Initial Investigation Kangaroo Creek Reservoir Modifications

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Initial Investigation

Kangaroo Creek Reservoir Modifications

Adelaide & Mt Lofty Ranges NRM Board

Internal Ref: 11029-2

Revision Date Approved Details A 17 June 2011 DJ Client Review B 18 July 2011 DJ Final

© Southfront 2011

Contents

1 Introduction 1 1.1 Torrens Taskforce 1 1.2 Investigation Scope & Objectives 1

2 Kangaroo Creek Reservoir Modifications 2 2.1 Flood Mitigation Scheme 2 2.2 Hydrology and Flood Plain Review 2 2.3 Potential Scenario 3 2.4 Flood Protection Assessment 3 2.5 Yield Assessment 4 2.6 Spillway Gate feasibility 5

3 Conclusions, Further Work 6

4 References 7

Tables Table 2.1 Modified Western Suburbs Flood Protection Standard 3 Table 2.2 Additional Kangaroo Creek Reservoir Yield 4

Figures Figure 2.1 Flood Frequency Comparison (Holbrooks Road) 4 Figure 2.2 Kangaroo Creek Reservoir Water Level (1980-2011) 5

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1 Introduction

1.1 Torrens Taskforce The outbreak of cyanobacterial blooms over successive summers from 1998 to 2005 on the Torrens Lake as well as the difficult to manage impacts and lack of clear solutions led to the establishment of the Torrens Taskforce in September 2006.

With the assistance of the and Natural Resources Management (NRM) Board, the Torrens Taskforce was established and led by Professor Don Bursill, a local scientist and water quality management practitioner.

The recommendations of the Torrens Taskforce from its first review are presented in Torrens Taskforce Summary of Findings (Torrens Taskforce, 2007), which was subsequently reviewed and updated to reflect progress in Torrens Taskforce Actions Update (Torrens Taskforce, 2009). One of the priorities of this review included:

• determination of the feasibility of an ‘amenity flow’ for the Torrens Lake (including what the source water should be) to reduce or eliminate blooms

The construction of large water storages and diversion structures in the upper Torrens catchment has ensured that, particularly during summer months, flows do not overtop the Gorge Weir except during extreme storm events. The provision of an amenity flow, through return of a low flow to the urban River Torrens by releases from Kangaroo Creek reservoir (or alternate sources) was proposed as a strategy which would enable a consistent turnover time in the Torrens Lake waterbody reducing the likelihood of blooms.

The 2007 Taskforce report noted that “a review of the yield possible from modifying Kangaroo Creek reservoir’s flood mitigation function would be worthwhile. In the early 1980’s approximately a quarter of this dam’s capacity was sacrificed for flood control purposes (approximately 6.5GL). The resulting River Torrens Flood Mitigation Scheme provided a 200 year flood protection standard. A review of the potential to recover some of the reservoir space (yield) is warranted. Such a review could result in a compromise between lowering the flood control performance of the River Torrens Flood Mitigation Scheme and provision of an allocation of local catchment water for urban environmental flows.”

This investigation has been commissioned by the Adelaide and Mt Lofty Ranges Natural Resources Management Board to provide some initial estimates surrounding the aspects described above, to inform further consideration of the feasibility of an amenity flow.

1.2 Investigation Scope & Objectives This investigation was required to provide the following assessment:

• A first order estimate to determine what additional flood risk would result if the reservoir’s capacity (full supply level and therefore annual yield) was incrementally increased

This investigation is based on a review of available reports and data. The collection of field data and execution of hydrological modelling is beyond the scope of this investigation.

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2 Kangaroo Creek Reservoir Modifications

2.1 River Torrens Flood Mitigation Scheme A number of floods from the River Torrens impacted on the early settlement of Adelaide, and in particular the western suburbs which remained undeveloped until the completion of the lower reach channel works (Breakout Creek) in the 1930s.

Construction of the Kangaroo Creek dam was completed in 1969. This dam was built at the time to provide the dual purpose of water supply (water from Kangaroo Creek is released into the River Torrens as required to maintain the level in , with the water being diverted at the Gorge Weir) and some flood mitigation benefits (understood to be a 1 in 60 year standard at that time).

The level of flood protection provided by the existing infrastructure was subsequently re- assessed, with high economic losses estimated to occur for any flood with a magnitude greater than a 35 year average recurrence interval (ARI) (Engineering & Water Supply Department, 1980).

In 1982-83, as part of the River Torrens Flood Mitigation Scheme, modifications were carried out to the Kangaroo Creek Reservoir (along with works along the River Torrens urban reaches) to ensure protection against a 200 year ARI flood.

2.2 Hydrology and Flood Plain Review After implementation of the River Torrens Flood Mitigation Scheme, a review of both the river hydrology (Lange Dames Campbell, 1995) and flood plain (SMEC, 1998) was commissioned to:

• Confirm the performance of the completed major works recommended in the 1980 Study • Utilise the additional streamflow and rainfall records within the catchment to increase confidence in peak flow estimates • Assess changes in floodplain behaviour due to changes to channel banks

The reviews were also able to take advantage of improvements in modelling techniques and computational capacity since the 1980 Study.

The hydrology review produced a conclusion of note that “the frequency of overtopping of the River Torrens in the western suburbs of Adelaide is estimated to be 1 in 150 years” - compared to the 1 in 200 years provided for in the works constructed following the 1980 Study.

This Lange Dames and Campbell report also highlights the impact that the Kangaroo Creek reservoir level (prior to the onset of a storm) has on downstream peak flows. Consequently this Study adopted a joint probability analysis, considering a range of ARI rainfall events and reservoir start water levels. Approximately 240 combinations were modelled using the hydrological model, from which the corresponding probability of each combination was used to assign an ARI to a given flood peak discharge, at several locations along the river.

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2.3 Potential Scenario This investigation seeks to assess the impact on both downstream flood protection, and reservoir yield, brought about by incrementally raising the spillway crest level to supplement the reservoir water supply storage by 1, 2, 3 and 4GL respectively.

2.4 Flood Protection Assessment To rigorously assess the effect of a change to the Kangaroo Creek reservoir outlet and compare this directly against the 1995 base case scenario would require an amendment to the hydrological model, execution of each of the combination scenarios, followed by a statistical analysis on the output. This is beyond the scope of this investigation, and hence a simplified approach has been developed to provide an initial indication of the impact of modifying the reservoir outlet condition.

It should be noted that due to the nature of the assumptions made, this approach is considered to overestimate the reduction in flood protection standard.

‘Volume – Gauge Height’ and ‘Discharge – Gauge Height’ curves for the Kangaroo Creek reservoir presented in the 1995 review have been reviewed, and from these it has been estimated that:

• A 1m increment in elevation above the current spillway crest level has an associated 1 GL increment in storage • A 1m increment in elevation above the current spillway crest level has an associated 32 m3/s increment in discharge rate

For the purpose of this assessment, it has been assumed that peak flood outflows from the Kangaroo Creek reservoir for a given ARI will increase by the above increments. It is acknowledged that this is an overestimate due to the mitigating effect of the higher outflow rate.

The peak flood flows in downstream reaches has also been assumed to increase by the corresponding increment. It is acknowledged that this may be an overestimate due to the potential for differences in timing between the peak discharge from the reservoir and the peak at a given downstream location.

The increased flood flows, at Holbrooks Road, corresponding to each of the Kangaroo Creek reservoir supplementary storage scenarios are presented in Figure 2.1. This indicates that the existing 150 year ARI protection (corresponding to a flow of 410 m3/s) would be reduced to a standard as summarised in Table 2.1 below.

Table 2.1 Modified Western Suburbs Flood Protection Standard

Supplementary Storage (GL) Protection Standard (years) 1 130 2 100 3 80 4 70

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900 Existing 800 1GL Supplementary Storage 700 2GL Supplementary Storage /s)

3 600 3GL Supplementary Storage 500 4GL Supplementary Storage 400

300 Peak Discharge (m

200

100

0 10 100 1000 ARI (yrs)

Figure 2.1 Flood Frequency Comparison (Holbrooks Road)

2.5 Yield Assessment The provision of additional water supply storage within the Kangaroo Creek reservoir will enable an increase in yield at the reservoir. An initial estimate of this increased yield has been undertaken through reference to the history of overtopping of the spillway crest, and corresponding flow volumes over the Gorge Weir.

The gauge record of daily water level readings at the reservoir has been obtained (refer Figure 2.2). This record indicates that over a 31 year period, reservoir levels have surpassed the current spillway crest level on 12 occasions (ie. there is a 39% probability of a spill event occurring in any given year).

With the exception of 1 event, each of these spill events exceeded a spill volume of 4GL, and consequently the additional average annual yield is equivalent to the product of supplementary storage and annual spill event probability. Estimated yields associated with incremental storages are summarised in Table 2.2 below. It should be noted that the reliability of these additional yields are relatively low.

Table 2.2 Additional Kangaroo Creek Reservoir Yield

Supplementary Storage (GL) Additional Average Yield (GL/yr) 1 0.4 2 0.8 3 1.1 4 1.5

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60 Kangaroo Creek Dam Level

Spillway Crest 50

40

30

Gauge Height Height (m) Gauge 20

10

0 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Figure 2.2 Kangaroo Creek Reservoir Water Level (1980-2011)

2.6 Spillway Gate feasibility The concept of providing flood mitigation storage via a rapid release during the onset of a storm was considered in the 1980 Study, rather than substituting water supply storage for flood mitigation storage, was discarded as follows:

“A method of flood mitigation sometimes used in the provision of spillway gates, and the operation of these gates such that, by prudent release of water an expected flood peak could be effectively reduced. Such a measure requires an efficient and reliable flood prediction system that would be capable of accurate prediction of inflows and thus allow appropriate releases to be made. While an uncontrolled free overflow spillway will not make a flood situation worse, an inappropriate gate operation can significantly worsen a flood peak.

The response of the catchment above Kangaroo Creek dam is fast. This reduces the opportunity for pre-release and increases the reliance on the flood protection system. It was concluded that the hydrological conditions existing in the catchment and the perceived operational difficulties precluded the provision of spillway gates as an option.”

(Engineering & Water Supply Department, 1980)

The 1995 hydrology review identifies that the critical duration (in terms of peak flow in the western suburbs), is 6 hours for all reservoir water level conditions, except for the condition where the reservoir is full in which case the 30 hour duration is critical.

For these scenarios where the 6 hour duration is critical, the conclusion that spillway gates cannot feasibly provide a benefit appears to be sound. Spillway gates may provide some benefit in the ‘reservoir full’ scenario, however determination of the extent to which this would be of benefit requires more detailed analysis.

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3 Conclusions, Further Work

This investigation has provided some initial estimates of the impacts resulting from modifying the Kangaroo Creek Reservoir outlet conditions. These results indicate that the level of flood protection provided by the River Torrens Flood Mitigation works completed in the early 1980s would be significantly reduced through the reallocation of a portion of flood storage to permanent water storage.

The expected community and political ‘tolerance’ for a reduction in flood protection standard is likely to be low. Of particular concern is that the hydrology review (Lange Dames Campbell, 1995) has already downgraded the level of flood protection provided by the flood mitigation works to a 150 year ARI standard, from the original 200 year ARI standard, and the potential for this standard to be further downgraded when another review is undertaken that takes into account intervening urban catchment development densification and design rainfall intensity increases associated with climate change.

It is noted that the flood protection standard impacts estimated by this investigation overstate the likely true impact, due to the simplified methodology adopted for this initial investigation. Should greater confidence be required in these estimates, it is recommended that a hydrological review be undertaken of a similar nature to the scope of the 1995 review.

The reallocation of a portion of the flood mitigation storage to permanent water storage was found to provide a moderate increase in average annual yield. For instance if a 100 year flood protection standard was accepted (from the current 150 year) an additional average annual yield of 0.8GL could be achieved. However, generally no additional yield would be achieved during the dry years (such as 1998, 2000) when an amenity flow has historically been desirable to manage cyanobacterial bloom outbreaks.

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4 References

Engineering & Water Supply Department (1980), River Torrens Flood Mitigation Study

Lange Dames Campbell (1995), River Torrens Flood Hydrology Study, prepared for Engineering & Water Supply Department

SMEC (1998), River Torrens Flood Mapping Study, prepared for SA Water

Torrens Taskforce (1997), Torrens Taskforce Summary of Findings

Torrens Taskforce (1999), Torrens Taskforce Actions Update

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