Otara Lake and Otara Creek: Report on remedial work to improve the natural environment of the lake and creek

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Otara-Papatoetoe Local Board

19 March 2013

Otara Lake update to the Otara-Papatoetoe Local Board Page 47

Otara Lake update to the Otara-Papatoetoe Local Board

File No.: CP2013/03906

Purpose1. To complete reporting to the Otara-Papatoetoe Local Board on studies undertaken by the

Auckland Council Stormwater Unit, within the Infrastructure and Environmental ServicesDepartment (I&ES), on Otara Lake including sediment dredging and disposal, and waterquality scoping.

Executive Summary2. Various studies and reports detailing the history of Otara Lake were commissioned by New

Zealand Electricity Department (NZEC)/Electricity Commission of New Zealand (ECNZ) andManukau City Council (MCC) prior to transition.

3. In 2011 these reports were summarised, along with additional work, by Golder and Associates

(Golder 2011). However, the Golder 2011 report did not provide an options assessment forsediment dredging and disposal, mangrove management, water quality, and on-goingmaintenance at Otara Lake.

4. A new report has been delivered Otara Lake Sediment Dredging and Disposal StudyWorking Report (Attachment A) to address these items. In addition, the Otara-PapatoetoeLocal Board resolved, at its meeting on 6 September 2011, That the Stormwater Unit ofAuckland Council undertake a scoping study of the current state of the water quality in OtaraLake.

5. This report includes a high level summary of the Auckland Council Working ReportACWR2012/007 in regards to options for sediment dredging and mangrove management atOtara Lake in addition to costings associated with a water quality testing programme for the

lake.

6. There is no budget allocated to implement any physical works at Otara Lake in the currentannual plan or long term plan.

RecommendationThat the Otara-Papatoetoe Local Board:

a) Receives the Otara Lake update report from the Stormwater Unit.

Discussion7. Otara Lake (as part of Otara Creek) is located to the east of SH1 and feeds into the Tamaki

River. Otara Lake was formed in 1968 by the construction of a tidal weir at the point whereOtara Creek discharges into the Tamaki River. The weir was constructed by the NZEC toprovide cooling water for the Otahuhu Power Station.

8. The construction of the weir altered the flushing characteristics of the historical creek, resultingin increased sedimentation within the man-made lake and an expansion of mangroves. Anumber of studies have been undertaken by ECNZ and MCC on Otara Lake usage, extent andsedimentation over time.

9. On 25 March 2010, MCC resolved: that the Council includes a detailed investigation on the

adverse effects, including the quantity and quality of sediment and water in the Otara Lakegenerated from the development of the Flat Bush Catchment as part of the Flat Bush Stage 2re-zoning and That this Otara Lake/Flat Bush Stage 2 rezoning study includes the options,


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timeframes and costs to remediate the sedimentation problem in the Otara Lake. That thecost of the study be incorporated into the costing of the Flat Bush Stage 2 rezoning project.

10. Subsequently, a report was commissioned from Golder Associates (Golder), and delivered toManukau City Council in March 2010 and presented to the Otara-Papatoetoe Local Board inSeptember 2011. This report summarised earlier reports undertaken by ECNZ and MCC, andreported on additional work undertaken. The report did not address options for sediment

dredging and disposal, mangrove management, or maintenance dredging. Consequently, tocomplete the MCC resolution, an additional piece of work was undertaken by AECOM(Auckland Council Working Report ACWR2012/007, attachment a).

11. There was no budget available for the sediment dredging and disposal options report in theFlat Bush Stage 2 rezoning project to give effect to the MCC resolution. Auckland CouncilStormwater Unit therefore decided to fund this additional piece of work from the StormwaterDevelopment and Technical Services budget.

12. Although the additional report provides information on possible sediment dredging anddisposal costs, it is noted there is no budget allocated to undertake the physical works in OtaraLake in the annual plan or long term plan.

Sediment dredging and disposal costs13. The sediment dredging and disposal report, ACWR 2012/007, considered dredging required to

provide an outrigger or waka ama course. Dredging of the entire area was not included in theassessment. Three dredging methods and two sludge drying methods were considered.Depending on the size of the course, the depth of dredging, and the dredging disposal costs,the initial works cost in the order of $3 to $18 million (see Table 1 below). Allowing areasonable depth for outrigger use to be 1.5 m, costs are likely to be in the order of $8 to $10million. This estimate excludes any resource consent costs.

Table 1: Volumes and cost estimates for each dredging scenario (ACWR 2012/007)

250 m long x 140 m wide 500 m long x 105 m wide

Volume (m3) Cost Volume (m3) Cost

1.0m water depth below weir

crest (RL1.3m)19,000 $2-5M 25,000 $2.5-6M


crest (RL0.8m)34,000 $3-9M 46,000 $5-12M


crest (RL0.3m)51,000 $5-13M 71,000 $7-18M

Note: Includes for dredging access channels. Costs range due to the three methodologiesinvestigated: cutter suction (with centrifuge or geobag), and barge-mounted excavation with air

drying.14. Golder (2011) estimated the annual sedimentation rate to be 0.03 to 0.06 m per year. At this

rate, maintenance of the course would cost $2 to $4 million every 10 years (refer table 2below).

Table 2: Approximate maintenance costs (based on removing 0.45m) at 10 year intervals(ACWR 2012/007)

Approx Volume(m

3)

Approx Cost ($)*

250 m long x 140 m wide 17,000 $1.8 - 4.3M

500 m long x 105 m wide 25,000 $2.6 - 6.3M

*The range of costs is a function of the dredging method employed.


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Water Quality monitoring costs

15. Auckland Council Stormwater Unit recommends that a reasonable sampling plan to determinethe current water quality would require monthly sampling at high and low tide for a period ofone year. Given the size of Otara Lake, 10 sampling points are recommended. In addition, itis recommended that water quality be sampled after 15 individual rainfall events. Aconservative estimate of sampling costs for this year long programme is $35,880 as per the

breakdown in Attachment B to this report.

16. Analytical laboratory costs are additional to the above. Contact recreation guidelines for salinewater quality recommend analysis for enterococci, which would cost in the order of $11,278.Note this cost is for enterococci only, it excludes environmental contaminants, and othercontaminants which are considered in freshwater contact recreation for example turbidity,ammonia and nitrate.

17. Therefore the total cost of a one year water sampling programme for Otara Lake is estimatedto be $47,158. There is no existing budget for this work.

Consideration

Local Board Views

18. It is noted that Otara Lake Improvements is a key project under the Otara-Papatoetoe LocalBoard Plan (Improving and Using Our Waterways Priority).

19. In addition the Otara-Papatoetoe Local Board identified, as part of their Key Objectives andActivities for 2011/2012, that an initiative that could be progressed from existing AucklandCouncil budgets was production of a report that details options, timing and costing toremediate Otara Lake. This report addresses that initiative.

Maori Impact Statement

20. This report is technical in nature and makes no recommendations in terms of intervention at

Otara Lake. Accordingly, there are no impacts to Maori from the present report. However it isnoted that this area is of significance to local iwi and any future plans for the area would needto made in consultation with local iwi.

General

21. Internal consultation at Auckland Council was undertaken between the Stormwater Unit andthe Environmental Monitoring team within the Policy and Planning Division.

22. No additional consultation has been undertaken. Additional communication needs to occurwith the City Transformation group who are responsible for the delivery of the Flat Bushproject. This should include the need for the submission of a business case to seek fundingfor any proposed works.

Implementation Issues23. This report is for information and planning purposes only. Otara Lake is not a stormwater

asset and there is currently no planned for budget for the scoped activities within Infrastructureand Environmental Services annual plan or long term plan.

24. Implementation would require liaison with Auckland Council City Transformation, AucklandCouncil Parks Department (adjacent landowner), Contact Energy (Adjacent landowner) and iwi(foreshore and seabed owners).


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Attachments

No. Title Page

A Otara Lake Sediment Dredging and Disposal Study Working Report 51

B Otara Lake Water Quality Sampling Scoping Study 97

Signatories

Authors Judy-Ann Ansen - Stormwater Technical Services Manager, Infrastructure &Environmental Services

Mara Bebich - Senior Relationship & Engagement Advisor

Authorisers John Dragicevich - Manager Infrastructure and Environmental Services

Carol McKenzie-Rex - Relationship Manager


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Otara Lake and Otara Creek: Report on remedial work to improve the natural environment of the lake and creek

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