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
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Power and Water Authority
Report ; 53/1989
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Technical Report WRA89053 Viewed at 03:02:00 on 18/02/2010 Page 94 of 139. I I I I I I I I I I I I I I I , I I I I omiet. A low. rocklill bund would be reeuired at the rock bar 70m downstream elf the dam to maintain the necessary tail water level for the dissipater. The capacilY of the low level outlet proposed and the storage volume of the dam up to the previous control spiilway level are both small in comparison to the volume of flow In a large nood hence the requirements for the control, main and auxiliary spillwavs and overall dam crest level are essentially unchanged from the "full" dam option. Table I and the attached explanatory notes detail the features of a "partially full" dam with a range of outlet capacities from 5 rn'fsec to 100 m3/sec compared with those of the prevlOusly examined "full" and "empty" dams. The following are the most significant points of variation. Effect on Tnyere-Akerte The whole site will be not be permanently inundated except during periods of rain and the permanent pool level can be expected to drop by at least 1 m due to evaporation for an average of 6 weeks per year. The average period of inundation per year above the permanent water pool is relatively independent of the outlet level but varies with the size of outlet. For a 20 mJ/s outlet capacity, the site would be inundated for an average of 5 days per year. The site is quite large and covers a range of levels, and a small pan wii! be inundated for longer periods. However the facility to lower the water level to as low as EL640m for particular purposes would mean that the site could be made free of water for short periods if necessary. Effect on Flood J'.fitigation The presence of the low level outlet signitlcantly reduces the peak nows for small to moderate isolated storms however the improvement diminishes for larger and longer duration storms. The perfonnance for large t100ds is similar to that for a "full" dam. The attenuation of t10ws results in longer periods of low now with the Wills Terrace river crossing likely to be closed for an average of 5 days per year for a 20 m3/s outlet. Effect on Costs The removal of control valves. gates and pits from the proposed outlet partly compensates for the increased pipe size required. There may be additional costs for improving the embank."I1ent stability under variable water level conditions which have not yet been induded. The estimated cost of the "partially full" dam is $21.57 million. Effect on Sedimentation The larger size low level outlets reduce sediment accumulation in the dam to as little as half the rate for the full dam. The lower downstream nows will also reduce downstream sediment degradation and hence reduce the volume of sand tor replenishment required to be transported from the sediment traps. These advantages are to some extent compensated by the increased difficulty in re-introducing bed-load sediments taken from the Todd and Colyer Creek sediment traps when they must be introduced to the river bed rather than the spillway channeL The 20 mJ/s outlet represents a compromise.