Case Study

Environmental impact assessments of projects require on-going multi-disciplinary surveys, where diverse data can be collected, stored and integrated at source, and evaluated at distant sites. The difficulties involved in such impact studies are indicated by considering one of several projects regarding the possibility of impounding the Purari River in Papua New Guinea.

Civil Engineering Impact Studies - Impounding the Purari River, Papua New Guinea

Over recent decades studies have been made to try to estimate the impact of impounding rivers in underdeveloped countries, and to assess the wealth and well being of relevant local populations.

Usually the proposed impoundment will provide hydro-electric production to service large industrial corporations, but may also allow new fisheries and/or new intensive agriculture, but these developments often turn out to be deleterious to pre-existing local economies downstream of the impoundment. Further, subsequent to an impoundment, few studies are funded to monitor environmental changes to the areas affected. To assess and monitor impacts in relatively unknown tropical areas which have poor access and seasonably unmanageable communications, is difficult, hazardous, and tedious. Many experts and many surveys are required. An understanding of the life styles and languages of local people and the way they have adapted to the environment, and its tremendous biodiversity is challenging to say the least. Between 1977-1982 some 40 research projects were carried out in relation to an impact assessment, and members of some of those teams were later involved in the development of Fireflower.

The Purari is the third largest river in Papua New Guinea. Its drainage basin covers more than 33,670 square km and its annual average discharge into the Gulf of Papua is 2,667 cubic meters per second, with annual maxima in excess of 10,500 cubic meters per second. It drains the highest mountains in the central cordillera through its many tributaries. Annual average rainfall in the mountain areas may exceed 8000 mm. The tributaries flow through very high mountainous country, until the main river leaves the foothills where one dam was proposed. Thereafter it traverses some 150 km of flat lowland areas before entering the sea through a deltaic plain. The lowlands are densely forested, marshy and swampy, infected with malarial mosquitoes and myriad parasites, and are still poorly known.

In the picture above, a leaden monsoon sky heralds more rain, and the banks of the Purari River in New Guinea are flooded some 45 km upstream from the sea. For much of the year, immense amounts of water from high mountainous catchments along the backbone of the island, back up downstream on the coastal plain, behind the rising sea tide. The river then floods parts of the plain. In these situations the waters are always fresh, but the forest which dominates these areas is tidal, and the forest floor floods and dries as the tide ebbs and flows each day. Here the depth of inundation at the river banks varies from a few centimeters to a meter according to the elevation and height of the tide, but during storm surges, flooding can increase the level to 3 meters and the flood can then temporarily inundate a further zone of the lowland plain.

During the monsoon, the area is still tidal but much is then permanently flooded and the forest floor depicted here remains under water for periods of 2-4 months and to depths up to 3 meters. The lowland forest vegetation is extraordinarily diverse with as many as 350 different tree species per hectare. Species common in salty tidal areas like Avicennia, Bruguiera, Rhizophora, Sonneratia, Hibiscus, Metroxylon and Pandanus spp. also occur in these fresh water swamps and may dominate the waterfront right down into the delta. An abundance of animal life swims, crawls, walks, flies and predates here 24 hours a day, although the spectrum of active species changes from dusk to dawn. Fishing bats hawk the waters at night, taking over from fishing eagles in the day. Photo - T. Petr