Key Topics: Stormwater

The Pukehāngi Heights Development Area lies with the Utuhina Steam Catchment and will discharge stormwater to the catchment.

The volume of stormwater runoff will increase with the urbanisation of the Pukehāngi Heights area due to the increase in impermeable surfaces.

Conceptual Stormwater Management Plans have been developed for each of the three sites within the Development Area:

A further report assessing flood risks has been prepared[4]. These reports have all been independently peer reviewed[5].

The general concept for each conceptual management plan is to manage stormwater quantity and quality following a “treatment train” approach, with a focus on source control (including ‘on-lot’) measures, distributing to a network of green open space that serves as stormwater attenuation areas including swales for attenuation and conveyance, and use of larger dry basins before discharging off the site.

Sunny Downs and Hunt Farm can both make appropriate on-site provision for stormwater management that meet these criteria using detention basins and control structures to manage runoff rates. An additional option that also could limit the on-site provision for stormwater management is the development of detention basins, ponds or control structures in Wrights Park.

Due to the site restrictions, detention basins or ponds are not a safe and practical option for Twin Oaks as any retained water body within the site will pose a risk to the existing downstream residential development. A combination of ground infiltration and on-lot detention tanks is a technically feasible option for reducing runoff rates to meet the criteria. Alternatively, Twin Oaks may be able to work with the adjoining Hunt Farm to provide access to retention areas with increased capacity to also take their runoff.

The proposed Structure Plan for the Pukehāngi Height Development Area shows the indicative location of stormwater detention areas and the main overland flow paths based on the Conceptual Stormwater Management Plans. Indicative detention areas for the Twin Oaks development have been identified on the Hunts Farm.

Finalising the design of the stormwater management system for the development is constrained by currently incomplete information on the Utuhina catchment. An effective Catchment Management Plan for the total catchment requires modelling that will enable consideration of issues such as future development, current flooding risk, and climate change. This work is underway, but not yet complete.

Development planning has therefore proceeded using interim assumptions about the level of attenuation to protect the proposed development and downstream areas from flooding. The preliminary design criteria set for stormwater management is to reduce the 1% AEP runoff to 80% of the pre-development flows.

This results in large areas needing to be set aside within the development area for stormwater detention basins. These areas will eventually vest in the council. The extent of on-site stormwater mitigation and the ultimate development yield cannot be fully confirmed until the catchment modelling and planning work is completed.

Given these uncertainties, the proposed plan change provisions require a catchment based Stormwater Management Plan (SMP) to be prepared prior to subdivision and development occurring. The issues addressed in the SMP are set out on the Performance Standards. This includes the need to address the potential effects of stormwater management measures, for example, the detention areas, on land stability and liquefaction;

Developers will also need stormwater discharge consents from the Bay of Plenty Regional Council to enable development to occur. This will require an assessment of water quality and quantity effects to be undertaken, and consideration of potentially affected persons.

Council is in the process of renewing the comprehensive stormwater discharge consents that it holds for the wider Utuhina catchment. To date, the application process has been approached on conservative design assumptions due to the lack of modelled data. Opportunities are being considered to enhance the discharge consent application process with modelled data and to widen the scope of the application to include the Pukehāngi Heights Development Area. This will ensure that issues are addressed effectively and efficiently on a comprehensive basis across the catchment, thereby simplifying the future development consent process.

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[1] Pukehāngi Road (Sunny Downs) development - Concept Stormwater Masterplan WSP Opus 2017

[2] Pukehāngi Road (Hunts Farm) development - Stormwater Masterplan Advice WSP Opus 2017

[3] Te Arawa Group Holdings Development, WSP Opus, Rotorua 2018

[4] Pukehāngi Heights Flood Risk Assessment, WSP Opus 2019

[5] Pukehāngi Plan Change Stormwater Technical, Review, Tonkin and Taylor, 2019