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DETERMINATION OF HYDRAULIC ROUGHNESS COEFFICIENT OF SOME VEGETATED SPECIES

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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).

 

 

1.2      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.

 

1.3      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;

  1. To determine the hydraulic roughness coefficient of some selected vegetation specie ii.  To identify the best flow resisting vegetation which can be used to control soil erosion

iii. To evaluate, analyse and compare the hydraulic roughness coefficient of vegetation determined with respect to soil erosion control

  1. To develop a relationship between depth of flow, drag coefficient, Reynold number, degree of submergence, vegetation density and the hydraulic roughness coefficient.

1.4 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.

 

1.5 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.

 

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.

 

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;

  1. To determine the hydraulic roughness coefficient of some selected vegetation specie.
  2.   To identify the best flow resisting vegetation which can be used to control soil erosion
  3. To evaluate, analyse and compare the hydraulic roughness coefficient of vegetation determined with respect to soil erosion control.
  4. 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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