SOME IMPORTANT ASPECTS IN CIVIL WORKS DESIGN OF A MINI HYDRO POWER PROJECT
By Eng Upali Mallawa Arachchi
Civil Works of a mini hydro power project have some distinct characteristics. One of the significant features is that the true contribution of the quality of Civil Works to the project’s performance is not adequately reflected in the capital cost component. Unlike in the case of Electro-mechanical Works where products of standard designs which can be directly incorporated in the power plant are available, location specific factors restrict the use of standard designs in Civil Works. Therefore, quality of civil works largely depends on a thorough understanding of site conditions, accuracy of surveys, involvement of expertise in different fields of civil engineering and more particularly, a very systematic design procedure which ensures an economical and workable design. Examination of the site to assess the relative merits of alternative solutions to location of components of the project would be the first step in the design process. Decision on the best alternative is taken based on economy and technical feasibility. In the detailed design too, a similar procedure should be adopted in selecting the most suitable type and size of the structure. Use of the most appropriate design parameters in both hydraulic and structural design is also vital in ensuring a good quality design. Management of the design process must ensure obtaining appropriate expert opinion and reference to literature to decide such parameters and recording them for reference. The Author highlights in this paper, some special considerations in designs and the methodology to achieve a good quality design of civil works of a mini hydro power project. It is expected that the paper would provide some useful guidelines to civil engineers and hydro power developers regarding the procedure to be followed in the design.
Hydro power generation means the conversion of potential energy available in water flowing in a stream to electrical energy with the use of a turbine. This process essentially involves diversion of water at a point of higher elevation and conveying it to the turbine installed at a lower elevation. The energy generated is directly proportional to the difference in elevation (available head) and hence the process of diversion of water should be done with great care to save the potential energy i.e. head loss should be kept to a minimum. A series of Civil Engineering structures are built for this purpose, namely, diversion dam, intake, head race channel, sedimentation tank, forebay tank, penstock line, power house and tail race.
The above set of structures is common for any hydro power project. There are some distinguished features in a mini hydro power generating facility (Capacity within the range 0.1 – 10 MW). Some of those features of interest are;
For the above reasons, strict controls during design phase of civil works is a very critical factor for the success of a mini hydro power project.
There is a well known phenomenon that the cost control be best achieved at the early stages of a project. From project feasibility to the construction stage, the freedom for cost control measures gradually diminishes. This phenomenon is applicable to mini hydro power projects as well. Next section of the paper deals with the special considerations and the procedure in design of Civil Works in view of achieving an economical solution appropriate to the prevailing conditions. Only the part of project feasibility aspects which has direct relevance to Civil Works design has been included in the paper as feasibility studies as a whole is a broad subject which should be dealt with separately.
Preliminary site studies as pre planning for a systematic design and design considerations in each component of the project are described separately in detail.
The initial and the most important part of the Civil Works design is the decision on appropriate location of each part of the structure. This has to be done to a considerable extent during the feasibility stage. The process should be continued with more accurate information as the designs proceed. Whole process can be divided into various sections namely Initial Investigation, Specialist Advice, Engineering Surveys, Sub surface investigations, Analysis of Alternatives and Conclusions. Each of these is described in detail in the following sections.
This is basically site visits with minimum amount of measurements. It consists of the following simple steps ;
This is done using contour maps or any other means of information. Availability of a significant head within a short distance is the main criterion (eg. 40 m head within 1 km distance). Distance to the nearest transmission line is also a factor to be considered.
It is generally economical to have the weir close to the starting location of the fall. Other considerations are;
- If the river is narrow, weir length is less, hence less cost.
- If the sides are high, less erosion problems
- It is better to have strong rock outcrops either side
- Foundation condition should be good. If different rock layers (foliations) are found, there is a possibility of water leakage underneath the weir.
Approximate difference in elevation between the diversion point and the toe of the fall should be measured. This can be done either using a level instrument or a water tube. Variation of the level of stream downstream of the fall should be examined. If the variation of the level is insignificant at a certain location, power house be best located at a relatively flat site (close to the stream) at that particular location.
In most cases, head race channel is constructed along a contour starting from the intake location. There may be several contours with the elevation of weir starting from intake location running in downstream direction. Penstock route is selected so that the head race channel contour is connected to the power house with a shortest length. Minimum cost of the combination of canal and penstock portions is the main criteria for selection of the best route out of the alternatives available. A ratio of 1 : 2 can be assumed as the ratio of per metre cost of channel : penstock under general circumstances. However, if any geotechnical problem exists, expert advice should be sought before selection of the best route.
It is possible to assess the general flood heights by local enquiries. However, expert advice should be obtained on flood with different return periods (25, 50 & 100 year).
During examination of alternative locations, it is essential that, land ownership, purchase price and associated legal problems are studied. Location of a particular structure may have to be changed due to problems of land availability.
2.3 Preliminary Cost Estimate of Civil Works
Preparation of preliminary estimates can be started after the initial site inspection. Such estimates –among other values- are an essential part of the project appraisal process. However, before starting estimating civil work costs, certain aspects related to operational characteristics should be finalised. Some important aspects are;
(a)Type of equipment (turbines and generators) based on the available head and equipment characteristics
(b)Hydrological Analysis and design flow based on the criteria of maximum power generation and load factor considerations
Method of determination of (a) and (b) above is beyond the scope of this paper and hence will not be discussed in detail.
Basic design decisions important for a preliminary estimate are;
This depends on the required head and effect of upstream inundation.
Type of material below surface to a reasonable depth should be investigated using a simple method such as trial pits.
The size of intake, headrace canal, sedimentation and forebay tanks and penstock, based on design flow, hydraulic considerations and site conditions. Such considerations are further illustrated later in this paper.
This depends on the equipment layout and specific water level requirements for the turbines.
Point of discharge of tail water is decided mainly based on river water levels and cross sections.
When the above aspects are finalised preliminary estimate can be made using cost factors (unit costs) determined using market rates. Some examples of such factors are;
- Weir, channel, penstock etc - per linear metre basis
- Forebay and Sedimentation tanks – per cubic metre (capacity) basis
- Power House - per square metre (floor area) basis
The following approximate values are useful in arriving at the cost factors;
Channels and tanks
Thickness of reinforced concrete walls can be taken as (150 + h/15 ) mm where h is the depth of the section from top (in mm). Reinforcements can be assumed as 0.10 tonnes and 0.075 tonnes for one cubic metre of concrete in tanks and channels respectively.
The cost per metre would be about twice the cost of one metre of head race channel.
The cost of the power house per one square metre of machine floor would be around Rs 40,000 for horizontal axis Francis type turbines. However, this may vary depending on the type of equipment and condition of the ground (presence of rock, hard soil etc).
If alternatives are available with respect to materials and technology, those factors can be computed for the most expensive alternative with respective probabilities, so that there will be greater flexibility during the detailed design stage.
Weir of a mini hydro power project is generally a gravity structure. Hydrostatic force for the highest flood condition, uplift forces due to seepage and impact of floating objects are the loading conditions to be considered.
The shape of the cross section of the weir is something similar to that of a large dam. A typical shape is shown in Figure 1. Scour protection measures should be adopted if the weir is founded on soil.
Analysis involves normal stability checks such as safety against sliding, overturning and bearing. Dowelling or keying into rock is normally done under the foundations as an extra precaution.
Other special considerations are the provision of minimum flow and silt removal pipes at appropriate points of the weir.
This is the structure which draws water from the river. This can be a channel, pipe or a tunnel depending on the site conditions. Pipes may not be ideal in high flows where very large diameter pipes are necessitated. Some important considerations in the design are;
Intake capacity should be adequate to draw the required quantity of water during normal design flow conditions. A control weir at the entrance and curved edges are some measures adopted in this regard (Figure 2).
The sediment movement pattern should be studied before making a decision in this aspect. However, a silt removal pipe at the entrance and several silt traps have to be provided at various points along the channel.
A flood condition results in a rise of water level at intake, which would give rise to a high flow in the head race channel. This is undesirable because spilling may occur along the channel path which can cause severe erosion on the sides of the channel. Hence flood water should be diverted from the channel path within the intake channel itself. A series of breast walls (walls above normal flow level) and spillway arrangements are suitable measures for this. Hydraulic jumps may occur in breast wall arrangement causing rise of water level in downstream of those walls. Spillways at each of these locations and also surface protection measures (by applying a rip rap) are some important elements to be designed. Spillway capacity can be computed taking the broad crested weir formula as a guide. Box sections can be used as an alternative to the open channel, which should be analysed using pressure flow principles.
Control gate is a very important component in intake structure. The following factors to be considered.
A trash rack should be provided near the entrance to avoid floating objects entering the channel. This generally consists of steel flat sections welded to a frame. Spacing between flat sections should be decided based on the size of trash expected in the stream (generally adopted value = 50 mm). Larger objects such as logs, bushes etc can be prevented from reaching the trash rack by having a screen mechanism upstream. To minimise the head loss, velocity of water through trash rack should be very small, preferably less than 0.5 m/s. Hence the total area of trash rack should be about 4-5 times the size of intake, and it should be ideally located upstream of the intake entrance. For design calculations, a condition with one half of the trash rack blocked with trash is generally considered. Cleaning mechanism (a rake operated manually/mechanically and a platform to collect and remove materials) is a must for a trash rack arrangement.
Main criteria of head race channel design should be minimising the head loss between intake and the forebay tank. Hence, the bed slope is kept at a fairly low value.
Some important considerations are;
This mainly depends on the terrain conditions. A trapezoidal section would be the most economical one in most cases, but this section needs a relatively flat terrain condition. Rectangular sections are common. Aqueducts have to be provided at places where bed level is considerably above the ground level. Manning’s equation is widely used in deciding on dimensions, bed slope etc. Box sections can be used where deep excavations may cause severe stability problems. Alternative materials available for channel construction are discussed elsewhere in this paper.
(b) Effect of Rain Water and Extraneous Items
Main problems that the rain water may cause to smooth functioning of head race channel are;
An effective catch drain system, adopting slopes appropriate to the ground materials, a filter medium behind the channel walls (specially in the case of thin linings) are some effective measures in overcoming these problems.
(c) Effect of Flood or Abrupt Shut Down
Spilling may occur during a high flood situation or a shut down of the power house operations. Adequate spillways should be provided along the channel (best at a location close to an existing stream) by removing the free board at those locations. Drainage path of the spill water should be protected using rubble paving or concrete.
(d ) Transitions
Transitions between different sections of channel should be made smoothly with gradual variation of dimensions (generally along a length not less than 5 times the change).
The purpose of sedimentation tank is the separation of fine material (silt) which would be harmful to the turbine from the water flow. Forebay tank is meant for providing some storage for operation of penstocks in conveying water to the turbines. Common practice is to combine both the functions in one tank. Two separate tanks would be required if the topography of the location of entrance to the penstocks is not favourable for a tank sufficiently long for sedimentation function.
Important consideration of sedimentation tank design are;
This is governed by the criteria that the tank should be capable of settling all materials with a particle size bigger than which is allowed to enter the turbine (This size should be obtained from the turbine manufacturer). Horizontal velocity of water through the tank should be maintained at a low value (around 0.2 m/s) to facilitate settling, hence the cross section of the tank is governed by this aspect. Length is determined by applying Stoke’s Law. A special shape (V or W) should be provided to the bottom part of the tank for accumulation of fine material. If the tank is combined with forebay, two sections should be separated with an internal weir to avoid passage of silt to penstocks.
Washout pipes with valves should be provided at the bottom to flush the tank periodically to remove sediments. Bottom of the tank should be adequately sloped (apprx 1:40) to facilitate removal. However, since these arrangements are not very effective, provision for manual cleaning should also be introduced. The outlet of the pipes should be directed to a water course with adequate protection from erosion.
Tank is a good location to have a trash rack, since the head loss will be very small due to low velocity of flow.
Some considerations for a forebay tank are;
Size of the forebay is generally governed by the minimum storage required. Quantity required for operation during time needed for would be a useful guide. Depth of the tank at the outlet area should be such that a depth of water of at least 1.5 times the penstock diameter should be available above the penstock pipes, to avoid vortex formation.
Covering of the tank to avoid foreign materials entering the penstocks would be very important.
It is always important to consider the stability of slopes around the tank. Appropriate slopes and proper drainage systems are essential.
Penstock anchors and supports are used to hold the penstock pipe (usually steel pipes) in position under various loading conditions. Anchors are provided at the places of change of direction (vertically, horizontally or both) of the pipe, whereas supports are provided at a predetermined spacing.
Anchor blocks are gravity structures built for resisting forces at a bend of penstock pipe. The following considerations are important in the design.
Factors considered in locating the anchors are;
Principal forces acting on an anchor block are;
This can be eliminated by introducing expansion joints at appropriate places (These are normally provided just after an anchor location).
- Force due to Dead Weight (Direction – vertical)
- Centrifugal force (Direction – perpendicular to the pipe)
- Force due to friction at supports (Direction – along the pipe)
- Friction between anchor block/ground interface
Friction coefficient takes a value between 0.3 and 0.6 depending on the material (soil – rock)
Certain components of these forces act as disturbing forces and other components along with the weight of the anchor block act as stabilising forces. Weight of the anchor block is determined using an appropriate safety factor (preferably around 1.5).
Bearing capacity of the ground should be correctly assessed for deciding the horizontal dimensions of the anchor block. Restrictions due to physical barriers such as rock outcrops, steep slopes, large trees or permanent structures should also be taken into account. Anchors on sloping ground would require benching of ground and stepping of the foundation. Dowels are provided for foundations on rock, as an additional precaution against sliding.
3.4.2 Penstock Supports
Spacing between the supports is determined by two main considerations, i.e. pipe stresses and convenience in handling. Pipe stresses to be considered are;
Support spacing should be such that the combined effect of these stresses should not exceed the allowable values.
Main forces acting on a support are reaction of the pipe and the friction force. The two forces are perpendicular to each other. Friction coefficient between pipe and the support depends on the material used at the interface. If properly lubricated rollers are used the coefficient can be reduced to 0.1. For normal steel concrete interface the value will be around 0.3, whereas rusting of steel may increase the coefficient to even 0.6. Direction of the friction force during expansion of pipe would reverse during contraction.
Supports can be designed as either gravity structures or framed structures. By having the sides of the structure parallel to the direction of the resultant forces, moment at the central axis of the base can be eliminated. Different types of structural forms are shown in Figure 3. The most economical type should be selected for a given situation.
3.5 Power House & Tail race
Design considerations of power house and tail race mainly depend on the following aspects;
Requirements for impulse type and Francis type differ considerably. In Francis type turbines, turbine runner is fixed inside a casing the water head will be the difference in water surface elevation at forebay tank and at just outside the draft tube. Equipment need not be installed above high flood level as the outside wall of the power house can be sealed to avoid inflow of flood water. In case of an impulse type turbine, water passing the turbine flows under gravity. Since the inflow of water under high flood conditions cannot be prevented, generators should be installed above the high flood level, to avoid damage under floods.
The head required for maximum efficiency of the equipment should have been finalised before commencement of detailed designs. Discharge point on the stream should be selected considering the possibility of maintaining the tail water level to obtain the head required.
Properties of material present underground should be studied to design the foundations and basement walls.
Provision should be made for overhead cranes, loading/unloading bays etc required for installation of equipment
Terrain conditions, physical and legal boundaries of site etc may cause limitations in design of layout, deciding structural form etc.
3.5.1 Power House
General structural form would be a framed structure similar to that of a two to three storeyed building with a basement. Equipment floor is generally a basement. Equipment foundation is a reinforced concrete raft.
The following loading conditions should also considered in addition to the normal dead and live loads;
This is generally about twice the dead weight. This torque has a lifting effect.
Stability of the whole structure under buoyancy conditions should be checked.
Weight of travelling overhead crane, sway, impact and braking loads are to be considered.
Depending on the terrain conditions and height of the building
Some other aspects to be considered are;
This can be an open channel or a closed conduit. Required level at the draft tube can be maintained by constructing a regulating structure (like a weir) at the location of discharge.
4.0 Material Selection and Detailing
Since the weir is a gravity structure, mass concrete with plums can be used. However, as forces from flowing water and impact from floating objects occur, it’s advisable to construct outer layers with concrete of strength of about 25 M Pa.
For channels (eg. head race), the alternatives available are;
(a) Earth and Rubble Masonry
From the above, earth canals are not preferred in mini hydro power projects for reasons such as difficult terrain conditions, maintenance difficulties and high possibility of sediment accumulation.
Rubble masonry can be used for a smaller canals of less than 1.5 m depth, but the main problem arises in protection from decay and erosion of mortar in the masonry wall. It is recommended that interior rubble masonry walls should be covered with a ferrocement coating to ensure durability of the structure.
(b) Unreinforced Concrete Linings
Canal sections with a thin (50 – 100 mm) unreinforced concrete lining is also an economical solution. However, this type requires a trapezoidal section (preferably with 1:1 side slopes) which occupies a larger area, and also reasonably good soil condition. Minimum strength of concrete to be used is 25 M Pa. Following aspects should not be ignored;
(c) Reinforced Concrete
reinforced concrete is the commonly adopted solution to larger canals in hilly terrain. It is sufficient to use provisions of BS 8110 in the design as the structure is designed for flowing water under a low head. Cantilevered or counterforted walls can be used. Pressure applied by the backfill should also be considered in the design. Critical load combinations are;
Stability of the whole channel under buoyancy forces at empty condition should also be checked.
It is necessary to provide expansion / contraction joints (with water stops) at appropriate intervals. Cover to reinforcement at the inner face should be high (about 60 mm) to withstand erosion effect of water.
In channel design, it is essential to limit the velocity of water flow to suit the material. For concrete, maximum velocity is in the region of 2.0 m/s. Special care should be exercised in selecting the value of Manning’s Coefficient. This coefficient for smooth concrete surfaces is about 0.015. It is always advisable to check the channel flow condition, by varying the Manning’s coefficient slightly around the assumed value.
The above conditions are applicable to the tanks except for the following;
4.3 Penstock Anchors and Supports
Appropriate materials for this are rubble masonry, mass concrete and reinforced concrete. Factors to be considered in making the selection are;
Recommended concrete types are;
4.4 Tail race
Rubble paving on a mass concrete lining can be used in tail race canal, if the location permits the use of a trapezoidal section.
Although the magnitude is comparatively smaller, Civil Works of a mini hydro power project contributes much to the viability of the project. All three aspects performance, safety and economy of Civil Works are equally important for successful operation. Major part of the controls of those aspects lye in feasibility and design stages. Good Civil Works design practice in a mini hydro power project requires the involvement of experts in the fields of hydraulics, hydrology, geology and structural design. Some important steps in the design process such as preliminary designs and estimates were discussed in the early part of this paper. Each component of a mini hydro power project has its special considerations. Missing of one aspect would lead to major problems at a later stage. Each of these considerations including loading conditions and parameters were dealt with in adequate details. This paper can be used as a guide for those involved in civil works in mini hydro power projects.
Author is specially thankful to Eng Samarakoon Banda of Zyrex Power Company Limited, for his valuable contribution in preparation of the paper.
Eng Upali Mallawa Arachchi , BSc.Eng. Hons., M.Eng.Const Mgt., PG Dip. H&T Eng., C.Eng., MICE (Lond), MIE (SL), MIHT(Lond). graduated from University of Peradeniya in 1981.
He is presently holding the post of Manager, Civil Engineering of Zyrex Power Company Limited.
Prior to joining Zyrex, he served as Chief Engineer of Road Rehabilitation Project - AKDA Group, Senior Engineer at Penta Ocean Wakachiku JV in Port of Colombo Extension Project, Engineering Consultant in Resources Development Consultants Ltd, Highway Design Engineer in Engineering Consultants Ltd, Engineer (Valuations & Claims) in AF- Joint Venture at Colombo – Kandy Road Rehabilitation Project (Phase 1), and as a Design Engineer, Resident Agent and Site Engineer in State Engineering Corporation.
Mr Mallawa Arachchi also serves as a Visiting Lecturer and an External Collaborator of Centre For Housing Planning and Buildings (CHPB) and Institute for Construction Training and Development (ICTAD).