Survey and Investigations for Hydro Power Projects

Planning for Survey and Planning for Survey and Investigations for Hydro ProjectsInvestigations for Hydro Projects

By,By,Saravanan.SSaravanan.S

Survey for Hydro Projects

Surveying is the technique and science of accurately determining the terrestrial or three-dimensional position of points, distances and angles between them. These points are usually on the surface of the Earth, and they are often used to establish land maps and boundaries. To accomplish their objective, surveyors use elements of geometry, engineering, trigonometry, mathematics, physics, and law.

Its principal modern uses are in the fields of transportation, building, land use, Water Resource developments, Monitoring and communications. Surveying is divided into the categories of plane surveying (mapping small areas) and geodetic surveying (mapping large areas of the globe).

Types of Surveying in Hydro Projects

Surveying is the very important and vital operation for river valley projects. Any river valley project to be planned and constructed is to be based on certain basic facts and figures. The process of collection of these facts and figures is termed as investigations.

Surveying for River Valley Projects can be Classified as follows,

• Topo Graphical Survey

• River Survey

• Reservoir Survey

• Site Survey

• Command Area Survey

• Communication Survey

• Flood Control Survey

Topo Graphical Survey

A survey that determines ground relief and location of natural and man-made features thereon is called as Topo Graphical Survey. 

The configuration of a surface including its relief and the locations of its natural and man-made features, usually recorded on a drawing showing surface variations by means of contour lines indicating height above or below a fixed datum termed as Topo Graphical Maps. The Survey of India is responsible for all topographic control, surveys and mapping of India.

The Topographical maps are playing very important role in the Planning of Hydro Projects from the Investigation stage to Post construction stages.

Uses of Topo Graphical Maps in Hydro Projects

The Topographical maps can be used for following purposes,

Planning for Locating the Project Components Planning for Fixing Reservoir levels. Planning for Alignments of the Access roads Computing the Quantities of Various works for Cost Estimations. Planning for Locating the Material Quarries for Construction Planning for Muck Disposal Area. Planning for Flood Protection works, etc. Planning for Resettlement works.

Total Station with Accessories

The total station is an electronic theodolite (transit) integrated with an electronic distance meter (EDM) to read slope distances from the instrument to a particular point.

Auto LevelThe Auto Level can be used to control the vertical Limit of the Survey area (i.e. Elevations)

GPS

The Global Positioning System (GPS) is a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites.

Instruments used in Topographical Survey

River Survey (Hydrographic Survey)

River Surveys cover longitudinal Section ( L-Section ) and Cross Section ( X-Section ) of the river on the upstream and downstream of the proposed Diversion structure.

The Depth of the River can be measured directly by Sounding rods, Lead Lines for Low depths (up to 30m) and the same can be measured / Calculated Indirectly by Echo sounder (Fathometer).

The River Cross sections are used to ascertain the Depth of flow, Foundation for the Diversion structure, Slope of the River bed and making river models for Tail water Rating.

Codal Provision for Hydro Projects

The L-Section on the upstream side shall extend from the axis of the

structure to the point up to which the back water effect is likely to

extend or up to the maximum water level ( MWL ) + 5 m whichever is

less. If any head works is situated within the reach, the L-Section

shall be taken up to the head works.

On the downstream side, the L-Section shall extend for 10 km from

the axis of the structure or up to the nearest head works whichever

is less. The leveling of the L-Section shall be done at 50 m or less

intervals along the fair weather deep channel.

The following items shall be indicated in the L-Section:

a) Date of survey of the particular reach and water level on that

day;

b) Deep pools and rapids, rock outcrops, etc; and

c) Maximum historical observed highest flood level ( H.F.L. ).

Codal Provision for Hydro Projects

The X-Section on the upstream side shall be taken at 200 m

intervals up to MWL + 5 m or 1 km on either side of the firm bank,

whichever is less, for a distance of 2 km from the axis of

structure and thereafter at 1 km intervals corresponding to the

length of the L-Section.

On the downstream side, X-Section shall be at 200 m intervals

and taken up to historical highest flood level + 1 m on either side

of firm bank for a distance of 2 to 5 km from the axis of the

structure depending upon the meandering nature of the river. An

X-Section shall also be taken along the axis of the structure. ‘The

leveling shall be done at 50 m or less intervals.

The longitudinal and Cross Sections shall be plotted to a scale

of 1 : 2 500 horizontal and 1 : 100 vertical.

Reservoir Survey

A reservoir survey is necessary to provide the basic data for

calculation of water storage capacity of a reservoir, for indicating

the limit of submergence areas and for locating the topographic

depressions or saddles in the reservoir rim which may cause

spilling or leakage of the reservoir.

The scale and contour interval for topographical survey for

reservoir survey depend on the extent of the area and the

topography of the site. The reservoir survey is made with a view to

examining all possible alternatives for its locations and to

eliminate such of the proposals which become unsuitable from

considerations of technical feasibility, economy and practicability.

Reservoir survey maps shall be made on scales ranging from I : 25

00 to 1:25000.

Site Survey A site survey is carried out to investigate the suitability of .a site

for a particular structure, such as dam site, power house or tunnel

site. A dam site survey should cover a sufficiently large area to

include all possible locations of the main dam, coffer dams,

spillways, separate outlet structures, and other appurtenances.

The scale of dam site survey maps may range from 1 : 1000 to 1 :

4000.

Power house site survey should include an area for possible

construction of plant sites, stockpile areas, warehouse, switchyard

areas and camp sites. The scale of maps may normally be from

1 : 250 to 1: 1000.

Tunnel site survey maps should be made along possible tunnel

alignments on scale 1 : 1000 to 1 : 10000.

Command Area Survey

The command area is the area which is likely to be benefited from the river valley project. The command area survey is very important in order to know beforehand the capability of the command area to make maximum use of the benefits of the river valley project. Sometimes the command area survey is also necessary to assist in designing the layout of main canals and distributaries for irrigation purposes.

The scale of command area survey maps should range from 1 : 10 000 to 1 : 1 5 000.

Communication Survey

The movement of construction equipment and material to a

project construction site will normally require the construction

of access roads, as most project sites are not accessible from

existing highways. Sometimes branch lines from the existing

railway map be required. Existing railways and highways

running through the reservoir area may have to be relocated

where these are liable to be inundated or submerged by a

reservoir. The surveys are required for this purpose and for

deciding the alignment and grades of railways and highways.

The surveys for the location or relocation of railways and

highways are normally performed by the field survey parties of

the railway or highways authorities who should be entrusted

with the determination of specification and survey instructions.

Investigation works for Reservoir SitesInvestigation works for Reservoir Sites

To Produce Efficient Design for Hydro Project Components and To Produce Efficient Design for Hydro Project Components and Execution, the Project site must be investigated completely, So Execution, the Project site must be investigated completely, So that surprises can be avoided during Construction. that surprises can be avoided during Construction.

Generally the Investigation works for the Hydro Project done in Generally the Investigation works for the Hydro Project done in following stages,following stages,

ReconnaissanceReconnaissance Preliminary InvestigationPreliminary Investigation Detailed Investigation StageDetailed Investigation Stage Post-construction StagePost-construction Stage

ReconnaissanceReconnaissance

In the reconnaissance stage, the objective of investigation is to bring out the overall geological features of the reservoir and the adjacent area to enable the designers, construction engineers and geologists to pinpoint the geotechnical and ecological problems which have to be tackled.

The scale of geological mapping for this stage of work need not be very large and the available geological maps on 1 : 50,000 or 1 : 250,000 scale may be made use of. It is advantageous to carry out photo geological interpretation of aerial photographs of the area, if available.

If a geological map of the area is not available, a traverse geological map should be prepared at this stage preferably using the aerial photos as base maps on which the engineering evaluation of the various geotechnical features exposed in the area should be depicted.

ReconnaissanceReconnaissance

A topographical index map on 1 : 50 000 scale should be used at this stage to delineate the areas which would require detailed study, subsequently.

To prevent an undesirable amount of leakage from the reservoir, the likely zones of such leakage, such as major dislocations and pervious or cavernous formations running across the divide of the reservoir should be identified at this stage of investigation for further detailed investigations.

Major unstable zones, particularly in the vicinity of the dam in tight gorges, should be identified at this stage for carrying out detailed investigations for the stability of the reservoir rim.

The locations for suitable construction material available in the reservoir area should be pin pointed at this stage so that after detailed surveys such materials can be exploited for proper utilization during the construction stage prior to impounding of reservoir.

The rate of silting of the reservoir is vital for planning the height of the dam and working out the economic life of the project. dependent on the type of terrain in the catchment area of the reservoir, the major geological formations and the ecological set up should be recognized at this stage to enable a more accurate estimation of the rate of silting of the reservoir.

ReconnaissanceReconnaissance

The object of preliminary investigation of the reservoir area is to collect further details of the surface and subsurface geological conditions, with reference to the likely problems identified during the reconnaissance stage of investigation by means of surface mapping supplemented by photo geological interpretation of aerial photographs, hydro geological investigations, geophysical investigations, preliminary subsurface exploration and by conducting geo-seismological studies of the area.

On the basis of studies carried out during the reconnaissance stage it should be possible to estimate the extent of exploration that may be required during the preliminary stage of investigation

Preliminary InvestigationPreliminary Investigation

The potential zones of leakage from the reservoir and the lateral extent of various features, such as extent of Aeolian sand deposits, glacial till, land slides, major dislocations or pervious and cavernous formations running across the divide, should be delineated on a scale of 1 : 50000.

The geohydrological conditions of the reservoir rim should be established by surface and sub-surface investigation

The extension of various features at depth, wherever necessary, is investigated by geophysical exploration and by means of pits, trenches, drifts and drill holes.

Major unstable zones along the reservoir identified during the reconnaissance stage and which are of consequence to the storage scheme should be investigated in detail at this stage by means of surface and sub-surface exploration. The areas should be geologically mapped in detail on a scale of 1 : 2 000.

Preliminary InvestigationPreliminary Investigation

On the basis of the studies carried out during the preliminary stage it should be possible to estimate the quantum of exploration which may be required during the detailed stage of investigation including the total number of holes required to be drilled and the total number and depth of pits, trenches and drifts as also the extent of geophysical survey which may be necessary.

The zones, which on preliminary investigation are found to be potential zones of leakage/seepage from the reservoir, and which due to other considerations cannot be avoided are geologically mapped on a scale of 1 : 2 000 and investigated in detail at this stage by means of a close spaced sub-surface exploration program. The purpose of this stage of investigation is to provide the designers sufficient data to enable them to plan the remedial treatment.

Detailed Investigation Stage Detailed Investigation Stage (Pre-construction stage)(Pre-construction stage)

The compilation and evaluation of various geological reports and drawings prepared during various stages of investigations should be done. The compilation should also contain details of remedial measures taken for the treatment of weak features present in the reservoir basin, rim and foundation of the dam, having relation to the stability of the reservoir rims, storage of the reservoir and seismic aspects etc.

Construction StageConstruction Stage

The object of post-construction geological investigations is to monitor the critical problems encountered during all the stages of investigation of the project and the effectiveness of treatment provided to those weak features.

It would be necessary to monitor the critical areas which are susceptible to landslides or to keep a watch on the areas where special treatment, such as that against leakage, has been provided so that necessary action, if warranted, can be taken in time.

Monitoring of the seepage in mines and quarries within the zones of the influence of reservoir, precision geodetic repeat surveys across fault planes or in slide prone areas and evaluation of micro seismic and seismic data from the network of seismological observatories has to be continued for a sufficiently long time till adequate reassurance in the matter has been achieved.

Post Construction StagePost Construction Stage

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