VubbLcation, n° 11? de. I'A&iocUvUon ïnteAnatlonaZe du Science* HydtologlqueA Sympoilum de Tokyo (Vicembie Î975) ENVIRONMENTAL IMPLICATIONS OF DAMS AND MAN-MADE LAKES H. BANDLER M.I.E. Aust., Turramura, N.S.W., Australia Preface Much of the investigation work mentioned has been and is being carried out by the Metropolitan Water, Sewerage and Drainage Board (M.W.S. & D.B.). The opinions expressed are not necessarily those of the Board but are obser­ vations of the author. Synopis The implications of the damming of a river and with it the creation of a man-made lake, extend over a very wide field. The technological considerations are usually given high precendence at the time of design and construction. Topography, geology and structural aspects will be the subject of investigations. The aspects of relocation of communities, the loss of valuable farming land, areas and monuments of tradition deserve serious consideration. The housing of the work force and the consequences of a construction village are hard to predict at the time of establishment. The newly created lake, once the dam is closed, will bring about a new environmental situation which at no time is static; changes occur in the chemical and biological make-up within the water body. The surrounding area from which the water enters the lake will respond to a variety of management techniques which will have an effect on flora and fauna as well as quality and quantity of water in the lake. The length of time of existence of the lake will be subject to the control of erosion to pre­ vent siltation. Making use of the lake for recreational purposes brings ad­ vantages as well as problems with it. Water operation policy has significant effects on the population downstream. Préface La majorité des travaux mentionnés ici sont actuellement entrepris -ou sur le point de l'être- par le Comité Métropolitain des Eaux, Egouts et Drainage (M.W.S. & D.B.). Les points de vue exprimés ne reflètent pas nécessairement les opinions de ce Comité ; il s'agit de remarques personnelles de l'auteur. 579 60.2 Résumé Les conséquences impliquées par le barrage d'un cours d'eau et, partant, la création d'un lac artificiel, ont une portée très vaste. Les considérations d'ordre technique sont en général prioritaires au moment de la conception et de la construction ; topographie, géologie et aspects structurels feront l'objet de diverses recherches mais le relogement des communautés, la perte de terre arable, la disparition de zones et monuments traditionnels méritent d'être pris en considération. Au moment de la construction, il est difficile de prévoir le logement de la main d'oeuvre et les conséquences éventuelles de la création d'un village car, une fois le barrage terminé, le nouveau lac donnera lieu à un nouvel environnement en constante évolution tant dans la composition chimique que dans la vie biologique de l'étendue d eau. La zone environnante dont les eaux pénétreront dans le lac va réagir à tout un éventail de nouvelles techniques qui affecteront la flore et la faune ainsi que la qualité et la quantité des eaux du lac. Quant à la durée de vie du lac, elle sera sujette à la lutte contre l'érosion qui évitera l'envasement. En outre, l'utilisation du lac à des fins de loisirs comporte avantages et inconvénients. Enfin, la politique appliquée en matière d'utilisation des eaux a une grande influence sur la population qui vit en aval du lac. Introduction Examination of Desirable Studies To supply drinking water to the community by the provision of water storage as a means of overcoming seasonal insufficiency dates back to antiquity. Such water impoundments have always had environmental impacts, but the implications have only more recently become recognised as significant with our present popula­ tion pressures and environmental awareness. The initial stage is the period of feasibility studies. During this period the decision whether to build the dam is established subject to political approval. Its location is determined, purpose, use, type and basic economics are generally fixed. The next stage is the period of planning, design and construction which continues up to and until the closing of the dam. As soon as the dam is closed reservoir filling commences and ecological instability begins. This instability still continues even after the dam is filled and is operative, but adjustments are more gradual and constant variations take place throughout the remainder of the life of the reservoir. Depending on the type of structure and the method of construction some of these stages will be overlapping. Feasibility studies of certain features can extend into the design and planning stage and some dams are being filled during construction. Minor changes in climatic condition, lack of rainfall, flood flows and excessive drawdown will have destabilising effects on the environment of the man-made lake which will be established by the dam. Certain major impacts will require study and consideration. The impounded lake, the dam, the catchment area and the river downstream have to be considered as eccosystems which are being subjected to change. Independent of whether the project is a large or a small one, many aspects of the system are subjected to rather sudden changes. During the early stagesof the feasibility study for the dam across the river channel, it will become apparent that the submergence of 580 60.3 the valley upstream will necessitate displacement of those people living and/or working in the area. Another socio-cultural impact will be the introduction of a group of persons who will be working on the construction of the dam. This com­ munity may, if the project is large, take up residence for long periods and estab­ lish local associations which will have considerable impact on the area. Finally, there is the population who will be resident or transient to maintain the struc­ ture and the lake for the remainder of the term of its existence and others who may choose to reside in the area. The creation of a new water body which had not been there before will be al­ most sudden in its evolvement when the dam is being closed and water fills the valley. The change from a terrestial environment to an extensive flat water sur­ face brings with it changes in micro climate, flora and fauna within the lake area. There will also be variations within the water body due to stratification at various dep ths. The dam will also change the regimen of the river flow downstream due to interception of run-off as well as human manipulation of draw-off or release of water. This will have more or less predictable hydrologie implications. The disturbance of the earth's crust, the additional loading imposed by the dam structure and the weight of water in the lake, can bring about seismic activity which needs to be monitored. None of the changes caused by the establishment of the lake can be reversed. Minor changes are constantly taking place in and around the water body due to in­ flow and outflow conditions. Maintenance of the lake to ensure its permanency, control of water quality and conditions of the catchment area as well as safety of the dam structure and the population in the vicinity and downstream will be envi­ ronmentally meaningful. Selection of Warragamba Dam for Examination It is proposed to examine some of the environmental implications, in retro­ spect, with regard to Warragamba Dam and Lake Burragorang in N.S.W., Australia. The dam was built for water supply to the city of Sydney. The construction exten­ ded from 1942 to 1960 if the time of early investigation is included. The physical details give an indication of the magnitude of the structure. (1) TABLE 1 Storage Capacity 2 057 000 Mega litres 452,500 Million Gallons Length of wall 350 m 1,150 feet Length of Spillway over Crest 95 m 310 feet Concrete in Wall 1.2 Million m3 1.6 million yard3 Maximum Height of Concrete 137 m 450 feet Greatest Depth of Water 104 m 340 feet Width of Wall at Base 104 m 340 feet Catchment Area 8,762 km2 3,833 miles2 Annual Average Rainfall 825 mm 32.5 inches Annual Average Evaporation 1,270 mm 50 inches Lake Area at Full Supply Level 75 km2 29 miles2 Maximum Length of Lake 52 km 32% miles Length of Foreshores 353 km 220 miles Hydro Electric Station 50,000 KiloWatts Estimated Cost (up to completion £35% Million in 1960) Water famines and serious shortages were a frequent occurrence since the establishment of the first white settlement on the Australian Continent at Sydney Cove in 1788. 581 60.4 The concept of using the Warragamba River as a source of drinking water for the population of Sydney was apparent at an early stage of colonial develop­ ment. (2) But only in August 1943 was it possible for construction of Warragamba Dam to proceed. The Dam, approximately 75 km (40 miles) from Sydney, was built in a steep sided gorge which was cut by the river in Triassic rocks of the Narrabeen group and the Hawkesbury sandstones. The Warragamba River flows have been recorded since 1880. These records show the irregular rate of flow which continues as inflow to the lake. By statistical analysis of river flow records and comparison with other methods, including that for maximum possible precipitation, a peak intensity of flood flow of 14,0003m /sec (500,000 cusec) was arrived at. The minimum flow for a period of 96 months is the basis for drought con­ siderations .