FOR INFORMATION ONLY. Gold Coast Seaway Channel Scour and Rock Wall Stability Investigation. R.B doc December 2011

Transcription

1 A part of BMT in Energy and Environment Gold Coast Seaway Channel Scour and Rock Wall Stability Investigation R.B doc December 2011

2 Gold Coast Seaway Channel Scour and Rock Wall Stability Investigation Prepared For: Prepared By: Maritime Safety Queensland BMT WBM Pty Ltd (Member of the BMT group of companies) Offices Brisbane Denver Mackay Melbourne Newcastle Perth Sydney Vancouver

3 DOCUMENT CONTROL SHEET BMT WBM Pty Ltd BMT WBM Pty Ltd Level 8, 200 Creek Street Brisbane 4000 Queensland Australia PO Box 203 Spring Hill 4004 Tel: Fax: ABN Document : Project Manager : Client : R.B doc Malcolm Andrews Maritime Safety Queensland Client Contact: Client Reference John Bendel Title : Author : Synopsis : Gold Coast Seaway Channel Scour and Rock Wall Stability Investigation Malcolm Andrews The Gold Coast Seaway provides essential all-weather access from the Broadwater to the ocean. Concern has been felt that the continual scouring of the entrance, particularly near the training walls, may lower the factor of safety to critical levels. This report details the methodology and results of the investigation into the entrance scour and the current factor of safety on the rock walls. REVISION/CHECKING HISTORY REVISION NUMBER DATE OF ISSUE CHECKED BY ISSUED BY 0 06/12/2011 DCP MJA DISTRIBUTION DESTINATION MSQ BMT WBM File BMT WBM Library REVISION PDF PDF PDF

4 CONTENTS I CONTENTS Contents List of Figures i ii 1 INTRODUCTION METHODOLOGY ENTRANCE SCOUR Analysis Results GEOTECHNICAL ASSESSMENT FUTURE MONITORING CONCLUSIONS 6-1 APPENDIX A: GEOTECHNICAL REPORT A-1

5 LIST OF FIGURES II LIST OF FIGURES Figure 3-1 Volume Change in the Seaway Entrance 3-1 Figure 3-2 Navigation Surveys Figure 3-3 Navigation Surveys Figure 3-4 Navigation Surveys Figure 3-5 Navigation Surveys Figure 3-6 Navigation Surveys Figure 3-7 Navigation Survey 2011 and Area for Volume Calculations 3-8 Figure 3-8 Difference Plots: & Figure 3-9 Difference Plots: & Figure 3-10 Difference Plots: & Figure 3-11 Difference Plots: & Figure 3-12 Difference Plots: & Figure 3-13 Difference Plots: &

6 INTRODUCTION INTRODUCTION The Gold Coast Seaway was constructed in 1986 to stabilise the Nerang River entrance and improve navigation. The works included constructing training walls south of the existing entrance, then simultaneously opening the new entrance and closing the old entrance. Immediately following the opening of the Seaway two scour holes began to develop within the entrance and there is concern that they may present a threat to the stability of the training walls. These holes have been continually monitored with regard to possible impacts on training wall stability. In 2001 WBM Oceanics investigated the stability of the training walls adjacent to the scour holes using Morrison Geotechnics as specialist sub-consultants. The conclusion from the investigation was that the training walls were geotechnically stable. BMT WBM has again been commissioned to review the stability of the training walls and have used Coffey Geotechnics as specialist consultants. This report details the methodology and results of a preliminary assessment of the current scour hole development and training wall stability.

7 METHODOLOGY METHODOLOGY The key assessments of ongoing entrance scour and training wall stability have been addressed by following investigations: Assessment of the historical navigation surveys and cross section profiles to give an indication of the trends of the scour holes both in a vertical direction (deepening) and spatially (broadening or migrating); Assessment of the stability and possible failure mode of the training walls at differing locations (foundation level and rock wall section changes) and for differing scour hole depths and side slopes; and The possible impacts of other factors such as sea level rise, increasing tidal velocities and redistribution of flows from the north and the south through the entrance.

8 ENTRANCE SCOUR ENTRANCE SCOUR 3.1 Analysis Historical entrance surveys and aerial photography have been captured as part of the monitoring program for the initial entrance works. These have been used to assess the historical changes in the entrance channel bed including the two scour hole developments. This has been done by creating digital terrain models (DTM s) on a GIS platform (MAPINFO) and initially colour coding according to depth. These plots are presented in Figures 3-2 to 3-7. Difference plots have also been made between each year as a further visual representation of the changes which are occurring. These plots are presented in Figures 3-8 and A further plot of the difference between 2001 and 2011 is given as Figure Results A volumetric analysis of the changes between survey dates in the entrance channel between Wavebreak Island and the eastern end of the breakwaters (refer Figure 3-7 for area) has been carried out using the DTM s. The results of this analysis, including the previous results from the 2001 report, are given in Figure 3-1 below. Figure 3-1 Volume Change in the Seaway Entrance The DTM plots and the volumetric analysis of the historical surveys indicate that the entrance scouring has slowed considerably from its historical rates. Annual fluctuations appear to be as high as 100,000m 3 ( ), and the recent average annual rate of scour is about 20,000m 3. The bed level and difference plots indicate that scour hole development occurred until about 1992 and extensive deepening of the main flow channel continued until about Since then the depth of the holes and the channel has slowed with some spatial extension of the channel deepening. The annual fluctuations are probably as a result of the conditions preceding the survey i.e. recent spring

9 ENTRANCE SCOUR 3-2 tides or flood flows could increase the depth and recent neap tides and some leaking of beach sand into the entrance could decrease depths. The scouring of the entrance channel is primarily related to the amount of flow through the entrance. Therefore, any factors, such as sea level rise, increasing tidal velocities and redistribution of flows from the north and the south through the entrance, which increase these flows (particularly in the north channel) are likely to exacerbate entrance scour with the impact related to the degree of the change. It is not known what lies at the base of the scour holes (approx. -20m LAT) but old geotechnical surveys of the entrance indicate that harder material (bedrock or indurated sand) exists at around this depth in the region. A thorough understanding of the sensitivity of the entrance to these changes could only be achieved by hydrodynamic and morphological modelling of the entrance. This modelling could also provide scenario analysis of any proposals to divert or re-balance flows around the entrance to reduce the asymmetry which causes the scour holes. Also, the presence of harder material could be included in the morphological modelling if this were carried out.

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