Chapter 1: introduction

5.5 Alternative Analysis of Project Layout

The Kabeli River has a number of potential HEP sites. Since the selection of the project location option is restricted within the geographic coordinates approved by DoED, the project location option is evaluated in terms of the project type and corresponding location of the layouts within the assigned geographical coordinates.Given the constraint of the geographical grid, run-of-the-river (ROR), peaking run-of-the-river (PROR) and storage projects options were considered for evaluation.

The storage project was rejected as the submergence area within the licensed geographical grid along the Kabeli River is small and could accommodate only a small volume of live storage to operate the project as a reservoir project.

Considering the ongoing power crisis, particularly in the dry season, run of the river cum peaking storage project was preferred against the ROR project. Even though, the loss of forest and agricultural lands was estimated higher for the PROR type project compared to the ROR, the option PROR was favored because it ensures peaking power production to meet the power crisis, though in a small way.

Based on the PROR concept; the location and the design of the various structural component layouts were analyzed. Geological factors and topographical conditions were considered while selecting the key structural sites and their respective layouts. In total, four alternative layouts were considered before the KAHEP, alternative IV, was finalized.

From the environmental perspective, this alternative was considered (i) the least damaging, (ii) minimizing private land acquisition, loss of forest area, wildlife habitat, number of affected families, (iii) having no displacement of population etc. Annex 5.1 presents the comparative assessment of the Alternative I (the worst alternative) with Alternative IV (the selected alternative) on environmental, social and technical aspects. The other alternatives (II and III) fall between the alternative I and IV. Section 5.6 below highlights on the location and designs of the various alternatives.

5.6 Location and Design Alternatives of Project Structures and Ancillary Facilities

Selection of location and design, particularly of the dam, powerhouse, settling basin, approach tunnel, surge shaft, penstock pipe, access roads, camps, and spoil disposal sites were considered and evaluated on the environmental grounds.

5.6.1 Dam Site

Alternative I (1998 Study): In this alternative, the diversion structure is proposed just downstream of a major bend in the Kabeli River. The right bank consists of a good rock but the left bank has colluviums deposits. Since the dam has to store peaking water, sealing and stability of the reservoir is questionable as it is unlikely to construct a guide wall to connect the rock face and the left bank. The colluviums may be prone to seepage and water logging. Water logging in the long run may jeopardize the stability of the reservoir. The immediate upstream of the dam is a wide flood plain and is suitable to propose a ponding reservoir. However, the full supply level of 569.2 m (as proposed previously) is lower by about 8 m in comparison to the Phawa Khola-Kabeli river confluence. If the dam is selected at this location, this head will remain untapped. The settling basin was proposed at the left bank of the Kabeli River on a steep hill toe which is almost entirely exposed to excavation. This area was selected in 1998 study by considering the good rock exposure suitable for tunnel inlet portal as well. However, the proposed settling basin requires hill toe cutting to locate the desander of about 3.000 m² surface areas. Enormous slope protection works are needed. The valleys are sharp with the depth of valley at least 10 m in cross-section. Additionally, the basin is in the vicinity to the Kabeli River and is not free from the risk of floods. Flood protection structures will be required. Therefore, this alternative analysis shows that the location of the settling basin is unfavorable and a better location should be explored.

Alternative II: This alternative has attempted to take advantage of rocks in both banks, to utilize the wide flood plain for the settling basin and to gain head. The diversion structure is proposed to shift about 400 m upstream of the dam axis of the Alternative I. This axis comprises of granite rock on both banks and the left bank immediately downstream of it is a wide flood plain suitable for placing the settling basin. However, the width has a negative side too; the axis is quite wide and the cost of headworks is likely to increase. Also, the river gradient in this location is locally gentler than the average river gradient of 1 in 90. Locating the dam axis in a steeper gradient would provide more room for the flushing head to flush settling basin for the same dam height than in the gentler gradient. Since, the Kabeli River is quite flat, this point remains vital in influencing the cost of the project.

Alternative III: The Alternative III is proposed as a modification of Alternative II in which the only difference is that the dam axis is moved upstream by about 250 m, and an inclined tunnel at the powerhouse side to serve as adit during construction and surge shaft during operation. This alternative, as the Alternative II, comprises of rock outcrops in both banks but the axis would be shorter by about 12 m. Also, the rock slopes are steep to prevent the increase in the dam length substantially as the height increases. The axis is at a bend of the river with the flow concentrated on the left bank (intake bank). Even though the bend effect may vanish after construction of the reservoir, the effect of the bend, if it prevails, should be favorable to the intake. Another prominent feature with this alternative is that the river gradient is locally steeper with about 1:60 gradient against 1:90 as the average river gradient. This gradient gives a gain of 2.5 m higher river bed as compared to the intermediate dam axis of the Alternative II. This head can be utilized for increasing the project head for energy production and flushing head for the settling basin. A dam with the same height as in the Alternative II would locate FSL at 575m thereby leaving only about 2 m of untapped head up to the Phawa confluence. The length effect, rock condition and gain in head are attractive features of this alternative in comparison to the Alternative II. This alternative, having a fairly wide dam axis, has a possibility for a surface diversion during construction if the dam is constructed in part by diverting the river on the next bank of construction. Also, if the diversion during construction is preferred through a diversion tunnel, this alternative is the best among all. The right bank of the river has granite exposure both up and downstream of the dam area and also comprises a curvature that makes possible to have a shorter diversion tunnel (360 m) than the de-watered portion of the river length (400 m).

Alternative IV: The Alternative IV is proposed as a modification to the Alternative III with the only difference being the change of the settling basin location from surface to underground and the necessary changes in the accompanying accessories. The headworks location at the upstream axis was concluded being the best among the available alternatives. Due to the flat river posing, the risk of the large amount of seepage water along the length of the coffer dam and a comparable cost of the coffer dam with that of the diversion tunnel, the diversion during construction was preferred for the tunnel option. A comparison of both surfaces versus the underground alternative was made. The underground option appeared to be cheaper, even though this option is associated with the risk of the little known underground rock mass. However, the risk of going underground will not be obviated even with the surface settling basin as the headrace tunnel in the same rock mass has to be constructed. Moreover, the settling basin was aligned with the favorable excavation direction in relation to the discontinuities and the risk of the Kabeli flood.

Environmental Comparison: The alternatives III and IV are same from the structure location aspect. The only difference between the two options is the configuration of the water way structure and desilting basin. Alternative III has a surface desilting basin while alternative IV proposes an underground desilting basin.

Out of four alternatives, the Alternative IV was selected based on the geological as well as environmental grounds. The alternative IV has advantages in locating the dam axis and in connecting waterway structures to favorable geological conditions to avoid the likely risks of land failure and stability of structures. Additionally, this alternative involves less damage to the forest land and the agricultural private lands compared to the alternative I, II and III due to underground location of the desilting basin.

Dam Design:

Among four alternatives of the dam design (concrete dam, rock-fill dam with concrete top and barrage), the barrage option is selected as the best alternative. Some of the environmental reasons for selecting the barrage type damming structure for KAHEP are:

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  easy mechanism for sediment sluicing,
  
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  best for reservoir longevity,
  
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  good for bed load exclusion,
  
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  least fluctuation in headwater levels as it can be controlled by the gates.
  

5.6.2 Powerhouse Site

Alternative I: The powerhouse in this option was proposed along the left bank of the Tamor River, at the corner of an apparently attractive flat terrace. The 1998 study selected this option and proposed to locate the powerhouse after removing the 20 m thick terrace deposit, although the extent of the excavation seems to be a big challenge. The location happened to be a remnant of an old landslide during geological investigations. Therefore, the area is vulnerable towards triggering slopefailures in response to a 20 m deep excavation along the slope.

Alternative II: The powerhouse in this option was proposed along the left bank of the Tamor River. The ridge downstream needed ridge protection against wash-outs during extreme floods. The ridge has outcrops of a phyllite rock in the upper part and a relatively flat terrace area in the lower part. This area, though has a risk from a small tributary called Piple Khola, was envisioned as a much safer location compared to the Tamor river flood risk. Piple Khola has a catchment of 5.4 km² and its risk could be viably controlled by constructing protection and retention structures. The benefit of this area for tailrace is that it can be drained out diagonally but not perpendicularly to Tamor River. The disturbance of Tamor River in excavation during construction of the tailrace will be minimized too. In this alternative, geological and topographical conditions were preferred to establish the powerhouse not directly on the bank facing Tamor River but on the ridge area against Tamor River.

Alternative III: Powerhouse location under Alternative III is the same as that of the Alternative II.

Alternative IV Powerhouse: The proposed powerhouse, as in the Alternative II, is a surface powerhouse located on the right bank of Piple Khola, Ward number 9 at Amarpur VDC, Panchthar district with about 93.10 m long tailrace canal following Piple Khola till Tamor River. The outside dimension of the powerhouse is 31.95 m long and 16.92 m wide. The excavated material will be used to construct the earthen bund at the Tamor side. In respect to the Tamor’s flood, the area lies on the leeward side of a phyllitic rock forming spur nose. The rocky spur nose can be effectively utilized as a flood protection structure. However, the area is adjacent to the flood plain of Piple Khola and needs flood protection structures.

Environmental Comparison: Alternative I was rejected as this alternative lies in the old landslide zone with potential risks of landslide rejuvenation on environmental grounds. Further, the site has a risk of floods from Tamor River. Alternative II, III, and IV has the same location and has the risk of Piple Khola erosion. The advantage of the site is that it is protected by the spur nose from the Tamor River flood. The difference in II, III and IV alternative is on the structural layout. With the proposed protection structures and the tailrace layout, the Alternative IV has the minimum risk of erosion from Piple Khola compared to the Alternatives II and III. Therefore, the Alternative IV is also selected based on the environmental grounds.

5.6.3 Approach Canal vs. Approach Tunnel

Regarding the approach canal and the approach tunnel options to link the settling basin to the intake, the tunnel option was selected considering the surface geological conditions of the canal option. Further, the choice of a canal vs. tunnel is related to the choice of the surface vs. underground settling basin. Since an underground settling basin was selected based on the geological stability grounds (refer section 5.6.4 below), the canal option was rejected.

Environmental Comparison: Environmentally, the tunnel option has the least risk of the ground stability compared to the canal option. Additionally, the tunnel option minimizes the land acquisition needs, affect on people and on the forest land area.

5.6.4 Alternatives for Settling Basin

There are two options for the settling basin: surface and underground settling basin. In case of a surface settling basin, a canal of 235m was required. It should be capable of carrying full discharge without sedimentation in the canal floor. The geological investigation of the site for the surface settling basin revealed the lack of bed rock up to 20m below the surface. So, a settling basin has to be constructed on alluvial deposits of Kabeli River which posed a great risk to the basin stability. The settling basin location is intersected by a small gulley named Tyapuje Kholsi. Kholsi can discharge only in monsoon but it is vulnerable for the whole structure because occasionally it brings huge debris during monsoon. So, for Kholsi protection a large protection wall is required, but in case of an underground settling basin, the site proposed for it contains granite of fairly good quality. Moreover, the settling basin was aligned to the favorable excavation direction in relation to the discontinuities. In addition, the risk of Kabeli flood, Kholsi debris, weak foundation and slope stability will be avoided.

For these reasons the underground settling basin was selected. In terms of number of settling basins, two potential options are studied: two or three basins. It t was found that the most appropriate option is two settling basins, factoring in the associated costs. The cost of three basins of 10.3 m width exceeded the cost of two basins of 15.8 m width. Hence, two basins of 15.8 m width were selected based on the financial grounds.

Environmental Comparison: The underground settling basin has the following environmental advantages against the surface alternative:

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  Minimize the private and forest land acquisition
  
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  Avoid the number of people affected
  
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  Avoid surface excavation and steep open cutting with the risks of land failure
  
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  Avoid additional protection structures
  
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  Avoid seepage water loss and basin structural stability
  

5.6.5 Penstock Pipe Alternatives

Regarding the penstock design, two options were evaluated: the exposed penstock pipe arrangement and the buried penstock pipe arrangement. In the exposed typet, four anchor blocks were fixed with spacing no more than approximately 56m. One bifurcation unit was also fixed near the powerhouse. In this option, as the tunnel outlet portal invert level is 535.75 m and the original ground level of that point is 567.009m, the huge amount of soil excavation took place while maintaining the final ground stability. For the buried penstock option with the same alignment and the same main anchor block configuration, the cost is cheaper than with the exposed option. The buried option also has a number of environmental advantages over the exposed one. Therefore, the buried option was selected.

Environmental Comparison: There are numerous advantages of the buried type penstock design against the exposed type:

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  It protects the penstock against adverse effects of temperature variations.
  
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  It protects the water from freezing due to the low air temperatures.
  
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  It protects the pipe from falling debris and trees.
  
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  It protects the pipe from tempering and vandalism.
  
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  It eliminates support piers.
  
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  Anchor blocks are not required if bends are small.
  
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  The landscape is not affected like in the surface exposed case.
  

5.6.7 Access Road to Headworks

Two alternative access road alignments to the headworks were identified that branch from the Mechi Highway at different chainages. The Alternative I is along the left bank of the river that branches from the highway at chainage 59+900 from Phidim. The Alternative II is along the hilly terrain that branches from the chainage 50+00 with approximate length of 7.4 km including a number of bends. The length of the Alternative I is comparatively less than that of the Alternative II. The alternative I will require opening of the new road corridor for its entire length, while the Alternative II is the existing motorable road along the edge of the forested area and only requires some upgrading. Also, the local communities of the area were demanding the existing road upgrades. Therefore, the Alternative II was selected for the access road to the headworks.

Environmental Comparison: The Alternative II has the following environmental advantages against the Alternative I:

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  Avoid acquisition of the prime agricultural land
  
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  Avoid economic displacement of additional households
  
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  Avoid agricultural production losses
  
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  Establish a project–people relationship as it meets the community request to upgrade the existing road.
  

5.6.8 Access Road to the Powerhouse

There were three options considered for the access road to the powerhouse: (i) Madibung Alternative, (ii) Panchami Alternative and (iii) Subhang Alternative. All alternatives connect the powerhouse site to Mechi Highway at different chainages. Among the three proposed alternatives, the Panchami Alternative II, starting from Bhanuchouk at chainage of 48+000 from Phidim, already had a motorable track opened by the communities, while other alternatives (I and III) will require opening of a new road corridor for the entire length. Further, there is a strong community demand for the existing road upgrades and the project will upgrade the total of 15 km if the Alternative II is selected as the project access road. Hence, the Panchami Alternative II was considered as the best alternative for the development based on the environmental grounds.

Environmental Comparison: Panchami Alternative II has the following environmental advantages compared to the Madibung Alternative I and Subhang Alternative Alternative III:

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  Avoid permanent acquisition of the forest and agricultural land
  
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  Avoid economic displacement of households
  
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  Avoid loss of standing forest vegetation and wildlife habitat
  
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  Avoid forest fragmentation
  
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  Avoid additional land degradation, erosion and landslides.
  
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  Establish project – people relationship as it meets the community demand for the road upgrading.
  

5.6.9 Alternatives to Camps at Powerhouse and Headwork

The ancillary facilities (temporary and permanent) could lead to the environmental degradation of the project area surroundings if the sites were not selected and fixed considering the local environmental conditions. To avoid the unforeseen environmental degradation and to ensure that the project contractors do not infringe on the local environment by making choices on the site selection to suite their benefits, he camp sites were selected and fixed for the project. The selection criteria for the camps (contractors, engineers and labors etc.) are primarily based on the followings:

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  Proximity to the worksites – the sites are as near to the construction work areas as possible, so the access to the construction sites is easy and within the walking distance;
  
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  The camp location does not interfere the construction works and also maintains safety of the individuals living at the camps;
  
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  The camp location should be at a distance from the local community residential areas;
  
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  The establishment of camps minimizes the loss of the standing forest vegetation and does not involve substantial landscape change; and
  
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  The camp location lies in a stable area free from landslide and flood risks.
  

Based on these criteria, the camp locations at powerhouse and headwork site were selected after the thorough field checks. The proposed camp sites meet the above criteria and the contractors will have to comply with the selected sites for the establishment of the camp facilities. The powerhouse camp site is located on the old alluvial fan of Piple Khola. On the ground of the observation made at the site from its geomorphic make up and the potential activities derived from the historical evidence of Piple Khola in the monsoon rainy season, the site is considered safe from the Piple Khola floods. For the headworks area, one temporary contractor/labor camp site will be located at Rajabesi village near the headworks area of Amarpur VDC.

5.6.10 Alternatives for Quarry, and Spoil Disposal Sites

Considering the availability of aggregates, there were two alternatives for the project. The first alternative was to mine the required aggregates from the bedrocks and the second alternative was to mine the aggregates from the flood plains of Kabeli and Tamor Rivers. The first alternative will involve direct impacts on the existing forest and agricultural lands, the main source of livelihood of the local people, whereas the second alternative will avoid such impacts on the land resource of public use but will have limitations on the mine operation in the rainy season. Based on the assessment, the second is the best alternative from the environmental and social stand point with a few measures to be implemented for the protection of the river water quality. The contractors will have to comply with the selected option and the sites, including with the mining methods for the aggregate extraction.

Considerations for the spoil disposal sites selection is based on the following criteria:

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  Avoid change of the productive land use;
  
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  Avoid erosion enhancement;
  
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  Maximize opportunities for the productive land development
  

In the local area context, the wide floods plains of the Kabeli and Tamor are the best suitable sites with measures to protect against the riverine erosion, and to plan for the productive land development. It is for this reasons that the elevated flood plain areas above the normal annual high flood level were selected for the spoil disposal and management. The contractors will have to comply with the selected sites for the spoil management.