Selection of Dam Site & Various Surveys for Site Selection

Selection of Dam Site & Various Surveys for Site Selection:

Objectives and Scope

The objectives and scope of the Survey was to enhance the WCD Knowledge Base by broadening the indepth studies already sanctioned by the Commission. The Survey aimed at complementing and supplementing the WCD work programme components in a way that added value and consequence to the arge dams debate.

This section of the report sets out the key Survey objectives and scope of work undertaken. It concludes with the limitations of the Survey.

 Objectives

The primary objectives of the Survey were:

·         to seek broader patterns and trends relating to the performance, social and environmental impacts, and decision-making aspects of large dams than is otherwise possible with only the 8-10 focal dam/basincase studies, thematic reviews, regional consultations, and submissions;

·         to provide a better indicator of regional experience than is currently available, through the emphasis on representation of all the major regions of the world in the sample; and

·         to allow a broad entry point to the emerging evidence from the WCD integrated Knowledge Base.

A secondary objective of the Survey was:

·         through use of the Survey questionnaires, to generate an awareness among large dam owners and responsible institutions regarding the minimum threshold and type of data that should be monitored and recorded accurately, responsibly and reliably

 Scope

       A variety of large dams of different types (eg storage, run-of-river), ages (eg dams from the 1930.s through the 1990.s), functions (eg water supply, irrigation, hydropower, flood control, multipurpose and other), heights, areas, ownership (eg public, state and private), and regional locations were included in the Cross-Check Survey. Each dam questionnaire provided date records with time series dimensions where feasible. Analysis was performed on the full set and on specified sub-sets of dams in the sample. Large dams have been added from previous studies or surveys but new data for these dams was collected.

The collection of data for all large dams in the Survey sample was conducted via a set of questionnaires including multiple categories and criteria for reporting. Previous experience and studies stating the inadequacy of questionnaire surveys for capture of intended information if too elaborate in design, influenced the nature of the questions posed in the Survey questionnaires. Information was often requested through specified lists aimed explicitly at minimising the level of detail required for reporting, and thus minimising barriers to provision of data and potential misinterpretation. Consequently, the scope n of the issues assessed in the Cross-Check Survey was purposely limited.

 Data Collection Procedure

Data for the focal and non-focal dams was collected in conjunction with the execution of the case studies, while information for dams drawn from other databases underwent supplementation and verification. The data for the complementary subset of large dams was collected by commissioned contributors and/or submitted by parties with access to the required information. The approach for developing the data records for the Cross-Check Survey has included research of published reports and secondary data sources available in the public domain, as well as contact with the following types of organisations by requesting information on standardised forms:

      I.        dam operators and utilities;

     II.         key government departments responsible for the project (eg Ministry / Department of Irrigation / Power of State or National government; or appropriate river basin development authority);

    III.         local institutions/universities that may have conducted research on the dam;

·         local NGOs/networks working on dams at the state / national level;

·        •NGOs/networks with a regional and/or international focus on dams issues;

  IV.         industrial/engineering firms with international experience and involvement in dams (with

               permission of the host country);

   V.        multilateral and bilateral agencies for data on large dams available with them;

  VI.          International professional associations (ICOLD, ICID, IHA, IEA) with access to data.

All submissions contributing to the final analysis of the Survey were required to include data accessed from at least two independent source documents. During the internal review process all contributors were requested to fill in a data source matrix to ascertain the degree of material referenced for each questionnaire. Moreover, contributors were requested to invite participation and cite any divergent perspectives from stakeholders where possible.

Site Selection

During the feasibility studies, the preliminary site selection will be dependent on the project

purposes within the Corps’ jurisdiction. Purposes applicable to dam construction include navigation, flood damage reduction, hydroelectric power generation, fish and wildlife enhancement, water quality, water supply, and recreation. The feasibility study will establish the most suitable and economical location and type of structure.

Investigations will be performed on:

1.      Hydrology

2.      Meteorology

3.       Relocations,

4.      Foundation

5.       Site geology

6.       Construction materials

7.       Appurtenant features,

8.      Environmental considerations

9.      Diversion methods.

Selection factors.

(1) A concrete dam requires a sound bedrock foundation. It is important that the bedrock have adequate shear strength and bearing capacity to meet the necessary stability requirements. When the dam crosses a major fault or shear zone, special design features (joints, monolith lengths, concrete zones, etc.) should be incorporated in the design to accommodate the anticipated movement. All special features should be designed based on analytical techniques and testing simulating the fault movement.

 (2) The topography is an important factor in the selection and location of a concrete dam and its appurtenant structures. Construction as a site with a narrow canyon profile on sound bedrock close to the surface is preferable, as this location would minimize the concrete material requirements and the associated costs.

(3) The criteria set forth for the spillway, powerhouse, and the other project appurtenances will play an important role in site selection. The relationship and adaptability of these features to the project alignment will need evaluation along with associated costs.

(4) Additional factors of lesser importance that need to be included for consideration are the relocation of existing facilities and utilities that lie within the reservoir and in the path of the dam. Included in these are railroads, powerlines, highways, towns, etc. Extensive and costly relocations should be avoided.

(6) The method or scheme of diverting flows around or through the damsite during construction is an important consideration to the economy of the dam. A concrete gravity dam offers major advantages and potential cost savings by providing the option of diversion through alternate construction blocks, and lowers risk and delay if overtopping should occur.

Outline of the survey methods:

Generally, a subsurface dam site is selected according to the following procedure:

1) Interpretation of satellite images and aero-photographs

2) Geological and topographical survey by preliminary exploration

3) Estimate of the geological structure by geophysical surveys such as electric

     soundings

4) Verification of the geological structure by test drillings and permeability tests

5) Estimate of the flow mechanism of groundwater by observation of groundwater level

Geological studies:

This is carried out to collect data regarding

1.   Geological features folds, faults, fissures etc their

            nature and extent.

2.   Water tightness of the reservoir basin

3.   Existing and potential slide area

4.   Assessment of valuable minerals in reservoir area

5.   Ground water condition

6.   Seismic conditions etc.

Bore hole logging and testing should be utilized to enhance the amount of information obtained from each hole drilled. Certain techniques work better in some environments than in others; thus, the following techniques listed must be utilized discriminately according to their applicability to the site conditions. Bore hole logging systems include caliper logs, resistivity logs, SP logs, sonic logs, radioactive logs, etc.

Foundation Studies:

Foundation Investigation for earth, rock fill and masonry dams includes investigation to determine properties of foundation soil, hidden weak spots and shear zones etc and depth of overburden.

 Usual methods of exploration include

◦ Bore holes

◦ Excavation of open pits

◦ Tunneling into the side of the valley

◦ Core drilling

Meteorological studies:

On the watershed plan, normal annual isohyets (rain fall contours) location of rain gauge stations, gauge and discharge sites and interprovincial boundaries are supposed to be shown.

·        Assessment of rainfall in the catchments

·        Collection and evaluation of data and frequencies

          of heavy rain fall

·        Mean monthly temperature,

Hydrological Studies:

Gauge and discharge observation, past flood, hydrographs to fix spillway capacity, rain fall runoff correlation studies etc.

·        Loss of storage capacity by silting is a vary vital issue for determining the useful life of the reservoir.

·        Sediment observation shall be carried for 3 years for suspended load, bed load and natural soil conditions including catchment characteristics from point of erosion.

Ground water investigations and permeability testing are necessary for several reasons. These investigations provide the basis for design of any dewatering systems required during construction. They also provide the data to evaluate the reservoir’s capability to impound water and to design seepage and uplift control required in the foundation beneath the dam and in the abutments. These data also provide the basis for making assumptions of uplift on rock wedges.