Amended Maintenance Dredge Spoil Management Plan Shute Harbour Marina Resort
Amended Maintenance Dredge Spoil Management Plan Quality Information Document Ref Amended Maintenance Dredge and Spoil Maintenance Plan GS10 Date 11 December 2012 Prepared by Reviewed by Vin Keneally Greg Betros Revision History Revision Revision Date Details Authorised Name/Position Signature Rev 0 12/12/2012 Formatted & PDF G. Betros Shute Harbour Marina Resort Page i 12 December 2012
Amended Maintenance Dredge Spoil Management Plan 1. Introduction Government Agencies raised a number of queries about maintenance dredging and spoil management in the August 2008 EIS. As a result of the significantly reduced scale of the project and in response to these queries a maintenance dredge spoil management plan has been amended as outlined in this Management Plan of the SEIS. Siltation Rates Siltation rates were established by modeling contained in the EIS as follows: 0.03metre (30mm) per annum in the eastern section of the marina basin 0.006metre (6mm) per annum in the western section of the marina basin 0.02metre (20mm) per annum in the access channel Siltation Volumes Marina Basin The average annual siltation volume as established in the EIS (for the larger project) is 3,000 cubic metres maximum per year which is equivalent to 1,500 tonnes of dry mass. The current amended development proposal for the marina has a significantly less footprint as follows: Original Marina Development Proposal Area: 269,386 sq metres Amended Marina Development Proposal Area: 168,558 sq metres Reduction in Marina Development Area: 100,828 sq metres As a result the average annual siltation volume is estimated at 1,878 cubic metres per year or a reduction of 1,122 cubic metres per year (or 37.4%) Access Channel The average annual siltation volume as established in the EIS (for the larger project) for the access channel is 446 cubic metres maximum per year. The current amended development proposal for the access channel has a significantly less footprint as follows: Original Access Channel Area: 22,300 sq metres Shute Harbour Marina Resort Page 1 12 December 2012
Amended Maintenance Dredge Spoil Management Plan Amended Access Channel Area: 18,000 sq metres Reduction in Marina Development Area: 4,300 sq metres As a result the average annual siltation volume is estimated at 360 cubic metres per year or a reduction of 86 cubic metres per year. Total Annual Siltation Volume The total annual siltation volume is estimated to be in the order of 2,500 cubic metres per year as follows: Marina Basin: 1,878 cubic metres Access Channel: 360 cubic metres Total : 2,238m cubic metres Add 10% safety factor: 224 cubic metres Total Volume per year: 2,462 cubic metres Allow: 2,500 cubic metres per year Over Excavation The marina basin will be over-excavated during the initial capital excavation by 200mm. As a result maintenance dredging will not be required in the first ten years of marina operations. Proposed Management Method The proposed Maintenance Dredging Management is to develop a purpose designed collection, holding, de-watering and disposal area at the west edge of the site and to contain and de-water the wet dredge spoil using Geotube Dewatering Technology. The location of the area is shown on the amended masterplan. It is proposed to dredge the marina basin and access channel using a small cutter suction dredge. The wet dredge material from the dredge head would be pumped via a floating pipeline to a permanent pipe connection in the sheet pile wall adjacent to the Indigenous Cultural Centre. At this connection point a non return valve will be provided to prevent any wet dredge spoil returning to the marina once the pumping has been completed. From the permanent connection point an underground pipe would convey the wet material to the Maintenance Disposal Area. The area would be contained by sheet piling and the floor graded to a catchment drainage system. Inside the contained area a series of Geotubes would-be laid out. The wet material would be pumped into each tube which inflates as it is filled. The principle of the Geotubes is to filter the water and retain any solids within the tubes. Once the tubes are dewatered the tubes and the solid material is removed from site. Shute Harbour Marina Resort Page 2 12 December 2012
Amended Maintenance Dredge Spoil Management Plan Maintenance Dredge Spoil Area Refer to appended plans; Masterplan number SEIS P6. Maintenance Dredge Spoil Area Detail Plan and Section number SEIS P19. The total holding capacity of the Dredge Spoil Area is 7,020 cubic metres calculated as follows: Geotube dimensions: 50 metres long x 6metres wide Total number of Geotubes required: 12 Geotube capacity: 450 cubic metres per tube Geotubes would be initially filled to a pre-determined height and pressure. The bottom layer of three Geotubes would be filled first simultaneously through a pipe manifold followed by the top layers. The total capacity of the initial fill is 5,400 cubic metres (12 tubes x 450 each) Once initial dewatering has taken place (usually within days), the tubes can be topped up with additional dredge material. The initial dewatering is estimated to be 30% of the total volume. Once refilled the total holding volume would be the initial volume (5,400 cubic metres) plus the refill volume (1,620 cubic metres) to a total of 7,020 cubic metres. Detailed consultation has been carried out with CRS Industrial Water Treatment Systems which is a division of Maccaferri. The proposed system has been deployed locally and internationally in a diverse range of industries and projects. The following technical information has been provided by the specialist contractor and is appended: Tencate Geotube information sheets and Case Studies pages 1-11 CRS: The Geotube Dewatering Process Information Sheet Technical data sheet CRS Case Study: North Canterbury CRS Case Study: Somerville Victoria Dredging Solutions Case Study: BHP South Australia Dredging Solutions Case Study : BHP South Australia Dredging Solutions Case Study: Queensland Shute Harbour Marina Resort Page 3 12 December 2012
Amended Maintenance Dredge Spoil Management Plan The key to the success of the system including the ability to dewater fine particles and sediments and the time required for dewatering is the conditioning of the fine sediments in line prior to the wet dredged material entering the Geotubes. The Conditioning is provided by dosing the wet material with inert polymer flocculants applied directly into wet dredge stream. During Construction Phase 4, site trials will be carried out by the specialist contractor to determine the most appropriate dosing system and rates. The flocculating agent and dosing rates will be tailored to suit the wet material. During maintenance dredging, the effectiveness of the dosing system will be monitored at sample points. Dewatering times are variable but averages are between a few days and up to three months. Settlement boxes will be installed and monitored within the marina to check if siltation predictions have been accurate and reliable and to enable maintenance dredging frequency to be adjusted to suit the actual site conditions. Dredging Frequency It is estimated that after the initial ten years (during which no dredging is required due to over-excavation of the basin) maintenance dredging will be required every 2-3 years. Discharge Water Quality The water quality discharging from the Geotubes will be tested during trials carried out in Construction Phase 4. The specialist contractor advises that in excess of 99% of solids are captured in the Geotubes. The clear filtrate from the Geotubes will be collected at the base of the Disposal Area and piped to a bio-retention basin before being discharged back into Shute Bay. Shute Harbour Marina Resort Page 4 12 December 2012
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