Wiri Pond 15 Upgrade Report

Wiri Pond 15 Upgrade Report on Infrastructure Auckland Grant Application, GHD Limited, 2001.

Project Description | Story Board | Proposed Works

Project Description

Wiri Pond Existing Configuration	Wiri Pond Overall Proposed Works
Click to enlarge map	Click to enlarge map

The "Wiri" Stormwater Treatment pond is sited in the Puhunui Stream near Inverell Ave, Wiri. It is an online water quality pond that has silted up and is in nccd of maintenance dredging. Due to the silt buildup the pond is of increasingly limited effectiveness

There is communitv pressure to have the silt removed from the pond and MCC wish to meet their obligations to improve water quality under the comprehensivie discharge consent for Puhunui Stream catchment. The pond as is does not meet current best practice for maintenance and design.

The Auckland Regional Council (ARC) will not issue a resource consent to undertake dredging of the pond unless flows can be diverted around the pond.

The need to undertake the de-silting and requirement to bypass flows around the pond during dredging presents an opportunity to undertake works to upgrade the existing stormwater pond to:

• Increase the pond size to provide greater sediment removal efficiency.
• Construct two new ponds, which will act as sediment forebay ponds to improve the sediment removal efficiency of the system.
• Construct a permanent bypass system of channels and pipes to enable stormwater flows to be bypassed when maintenance of the ponds is rcduired and thereby satisfy ARC requirements.

The project will have a positive impact on water quality in the Puhunui Stream und is an integral part of MCC initiatives for improving the water quality in the Puhinui Stream and the Manukau Harbour. Adjacent works such as the Everglade and Pacific Arena Ponds will complement this project.

The main works are programmed to be undertaken this financial year (2001/2002)with planting to be undertaken the following financial year.

These works will:

• Improve water quality in the Puhunui Stream and hence water discharged to the Manukau Harbour.
• Improve the appearance of the existing pond in terms of removal of wecds and improved water clarity
• l'rovide additional landscaping of the Surrounding reserve.
• Provide fish passage to facilitate fish movement upstream of the pond.

ARC are supportive of the proposed works and have had input into the design concept adopted. A resource consent has been lodged and a section 92 request received and responded to. Consent approval is expected by the end of Scptember. MCC consent has already been obtained for the works.

Story Board

Looking Upstream from Pond 15 at the Puhinui Stream. Proposed Forebay to left of channel.

Sediment buildup at outlet of 2.0m dia pipe. Proposed forebay between pipe outlet and pond will capture this in future.

On 20 April 1993. the ARC issued a comprehensive discharge permit (No. 928511) to the Manukau City Council (MCC) for the Puhinui Stream Catchment. Since 1993 MCC 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 Inverrel Ave, Wiri. Pond 15 is an important component of the Puhinui Stormwater Catchment Management Plan. The pond fulfils the following functions:

• An aid to the improvement of stream water quality, by means of de-silting, oxidation and the provision of wildlife aquatic areas.
• And the provision of a landscape water feature, in a park-like setting.

Pond 15 has a surface area of 7,100 m² approximately. In the middle of the pond is an island covered with native and exotic trees. At the edge of the pond is a rock wall that rises to about 300mm above the water level. Pants of the pond are covered with weeds and other vegetation. The pond is fed by a concrete lined channel (3.5m wide x 0.450m deep) that forms part of the Pullinui Stream and 2.0m diameter pipe from upstream of Great South Road.

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.

The existing situation is illustrated on drawing 17395/01 in Section 3 and includes photographs ofthe site.

The deposition of silt in the pond has now reached the stage where it is adversely affecting the operational efficiency of the pond. 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.

MCC is proposing to upgrade the sediment removal efficiency of Pond 15 by removing a considerable level of silt and weeds from the pond enlarging the pond and by establishing additional forebays to the east and south of Pond 15.

The objective of the proposed works is to also provide 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 minimising the effect on the environment.

Proposed Works

Pond 15 Outlet Weir. Vegetated Island in centre of view.

Downstream channel between outlet and Wiri Road Station Road Culverts (view upstream)

Description of Proposed Works

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

Cleaning of this pond will require the removal of about 7,000 m³ of sediment. In addition to the removal of sediment, it is also proposed to increase the surface area of the pond by some 1,200m². This additional area will be provided to the northcast of the pond.

Proposed Eastern Forebay Pond

The Forebay will be located to the east of Pond 15. This will treat flows from a 2.0m pipe from a 35 ha catchment upstream of Great South Road.

This new pond will have the following characteristics:
Surface area: 735m²
Volume:  800m³
Depth: 1.5m

Proposed Southern Forebay Pond

This forebay is located to the south of Pond 15 and adjoins the eastern side of the Puhinui channel. Low flows in the Puhunui will be diverted into the forebay rather than flow along the concrete lined channel.

This new pond will have the following characteristics:
Surface Area: 2,800m²
Volume: 2830m³
Depth: 1.5m

Weirs and Diversion Channels

To have the ability to build the proposed sediment forebay ponds now, to clean Out the main pond and to maintain all ponds in the future, a system has been Hcveloped to allow diversion of water also from the facility being maintained or developed. This system consists of weirs and bypass channels with prurision for fish passage.

Landscaping

The proposed landscaping of the area, in the immediate vicinity of the ponds, will form an integral part of the improvement works. A copy of the Landscape Plan nd the landscaping objectives is contained in Appendix B.

The Landscape Plan has been prepared in accordance with the requirements of the Council's 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.

The Receiving Environment

The initial receiving environment for any runoff is the Puhinui Stream, which runs through the subject site and the adjoining land, northwards via a concrete channel. North of the subject site, the Puhinui Stream flows in a northwesterly 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 interidal 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 fo international migratory and New Zealand endemic wading birds including a number of threated species.

Notvithstanding the statement above, the Puhinui Stream is known to be significantly degraded. MCC have embarked upon a programme to improve the water quality in the Stream.