Kings Forest Development - Mortons Urban Solutions

ClientProject 28 Pty LtdLocationAustralia, NSW
ConsultantMortons Urban SolutionsContractorRoger Brisbane

Project Summary

First phase of a 4,500 lot master planned community in Kings Forest.

The Kings Forest site is on the far north coast of NSW, approximately 20km south of the Queensland/NSW border. Kings Forest is identified within the NSW State Government’s Far North Coast Regional Strategy and Tweed Shire Council’s adopted “Tweed Urban and Employment Lands Release Strategy 2009” as one of the largest contributors for the provision of new housing and employment within the Tweed Shire in the next 25 years (Source: Tweed Shire Council).

The Challenge

In November 2006, Kings Forest was listed as a State Significant Site under the State Environmental Planning Policy (Major Development) 2005. This had the effect of nominating the applicable zoning which applies to the whole site, as well as nominating specific controls which affect all future development at the Kings Forest site (Source: Tweed Shire Council).

Kings Forest comprises around 866 hectares of flood affected flat land. The primary constraints for the Mortons team included cultural heritage, SEPP 14 wetlands and groundwater dependent ecosystems, perched groundwater, and protected fauna.

The development of this land for residential, commercial and institutional purposes includes approximately 4,500 home sites, community facilities, and a commercial precinct.

Flood plain management and flood immunity presented significant obstacles to development of the site, with initial studies indicating the necessity to import approximately 1 million cubic metres of material (representing a potential development cost up to $20 million).

The first phase of the development incorporated 431 dwellings, estate major access, detailed grading and stormwater analysis of all interdependent areas, and full site preliminary grading.

Initial modelling of the site based on site constraints indicated areas potentially available for development, with the first phase comprising approximately 64 hectares of flat land, the largest portion of which being approximately 850m wide x 850m long.

In order to perform grading of such a flat site, the traditional approach would be to create a shield using the road network with the centre of the site being graded outward towards the perimeter over roughly 425m. Such a grading would require a central area roughly 2.2m higher than the perimeter (at 0.5% nominal grading), requiring an import of approximately 200,000m3 of material in the first phase. The traditional alternative would be to construct a central drainage channel.
Local haulage costs and sourcing material were prohibitive costs, so a solution that minimised import was highly desirable. The construction of a swale within the site with corresponding duplication of roads and resultant loss of yield were economically undesirable outcomes. Thus, the Mortons team was required to find an optimum solution meeting the requirements of QUDM (minor/major storm flow paths, etc.), and maximising yield and minimising import.

The Solution

The team initially established a largely flat site with saw-toothed roads, which went some way to reducing import and maintaining major storm flow paths, necessitating a piped drainage system in excess of the “minor storm” requirements.

They determined to change their perspective of the site from a water shedding development to one behaving more like a mitigated release reservoir. The challenge there was managing stormwater objectives over an effectively flat large site without causing nuisance or risk to property or people.

Infiltration and storage swales were utilised on the perimeter of the site within an area permitting earth works, but this area could contain no infrastructure. This effectively managed flows around the perimeter; however, it was the centre of the site that presented a more complicated issue.

The site has a central spine road servicing the whole development, and so leant itself to the design of a central drain, meaning Mortons could halve the distances for overland flow to travel. This also provided an opportunity to manage flood immunity and stage outflows, and provide at source infiltration.

The first phase of the development comprised 15 inter-connected basins fed variously by daylighting and surcharging outlets, incorporating infiltration, and multiple bi-directional outlet pipes and weir outlets. The surcharging outlets were drained to daylight via a 300 diameter pipe, to ensure even the lowest flows were infiltrated to meet water quality objectives.

Treatment of the site in this way was only possible because of the powerful bifurcating pit, basin outflow and infiltration capabilities within the Dynamic Drainage Analysis component of 12d Model software.

Using a combination of multiple outlets, basin links and channels, basins formed part of the conveyance system, permitting design of the central road with no longitudinal grade, thereby significantly reducing earthworks import and achieving the lowest practical level for secondary road connection.

The result of this design was that Mortons negated the requirement for import and achieved a spoil of approximately 60,000m3 which could be used in other areas. The net improvement of earthworks was roughly 260,000m3 or $5.2 million in Phase 1 alone.

The Result

This design philosophy of utilising the site’s reservoir potential will continue to be used to reduce the necessity for import as Mortons progresses through the remaining phases of the development.
Extensive consultation was being undertaken with Tweed Shire Council after this first phase, including an in-house presentation to discuss what could be considered at the extreme end of “conventional” stormwater systems.

This innovative approach could have an impact on many future projects of this type.

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