Appendix H: Preliminary ecological assessment · Message Ref: New Aotea Reservoir – Ecology preliminary assessment . Project No: W18009 . Dear Paul, As requested, this memo provides

Appendix H: Preliminary ecological assessment

W17071_Aotea_eco_scoping_assessment_20180411_reviewSF.docx page 1

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Attention: Paul McGimpsey

Company: Connect Water

Date: 15th July 2018

From: Tessa Roberts & Stephen Fuller

Message Ref: New Aotea Reservoir – Ecology preliminary assessment

Project No: W18009 Dear Paul,

As requested, this memo provides a preliminary assessment of the potential for ecological impacts from the construction and operation of the proposed new Aotea Reservoir.

Introduction A BML ecologist was asked to carry out an ecological survey and provide a preliminary assessment on the potential impact of construction and operation of a proposed new water supply reservoir in the Aotea subdivision, to be located to the south of the current reservoirs at the end of Queen Charlotte Drive, Aotea, Porirua.

The purpose of this work was to determine whether a full assessment of ecological effects is required. It should be noted that the work undertaken and described in this memo does not constitute an assessment of ecological effects.

Proposed works The proposed new reservoir would have a 11.3 million litres storage capacity and would be approximately 38m in diameter and 12m in height and partially buried. It would sit within the existing construction zone of the Aotea Block residential development. The construction site currently compromises flat worked earth on a ridgeline.

The operation of the reservoir would require the provision of an overflow structure which is proposed to discharge to land, in the upper headwaters of a small stream locally known as Grundy’s Gully. It would discharge by way of an overflow structure which would be designed to attenuate flow and prevent scour of the ground below it.

Two existing reservoirs are present near this site. These existing reservoirs have a (combined) overflow discharge into the same watershed. It is proposed that the new reservoir would discharge to the same general area, with the new overflow structure being relocated 1-4 m further down the gully than the current overflow structure.

Memorandum


The discharge point would be to dry ground within a headwater depression. The discharged water will flow down an ephemeral/intermittent channel approximately 200 m to Grundy’s Gully wetland. Any overflow will be of potable water and so there is no risk of contamination.

Study method The site was visited by an ecologist (Tessa Roberts) on the 17 January 2018. The weather at the time was showery and it had rained the day before. However, prior to this there had not been significant rain for 22 days. The water levels in the waterways and wetland described below are therefore at their typical summer low.

Upper Grundy’s Gully and the surroundings were walked, with notes and photographs taken on the signs of erosion or sediment deposition, as well as presence of native species and vegetation structure.

The distance that overland flows will travel before reaching a waterway was determined.

In addition, material relevant to the site was reviewed. This included the existing ecological reports from the Aotea subdivision and plant species lists of the area.

Site description The flow path of the existing discharge into Grundy’s Gully can be considered as three discrete sections (see figure 1):

1. Upper Grundy’s Gully headwater: this is where the proposed discharge enters and extends from the discharge point into a small vegetated ephemeral channel before being deposited into the reconstructed wetland, and then culverted under Waitaria Terrace to lower Grundy’s Gully. This upper basin is approximately 1.2 ha in area, consisting of an outer edge of kanuka, gorse and mahoe communities, the understory is sparse and appears dry with scattered kawakawa and fern. At the centre of this is a reconstructed wetland which has been planted in native carex and cyperus species. This wetland has been developed as part of the Aotea Block subdivision, in part to filter runoff, in part as mitigation for effects. It has been planted with native carex species

There is currently no obvious scour of this channel below the existing overflow discharge sites.

2. Lower Grundy’s Gully: sits below upper Grundy’s Gully approximately 340 m below the discharge point. A small ephemeral / intermittent channel emerges from the culvert and runs for approximately 200 m down valley. The waterway, receives flow from upper Grundy’s Gully, as well as from two smaller gullies’ to the north, via culverts and distributes this flow through a large culvert into Aotea Lagoon adjacent to State highway 1.

The vegetation within Lower Grundy’s Gully is also known as Aotea College Bush and is identified as a site of ecological significance on Porirua City Council’s Ecosite areas1. The vegetation here is 2 ha in area and contains a mix of tawa-titoki-seral native broadleaved forest, with emergent pine and macrocarpa. Tonkin and Taylor have previously assessed lower Grundy’s Gully as containing high ecological value and significance, as part of the Aotea Block residential development 2.

Aotea Lagoon: sits 760 m below the discharge point is a constructed wetland. This flows into Aotea lagoon and out into the Onepoto arm of the Porirua harbour, just north of Porirua Stream.

1 Boffa Miskell Ltd, “Inventory of Ecological Sites in Porirua City” (Prepared by Boffa Miskell Ltd for Porirua City

Council, 2001). 2 Tonkin & Taylor Ltd, “Aotea Block Master Plan: Ecological Assessment” (Carrus Corporation Ltd, October 2011).


Potential for ecological impacts

Construction

The proposed reservoir site lies on existing earthworks associated with the Aotea subdivision. Therefore, there will be no direct ecological impacts of the construction of Aotea reservoir within the construction footprint and given its location on a ridgeline, and assuming appropriate sediment management on site, there will be no indirect effects of construction.

Operation

There is the potential for indirect ecological impacts if overflows from the reservoir discharge into Grundy’s Gully cause bank scour, both within the gully itself and where it passes through the constructed wetland.

Looking at the current waterway between the discharge point and the constructed wetland there is no obvious scour resulting from the current regime of overflow from the existing reservoirs. On the basis that the proposed flow attenuation device is installed to control the rate of flow, in conjunction with appropriate erosion and scour protection, we do not believe there is a risk to the waterway between the discharge point and the wetland.

The constructed wetland appears capable of absorbing additional water in an overflow event, and the there is no risk of contamination as the water will be potable. There is no apparent damage occurring currently under this discharge regime to Grundy’s Gully. Therefore, we do not believe there is a risk to the wetland from periodic discharge.

The wetland will also further attenuate and slowly release water downstream to Aotea Lagoon reducing the risk of scouring at the discharge point within the lagoon. Therefore, we do not believe there is a risk to Aotea Lagoon from periodic discharge.

With the buffering of Grundy’s Gully wetland and Aotea Lagoons, there will be no adverse effect on the coastal marine area. Any discharges will mirror a normal rainfall event.

Conclusion With appropriate design of the overflow structure to restrict rate of flow and avoid erosion and scour, we consider that construction and operation of the proposed new Aotea reservoir would have no impact on the ecology of the area.

• There will be no impact on significant indigenous vegetation or significant habitats of indigenous species;

• There will be no discharge of contaminants;

• The occasional short term flow of potable water will not adversely affect any of the freshwater receiving environments including:

o Grundy’s Gully wetland;

o Aotea Lagoon; and

o The waterway that connects them.

• Aotea lagoon will buffer the coastal marine area from any small, short term changes to the freshwater regime.

• The absence of adverse effects means that no mitigation is necessary to safeguard habitats or ecosystems, and no monitoring of ecological systems is required.

Therefore, we consider that a full ecological assessment is not necessary.



Photo 1: The area where the overflow discharge point will be relocated (water channel runs from the right of the photo down into Grundy’s Gully below.

Photo 2: The restored Upper Grundy’s Gully with planted wetland. Photo taken from Waitaria Terrace looking South-West showing current reservoir A on the skyline. Note the wetland area in the centre, which is currently dry. Red arrow

indicates area where overflow discharge will be deposited.


Photo 3 (left): The scruffy dome water intake at the northern end of Upper Grundy’s Gully.

Photo 4 (right), The scruffy dome water intake at the southern end of Upper Grundy’s Gully.

Photo 5: The entrance of the watershed 3m above the wetland within Upper Grundy’s Gully. Note lack of vegetation

within the watershed depression.


Photo 6: Showing wetland at time of peak low rainfall, a small pool of standing water was found towards the Northern point of the wetland, however the majority of the area was dry.

Photo 7: Showing weedy edge between remnant Manuka and restored wetland area.


Photo 8: Lower Grundy’s Gully or Aotea College Bush (showing Aotea College in background). Taken looking north from

Waitaria Terrace.

Photo 9: Standing water, surrounded by exotic aquatic macrophytes and native

Carex sedges.

Documents

Appendix H: Preliminary ecological assessment · Message Ref: New Aotea Reservoir – Ecology preliminary assessment . Project No: W18009 . Dear Paul, As requested, this memo provides