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 128 of 139. I I I I I I I I I I I I I I I I I I I I level in the dam exceeds the control spillway level EL658.0. This only occurs once on average every 3 years. For smaller more frequently occuning floods. discharge occurs through the low level ouUet and it doesn't have the ability to transfer sand from the spillway channel. The advantage of the "partially full" dam is that it provides greater attenuation for most floods. As bed degradation increases exponentially with flow, the greater attenuation would mean a reduced rate of bed degradation. Bed degradation, if left unchecked, is estimated to occur at a lesser rate than the 12,OOOm'/annum for the "full" dam. It should be noted that sediment transport rates can not be estimated exactly but the nett effect is expected to be that less sand and gravel would need to be carted from the sediment traps to the spillway channel for the "partially full" dam. The rate actually carted would be determined by monitoring bed levels. If monitoring of river bed levels shows that discharges through the control spillway are not frequent enough to move the required volume of sand from the spillway channel into the river, then some supplementary work may need to be carried out within the river channel downstream of the dam. River "Freshes" The partially full" dam would not have any significant impact on the frequency of "freshes which occur in the river on average every 6 months or so, but the size of flow would be affected. Because the capacity of the low level outlet from the dam would be 20m3/s, the contribution of the Todd River to these regular freshes" would be limIted to 20m3fs for the smaller more frequent floods. Catchments on the Todd River downstream of the dam and the Charles River would continue to contribute to these "freshes unaffected by the dam. To put these tlows in perspective, a flow of 20m3/s would represent a flow O.4m deep over Wills Terrace, sufficient to close the causeway. Larger flows could be expected on average once every three years when discharge would occur through the control spillway from larger floods. Wills Terrace Causeway The "full" flood mitigation dam would have had little impact on the expected time of closure of Wills Terrace causeway. The 'partially full" dam would release floodwater at a reduced rate but for a longer time. This in turn means that Wills Terrace would remain closed for a longer time, on average an estimated 5 days per year unless the capacity of the causeway was increased. to
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