1.6 Site assessment and selection of type of dam1.6.1 General site appraisalA satisfactory site for a reservoir must fulfil certain functional and tech-
nical requirements. Functional suitability of a site is governed by the
balance between its natural physical characteristics and the purpose of the
reservoir. Catchment hydrology, available head and storage volume etc.
must be matched to operational parameters set by the nature and scale of
the project served. Technical acceptability is dictated by the presence of a
satisfactory site (or sites) for a dam, the availability of materials suitable
for dam construction, and by the integrity of the reservoir basin with
respect to leakage. The hydrological and geological or geotechnical
characteristics of catchment and site are the principal determinants
establishing the technical suitability of a reservoir site. To these must be
added an assessment of the anticipated environmental consequences of
construction and operation of the dam, alluded to in Section 1.2.4. They
are not considered further here.
The principal stages involved in site appraisal and leading to selec-
tion of the optimum dam site and type of dam for a major project are as
indicated schematically in Fig. 1.6.
The considerable time which can elapse between initial strategic
planning, with identification of the project requirement, and commence-
ment of construction on site will be noted. A significant proportion of that
time may be attributable to the ‘political’ decision-making processes and
to arranging project funding.
In the reconnaissance phase, which may extend over a substantial
period, the principal objective is to collect extensive topographical, geo-
logical and hydrological survey data. Large-scale maps and any records
already available provide the starting point, but much more detailed
surveys will inevitably be required. Aerial reconnaissance, employing
modern sensors in addition to the traditional photogrammetric survey
techniques, has a particular rôle to play in the preparation of accurate and
large-scale site plans (e.g. 1:5000 and larger). In the hands of an experi-
enced engineering geologist as interpreter, aerial surveys also provide
valuable information on geology, on possible dam sites and on the likely
availability of construction materials. Hydrological catchment and river
surveys are directed to determining rainfall and run-off characteristics, and
assessing historical evidence of floods etc.
The feasibility report prepared at the conclusion of the reconnais-
sance phase assembles and interprets all available information, data and
records, and makes initial recommendations with respect to the technical
and economic viability of the reservoir. Options with regard to the loca-
tion, height and type of dam are set out, and comparisons drawn in terms