South Beach, a Breakwater and a Greene. Substantial excavation/dredging is required to create the Boat Harbor, much of which Is within an area of actual and potential acid sulfate soils. A range of modifications were made to the original Boat Harbor design to enhance the technical, environmental and social outcomes. These modifications included rotation of the Boat Harbor platform (as one component of the management of acid sulfate soils), re-design of the Breakwater and Grosses, and deletion of a proposed Boat Harbor flushing pipe system.
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An extensive Construction Environmental Management Plan (KEMP) and Operational Environmental Management Plan (MEMO) were developed to ensure achievement of the enhanced outcomes. Construction of Stage 1 of the Boat Harbor commenced in early 2013. Keywords: coastal structures, acid sulfate soils, water quality, environmental management plans. 1. Introduction Shell Cove is a large scale, masterminded, beachheads, urban development originally approved In the asses located on the New South Wales South Coast in the Shallower City region (Figure 1).
The project comprises the development of approximately 3,000 dwellings. A Hampshire 18 hole golf course, a 300 berth Boat Harbor, commercial development including a town centre, hotel and business park and associated open space, environmental and social provisions. Changes to statutory legislation since the original design. The Boat Harbor element of the project is includes an Inner Harbor, Outer Harbor, Entrance Channel linking the Boat Harbor to the sea across Shallower South Beach, a Breakwater and a Greene.
Substantial excavation/dredging is required to create the Boat Harbor, much of which is within an area of actual and potential acid sulfate soils. Patterson Britton & Partners (Patterson Britton) were appointed by Australian Corporation (NEWS) Pity Ltd (Australian) in May 2003 to evolve the conceptual design for the Boat Harbor prepared in 1996, which formed the basis for the original Development Application (DAD), to a final detailed design and documentation stage.
The Brief prepared by Australian included a Value Management protocol involving review of all elements of the original design in respect of, among other issues, availability, cost implications, environmental and ecological outcomes, aesthetics, risk management, construction schedule, impact of the conditions of consent and Figure 1 Shell Cove Location Plan The review conducted by Patterson Britton led to recommendations to modify the original design in a number of ways.
The recommendations were accepted by Australian and led to preparation of a technical report to support a Section 96 (ass) modification application under the NEWS Environmental Planning & Assessment Act, 1979 to modify the original consent . The key modifications included: rotation of the Boat Harbor platform; design modifications to the Breakwater and Grosses; deletion of the Boat Harbor flushing pipe system. Construction of Stage 1 of the Boat Harbor commenced in early 2013.
Completion of the Boat Harbor is scheduled for before 2020. 2. Rotation of the Boat Harbor Platform The method of management of acid sulfate soils (ASS), comprising both actual acid sulfate soils (AAAS) and potential acid sulfate soils (PASS), proposed in the Environmental Impact Statement (IIS) involved reburial of the materials, both below the bed of the Boat Harbor (by over-excavation) and within burial pits adjacent to the Boat Harbor under the future land platform.
Removal and transport of the materials to the disposal areas was reposed to be by use of a cutter-suction dredger pumping the ASS as a slurry through a pipeline. This wet method of removal and transport of ASS was proposed in order to reduce the risk of oxidation of the ASS.
Two main categorical issues were identified with the ASS management strategy proposed in the IIS based on field and laboratory investigations undertaken by Coffey Consciences on behalf of Patterson Britton: the lengthy time necessary to achieve adequate consolidation of the hydraulically placed ASS material in the burial pits under the future land platform and hence the time delay public and private infrastructure, estimated to be from 4 to 70 years; the shallower rock levels determined over the site compared to those adopted in the IIS, which substantially restricted the opportunity for reburial of ASS below the Boat Harbor and significantly increased the required lateral extent of burial pits under the future land platform thereby exacerbating consolidation issue above. Rotation of the Boat Harbor by approximately 1 50 in an anti-clockwise (westerly) direction was proposed in order to reduce the quantity of ASS that required excavation and management (Figure ). In addition, it was proposed to ‘consolidate and cap’ (not remove) insist ASS located outside (to the east) of the footprint of the rotated Boat Harbor. These avoidance measures reduced considerably the quantity of ASS to be excavated and managed, from the figure of approximately 640,000 mm in the IIS to a figure of approximately 340,000 mm. This led to considerable cost savings and reduction in environmental risk.
Figure 2 The location of the original Boat Harbor platform (in blue) and the rotated platform (in green) superimposed on the lateral extent and thickness of ASS on site. The reduction in the quantity of ASS to be excavated and managed also meant that it was possible to avoid the need for creation of burial pits located under the future land platform. This had the major advantage of De-coupling the consolidation behavior of the excavated ASS from the schedule for development of the land platform. The rotation of the Boat Harbor platform also provided an advantage for availability and construction sequencing. The western section of the Inner Harbor was now able to be excavated in non ASS, immediately winning material for strategy and delaying the introduction of ASS management measures.
Testing was also undertaken to determine the rate of oxidation of the clayey textured ASS when exposed to air. This involved recovery of representative samples and monitoring in the laboratory of the acidification of the samples over time. This testing showed that it would be possible to adopt an ‘in the most’ technique for excavation and transport of the ASS, using conventional earthmoving equipment, rather than use of a cutter-suction dredger pumping the ASS through a slurry pipeline. This significantly reduced the estimated consolidation times for the ASS placed below the over-excavated Boat Harbor, again achieving significant schedule benefits.
Drilling and blasting may be necessary in order to achieve sufficient storage capacity for the ASS below the Boat Harbor, beyond the base of the repairable rock. Studies have shown that drilling and blasting would satisfy vibration criteria and, with suitable controls in place, air overpressure criteria. The Northern Greene and Southern Greene had several purposes; to protect the channel and berthing areas from wave action, to prevent sediment infilling of the channel (Northern Greene) and to maintain a sandy beach south of the entrance (termed Harbor Entrance Beach). The Northern Southern Grosses were approximately 230 m long and 180 m long respectively and extended out to a common water depth of approximately -4 m ADD.
The primary armor on the Breakwater and the Southern Greene included artificial concrete armor units (hangars) in addition to rock. Primary armor was located along the crest of each entrance structure. As such, pedestrian access along the structures would not have been possible. Breakwater and the Northern and Southern Grosses with the aim of achieving a functional design outcome at least equivalent to the IIS design (egg wave height attenuation and overstepping performance), but having a reduced environmental impact and providing improved public access. The proposed modifications were supported by the results of an extensive program of numerical modeling, two dimensional (ID) and ID physical modeling, analytical calculations, and a specialist navigation assessment.
A wave rider buoy was installed locally and utilized in the calibration of the numerical and physical modeling. Wave rider buoy data was also available from the offshore Port Kamala and offshore Sydney wave rider buoys. The proposed breakwater modifications were (Figure 3): Rooney Early findings from the Stage 1 excavation works indicate that the rock below the Boat Harbor may be repairable to a deeper level than initially thought, at least in the western section of the Inner Harbor, reducing dependency on drilling and blasting. Design Modifications to the Breakwater and Grosses The entrance configuration for the Boat Harbor proposed in the IIS comprised a Breakwater, a Northern Greene and a Southern Greene (Figure 3). Alignment and shortening of the Breakwater by 50 m, thus reducing the visual impact of the structure; Breakwater rather than a conventional rubble mound design so as to avoid use of artificial concrete armor units such as hangars, thus reducing the visual impact of the structure and allowing efficient use of local quarry materials; deletion of the Southern Greene, thus reducing the extent to which structures cover natural rock reef ecology, reducing the visual impact of the entrance structures, and reducing the amount of construction activity in the vicinity of Aboriginal maidens located south of the entrance channel; 3. The Breakwater was approximately 565 m long and extended offshore into a water depth of approximately -10 m ADD.
It had two main reposes; to protect the entrance navigation channel and berthing areas from wave action and to provide a physical barrier to sediment movement which would otherwise cause infilling of the channel. Provision pedestrian access maintenance vehicle access along the crest of the Breakwater and the Northern Greene, thus enabling greater public accessibility and providing straight-forward access for any required maintenance following severe storms; Figure 3 Proposed modified breakwater and Greene design (in black) superimposed on the IIS Figure 4 View of the ID physical model testing of the entrance configuration inducted in the wave basin at NUNS Water Research Laboratory slight modification to the head of the Northern Greene, thus providing greater wave protection to the Access Channel and to the boating facilities located upstream.
Construction of the Stage 1 section of the Breakwater and Northern Greene, which are located across the sandy beach above high water mark, is now underway. These works are being constructed within a temporary steel sheet pile cofferdam which incorporates temporary rock and sand ground anchors. Deletion of Boat Harbor Flushing Pipe System The IIS proposed a flushing pipe system for the Boat Harbor on the basis that, adopting conservative assumptions, a pump may be required at the ultimate urban development stage to assist natural tidal flushing of the Boat Harbor and ensure adequate water quality. The concept of the flushing pipe system was to pump ocean water into the Inner Harbor of the Boat Harbor 50% of the time, during ebb tides.