Deforestation in power line construction in the Central African Region

Power transmission lines in forest regions like the southern parts of Cameroon are subjected to numerous failures arising from natural hazards, including earth faults and line ruptures provoked by swaying or falling neighbouring trees and their branches. To pre-empt this problem, those trees which represent a potential threat to the operation of the power line must be eliminated. Deforestation during line construction phase therefore becomes inevitable, and this leads in turn to the destruction of flora and fauna. The need hence arises to implement a deforestation strategy during power line construction which limits the negative impact of loss of forestry and wildlife resources on the environment to an acceptable level. In this paper a method is proposed which limits the level of destruction of vegetation and respects modern environmental standards during the construction of power lines through dense forest regions. It is shown that the required right-of-way depends on the quantity of power to be transmitted, on the voltage level chosen for the transmission, on the type of accessories used for the line construction and on the relief of the line track. Consequently, the relevant parameters for deforestation have been identified, listed and analysed. This leads to a good overview of the required deforestation level in the design and realisation of a power transmission line. The environmental impact assessment of transmission line projects can hence be better quantified and compared in aspects that relate to the protection of trees in the fight against global warming and desertification.

run through dense protected forest areas.Apart from the fact that dense forest vegetation renders access to the installations and maintenance work extremely difficult, recent statistics show that over 80% of supply disturbances on power transmission lines are singlephase line-to-earth faults provoked by felling of trees or sawing of tree branches (Désiré, 2004).These provoke short circuits and cause line ruptures.It therefore becomes indispensable for supply reliability to get rid of trees along the power line which represent a potential danger to it.The trade-off is of course the destruction of flora, and possibly fauna too.
Fragmentation is a severe threat to tropical rainforests.However the habitat loss and less extensive fragmentation caused by roads can also be a threat, not only through allowing access to remote areas, but also through a series of insidious associated impacts.These include abiotic and biotic edge effects adjacent to road clearings, the disturbance impacts caused by vehicle operation, invasion by weeds, feral and alien fauna and disease, and faunal mortality from vehicle collisions.
In combination, these can create a significant barrier to movements of rainforest biota.Impacts can be ameliorated through clever road design and sustainable vehicle operation (Goosem, 2007).
A compromise needs therefore to be sought that takes care of the aspects of improved reliability in power supply, while also limiting the destruction of the environment to acceptable modern standards.Using objective and quantitative methods to circumscribe the exact minimum of unavoidable deforestation is the subject of analysis of this paper.
In the first part, a survey of the complete deforestation process is done.The term deforestation is defined, and deforestation techniques are presented.The factors influencing these techniques are treated.The actual reasons for tree-felling are dealt with and a general formulation derived from there to serve as a rule for the establishment of a security corridor during deforestation for line construction.
An analysis of the relief of the zone allows the proper specification of the needed security corridor, even for areas with difficult landscape.

METHODS AND ORGANISATION OF THE ACTIVITY OF DEFORESTATION IN POWER LINE CONSTRUCTION
In Laurance et al. (2009), linear infrastructure such as roads, highways, power lines and gas lines are omnipresent features of human activity and are rapidly expanding in the tropics.Tropical species are especially vulnerable to such infrastructure because they include many ecological specialists that avoid even narrow (<30m wide) clearings and forest edges, as well as other species that are susceptible to road kill, predation or hunting by humans near roads.In addition, roads have a major role in opening up forested tropical regions to destructive colonization and exploitation.
Deforestation is the act of felling of trees over a large area as could, for example, be needed for the right-ofway of a long power transmission line through the equatorial forest.This can be done in an intuitive manner on site or could be conceived to follow a pre-defined pattern.Whatever the case, both technical and environmental constraints must be given focused attention.The technical constraints include transmissible power, voltage level and the accessories used.Environmental constraints in their turn should include existing settlements, their cultures, river paths, track landscape and community construction standards.In order to undertake a systematic description of the line construction track and scientifically formulate and specify the "level of deforestation", the following part-corridors are defined as shown in Figure 1: A d : Deforestation Reference Axis or Power Line Axis (This coincides with the lengthwise centre of the transmission line).L c : Central Lane (The central lane is the corridor in which the power line will actually be positioned.The vegetation here must be reduced to less than 20 cm of height to permit the easy unraveling of conductors during their laying).C: Main Track Corridor (This is the central lane plus the flanking security zones.This area should have vegetation of not more than 30 cm of height to allow for the free movement of construction equipment).L d : Deforestation Limit (The vegetation can be allowed to be more than 30 cm of height beyond this area).n: Out-of-corridor tree-distance to deforestation limit (The out-of-corridor area is the one beyond the deforestation limit).Any tree within this zone that is considered dangerous for the line operation must be cut down).Z s : Flanking security zones (A security margin to be determined based on voltage level).L s : Main Corridor Security Margin.d a : Average Distance between an out-of-corridor tree and the power line axis.h c : Maximum out-of-corridor tree-height.
The determination of the sizes of the various corridors defined above depends firstly on the voltage level, which prescribes the accessories to be used and is in turn chosen based on the transmissible power, and secondly on whether the line is single-phase or three-phase.Generally in Cameroon, the single-phase systems are medium-voltage lines used for primary distribution and running through rural areas, while the three-phase systems are for transmission voltages above 90 kV that are used to transport bulk power to urban areas.Practical measures to reduce the negative impacts of roads and other linear infrastructure on tropical species are also highlighted (Laurance et al., 2009).

USING THE LINE REACTANCE TO DETERMINE THE CENTRAL LANE
The design of every transmission line requires that the quantity of power to be should first be specified.For short and medium-length transmission paths, the line resistance is small and often neglected compared with the line reactance.The power that can be transported over a lossless transmission line is given by the following formula (Désiré, 2004) and Lavanchy (1952): Where: x is the reactance of the transmission line θ is the transmission angle The maximum transmissible power is therefore inversely proportional to the reactance of the line only, when the sending-end and receiving-end voltages are fixed.Hence, the reactance of a given line can be used as a guiding parameter for the determination of the right-ofway.
Considering, for example, that the specific linear inductive resistance of an overhead electric line can be calculated using the following formula (Levêque, 1996a): Where: D m is the average distance between conductors in mm d is the diameter of conductor in mm  r is the magnetic permeability of the conductor material And that: For a given conductor length and known specific linear reactance of the conductor material, the line reactance can be calculated as (Levêque, 1996b): Where: x 0 is the specific linear reactance in /km; l is the length of conductor of an electric line.
Then the reactance of each power transmission line can be seen to depend on the ratio of conductor-spacing to conductor-diameter, as well as the length of the transmission line.The former determines the width of the right-of-way, while the latter is responsible for the length of the right-of-way.For a given maximum transmissible power and voltage level, the line reactance can be quickly determined.From the line length and conductor diameter d, the conductor spacing D is also determined.
Hence the central lane L c defined earlier is known.Treefelling must be within this L c area.
The central lane will also vary depending on the layout of the conductors.In practice, the average distance between the conductors is fixed by the type of accessories chosen and how the conductors are suspended on these accessories.The area covered by the suspension accessories is largest and leads to maximum destruction of vegetation, when the conductors are displayed horizontally as shown in Figure 2.  (Lavanchy, 1952;AES-SONEL, 2004a).

Conductor layout
Average distance Depending on how the conductors are placed in space, their average distance can be determined using the formulas in Table 1.

AIM OF DEFORESTATION AND ITS PROCEDURES
Generally in power line construction, the aim of deforestation is to secure the future energy transportation from the generation site to the consumption centers at the lowest possible cost.This entails: i) Keeping the central lane free for movement of equipment; ii) Protecting the central lane against any possible tree growth; iii) Minimizing the number of times tree-cutting under the power line is carried out during operation.
Such interventions would require qualified human resources and appropriate equipment, thereby leading to increased running costs.

Deforestation parameters
The following formulas can be used to predict the total right-of-way to be created by the deforestation exercise: Hence, the entire deforestation surface can be simply Roger et al. 269 calculated by multiplying the determined right-of-way by the length of the transmission line, as follows: L is the length of the entire transmission line.This gives a handy figure for the environmental destruction causable by the line construction and is a basis for the comparison of similar projects in their environmental impact assessment studies.
Below are parameters used in calculations to determine the choice of the way the deforestation can be carried out: A d is the Deforestation Reference Axis or Power Line Axis L c is the Central Lane C is the Main Track Corridor L d is the Deforestation Limit n is the Out-of-corridor tree-distance to deforestation limit Z s is the Flanking security zones L s is the Main Corridor Security Margin d a is the Average Distance between an out-of-corridor tree and the power line axis h c is the Maximum out-of-corridor tree-height.

Deforestation equation
After deforestation works, it can be asserted that the energy transport line is viable and secured, as the cutting of trees represents no danger anymore.This security is assured by the deforestation equation (D.E.), which is given by the following relation (Désiré, 2004;AES-SONEL, 2004a, b).
Where: h a is the dangerous out-corridor tree height d a is the average distance between an out-corridor tree and the power line axis C is the main track corridor.
Considering that some out-corridor trees are grouped while others are dispersed, then the average distance between an out-corridor tree and the power line axis can be calculated as: d aep is the average distance of dispersed trees; d ar is the average distance of grouped trees.From Equation 7, it is seen that all the maintained outcorridor trees considered not to be dangerous have a height less than the average distance of an out-corridor tree, increased by half of the main track corridor width.

Eccentricity
When the electric power transmission lines pass through an inclined terrain, it is necessary to take the eccentricity into account.Eccentricity here refers to the movement, due to gravity, of the tree stem from its stump when falling down a slope.In this case, a safety margin (sm) is introduced, such that sm є] 0, 3], in meters.
For such an inclined terrain as shown in Figure 3, D.E. becomes: Where cosθ is the angle of the inclined plane.This formula exposes the fact that the maximum outcorridor tree height h c is bigger for the inclined plane than for level land because of the introduction of the safety margin (sm).Also, trees at the top side of the inclined plane must be subjected to very regular controls to ensure that they do not grow to a height bigger than h a as given in Equation 9.Such trees must be felled, if they attain the height h a value given in Equation 9. Deforestation on an inclined plane is therefore more than would be necessary on normal level land.
Figure 4 shows the dependence of the maximum outcorridor tree height (h a ) on the slope of the inclined plane (cosθ).As an example, the average distance d a between an out-corridor tree and the power line axis is taken here to be 12 m, while the corridor width C is in turn taken to be 20 m.This curve exposes qualitatively that, as the inclination of the slope of the terrain increases, that is, cos decreases, the dangerous out-corridor tree height also increases rapidly.
These interrelationships have been furthermore applied to the case of a 30-kV line with a main track corridor C of 40 m and a central lane of 8 m.The out-corridor in this case is unlimited.Also there are some grouped trees at a distance of 34 m while some dispersed trees are at a distance of 40 m.Applying Equation 8, the average distance between an out-corridor tree and the central axis of the power line will be 37 m.In this case, the height of an out-corridor tree has to be less than 17 m (that is, applying Equation 7).All out-corridor trees of height 17 m or greater are dangerous to the power line, and must be cut down.The value of the security zone of such a power line is therefore 16 m.
Rainforest species sometimes use regrowth connections along gullies to cross the powerline corridor.Mitigation of the fragmentation effects caused by powerline grassy swathes can best be achieved by strengthening extant canopy connections in regrowth gullies, and by establishing new connections across the clearings (Goosem and Marsh, 1997).

CONCLUSION
A direct interdependence between the right-of-way of a projected power transmission line and the maximum transmissible power has been established.This permits the design Engineer to quantify the area of forest and vegetation to be destroyed by a power transmission undertaking.Hence a tool is provided for comparative environmental impact assessment studies, which today constitute a compulsory part of multilaterally funded power line construction projects.
Furthermore, a direct relationship between the transmission voltage level and the height of out-corridor trees to be felled has been given as a mathematical formula.This permits a quick identification and assessment of the trees in a project area that need to be felled.This study reveals in parallel that trees standing on an inclined plane need a wider security margin away from the power line so that falling trees do not temper with power system operation.
It is finally evident from the results obtained that this tool helps to reduce the destroyable forest area, since forest destruction no longer needs to be arbitrary and disorderly.
In agreement with the work of Laurance et al. (2009), tropical forests mainly exist in developing nations like those in the Central African Region, which are being transformed by ongoing industrialization, population growth and natural-resource exploitation.Electricity transmission lines, roads and other linear infrastructure are rapidly expanding in many of these countries and have a key role in opening up forests to hunting, illegal mining, land speculation and destructive exploitation.The problem is usually aggravated by the kind of attention given by the governments and other stake holders, to the environmental impact assessments conducted before many huge electrification projects.

Figure 1 .Figure 2 .
Figure 1.Areal view of the line construction path.

Figure 3 .Figure 4 .
Figure 3. Areal view of the security corridor of the inclined terrain.

Table 1 .
Average distance between conductors