Small Hydro Power Site Selection
- Topographical Consideration
Identification of a suitable small hydro power (SHP) site is a prerequisite for the safe functioning and long lasting performance of any SHP project. Site selection requires general understanding of various aspects such as; topography, geology, hydrology, power requirement, accessibility, socio-economic constraints, environmental protection policies, availability of funds, climatic conditions etc. In other words, all these factors form the general selection criteria for a suitable SHP site. Though, among all these factors, topography and geology of the area are the two main technical factors which significantly affect the site selection, design, construction and performance of SHP project. Ideally, no site is assumed to be suitable from topographical or geological point of view, as there may always be some problems associated to topography or the geological setup of the area. However, identification of possible adverse and unfavorable conditions in the project area, during initial stages of site selection, helps in adopting proper remedial measures so that the project do not pose any problems during the construction or performance stage.
Topography, in general is the land-scape of the area. It defines the general ruggedness of the area, general elevation difference within the valley and steepness or gentleness of the valley slopes. Topography of the area is an important factor and is the key component for the identification of a suitable site.
Availability of Head
For power generation, two factors are essential one is availability of discharge and other is the head. For a given site, discharge is mainly controlled by the hydrology and catchment characteristics whereas, head is a function of topography of the area. Head can be defined as the vertical elevation differrence between the locations from where the water will be dropped over to the location of turbine. In general, the availability of head will mainly be governed by the gradient of the stream. It implies that if the gradient of stream is high more head will be available with relatively smaller water conductor system. Gradient of the stream will be controlled by the general topography of the area. Therefore, to select an economically suitable site it will be preferred to select the stream where it has steepest gradient. As already mentioned, relatively more gradient of stream will ensure high head with smaller water conductor system, thus the cost for the water conductor construction will be less.
Other topographical factors which are important to ensure good available head are the presence of flat and stable area for housing powerhouse and suitable location for forebay tank. Thus, while assessing the availability of head, we need to consider shortest water conductor length with suitable locations for forebay and powerhouse site. However, apart from the topographical consideration the suitability of powerhouse and water conductor system has to be assessed from engineering geological point-of-view.
Diversion Weir and Intake Structure
Diversion weir is a structure which is constructed across the stream to divert water into the intake structure of the water conductor. From topographical point-of-view, diversion weir has to be constructed where the valley width is relatively less. Thus, the cost for construction of diversion weir will be less as compared to the case where the valley width is more. Other factor which is important while selecting the diversion weir site is to avoid location just downstream of confluence of cross streams. Such locations may probably be subjected to excessive erosion and depositions of stream debris, which not only will damage the diversion structure but will also, affect the water conductor by bringing in excessive silt into it. The curvature of the valley is also an important factor. Thus, as far as possible, diversion weir site must be selected where the stream course is more or less straight. If the diversion weir is selected at a location where the stream meander forming a curvature, concave on one side and convex on other side of the valley. These convex locations generally poses problem during operational stage. Concave curvature sides during high flood times are subjected to toe erosion which may result into slope stability problems whereas, on convex valley side river debris will be deposited. Thus, the intake structure if it is constructed on concave side will be subjected to undercutting by the stream water and may also be damaged by the slope stability problems. If the intake structure is constructed on the convex side of the valley it will face problems due to deposition of stream debris. However, geological conditions and slope geometry prevailing at concave curvature sides will be a controlling factor for slope instability.
Desilting Tank, Headrace channel and forebay site
Desilting tank is an important component of water conductor system which is primarily constructed to remove or trap the silt which may come along with water from the diversion and intake structure. The length and dimension of desilting tank will depend mainly on the particle size of the silt which it has to trap. From the topographical point of view suitable flat or gentle slope area must be available for the construction of desilting tank. The selected site must be free from any seasonal or perennial cross drainage. Headrace is the component of water conductor system which carries water from desilting tank to the forebay. The choice for the open channel or steel pipe will mainly depend on the steepness of the slope through which headrace has to be aligned. Generally, over the gentle slope open channel will be more feasible whereas, for steep slopes steel pipe will be more preferable. Generally, steep slopes require more rock cutting for open channel and pose difficulty for access during construction. Steel pipes can be prefabricated and can easily be placed over the steep slopes as compared to the open channel. Further, headrace pipes can easily be flanked over two saddle structures to pass any cross drainage along the headrace alignment. However, in case of open channel an aqueduct has to be constructed to pass through the cross channel.
Forebay is a structure which is constructed mainly to provide stable discharge to the penstock and to accommodate surge due to emergency closing of turbines. From topographical point of view flat or gentle slope with required dimensions must be available for the forebey. Besides, location for spillway must also be available to spill off any excess water from the forebay.
Penstock is a steel pipe through which water is communicated from the forebay to the turbine. In order to minimize the frictional forces the penstock alignment must be straight with least bents. Each bent in the penstock pipe will result into head loss due to the frictional forces which will develop along the bent in the pipe. Therefore, while selecting the alignment for penstock efforts must be made to locate it along the ridge and to avoid any valley depressions along the alignment. Although such minor depressions can be accommodated by providing suitable height to the saddle pillars on which penstock pipe will be placed. As far as possible any cross drainage lines should be avoided along the penstock alignment, particularly the saddle pillars should not be placed close to such cross drainage. Ideally, to minimize the head losses due to frictional forces the penstock alignment must be selected through the steepest portion of the slope. However, it has to be ensured that the geological conditions along the proposed alignment are suitable. If unfavorable conditions are investigated proper remedial measures must be adopted before the installation of penstock.
From topographical consideration the powerhouse must be located over the flat or gentle slope. The powerhouse must be placed safely above the high flood line of the stream. As far as possible the stream course must be straight close to the powerhouse site. Any curvature of the stream bank near to the powerhouse site may result into slope toe erosion which ultimately affects the stability of the slope and the safety of the powerhouse. Though, the bank toe erosion will depend on the geological conditions. If the geological conditions favor the possible toe erosion proper stream bank protection has to be provided. Another important point related to the powerhouse location is to place powerhouse at such a location where there is no threat of slope stability problems such as; slope mass slides, rock falls or topples. While making a initial selection of powerhouse site surface manifestation of instability of slope must be observed.
General topographical considerations
For site selection following general topographical consideration must be followed;
-To select an economic suitable site it will be preferred to select the stream where it has the steepest gradient. This is to ensure high head with relatively smaller water conductor.
-Avoid locations which shows field manifestations of instability of slopes; such as;
Presence of scarp faces on the steep slopes.
Removal of toe support by river erosion.
Presence of evidences of slope distress, such as, development of cracks, bulging of slope face and other such features.
Presence of zones of excessive seepage.
-Avoid locating diversion weir and powerhouse at or just downstream of confluence of main stream and cross streams.
-Headrace alignment should be selected where the slope is relatively stable, gentler and has to pass through minimum cross drainage.
-As far as possible the penstock alignment must be straight and should pass over the ridge. Valley undulations must be avoided to minimize the head losses.
-Powerhouse site must be located on relatively stable flatter land and above the maximum flood line. The powerhouse location must be safe from toe erosion by the stream.
Strategies to Deal with Uncertainties
– A perspective
Knowledge of geologic principle and processes forms the basis of Engineering geology. As an engineering geologist our main responsibility is to make an assessment of ground conditions to provide; a safe foundation to engineering structures, stable natural and artificial slopes and firm underground openings. How good we are in making precise assessment of ground condition demonstrates our competency in making sound judgment. Unlike other discipline of science, where general rules and principle are well defined, in engineering geology most of the times such general rules or principle may not be applied directly, it will require a thorough judgment before applying general rules and principle or making any inferences from the findings.
The design requirement for an engineering structure may need an absolute design value for a bearing capacity or for settlement potential of the foundation material. For such a requirement, most of the times, we need to deal with foundation material which may be heterogeneous. There would be a wide variation in mineralogical composition and engineering properties of such a heterogeneous foundation material. Things would be very simple if only qualitative assessment of ground conditions would be required. In fact as an engineering geologist we need to translate geologic findings into quantitative terms which can directly be applied to engineering design needs of a structure. Therefore, while dealing with such uncertainty it is necessary that whatever experimental, analytical or empirical techniques are applied it must be well supported by logical judgment and past experience gained from similar type of foundation material and ground conditions.
Landslides, most devastating natural hazards are triggered by natural factors such as; rainfall/ snow or earthquake activities and by human activities such as improper or poor road construction or inadequate road maintenance in mountainous terrain. Identification of such slope instability problems in the initial stage of planning and investigation of engineering structures may lead to evolve possible remedial measures which may either be adopted to improve the slope stability or such problematic slopes may be avoided if identified in the initial planning stage. Any successful design of the slope will depend on geologic information and its site characteristics; such as, slope geometry, properties of slope material, discontinuities orientation, groundwater conditions and possible triggering factors. Choice for correct slope stability analysis technique will mainly depend on both site condition and potential mode of slope failure.
While conducting slope stability analysis we need to deal with many uncertainties; is the technique selected for analysis is correct, is the assessment of properties of slope material such as; shear strength parameters and bulk density are representative, is the groundwater conditions considered for the analysis is actual etc. The uncertainties related to slope stability analysis can be broadly classified into; parameter uncertainty – material properties used as input for analysis, model uncertainty - due to the limitation of the theories and models used in performance prediction and human uncertainty - due to human errors and mistakes. The success of any slope stability analysis will depend on correct selection of analytical technique and correct assessment of slope parameters used as input for the given analytical technique. Further, safe slope design require a sound judgment on various aspect related to slope parameters and on existing and anticipated adverse conditions for which the slope would be subjected.
Investigation for underground structures is different from other engineering structures because the principal construction material is the rock mass itself rather than an engineered material. For underground structure an intelligent thought process and processes are required as uncertainties exist in the properties of the rock material and the manner in which the groundwater will interact to the rock mass. These uncertainties must be overcome by sound investigation, flexible design, safeguards during construction and anticipated adverse conditions during the performance stage of the underground structure.
Thus, for any engineering application it is essential that our effort to analyze ground condition must be guided by sound judgment for various engineering properties and anticipated adverse interactions between the surrounding conditions and the engineering structures. The safe performance of any engineering structure or underground opening can only be guaranteed if as an engineering geologist we realize the necessity of sound judgment, logical interpretations, thorough understanding on anticipated adverse conditions and confident experienced based decisions.