ANALYSIS AND ASSESSMENT OF CULVERTS DISCHARGES ON THE PHYSICAL

ENVIRONMENT: - A CASE STUDY OF OMU-ARAN EXPRESS-WAY TO OKE-ONIGBIN TOWN IN IREPODUN LOCAL GOVERNMENT AREA OF KWARA STATE.

 GanaA.J.and BraimohO.S.

Department of Civil Engineering, College of Science and Engineering,

 Landmark University, Omu-Aran, Kwara state, Nigeria

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ABSTRACT

The express-way from Omu-Aran to Oke-Onigbin town is expressing a very high rate of soil erosion due to several causes. Most of the causes are of anthropogenic in origin; and one of this origin is the undertaking of expansion and improvement of the express-way infrastructure to which the two towns are subjected to. The principal objective of this study is to access the magnitude of the eroding effect of discharges from the express highway culverts between the two towns; with the formation of wide and deep gullies along the express-way. The study also assessed natural streams and longitudinal drains, and drowned final conclusion.

Key words: -Soil Erosion, Culverts discharges effect, Omu-Aran express-way to Oke-Onigbin town.

 

INTRODUCTION

The express way between Omu-Aran and Oke-Onigbin has been experiencing a significant environmental degradation for a period of time. Considering the case of soil erosion, natural as well as anthropogenic interventions have also contributed to the rapid rate of loss of naturalresources within the two towns. The provision of infrastructural facilities in term of small bridges and culverts between the two town on the express way have several times witnessed a non-functional behavior of run-off especially during rainy seasons, thereby making the road uncomfortable for vehicles and other transporters. It is for this reason that this analysis and assessment is initiated.

 

MATERIALS AND METHODS

In carry out the study, field trips to the sites and discussions with people within the twotowns were the major tools employed. In order to estimate the magnitude of culverts discharges during rainy seasons and soil erosion dimension of gullies, a detailed engineering analysis was also added: in addition, a preliminary economic evaluation was also made in order to estimate the loss by the farmers during rainy seasons.

 

Soil Erosion

Erosion of soil commences as a result of disturbance of thenatural state of the ground due to activities within the two towns like road construction. As the natural condition of the land begins to bemodified, it marks the beginning of a race between soil erosion and growth of vegetation. Activities during road construction can upset the delicate balance between stabilizing factors; such as vegetation and running water. The extent of vegetation collar in these towns directly proportional to the run off experienced.

     In the hydrological analysis, especially for drainage structures; it is recognized that there are many variable factors that affect floods. Some of the factors that need to be recognized and considered on the individual site-by-site basic are he following:-

  1. Rainfall amount and storm distribution.
  2. Catchment area size, slope and orientation.
  • Ground cover
  1. Slope of terrain and stream
  2. Storage potential (overbank,pounds,wet lands,reservoirs,channels,etc, and

 

  1. Catchment area development potential.

            One of the principal factors for the causes and aggravation of soil erosion along this express-way deforestation; which is due to the high intensity of rainfall every year coupled with high deforestation rate and susceptibility of the soil to erosion. The magnitude of soil erosion has increased at an alarming rate over the years and not much effort has been made to reverse the tend.

 

Road and Transversal Erosion

Surface hydrology along this express road reveals that the drainage pattern(erosion phenomenon) occurs in both the longitudinal as well as the transverse direction, side ditches convey water in the longitudinal direction until they meet at culverts inlets or changed into turnouts. In the transverse direction, water is discharged into the adjacent land either by the turnouts or by the cross drainages structures (i.e. culverts)

      In the conventional highway engineering, the environment impact of roads, especially erosion of adjacent land due to discharge from culverts has not been well treated in many parts of the world. Rather, highway engineers have been concerned more about protecting the pavement structure against the jeopardy; resulting from water. However,kadiyah (1997) made an attempt to include environmental considerations in highway designprocess and unlined the necessity of the preparation of environmental impact assessment (EIA), but at the end, it was discovered that is focus on environmental consideration included only noise pollution, Air pollution, Visual intrusion and defrauding the aesthetics and the community impact.

 

Observed gullies

All the observed in the cause of the study are by a trapezoidal cross – section and the trapezoidal sections in the table below:-

No

Station

Culvert type

Approximate Gully Size

Length

Average width,m

Area,m2

Average

Vol.or soil loss m3

1.     

8+ooo

Single pipe

10

0.5

5

0.5

2.5

2.     

13+000

slab

100

2.00

200

0.5

100

3.     

17+000

slab

70

3.00

210

1.0

210

4.     

19+000

Double pipe

180

5.00

900

2.0

1800

5.     

22+000

Signal pipe

100

2.00

200

0.5

100

6.     

26+000

Signal pipe

90

5.00

450

2.5

1125

7

32+000

Signal pipe

200

4.00

800

2.0

1600

8

36+000

Signal pipe

150

4.50

675

2.5

1687.5

      9

37+000

Signal pipe

150

5.00

750

3.0

2250

    10

38+000

Signal pipe

100

0.50

50

0.2

10

    11

45+000

slab

150

3.00

450

2.0

900

    12

48+000

slab

50

2.00

100

1.5

150

    13

61+000

Signal pipe

30

1.50

45

0.5

22,5

    14

72+000

Signal pipe

10

1.00

10

0.2

2

    15

80+000

slab

80

3.00

240

1.0

240

    16

83+000

Signal pip

40

0.80

32

0.4

12.8

    17

104+000

Triple pipe

200

10.00

2000

7.0

14000

 

 

Total

1710

 

7117

 

24212.3

 

 

 

 

 

 

 

 

 

 

As shown in the table above, the average length, width and depth of the gullies are 101.3 and 1.6 meters respectively and the average area and volume are 419m2 the total stretch (with about 120 culverts) of Alemgena-Butajira section:

    Hydrological design of culverts

The problem of soil erosion is not the only factor that has affected the express-way between the towns alone, the design and construction of culverts along the express-way also contributed to a greater extend. It was expected that the problem of erosion in relation to discharges from high way culverts be combined during design and construction; but it was omitted by the designed professionals.

The major factors contributing to the formation of gullies at downstream of culverts were observed at least to be following:

  • The land upstream of the culvert sites in most cases is not stabilized. The soil is loose, overgrazed or not covered by vegetation to lessen the amount of incoming flow.
  • No engineering or bioengineering measures are taken to reduce the amount or the speed of the approaching discharges (well before culvert inlets).the measures observed are only to protect the culvert inlets and are not extended to the upstream side.
  • No energy dissipaters of any kind are installed at the downstream side of the culvert for stretches beyond culvert outlets. The structures observed for only protect side against being undermined by the outflow.

 

CONCLUSION AND RECOMMENDATION

On the basic of the field survey methodology employed, the following conclusion and recommendation are made

 

  1. It is evident that as far as there exist significant amount of rainfall that is capable of erodibility, it is possible to significantly reduce the magnitude if proper and sustainable measures are taken by involving all the stake holders.
  2. An important past of the control of erosion is by velocity of flow. This may be achieved by directing the water over gentle, rather than steep slopes wherever possible
  • Other methods of controlling erosion includes using grass sod, mulching and the placement of gravel or stone particles two heavy to be picked up by the water flow.
  1. Culverts design:- to remedy the challenges of culvert design on the express-ways, standard methods for design should be adopted for the design construction.
  2. Some of the existing culverts are observed as inadequate to accommodate the flow. Therefore, the following consideration may be made during covert design procedures:-
  3. Adequate cross-section should be provided to accommodate the expected discharge.
  4. A comparison between the installation of either multiple culverts at a single site or separate single culvert at different sites should be made depending on the terrain and hydrological situation in the area.
  5. Culverts may be closely spaced to reduce individual discharges. What are highly concentrated into adjacent lands.
  6. The following culvert design methods are sui6able for areas like this express-ways and can be adopted:-
  7. The rational methods:-
  8. The SCS methods:-

The rational methods than 50 ha (0.5 km2) as expressed by the following formula (ERA, 2001a):

                             Q=0.00278CIA

Where:

Q=maximum rate of runoff, m3/s

C=runoff coefficient representing a ratio of runoff to rainfall. C values are given depending on the surface condition of the area under consideration. Other features of C are described in box 2.1.

 

I= average rainfall intensity for a duration equal to the time of concentration, for a selected return period, mm/hr

A=catchment area tributary to the design location, ha.

However, for less frequent and higher intensity storms, the rational formula is modified by introducing a frequency factors as follows.

                                                Q=0.00278CCfIA

Where:

Cf=frequency factor

Note: the product of C and Cf shall not exceed 1.0.

In box 2.1, the basic assumptions for the rational methods are stated.

The SCS methods

If the catchment area is greater than 50 ha, calculate the rates of runoff, the unique develop by the U.S. soil conservation service (SCS) is used. It requires the same basic data as the Rational Methods: catchment area, a rainfall. The SCS approach, however, is more sophisticated in that it considers also the time of distribution of rainfall, the initial rainfall losses to interception and depression storage, and an infiltration rate that decreases during the course of the storm.

The SCS runoff equation is therefore a method of estimating direct runoff from 24-hours or 1-day storm rainfall. The equation is (ERA, 2001a):

Where:

Q=accumulated direct runoff, mm

P=accumulated rainfall (potential maximum runoff), mm

Ia=initial abstraction including surface storage, interception, and infiltration prior to runoff, mm

S=potential maximum retention, mm.

 

The SCS runoff equation is therefore a method of estimating direct runoff from 24-hour or 1-day storm rainfall. The equation is (ERA, 2001a):

                             Q= (P-Ia) 2 / (P-Ia) + S     (2.3)

Where:

Q=accumulated direct runoff, mm

P=accumulated rainfall (potential maximum runoff), mm

Ia=initial abstraction including surface storage, interception, and infiltration prior to runoff,mm

S=potential maximum retention, mm

 

The relationship between Ia and S was developed from experimental catchment area data. It removes the necessity for estimating Ia for common usage, the empirical relationship used in the SCS runoff equation is (ERA, 2001a):

                             Ia=0.25                                               (2.4)

Substituting 0.2S for Ia in equation 3.3, the SCS rainfall-runoff equation becomes (ERA, 2001a)

                                      Q= (P-0.2S) 2/ (P+0.8S)       (2.5)

Number, CN. CN has a range of 0 to 100 and S is related to CN by (ERA, 2001a)

                                      S=1000/ (CN-10)                (2.6)

Curve number shall be selected only after a field inspection of the catchment area, a review of cover type and soil maps.

 

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(UNECA).(1982). United Nations economics commission for Africa.ROAD MAINTENANCE HANDBOOK practical guidelines for road maintenance in Africa volume 1 maintences road maintence of road maintence of roadside area. Drainage structure and traffic control devices (1stEd). Cologne. Carl duisberggesellschafte.V

 

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