HYDROPOWER SITES
The first stage in building a hydropower plant is to find a suitable site. This may appear obvious, but it is important to realize that hydropower is extremely site specific. Not only does it depend on a suitable site being available, but the nature of the project will depend on the topography of the site. You cannot have a hydropower plant without a suitable place to construct it. In the case of large hydropower projects (> 10 MW in capacity), sites will often be a long way from the place where the power is to be used, necessitating a major transmission project too.
A successful hydropower project requires a river with suitable hydrological conditions. The amount of energy that can be taken from the river will depend on two factors: the volume of water flowing along it and the drop in riverbed level (normally known as the head of water) that can be exploited. The available power increases with the volume of water while a steep riverbed carrying a fast- flowing river will generally yield more electricity than a slowly descending, sluggish one of similar size. For a given volume of water, the energy available will depend directly on the head height, or drop in water level, that can be utilized and this is normally larger the steeper the riverbed.
This does not mean that slow-flowing rivers are not suitable for hydropower development. They often provide sites that are cheap and easy to exploit. In contrast, steeply flowing rivers are often in inaccessible regions where exploitation is difficult.
Hydropower sites vary in potential from a few kilowatts to many hundreds of megawatts. Occasionally sites will yield thousands of megawatts. The largest single developed site in the world is the Three Gorges Dam on the Yangtze River in China, with a generating capacity of 22,500 MW. Probably the largest unexploited site is on the Congo River in Africa where a multiple barrage development is estimated to be capable of supporting up to 35,000 MW of generating capacity. This is exceptionally large; most are smaller. Large projects of this type, where developed, are likely to be multipurpose projects involving flood control, irrigation, fisheries, and recreational usage, as well as electricity generation. Smaller projects may be multipurpose or they may simply generate electricity.
In choosing a site, hydrology is important, but so too are geography and geology. Given a river capable of supplying energy, the optimum site or sites for extracting this energy will be determined by the geography. Once a site has been identified, an extensive geological survey will then be necessary to deter- mine the underlying structures. Many hydropower plants are physically massive and can generate enormous pressures, leading to stresses in underlying strata and potential fractures. These can be disruptive if possible faults are not identified before work begins. Where large reservoirs are involved, more stress can be created, and this has in some cases led to the generation of seismic tremors as underlying strata react.
How does one set about locating a hydropower site? Many countries have carried out at least cursory surveys of the hydropower potential within their territory and provisional details of suitable sites are available from the water or power ministries. Sometimes much more detailed information is available but this cannot replace an onsite survey. Indeed surveys carried out as part of a feasibility study form an integral of any hydropower scheme. For a large scheme a feasibility study may account for 1% or 2% of the total cost. For smaller schemes it can reach 50%.