grass 60
Figure 4.4b: Variation of Hydraulic roughness coefficient (Manning n) with vegetation density (Dv) for Guinea grass 61
Figure 4.4c: Variation of Hydraulic roughness coefficient (Manning n) with vegetation density (Dv) for Spear grass 61
Figure 4.5a: Variation of Drag coefficient with Reynold number for Bahama grass 62
Figure 4.5b: Variation of Drag coefficient with Reynold number for Guinea grass 63
Figure 4.5c: Variation of Drag coefficient with Reynold number for Spear grass 63
LIST OF TABLE
Table 2.1: Recommended side slope for open channels 12
Table 4.1a: Experimental data on hydraulic roughness coefficient of Bahama grass 47
Table 4.1b: Experimental data on hydraulic roughness coefficient of Guinea grass 48
Table 4.1c: Experimental data on hydraulic roughness coefficient of Spear grass 49
Table 4.1d: Experimental data on hydraulic roughness coefficient of Control (No vegetation) 50
LIST OF PLATE
Plate I: Channel setup 43
Plate II: Measurement of Stem Height 75
Plate III: Measurement of Depth of Flow at The Control Channel 76
Plate IV: Measuring the Discharge at The Downstream 77
NOTATION
Symbol Description SI unit
Re Reynolds number –
Fluid density kg/
v Velocity m/s
R Hydraulic radius m
µ Dynamic viscosity Ns/m2
Kinematic viscosity m2/s
Fr Froude number –
V Mean velocity of flow m/s
g Acceleration due to gravity m/
D Hydraulic Depth m
b Bottom width m
T Top width m
d Depth of flow m
side slope angle
S Average slope m/m
V Flow velocity m/s
A Cross-sectional flow area m2
P Wetted perimeter m
n Manning hydraulic Roughness coefficient –
f Darcy-Weisbach hydraulic roughness coefficient –
A base value of n for a straight, uniform, smooth channel
in natural materials –
A correction factor for effect of surface irregularities –
A value for obstruction –
A value for vegetation and flow condition –
m A correction for meandering of the channel –
C Chezy’s coefficient m1/2/s
Bottom slope m/m
FD Drag force exerted on the vegetation N
FG Gravitational force N
FS Surface friction of the side wall and bottom –
CD Drag coefficient –
L Vegetated reach m
T Vegetation height m
Vegetal areas coefficient representing the area fraction
per unit length of channel –
AL Total frontal area of vegetation in the channel reach L –
T Height of the vegetation m
Ap Projected area m2
Frontal area of the vegetation m2
The unit’s term –
l Side length m
z side slope –
Y/T Degree of submergence –
CHAPTER ONE
INTRODUCTION
1.1 Background of Study
Approximately 40% of the world agricultural land is seriously degraded (Suresh, 2013). According to the survey report, an area of fertile soil equal to the size of Ukraine (233062 sq. mi) is lost every year because of draught, deforestation and climatic change. In Africa, if the current trends of soil degradation are continued, then the continent might be able to feed only 25% of its population by 2025 (Suresh, 2013).
Hydraulic roughness coefficient is a critical parameter reflecting soil erosion and runoff process and is influenced by many factors. During tillage, water erosion process on cultivated lands are often affected by soil micro-relief features (Zi-cheng Zheng et al, 2012).
Soil erosion is a process of detachment, transportation, and deposition of soil particles by wind or water. Soil erosion is an environmental hazard usually associated with agriculture in many parts of the world (Ogunlela and Makanjuola, 2000). In Nigeria, especially south eastern part of the nation, water is more prevalent cause of erosion than wind. Water erosion is seen as a function of the erosivity of the rain and erodibility of the soil. Erosivity is the potential ability of rain to cause erosion process. The erosivity of rainfall is the input force required to detach soil particle. Soil erodibility is the vulnerability or susceptibility of the soil to erosion.
According to Ogunlela and Makanjuola (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 vegetative lining. Water erosion decreases exponentially with increased vegetation root density. The soil cover is made up of vegetation either in live form or in mulch form or by putting impervious materials such as stones etc. Among all means of erosion control, vegetation provides a most suitable and economical cover on the ground surface to reduce water erosion. The soil without adequate vegetative cover are extremely exposed to degradation by the sheet erosion.
Vegetation offers resistance to flow, a property referred to as hydraulic roughness coefficient. This resistance depends on the vegetation characteristics such as vegetation specie, distribution, flexibility, degree of submergence, density and as well as flow characteristics including flow area, depth, and boundary characteristics. Most times, this vegetation is seen growing naturally on the river banks or the beds of flow channels or where it is being planted by man. It is classified by its shape and location where it grows. The vegetation growing in a channel consists of aquatic plant which according to Manal et al (2006), may be divided into four categories; emergent, submergent, floating-leaf, and free floating vegetation in a river channel or agricultural land. These provide both benefits and problems. From an environmental point of view, aquatic plants are essential parts of natural aquatic systems and form the basis of waterbody’s health and productivity. However, from engineering point of view, vegetation can improve the strength of stream/river bank materials, soil through buttressing and roof enforcement (Manal et al, 2006). The excessive growth of vegetation results to retardation, that is reduction in hydraulic flow or capacity and flooding. Thus, the capacity of flow can be increased by total or incomplete removal of vegetation which may lead to erosion of agricultural land, decrease infiltration thereby reduces the soil water and increases sediment load carried by flowing water. It is of importance to know that when growth of vegetation in a channel or agricultural land is not restricted, it will lead to a total loss of hydraulic capacity.
The roughness coefficient of a vegetation is the degree of the roughness of a vegetation and it is defined as the extent of its resistance or retardance of flow of water. The hydraulic roughness coefficient of an open channel is widely dependent on the hydraulic and flow parameters of the channel.
The roughness coefficient varies with the type of vegetation and for a particular plant, it varies with flow depth, slope, and shape of a channel (Ogunlela and Makonjula, 2000).
Zic et al (2009) showed that, the determination of the roughness coefficient tends to be complex in the hydraulic open channels flow. Its determination requires the knowledge of hydrology, statistics, hydromechanics, hydraulics, geology, and mechanics (Zic et al, 2009).
- Statement of Problem
In Nigeria, especially south eastern part of the country with (rain forest vegetation) water erosion has been limiting agricultural production as result of land degradation. Most of the cultivated or arable land has been degraded by action of water erosion. In most areas, rain storms increase in both frequency and intensity giving rise to more runoff and less infiltration into the soil due to the fact that the land is bare or disturbed by human activities. Thus predisposing the area to water erosion.
In some areas where the vegetation growth is restricted or limited, there is an increase rate of sediment load carried by flowing water and flooding because the velocity of flow is not disturbed, the water table is lowered; the quantity of water available for entering into the soil is decreased thus reducing the supply of water to replenish the groundwater wells and their yields are also reduced (Onwualu et al, 2006).
Most of the vegetation species that are within the reach of resources farmers in Africa especially in Nigeria that can be used to resist the flow of water and increase infiltration thereby reducing water erosion have not been studied and evaluated to determine their hydraulic roughness coefficient.
- Aim and Objectives
The aim of this project is to determine the hydraulic roughness coefficient of some selected vegetated species as means of controlling soil erosion.
The specific objectives are;
- To determine the hydraulic roughness coefficient of some selected vegetation specie.
- To identify the best flow resisting vegetation which can be used to control soil erosion
- To evaluate, analyse and compare the hydraulic roughness coefficient of vegetation determined with respect to soil erosion control.
- To develop a relationship between depth of flow, drag coefficient, Reynold number, degree of submergence, vegetation density and the hydraulic roughness coefficient.
- Justification
The small farmers in the country contribute about 90% of the farmer nation’s food production (FMARD, 2011). Most of these farmers cultivate land that have been degraded or destroyed by water erosion. The land available for commercial agriculture is no longer adequate to produce and satisfy the national food demand.
Most of the research works have been carried out to determine the flow retardance of the plants used in vegetal waterways. This has however, received little attention in Nigeria and other parts of Africa (Ogunlela and Makanjuola, 2000).
Other reasons that make vegetal control more desirable is its low initial cost, less skill requirement in designing and construction, ability to multiply over the years and aesthetic advantages (Temple, 1987).
For these reasons the need to determine and evaluate the hydraulic roughness coefficient of some African grasses become imperative.
- Scope of Study
This research work is being geared towards determining the hydraulic roughness coefficient (n) of some vegetation which are easily available in south eastern part of Nigeria.
- 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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