Flood Warning and Damages in Alice Springs: Part 1 Executive Summary. Part 2 Tangible Damages Part 3 Intangible Damages & Emergency Procedures
Handmer, John; Smith, D. I.; Greenaway, Mark
E-Publications; E-Books; PublicationNT; Report ; 53/1989
Made available via the Publications (Legal Deposit) Act 2004 (NT).
Power and Water Authority
Report ; 53/1989
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Technical Report WRA89053 Viewed at 03:02:00 on 18/02/2010 Page 98 of 139. I I I I I I I I I I I I I I I I I I I I DESCRIPTION OF "PARTIALLY FU'LL" DAM The propo,ed dam lS similar to that described in the EIS, the pnncipal difference being that instead of permanently storing water up to EL 658, it would empty after the passmg or each 1100d down to a small pool of water at EL 645. Through evaporation the water level wii! drop naturally below EL 645, and can be lowered manually to EL 640. Thus the sacred site at Tnyere-Akerte would no longer be permanently inundated. Some parts of the site would be inundated for varying periods depending on their level. The arrangement of the dam is shown in F 1 & '1 19 ~ .... A low level outlet comprising a circular steel lined concrete conduit would be positioned under the main dam adjacent to the eastern abutment to drain the dam after the passmg of the tlood. The inlet would be arranged so that the dam would continue to drain down to EL 645, ie. only some 9m above bed level. This small pool of water would have an area of approximately 16ha and a volume of 360l\IL and would provide a supply of water to heip prevent the core of the dam drying out. By way of comparison this storage volume is signiticantly less than one peak week \\Illter consumption for Alice Springs. A small solar powered pump would pump water to the top of the upstream filter of the dam. and allow it to trickle down. If lert completely dry, the core may dry out and crack following prolonged dry speIJs which could have implications for dam stability. An energy dissipater is required at the downstream end of the low level outlet. This is expected to comprise a stilling basin constructed of reinforced concrete, and set below the bed of the river (refer Fig 2). A low weir approxImately 1 m high and constructed on a rock bar 70m downstream of the dam would provide a sufficiently high tailwater level for the operation of the basin. Being constructed of permeable rockfill, it would be free draining following the cessation of flow from the dam. The upstream end oi the low level outlet would be provided with trash racks to prevent the entry of large debris into the conduit. The low level outlet would be uncontrolled (ie. no gates and vaives) for levels above EL 645 and the now of water through the outlet would be controlled purely by the water level in the dam. The capacities quoted for the various options examined represent the now through the conduit when the water level is at the control outiet level EL 658. Capacities considered are 5, 10, 20, 50 and 100 cubic metres per second. It would reach this level on average once or twice a year for the smallest oulets or as little as once in 5 years for the largest option. A lower level drain would be provided to permit lowering of the water level to approximately EL 640.00 for t10w by-pass purposes during construction. This outlet would be valved off after completion of the dam however the facility would be retained to permit temporary lowering of the dam water level for special purposes.