Report On Upgrading of Puhinui Stream

Resource Consent Application to the Auckland Regional Council and Manukau City Council. Report On Upgrading of Puhinui Stream - Wiri Pond 15, GHD Ltd, 2001

Introduction | Proposed Works | Construction

Introduction

On 20 April 1993, the ARC issued a comprehensive discharge permit (No. 928511) to the Manukau City Council for the Puhinui Stream Catchment. Since 1993, Manukau City Council has been progressively installing stormwater ponds to control the discharge of pollutants into the Puhinui Stream. Pond No. 15 (referred to as Wiri Pond) was constructed about 20 years ago and is located to the west of Inverell Avenue, Wiri. The Council is proposing to increase the sediment removal efficiency of Pond 15, by removing a considerable volume of silt and weeds from the pond, enlarging the pond and by establishing additional forebays, to the east and south of Pond 15.

The Council has engaged GHD Ltd., to prepare an Assessment of Effects on the Environment (AEE) for the proposed works, and to obtain the required consents from Auckland Regional Council and Manukau City Council.

This document comprises the applications for consent, and includes the required AEE. It meets the requirements of the Fourth Schedule to the Resource Management Act 1991 (RMA), the Proposed Auckland Regional Plan: Sediment Control, and the Manukau City Council Proposed District Plan.

Site Description

Site and Locality

As illustrated on the Manukau City Council planning map contained in Appendix A, the site is a public reserve and is contained within the eastern bank of the Puhinui Stream, from the rear boundary of 38 Inverell Avenue, Wiri to north of pond 15. All of the land affected by the proposed works is zoned Public Open Space 5 under the Proposed District Plan and has the street address of 52R Inverell Avenue, Wiri. The major feature of the site is the existing Pond 15, which has an approximate surface area of 7,100 metre 2 . In the middle of the pond is an island, containing native and exotic trees. At the edge of the pond is a rock wall, rising to about 300mm above the water level. Parts of the pond are covered with invasive weeds and other vegetation.

The pond is fed by a concrete lined channel (3.5 metre wide x 0.450 metre deep) that forms part of the Puhinui Stream, and a 2.0 metre diameter pipe serving part of the upstream catchment.

There is an area of exotic trees at the southern end of the site, and a scattering of larger gums and macrocarpa along the banks of the stream.
A children's playground has been recently relocated to the southern side of the footpath, from Inverell Avenue. The only other structures of significance, on the site, are footpaths, and footbridges over the weirs to pond 15.

All of the land adjoining-the Public Open Space 5 zone is zoned Main Residential under the Proposed District Plan. This residential land is predominantly occupied by -single storey houses, about 20 years old. Some sites have been developed as medium density housing. For example, 54 Inverell Avenue has a site area of 4919 metre 2 and accommodates 9 single storey houses.

To the north-east of pond 15 is a large property, 3.4 ha in area, which is occupied by the Bethesda Home and Hospital.

The Receiving Environment

The initial receiving environment, for any runoff, is the Puhinui Stream which flows through the subject site and the adjoining land, northwards, via a concrete channel. North of the subject site, the Puhinui Stream flows in a north-westerly direction, through the Wiri industrial area, then heads south to the Puhinui creek and the Manukau Harbour, west of the Weymouth Peninsula. The Puhinui Creek is zoned Coastal Protection 1 under the Auckland Regional Plan: Coastal. The Creek is listed in Schedule 3 of the Plan and is described as follows:

Area of intertidal banks and shell banks forming a complex habitat for a variety of animal and plant communities. The extensive gently-graded sand flats (27a) support dense populations of intertidal sand flat organisms and are an excellent feeding ground for thousands of international migratory, and New Zealand, endemic wading birds including a number of threatened species.

Notwithstanding the statement above, the Puhinui Stream is known to be significantly degraded. Manukau City Council has embarked upon a programme to improve the water quality in the stream.

Revised Pond Sediment Removal Capabilities

Area Affected By Proposed Works
  1. Wiri Pond Upgrade Existing Pond Configuration
  2. Proposed Layout

 

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Area Affected By Proposed Works

Click maps to enlarge

1.

Existing Pond Configuration

2.

Proposed Layout of Pond Upgrade

The objective of the proposed works is to increase the sediment removal efficiency of the existing pond 15, so that the water quality of the Puhinui stream, and the Manukau Harbour, is enhanced.

The Existing Situation

Pond 15 is an important component of the Puhinui Stormwater Catchment Management Plan. The pond fulfils the following functions:

a) an aid to the improvement of stream water quality, by means of desilting, oxidation and the provision of wildlife aquatic areas,
b) flood attenuation, and
c) the provision of a landscape water feature, in a park-like setting.

The existing situation is illustrated on Drawing 17395/O1 in Appendix B. The deposition of silt in the pond has now reached the stage where it is adversely affecting the operational efficiency of the pond. In accordance with the project brief, the objective is to determine a practical method of removing the silt, both at the present time and in the future (for the purpose of pond maintenance), whilst at the same time providing for the existing stormwater flows into the system and minimizing the effect on the environment.

The average depth of pond 15 is about 1.75 metres, of which between 1.00 metres and 1.10 metres is currently occupied by silt. At best, even with all of the silt removed and the existing pond functioning with complete "efficiency", the maximum sediment load that may be removed from the receiving waters is about 30%. This compares with the desired sediment removal efficiency of 75%, contained in the Auckland Regional Council TP10.

However, in order to achieve a 75% level of sediment removal, for an upstream catchment area of about 1050 hectares in total, a pond volume approximately 10-15 times the size of the existing pond 15 would be required.

The Existing Pond (Pond No. 15) - Upgrading and Extension.

Cleaning of this pond will require the removal of about 7,000 metre3 of sediment. The manner in which this sediment will be removed is set out, in section 4.2 of this Report.
In addition to the removal of sediment, it is also proposed to increase the surface area of the pond, by some 1,200 metre2. This additional area will be provided, to the north-east of the existing pond 15.

Earthworks for this pond extension will amount to about 3,000 metre3, of which 2,500 metre3 will be part of the new permanent pond water volume.

Proposed Eastern Forebay Pond

This new forebay will be located to the east of existing pond 15. The eastern boundary of the forebay will be located about 12 metre from the residential property at 54 Inverell Avenue, and the Bethesda Home and Hospital.
This new pond will have the following characteristics:
Surface area:           735 metre2 approx.
Volume:                    800 metre3 approx.
Depth:                      1.5 metre approx.

To form the new forebay pond, earthworks of approximately 1500 metre 3 will be required.

Proposed Southern Forebay Pond

This new forebay is located to the south of existing pond 15, and adjoins the eastern side of the existing Puhinui channel. In order to form the pond, it will be necessary to undertake earthworks close to the rear boundary of the residential properties at 40 and 42 Inverell Avenue.

The new pond will have the following characteristics:
Surface Area:           2,800 metre2 approx.
Volume:                   2830 metre3 approx.
Depth:                     1.5 metre approx.
To form the new forebay pond, earthworks of approximately 3,500 metre3 will be required.

Weirs and Diversion Channels

In order to build the proposed sediment forebay ponds, to extend and de-silt the existing pond 15, and to maintain all these facilities in the future, a system has been conceived which will allow for the diversion of stormwater around the facility being maintained, or developed.

Both of the new forebay ponds have a low flow level of Reduced Level = 21.0 metre A.O.D., i.e. 0.5 metre above the water level of existing pond 15.

The following weirs and channels are required:

  1. Main Puhinui Stream Channel to new Southern Sediment Forebay.
    • A drop of some 800mm is required, such that water from the existing Puhinui channel can flow into the proposed southern sediment forebay. A fish ladder, or similar approved, is proposed at this location.
    • When the new southern forebay pond is under maintenance, a stop-log device will be temporarily installed, to direct flow, in the Puhinui Stream, back down the existing channel.
  2. New Southern Sediment Forebay Pond to existing Pond 15
    • A weir, at Reduced Level 21.0 metre, will control the water level in the southern forebay pond. From this weir, a channel will return water back to the existing Puhinui channel. A fish ladder, or similar approved, is also proposed at this location.
    • When existing pond 15 is under maintenance, a stop-log device, above the weir, will be temporarily installed to direct flow in the southern forebay pond into the 2.00 metre diam. pipe, at manhole "E", and into pipeline "E to D" and, thence, into the new eastern forebay pond.
  3. New Southern Sediment Forebay Pond to New Eastern Sediment Forebay Pond
    • At present, stormwater run-off, from the Great South Road vicinity, discharges via the southernmost 2.00 metre diam. pipe, into the main Puhinui Stream channel at point "H". This flow will be diverted, at new manhole "E", to the new eastern forebay pond at "D".
    • Stop-logs are to be permanently installed, in manhole "E", to divert flow in this 2.00 metre diam. pipe, into the new eastern forebay pond. These will only be removed when flow from the new southern forebay pond is to be diverted to the new eastern forebay pond.
    • When the existing pond 15 is under maintenance, the water level in the new southern forebay pond will rise, and discharge down the pipeline "E to D", to the new eastern forebay pond.
  4. New Eastern Forebay Pond
    • At times of low flow, stormwater will discharge into the existing pond 15, at point "J".
    • At higher flows, the water level in the new eastern forebay pond will rise, overflow into manhole "C", and then discharge through the 900mm. diam. pipeline, "(B)-(A)". If the capacity of this pipeline is exceeded, flow across a new weir structure at "B" will occur, and, via a proposed swale between points "(B)-(A)", enter the Puhinui stream downstream of point "(A)".
    • During maintenance of the new eastern forebay pond, a stop-log device will be installed at manhole "C", and the flow diverted down pipeline "(C)-(B)-(A)".
    • During maintenance of the existing pond 15, a stop-log device will be installed at point "J", to prevent water entering pond 15. The water level in the eastern forebay pond will rise, and flow down pipeline "(C)-(B)-(A)", and/or over the weir at point "(B)".
  5. Main Pond
    • At the outlet "K", a new fish ladder, or similar approved, is to be constructed.
    • At point "M", the level of the existing (informal) spillway from the main pond is to be raised. This will allow pedestrian access between Trevor Hosken Drive and Nlanukau City Centre, in all but the severest of weather.
    • Minor modifications to the earthworks, around the existing outlet, will be made to increase the capacity of the existing weir detail.
  6. New By-Pass Channel "B-A"
    • A bridge is to be constructed, over the by-pass swale, to allow access for pedestrians, between Trevor Hosken Drive and Manukau City.
    • The swale is to be grassed and landscaped with low shrubs. It is expected that water will flow down the swale a number of times per annum, for limited periods of time.

Assumptions

For the purpose of estimating pond efficiency and annual sediment removal, a sediment generation potential of 1 tonne / ha / annum has been adopted for the following calculations.

Sediment removal efficiency has been adopted, using information obtained from publications TP10 and TP4.

Existing sediment loading

The existing pond 15 has a sediment removal efficiency of about 30%. Total catchment sediment generation is expected to average 1 tonne/ha/annum. The total tonnage arriving at the pond is expected to be 1050 tonnes / annum.

Since the construction of Pond 2000 and the Hospice Pond (in year 2000), the sediment generation is expected to be about the same, as development in this catchment continues, but in time, as this young urban catchment matures (say 5 years), the sediment arriving at pond 15 will decrease by (48ha x 1 tonne/ha/annum x 75% removal efficiency) = 36 tonnes / annum.

Hence, the existing Puhinui stream has an estimated sediment load of 1050 tonnes/ annum, reducing to 1015 tonnes/annum, as development of Totara Park becomes more established.

The theoretical sediment removal efficiency of about 30% has the potential to remove 315 tonnes per annum. However, the pond is mostly full of sediment and, hence, we expect between 20 and 100 tonnes of new sediment to be added annually.

Other works to reduce sediment loading.

SH20 - SH1

SH20 - SH1 (Transit New Zealand) Project has a 16 ha catchment feeding pond 15. When the project is complete, in the year 2007 (or sooner), this catchment will be diverted downstream of Pond 15.

The sediment generation potential and removal, through a series of new ponds, has been reported in the AEE accompanying the resource consent applications submitted for that project. Significant improvements to the future sediment capWre as a result of the SH20 - SH1 project, and other ponds being constructed on Manukau City Council initiative as part of the overall SH20 - SH1 project, will see a further significant reduction in the sediment volumes contained in the Puhinui Stream.

Everglade Pond

Construction of the Everglade Pond is programmed to occur in the year 2001/2002 construction season. This pond has an 80 ha catchment, and a theoretical sediment removal efficiency of 75%.

Sanctuary Pond

This 80 ha catchment has no current treatment. A sediment forebay is proposed upstream of the motorway. Expected efficiency of the proposed pond is 30%.

Pacific Arena Pond

This 40 ha catchment has no current treatment. A water quality feature and pond is expected to have a sediment removal efficiency of 40%. This facility is expected to be built within the next 5 years.

Great South Road Catchment.

This 35 ha catchment has no current treatment. This catchment run-off will be diverted to the new eastern forebay pond. All of the stormwater from small storm events, and part of the stormwater from larger events, will be diverted to the existing pond 15. As such, there will be treatment for this catchment for a significant part of the annual runoff.

Revised Sediment Removal

The existing pond 15 and sediment forebays, have an equivalent reduced catchment of 750 ha. As such, the enlarged ponds have an increased theoretical efficiency of 43%. The sediment generation of 750 tonnes per annum will be captured, in part, by the enlarged pond i.e. estimated capture = 322 tonnes per annum ....750 x 0.43 tonnes.

In addition, the balance of the catchment i.e. (1050 - 750 - 16) = 284 ha, will have a sediment generation potential of 284 tonnes per annum. Upstream treatment devices are expected to capture 128 tonnes, leaving 156 tonnes to flow into the new eastern sediment forebay pond. Most of this sediment will be directed to the existing pond 15, with the peak of storm events being diverted onto the swale by-pass. Of this, we expect a capture of a further 15%, or 24 tonnes, per annum.

Thus, the potential sediment removal of the existing pond is (322 + 24) = 346 tonnes per annum.

Total effect on the Puhinui Stream

Based upon the assumption that the existing 1050ha catchment generates some 1050 tonnes per annum (1 tonne/ha/annum), the sediment load to the current pond 15, is 1050 tonnes per annum.

As the pond is full of sediment/silt and removes an estimated 20 - 50 tonnes per annum, the downstream load in the Puhinui stream is about 1000 to 1030 tonnes per annum.

As a result of this development i.e. pond 15 upgrade and de-silting, the downstream sediment load is expected to drop, by about 346 tonnes per annum.

As a result of the Everglade, Sanctuary, and Pacific Arena Ponds, an additional 128 tonnes per anum is expected to be captured.

Thus, the new sediment loading, in downstream Puhinui, is expected to drop from the present 1030 tonnes per annum, to about 556 tonnes per annum.

Earthworks Quantities

The total area of disturbed ground will be approximately 12,000 metre 2. Preliminary earthworks quantities, derived from the present preliminary design and construction drawings, are as follows:

(i) Cut to Fill: 3,000 metre3 approx.
(ii) Cut to Spoil (excluding silt removal): 10,000 metre 3 approx.
(iii) Silt volume to be removed: 7,000 metre 3 approx.
Total =20,000 metre3 approx.

Landscaping

The proposed landscaping of the area, in the immediate vicinity of the ponds, will form an integral part of the improvement works.

The landscaping component is to be undertaken by Natural Habitats (Landscaping consultants), and a copy of their Landscape Plan (Concept) and the landscaping objectives, are contained in Appendix C.

Wherever possible, all existing trees are to be retained in their present location, such trees having been surveyed and identified beforehand, as part of the topographical survey of the site. Any small trees and shrubs that cannot be retained in their present location will, if practical, be removed and relocated to a new position close at hand.

The Landscape Plan (Concept) has been prepared in accordance with the requirements of, and after discussion with, members of the Manukau City Council Parks Department. A major objective of the Landscape Plan is to retain the existing open space appearance of the reserve. To achieve this objective, the planting of grasses, small shrubs and fruit bearing trees has been maximised.

In all probability, the bulk of the planting envisaged to form part of the Landscaping physical works, will take place after completion of the Defects Liability Period to the civil engineering Contract. However, certain elements of the landscaping works will be incorporated into the civil contract, in particular the establishment of the grassed areas, after the earthworks have been completed, with defined grasses, if specified by the Landscaping Consultants.

Construction Methodology

As illustrated on the drawings contained in Appendix B, completion of the civil component of the pond upgrade works is to be undertaken in two distinct phases, as follows:

(a) first, construction of the by-pass system around existing pond 15 will be undertaken, and secondly...
(b) enlargement and de-silting of existing pond 15.

Set out below are the basic activities, and the order in which they will be constructed. However, there maybe some adjustments as a result of discussions between the successful Contractor, and all interested parties, at the pre-construction meeting, for example.

The new by-pass system

This consists of two new sediment forebays, one to the east and one to the south, connected by a network of gravity stormwater pipelines.

Although the works are to be undertaken in the summer months, it will be essential that the existing pond facility be operational during the construction period. Therefore, construction of the by-pass system shall proceed at the downstream end of the Puhinui stream. The existing open channel flow of the Puhinui stream, through the existing pond 15, will not be interrupted during this period of the physical works.

Construction of the pipeline "(A) to (B)", provisionally sized at about 900mm. diam., will be by conventional open-cut methods. In conjunction with this activity, construction of the pipeline between "(B) and (C)" will be completed, adopting a similar construction technique.

It is expected that the new eastern forebay pond will be substantially complete, before the existing 2.00 metre diam. pipeline is removed. This would allow work on the new forebay to be undertaken "off-line".

The temporary diversion of the inflow, from the upstream catchment into existing stormwater manhole "(C)", will enable the balance of the construction of the new eastern forebay pond to be completed "off-line".

The removal and temporary storage of some of the existing 2.00 metre diam. stormwater pipes will be required, prior to re-use on the proposed pipeline between "(D) and (E)".

It is intended to retain the connection, between the new eastern forebay pond and the existing pond 15, utilising a twin culvert cell configuration, with new 1200mm. diam. pipes.

At present, the existing 2.00 metre diam. pipeline, "(F) to (E)", is discharging directly to the open channel of the Puhinui stream, above the existing pond 15 at "(H)". This flow will be re-directed into the new eastern forebay pond, by means of a new stormwater 2.00 metre diam. pipeline "(D) to (E)". This pipeline will be utilising, as far as practicable, pipework removed from the system downstream of manhole "(C)". The proposed pipeline "(D) to (E)" will be a gravity pipeline, built by open-cut methods.

Upon completion of pipeline "(D) to (E)", and construction of the stormwater manhole at "(E)", the only direct flow into the existing pond 15, via the existing open channel of the Puhinui Stream, will be the existing 675 mm. diam. pipeline, discharging at stormwater manhole "(G)". This connection to .the open channel will remain in place, due to the minor nature of the catchment which this pipeline serves.

Temporary diversion of the 675mm. diam. pipe, into the new southern forebay pond, will be required during future maintenance of pond 15.

Construction of the new southern forebay pond can now proceed, together with the diversion facility at the pond inlet. A new weir structure, at the downstream end of the pond, is to be constructed over an existing 300mm. diam. trunk sewer main. From this new weir, a new open channel is to be constructed, to re-direct the flow back into the existing Puhinui channel, upstream of the existing pond 15.

Some landscaping/fill is required in the creation of the swale and elevated footpath, adjacent to the new stormwater pipeline "(A) to (B)". However, the balance of material removed, from the excavation of both of the new forebay ponds, and surplus spoil from the trenches, is likely to be carted off-site.

During the execution of the above portion of the civil works, it is anticipated that, at no time, will there be more than 100 metres of trench open at any one time and, at the discretion of the Engineer, this figure could be reduced if, for example, the safety of the public is deemed to be at risk.

Completion of the by-pass system will now enable diversion of the flow, from the Puhinui stream, into the new southern forebay pond.

Existing Pond 15 Remedial Works

The existing facility has had no maintenance since its construction, some 20 years or so ago. There is a maximum of 600 mm of water in the pond, above some 1.10 metres of silt and /or sediment.

Hence, de-watering of the pond will be necessary, before sediment removal or the pond enlargement activity can proceed.

This component of the civil works has been the focal point of discussion between GHD Ltd. and Mr. Dene Andre, of the Native Freshwater Fish Society, who has expressed concerns about the effect that this activity may have on fish species present in the existing pond. As a result of a meeting held on site on 9 July 2001, a basic conceptual methodology has been agreed with regard to the removal of such fish. Such methodology may be refined on site, due to adjustments recommended by others, or due to circumstance.

De-cantinglRemoval of the Supernatant Water

This operation, which should remove the greater part of the water present in pond 15, shall be undertaken at the downstream end of the pond, immediately upstream of the existing weir.

A bund, of some pre-determined shape and dimension, shall be formed around the weir, consisting of robust but clean permeable material, and with an allowance for fish species to enter within it, during the de-canting process. Existing sediment contained within the bund shall be removed, to form a "collection well" in which fish may be contained.

The existing weir consists of broken rock and/or rip-rap, and the slow release of pond water may be achieved by the incremental removal of the rock, say in depths of the order of 150 mm., or so. The drawdown process itself is likely to contain, within the water, any fish species present in the pond, but which can accumulate in the well device. At this time, removal of the fish , by the Native Freshwater Fish Society and, no doubt, representatives of the Contractors' staff as well, can take place, prior to temporary storage, and relocation of the fish, in new waters of the Societies' own choosing.

Construction of a Pump Well in Pond 15.

Upon removal of the supernatant water from the pond, a pump well shall be created, contained within a rock bund of radius 10 metre to 15 metre, adjacent to the existing Puhinui stream inlet. The rock should be clean and gap-graded, to allow for the free passage of water. The top of the rock bund should be to Reduced Level 20.5 metre i.e. normal pond low water level.

The creation of a pumping well, within the bund, will enable residual water in the exposed sediment and silt to be released, contained and removed into the newly constructed southern forebay, thereby such water receiving some clarification before discharging to the Puhinui stream, further downstream.

Sediment removal

Upon completion of the pump well and rock berm, removal of the sediment may proceed, working outward from the berm and into the pond 15 proper.

Initial entry into the pond is expected to be from the rock berm itself, and from points along the perimeter of the pond. A long-reach excavator has a potential reach of 15 metre to 18 metre. However, the dimensions of the pond are much greater than this.

Excavation of the sediment, from the perimeter of the pond 15, will allow residual water, and moisture, to flow into the pump well. It will also allow sediment to consolidate and dry out, in those areas where the Contractor can maintain a low level of water content, within the sediment.

A total of about 7000 metre3 of sediment is expected to be removed from pond 15. A production rate of 1 truck every 10 minutes, removing some 600 metre3/day, would indicate a scheduled de-silting activity of 15 days. Allowing for wet weather events, etc., during this process, a total duration of 6 weeks is expected for completion of this second phase of the civil works Contract.

This excavated sediment material may need to be spread, and dried, before being removed to spoil. This "spreading and drying" will occur on land to the north and west of the site, i.e.- on Manukau City Council ground, adjacent to the Rally Track.

Negotiations are also underway, with Transit New Zealand, for use of the sediment material as bulk and/or landscaping fill, as part of the SH20-SH1 project.

Some of the sediment/silt may be used for pond landscaping purposes, immediately below the topsoil layer. Samples of silt have been analysed in the laboratory, to determine the presence of heavy metals and/or chemicals. No deleterious material was detected in the samples taken. Additional data, confirming this, is attached as Appendix D.

Upon completion of the sediment removal from pond 15, any accumulated deposits in the new southern forebay, resulting from this de-silting activity, will removed.

Completion of the upgrade to, and sediment removal from, pond 15 will enable the water flow into the new southern forebay, to be re-directed towards pond 15, thus enabling the Puhinui stream to receive the fully increased sediment removal benefits of the modified pond system.

After completion of the Defects Liability Period, which shall be not less than 6 months duration, landscaping of the site will proceed, essentially in accordance with the concept plan, attached in Appendix C.

Programme of Works

It is proposed to commence physical works, on site, on lst October 2001.

The next New Zealand rally is due to commence on 20/21 September 2001 (provisionally), and to avoid conflict with this annual event, no access to the site will be allowed until the last week of September 2001 e.g. for site establishment, etc.

The by-pass pipework and swale are required, complete, before the diversion of any stormwater can proceed. The swale shall be stabilised/grassed first.
Completion of the by-pass network, including the new pipework, construction of both new forebays, and much of the support infrastructure etc., is expected to be complete, by Christmas 2001.

De-watering of the existing pond 15, enlargement and de-silting etc., is programmed to take an additional 6 weeks. Completion of the civil works is expected, therefore, by the end of February 2002.

A maintenance (Defects Liability) period of at least 6 months will follow the above.
The contract to complete the landscaping will proceed, in September/October 2002.

Sediment Control

A Sediment Control Plan, Outline Plan Of Works shall be prepared by the Contractor in his Methodology, submitted with his tender.

A detailed "Sediment Control Plan" shall be prepared by the successful Contractor, on tender award. This plan shall be submitted to the ARC for its approval. Once the plan is approved, it shall be adhered to throughout the period of the construction works.

Erosion Control

General Measures

Soil erosion of the site, during the period of the physical works Contract, will be kept to a minimum by the application of the following measures:
Programming the works sequentially, so as to minimize the amount of bare soil, at any one time,

(ii) Reducing the amount of external run-off entering the site, by the construction of cut-off trenches, and re-directing the run-off safely away from the working area,
(iii) Minimising the length of pipe trench to be open at any one time (not to exceed 100 metres), and to complete the pipelaying and backfilling activities as soon as is practically possible,
(iv) Complete the topsoiling and grass seed activities in a methodical manner, as soon as is practical after the proposed works have been undertaken.
(v) By the adoption of any other systems and methods that present themselves during the execution of the physical works, in order to accomplish the standards and controls necessary for erosion control.

Progressive Stabilisation

The by-pass swale will need to be grassed, and have erosion protection, to enable this to be used for water conveyance. A programme to get this item constructed, and stabilised, before use will form part of the tender process.
For the remainder of the project, progressive stabilisation will be a requirement of the Contract, as set out in the specification.

Dewatering Control

To achieve successful de-watering of the existing pond 15, and in a manner that will prevent elevated concentrations of sediment entering the receiving waters, will require careful execution and attention.

Monitoring

Turbidity monitoring of the Puhinui stream is recommended to ensure that the discharge of sediment, into the receiving environment, is within tolerable limits.

Sampling, by the site foreman, on a regular basis (say, l0am and 3pm i.e 2 times/day), will occur when machines are working in a situation, where the risk is paramount. Samples shall be taken, upstream and downstream of the site, and retained on site for 7 days.

Samples can be analysed for turbidity, should there be a potential problem present at the site. The taking of samples for turbidity shall be at the discretion of the Engineer and/or the ARC. Readings are to be recorded in the Environmental Site Diary.

It is recommended that the turbidity should not increase by more than 30%, at any time during the Contract. If the increase is more than 35%, it shall be reported, by the Contractor, to the Auckland Regional Council and the Engineer, by facsimile, the same day, with an explanation as to why the readings are so high, and what systems are in place to mitigate the situation.

The proposed monitoring, as set out above, shall be reviewed by the Engineer and ARC on a regular basis throughout the Contract, to ensure that the construction methodology is sympathetic in controlling adverse effects.