A Case on Planning and Operation of a Small Hydro Power Project With Minimum Negative Effects to the...

8/10/2019 A Case on Planning and Operation of a Small Hydro Power Project With Minimum Negative Effects to the Environ

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A Case on Planning and Operation of a Small Hydro Power Project

with Minimum Negative Effects to the Environment

Samarakoon Banda. (B.Sc.( Eng.) Hons, P.G.Dip.( H&T Eng.), C. Eng., MIE(SL) )

VS Consulting (Pvt) Ltd, No. 171-5/1, Fourth Floor, Keen Square

Kensey Road, Colombo-0 8, Sri Lanka.

E-mail: [email protected]

Abstract

Bulathwatha small hydropower project at Haldumulla in Badulla district in Sri Lanka is a run-of-the-river small

hydropower scheme commissioned in May 2013. It has a generation capacity of 3.8 MW. The expected annual

energy generation is 12.6 GWh. The electricity generated is fed to the national grid. The project covers a length of

2.1 km from intake to the powerhouse. Lands within 1.6 km of project length were under the control of Forest

department of Sri Lanka and remaining lands were owned by the villagers. The approval for the project had been

granted and the lands were released by the Forest Department subject to several conditions applicable to construction

and operations . The construction had to be carried out with minimum damage to the forest. Removal of old, giant

trees were not permitted. The water way trace (head race canal & penstock) to the power house had to pass through

a village and many agricultural lands. It was necessary to carry out the construction works without any adverse impact

on the livelihood of the villagers and with bare minimum damage to the agricultural lands. Further, there were many

wild animals in the project area due to existence of the forest reserve. Therefore, project planning was done giving

due consideration to these social and environment elements, specifying mitigation measures and monitoring plans.

This paper describes how the project planning and selection of structures were made to minimise the negative

environmental effects during the construction and operation stages of Bulathwatha small hydro power project.

1. Introduction

Hydropower is certainly the largest, most mature application of renewable energy. Based on the generation capacity,

hydropower projects are classified as Large, Medium, Small, Mini or Micro. Projects within the range of 500 kW and

10 MW are considered as Small Hydropower Projects (SHPs) in Sri Lanka. There is a trend in developing more Small

hydro schemes, as it is an environment friendly source of energy.

Most of the SHPs are run-of-the river schemes, which means that the turbine operates only when water is available

in the river. The river inflow is not regulated using a reservoir. Even though an SHP is a clean environment friendly

source of energy, there are many negative environmental impact during construction of a SHPs. The environmental

effects of an SHP are unique to the project. In some cases, very attractive SHPs cannot be implemented due to

environmental considerations. Hence, the environmental sensitivity of a SHP project have to be properly identified by

the approval agencies, stakeholders, environmentalists and the design engineers. The project approval can be given

with conditions to minimise negative impacts to the environment rather than abandon the project in the name of

negative environmental impacts.

The negative impacts to the environment can be minimized while implementing a project, by identifying the negative

impacts and incorporating them in the project designs, construction & maintenance. The SHPs project design


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engineers have a major role to play in evaluation of various structural types & selection of the most suitable structural

types to meet engineering & environmental challenges.

2. The main structures of a Small Hydro Power (SHP) project

The main structures of a small hydro power project and their functions are given below . The figure 1.0 depicts a

layout of a small hydro power project.

Weir a structure built across the river to divert water in to the intake

Intake a structure built at one of the banks to abstract the water in to the Head race canal

Head race canal or pipe a structure built to deliver water from the Intake to the Forebay tank with a

minimum head loss

De-sand tank a structure built to remove the sand particles which are harmful

to the turbine runner, and constructed in the HRC path at a suitable location.

Forebay tank a structure built to store water and serves as the intake to the penstock

Spill way a structure built from the Forebay tank up to a nearest water way to direct water coming in when

there is no power production Penstock - delivers water from the Forebay tank to the Turbine in the power house

Power house Houses the turbines & generators

Tail race delivers water from the power house back to the river

Fig.1.0

3. Planning stage of a small hydro power project

At the planning stage of a hydro power project, the following documents or information are having significantimportance to carry out the layouts of the project and the structural engineering designs to minimize the negativeeffects to the environment. The main set of documents can be named as,


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a. Environment approval conditionsThe impacts of the project to the environment are discussed at the environmental investigation report. Thenegative effects of the project are identified during the environmental study stage. Conditions related tomitigation of those negative effects are given in the project approval letter as conditions to be fulfilled by theproject proponent. The planning should be done to meet the conditions given in the approval letter.

b. Flood volumes

The design flood volumes at the weir and the power house are necessary in planning and designing the weir.The food levels of the river shall not be changed after construction of the weir across the river to a level whichaffects the houses, river crossings etc. In order to determine the flood level increase at the weir and the powerhouse after construction, a hydrology report shall be obtained from a hydrologist, which gives the 100 yearand 1000 year flood volumes.

c. Project area detailed survey plan with contours

To plan a small hydropower project, a detailed contour survey is essential. The detailed contour plan canindicate the ground levels as well as the existing structures, road network, locations and objects which shouldbe protected or avoided during implementation stage of the project. Further, the detailed contour survey plancan be used to develop a project lay out with optimum cost.

d. Geological investigation report

Excavation of slopes would be required to build the structures of the project. Also, the weir has to be

constructed on the river bed across the river. It is necessary to safeguard the structures against water

seepage, settlements, slope stability and foundation failures. In order to determine the slope stability and

strength of the soil or rock at the foundation level of the structures, geology of the project area shall be studied

and the findings of the geological investigation shall be presented in a geology report. Recommendations in

the geology report shall be passed to the structural design engineer to have an optimal design solution.

e. Construction material availability and access roads

Availability of construction materials in close proximity to a site can reduce the cost of construction. Also, the

access roads to access the project, head race canal, penstock and other structures locations are necessary. If

they are already present, the construction cost and the environmental effects due to new roads construction

can be reduced. Hence, the detailed land survey shall be carried out to pick the road network in the project

area or to plan the new road network to be used during construction.

3.1 Planning of Bulathwatha SHP project - weir and intake

The project approval conditions of Forest Department of Sri Lanka being the project approval body had the following

conditions with respect to weir construction.

(a) Maximum height of the weir had been restricted to 1.17 m.

(b) The environmental flow had been decided as 780 liters per second.

(c) The silt deposit behind the weir had to be flushed out weekly.

As the weir height had been restricted to 1.17 m and the river width was 20.6 m , the 100 year flood flow was not a

serious issue in this case. The flood water rise due to 100 year flood flow was about 2.85 m above the weir top. The

100 year flood water level change due to weir construction was estimated as 1.2 m. As the weir height was very short

& continuous minimum environmental flow release was 780 l/s, a considerable negative impact was not created due to

construction of the weir.

As the weir height was restricted to 1.17 m, the sedimentation behind the weir can significantly reduce the water flow


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in to the intake. Considering the operational aspects and the environmental aspects, two silt removal gates of 1m x 0.6

m were provided to the weir. One gate was located near the intake and the other gate was located at the mid of the

river to remove the silt frequently. Normally, only a single silt removal gate is provided at the mid of the weir in most of

the projects of similar capacity.

A 10 m wide side intake with a flow depth of 0.9 m was designed to abstract the designed flow in to the head race

canal. A small drop weir was constructed in front of the intake opening to prevent sediment flow in to the intake. Theminimum flow pipe was located along the small weir to take the bed material with the continuous minimum flow

release.

3.2 Head race canal planning

The project approval conditions of Forest Department had the following conditions with respect to canal construction.

(a). Tree cutting should be minimum

(b). Slope stability must be ensured by engineering designs

(c). Over passes should be provided at every 50 m for terrestrial animals over the canal

The total head race canal length was 1620 m. Initial 300 m of the head race canal was in the forest department

reservation. It had very old rare types of trees like Kubuk (Terminalia arjuna). The second section of the head race

canal (960 m) was in agricultural lands of individual village farmers. The third section of the head race canal (360 m)

was again in forest department reservation.

The first step in the design of the head race canal was commenced with preparation of detailed contour survey of the

project land area. All the trees to be preserved in the canal trace were marked in the detailed survey plan. The head

race canal alignment was selected using the detailed contour plans of the head race trace avoiding the large trees as

much as possible.

In meeting the condition (c) as given above, it was decided to construct a box canal through the initial 300 m section

of the forest lands and to back fill over the canal and to reinstate the ground to the original condition to provide

unhindered pass over the canal for the terrestrial animals. After reinstating the excavated ground over the canal, the

slope stability issues were also automatically resolved. The box canal was provided with inspection manholes at 100

m to inspect the canal during periodic maintenance.

Photo. 1.0 - Concrete box canal constructed in first section of head race canal


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The second section of the head race canal (960 m) and the De-sand tank were through farm lands of the villagers.

Out of 960 m, 300 m was through the paddy field and the balance section was through the cultivated farm lands. The

design of the head race canal in this section had two main challenges other than the approval conditions. They are,

(a) 11 m high aquaduct of 60 m long required to be constructed over a valley. (b) There had been no land acquired to

construct a spillway from the Forebay to the river. The forebay tank spills over when the turbine shuts down, and it is

necessary to have a spillway from the forebay to the river to send the spill water. Although a decision was taken to

acquire the lands for the forebay spill way, it was not possible to acquire additional lands than approved area from theForest department.

Hence, to address these issues, the following three options were considered for the head race canal in the second

section between the De-sand tank and the forebay tank.

(a) A concrete open canal from the forebay tank with sufficient free board to bring water back to the De- sand

tank when the turbines shut down;

(b) A concrete box canal as a pressure conduit with surge tank, instead of a forebay;

(c) A buried Glass fiber Reinforced Plastic (GRP) pipes as the head race pipe with surge tank, instead of

forebay.

The three options were ranked as follows considering environmental considerations, cost and the construction time.

The cost of the GRP pipe construction was higher than concrete canal, but selection of best alternative did not

consider the construction cost alone. The environmental considerations also were taken in to account and selection

was made accordingly.

Factors Concrete Open canal Concrete box canal GRP pipes

No Restrictions onmovement to Villagers &Terrestrial animals

1 2 3

Safety to terrestrial animals 1 3 3

Slope stability 1 2 3

Excavation volume 2 2 1

Removal of trees 1 2 3

Construction cost 2 2 1Constructability over thevalley

1 2 3

Water tightness 2 1 3

Durability 2 2 3

Construction time 2 1 3

Damage proneness if notburied

2 3 1

Head loss 2 2 3

Maintenance 1 2 3

18 26 33

Key : 1- Poor , 2- Good, 3- Very Good

Out of the three options, GRP pipe construction obtained highest rank, and hence considered as the best environment

friendly option in Bulathwatha SHP project. So, the construction of second section of the head race canal used GRP

pipes as the construction material.

The construction of GRP pipes in Bulathwatha project was easy and a good construction progress was achieved as,

a) The diameter of the head race pipes were 1.6 m, 1,4 m and 1.3 m;

b) The technical assistance and solutions for pipe installation was given by HOBAS, Austrian GRP pipe

manufacturer;


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c) There were no steep slopes in the head race pipe trace other than four valleys which require open above

ground pipe laying;

d) The rock blasting was minimal . Excavation was not a problem as excavator could use;

e) As the head race pipe trace was close to village road, pipe transportation to the head race trace was

easy.

Photo.2.0 Photo. 3.0

Photo.2.0 - GRP pipe in a deep cut

Photo.3.0 - GRP pipe as a siphon across a valley instead of a 11 m high aqueduct (pipe diameter 1.6 m)

3.3 Selection of a Surge tank instead of a Forebay

Having a Forebay tank with a spill way is usually the head pond arrangement at the upper end of the penstock. The

forebay tank stores water, serves as the intake to the penstock and caters the sudden water demand of the turbine.

But, a forebay tank is not constructed to store, when a operating turbine shut down. Hence, the water entering theforebay starts to spill over the forebay tank walls during a turbine shutdown. To prevent damage to the ground at the

forebay, the spill water shall be directed safely to a river or to a natural stream using a spill way.

The land required for the spill way of Bulathwatha SHP project was approximately 2000 m2 at the shortest route. The

shortest forebay spill way route was through the forest department lands. The land necessary for the spill way at the

forebay had not been acquired from the forest department at the land acquisition stage. Possibility of the location of a

spill way between De-sand tank and the upper end of the penstock was investigated and found as not feasible due to

land acquisition difficulties, environmental considerations and higher construction cost.

As an alternative to the forebay tank which requires a spill way, a surge tank was considered as a feasible option in

Bulathwatha SHP project. A surge tank is a circular, tall structure. The height of the surge tank is selected such that

water will not spill over its top wall during turbine shut down and to have a sufficient volume of water required forsudden turbine opening .Correctly designed surge tank can handle the water demand of the turbines and the

oscillations of the water surface within the surge tank without any spill. Therefore, the negative environmental effects

like removal of trees, excavation for the spill way construction, blasting of rocks, erosion and threat to the lives of

terrestrial animals and people could be avoided. In Bualthwatha SHP project, it was possible to avoid negative

environmental impacts due to forebay spillway by selection of a surge tank as an alternative to forebay cum spillway

solution.


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An SHP with an open head race canal , forebay tank, spill way at forebay, steel penstock etc. needs the following

personnel to manage & maintain the structures ( Table -01). The necessary management and resources are given in

the table.

But, Bulathwatha SHP project is having GRP pipes instead of concrete open canal, surge tank instead of forebay

tank and no spill way hence, no significant maintenance requirements ( Table -02) . Also, the penstock was

constructed using GRP pipes. Hence, the selected structural arrangement minimizes negative environmental effects

and saves the maintenance cost during the project life time.The two tables below compare the management of thestructures during operation phase of the project.

Table -01 Management of structures (Head race canal / Forebay / Spillway)

Structure Management Resources necessary

Head race canal Road along the HR canal,

safety fences, over crossings,

weeding

Drainage structures, side slopes

1 person on regular basis,

periodic maintenance funds

Forebay tank Trash removal, trash rack, silt

removal

2 persons on regular basis.

Spill way Erosion of the structures due to

high velocity

Periodic maintenance funds

Penstock Painting, adjustment of

expansion joints

Periodic maintenance funds

Table -02 Management of structures (Buried head race pipes/ Surge tank)

Structure Management Resources necessary

Head race pipe ( GRP) buried No maintenance Nil

Surge tank No management Nil

Spill way No spill way & No management Nil

Penstock ( GRP) buried No maintenance Nil

5. Conclusion

Factors to be considered in selection of structures of a hydro power project which had to be constructed in

environmentally sensitive area in line with the project approval conditions were discussed in the paper. The

awareness of the project approval agency and their projects approval conditions by the stakeholders as well as design

engineers can ensure minimum negative impacts to the environment. The logical selection process of structures

based on engineering, environmental and operation & maintenance aspects at the design phase of a project ensure a

viable power project. This aspect was elaborated using the examples drawn from Bulathwatha SHP where alternative


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structural forms were used instead of traditional head race canals and forebay tanks. The possibility of achieving

savings on maintenance costs through adopting environment-friendly structural solutions was also highlighted. It is

expected that the concepts discussed in the paper would be beneficial to design engineers and developers of Small

Hydropower Schemes to be developed in environmentally sensitive areas.

6. References1. Bulathwatha small hydropower project approval letter by Forest Department, Sri Lanka

2. Standards/Manuals/Guidelines for small hydropower development Alternate hydro energy center, Indian

Institute of Technology, Roorkee

3. AWWA-M45-Fiberglass Pipe Design

4. HOBAS pipe laying instructions (http://www.hobas.com)

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