The main aim of all water resource development is to improve the economic and environmental conditions for human living. A water resource project may serve one or more purposes and, accordingly, can be either single-purpose or multipurpose. In most cases, a project would be of a dual or multipurpose type. As such, the entire project needs to be investigated as a unit before the design requirement of a single component, such as a dam, can be finalized. A water resource project may serve one or more of the following purposes:
(i) Irrigation,
(ii) Power development,
(iii) Flood control,
(iv) Industrial water supply,
(v) Domestic and municipal water supply,
(vi) Recreation,
(vii) Fish and wild life preservation and promotion, and
(viii) Navigation.
In almost every water resource project, dam and reservoir are key components of the project. Dams impound water, divert water from a stream, or raise the water level. In exceptional cases, dams may be constructed to impound water-borne sediments and water having a damaging chemical quality. Dams contribute immensely in reducing poverty and impacts of floods and droughts besides rejuvenating rivers in dry season.
Dams also enable recharge of ground water and growth of more biomass. A reservoir is a fresh-water body created or enlarged by the building of dams, barriers or excavations (1).It is seldom that a water resource project consists of only a dam and reservoir facility. In a flood control project, levees and other channel control works, besides the dam and reservoir, are usually desirable.
Water resource projects for power development and water supply (for irrigation, domestic, municipal, and industrial purposes) have a combination of project components to accomplish the desired objectives. Therefore, dams must be planned, designed, and constructed to operate efficiently and harmoniously with other components of the project to achieve maximum benefits at minimum cost.
The economic, environmental and social feasibility, and justification of dam must be examined in combination with those of other project components, and the total project must be evaluated and judged for its feasibility. If the evaluation of a project proposal does not show justification for its construction, it may be dropped or, alternatively, revised and updated with possible justification at a later time. A water resource project should be planned bearing in mind probable physical, economic, and environmental effects.
PHYSICAL FACTORS
Except for flood control projects, availability of sufficient water is essential for all types of water resource projects. In flood control projects, the sudden excess of water is the problem. The source of water is the surface runoff resulting from weather phenomena which are understood only in a general way. Weather conditions can be predicted only as seasonal probabilities. Weather predictions for shorter periods (a few hours or days) can, however, be made with more reliability. Historical measurements of stream flows and rainfall are considered the best available means for forecasting stream flow supplies for water resource projects. At sites where no measurements or only a few measurements have been made, reliable correlation methods are used to estimate stream flow statistics.
There is always some risk involved in building a project either too large or too small at sites of major streamflow measurements. In such situations, alternatives of staged development or other means of adjusting the project size and scope may have to be considered. A flood occurring once in 100 years or less may cause enormous damage. Therefore, stream gauging records of 10, 20 or 30 years, though useful to some extent, are inadequate for flood control projects and spillway design for large dams. Besides, actual measurement of peak flood flows is difficult even if the stream is being gauged. Some other methods of estimating the magnitude of peak floods are invariably used for the planning of such works.
Computation of the stream flow based on high water marks and flood channel dimensions is one such method. Alternatively, stream flow (or runoff) estimation can be based on actual measurements of amount and duration of high rainfall at rain gauge stations in the catchment area upstream of the dam site. The latter method considers factors such as principles of precipitation as affected by stream characteristics in the region, and the catchment characteristics (location, shape, vegetative cover, and geological structure). Extremely large floods are also extremely infrequent floods. Hence, the planner’s judgement is crucial in deciding the size of the flood to be controlled by the project.
Two main factors which determine the site of a water resource project are the areas needing water and the location where water supply is available for development. For economic reasons, the water source must be near the place of use so as to save on cost of conveyance. Also, the source should be at higher elevation than the service area to avoid pumping. In case of projects where water is stored only for the purpose of flood control, there is no conveyance cost involved.
ECONOMIC CONSIDERATIONS
The cost of a water resource project includes capital investment for constructing the project facilities and the annual or recurring expenditure for operation and maintenance (including replacement) of the project. The capital cost includes the costs of planning, investigations, designs, and construction besides the cost of acquiring rights to the use of water, litigation, and rehabilitation of the affected people. The capital cost also includes the interest on the money invested during construction and up to the start of the project. The benefits likely to be received from a water resource project are widely distributed.
As such, the investments on the project cannot be compared with the benefits in terms of monetary units. However, the benefits are expressed, as far as possible, in terms of monetary units and the investment and operational costs are thus compared with the benefits. It is difficult to quantify some types of project benefits. For example, in an irrigation project, the benefits extend beyond the farmer through a chain of related activities to the people of the area. Social benefits (such as protection against loss of life by floods), recreational benefits, etc. are also difficult to estimate in monetary terms.
However, benefits of municipal and industrial water services and hydroelectric power generation can be easily estimated by working out the cost of producing the same results by another reasonable alternative arrangement or by determining the market value of the product. Benefits from a flood control project can be estimated by working out the reduction in flood damages in agricultural, residential, commercial, industrial, and such other activities. The value of the land protected from floods increases and this fact should also be included in the benefits of a flood control project.
ENVIRONMENTAL EFFECTS
A well-planned water resource project should be desirable from economic, social as well as environmental considerations. It should, however, be noted that some of the project components, notably dams and reservoirs, cause adverse environmental effects in the regions of their direct influence.
While trying to achieve major project objectives of a water resource project, the planner must examine alternative plans of dams and reservoirs to minimize adverse environmental effects. Environment is best defined as all external conditions which affect the existence of all living beings. Different living beings affect one another, and the environmental requirements of different living beings are interrelated. Besides, it is generally not possible to evaluate environmental effects in economic terms.
In case of pollution of water and air, however, it is possible to estimate economic loss to some degree. In addition, it is difficult to assign a degree of importance to various environment conditions likely to be judged differently by different persons depending upon their own viewpoint. For example, the people of a hilly region will have a different viewpoint regarding the siting of a dam from those living in the plains where land is inundated during floods and wells go dry during drought
SELECTION OF A PROJECT PLAN
Planning may be defined as the systematic consideration of a project from the original statement of purpose through the evaluation of alternatives to the final decision on a course of action. Planning of water resource project begins with some definite idea about its main purpose.
It is usually economical to have a multipurpose rather than a single-purpose project. From economic considerations, the best project plan is the one for which the ratio of combined project benefits and the total project cost is maximum. The time required to construct a dam and then to first fill the reservoir before the start of the project operation is usually very large (several years) and, hence, the interest on the investment up to the start of the project operation should also be added to the investment costs.
The cost of a dam and other major project features and also the benefits for at least three different sizes (the smallest, the largest and an intermediate) of the project are worked out. Using these computations, size-benefit and size-cost curves for different possible functions are prepared. A proper analysis of all this information would yield the size and functions of the project which would result in maximum benefit-cost ratio.
INVESTIGATIONS
The basic data, usually required for planning of dams and reservoirs, can be grouped in the
following categories (1):
(i) Hydrologic data: Stream flows, flood flows, evaporation, sedimentation, water quality, water rights, and tail-water curves.
(ii) Geological data: Reservoir sites, dam sites, and construction materials.
(iii) Topographic surveys: Catchments, reservoir sites, dam sites, and borrow areas.
(iv) Legal data: Water rights.
(v) Reservoir site cost data: Land acquisition, clearing, and relocation.
(vi) Environmental factors: Fish and wildlife, recreation, scenic, historical, and archaeological.
(vii) Economic data: Economic base for area benefited, crop data, land classification, and market data for various purposes.
The desirable quality of these data would depend on the level of investigations. Investigations for a water resource project are generally carried out in three separate steps (or levels or stages): reconnaissance (or preliminary), feasibility, and pre-construction.
Reconnaissance (or Preliminary) Investigations
The main purpose of such investigations is to screen out the poorer alternatives and to decide the types and amounts of more expensive and time-consuming data (such as stream flow records, topographic mapping, and so on) which need to be collected for making feasibility investigations of the remaining selectable alternatives.
A reconnaissance survey will identify the scope of a project plan with respect to its geographical location, project functions, approximate size of its various components, likely problem areas, and time and cost of conducting feasibility investigations.
Feasibility Investigation
The aim of the feasibility investigation is to ascertain the soundness and justification, or lack of these, of different alternative plans chosen after carrying out preliminary investigations. The analyses need to be of high accuracy and dependability so that the reliability of results, on the basis of which the final selection of the project plan is made, may not be questioned.
It should, however, be noted that the feasibility investigation does not mean the end of the project planning. Some minor changes are always required to be made for various reasons during final designs before construction, during construction, and even during project operation.
Pre-construction Investigations
The final adoption of provisionally selected project site and its size and functions begins after the project has been approved and funded for construction. It is essential that final designs consider any new information which might have been obtained or received during the time interval between feasibility investigation and the final design.
For example, an extreme low runoff season or a flood of large magnitude might have occurred during this intervening period and this may necessitate changes in the estimates of the critical dry year project water supplies or of the frequency of occurrence of a flood of given magnitude.
CHOICE OF DAMS
Most of the dams can be grouped into one of the following two categories:
(i) Embankment dams, and
(ii) Concrete dams.
Embankment dams include earth-fill dams and rock-fill dams. Concrete dams include gravity dams, arch dams and buttress dams. Preliminary designs and estimates will usually be required for different types of dams before one can decide the suitability or otherwise of one type of dam in comparison to other types. The cost of construction is the most important factor to be considered while making the final selection of the type of dam.
Besides, the characteristics of each type of dam, as related to the physical features of the site and its adaptation to the purposes of the dam, as well as safety, and other relevant limitations are also to be considered for selecting the best type of dam for a particular site. The following are the important factors which affect the choice of the type of dam:
(i) Topography,
(ii) Geology and foundation conditions,
(iii) Material available, and
(iv) Size and location of spillway.
