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Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 1 “ASSESSMENT AND REPORTING OF ACOUSTIC CLEARANCE SURVEYS.

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Presentation on theme: "Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 1 “ASSESSMENT AND REPORTING OF ACOUSTIC CLEARANCE SURVEYS."— Presentation transcript:

1 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 1 “ASSESSMENT AND REPORTING OF ACOUSTIC CLEARANCE SURVEYS IN DEEP-DRAFT NAVIGATION PROJECTS” Presentation Highlights: 1 – Establish Channel Depth 2 – Typical Dredging Section (Current & Proposed) 3 – Criteria for O&M Channel Acceptance 4 – Criteria for Multi-beam Cell Depth 5 – Criteria for Rounding Depths Furnished to the Public 6 – The EC Is Evolving – Comments Are Still Being Accepted Possible Effects of Draft EC xxxx (under review)‏

2 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 2 1 to 2 ft Establish Channel Depth Usually 0 ft { Justification for advance maintenance should describe historical shoaling rates, frequency of dredging, and cost analysis. DREDGING TOLERANCE 2 ft * How do you establish underkeel clearance? *

3 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 3 Dredging Tolerance: 1’ Allowable (Paid) OD 1’ Allowable (Non-Paid) OD Current Typical Dredging Section (O&M Dredging) Dredging Limitations: “Contour” Payment for In-Channel OD No Side-slope Dredging BUT Payment for material that sloughs off of side slope No Payment for Side-slope OD

4 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 4 Unclear Aspects (especially in the SF Bay area): Side-slope Dredging Side-slope Over Depth Non-pay Over Depth Total Volume of Dredged Material (Pay & Non-Pay) New: Survey Uncertainty Tolerance EC xxxx Typical Dredging Section (soft bottom maintenance dredging)

5 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 5 Required Channel Depth Allowable Over Depth Estimated Survey Tolerance Material falling within the survey uncertainty tolerance window need not be removed. The survey tolerance is defined as the estimated repeatability or reproducibility of the statistical average of multiple acoustic measurements made over a finite area or cell, and at a specific project site using the same or different measurement systems. Criteria for O&M Channel Acceptance * *Since the statistical computations are complex, practical engineering judgment necessitates that an estimated "average survey tolerance" be assigned to specific surveys.

6 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 6 Depths in a Cell Average Minimum Criteria for Multi-beam Cell Depth San Francisco District Plans to Acquire Multi-beam Survey System in FY09 EC xxxx specifies using Average Cell Depth (minimum depth in a cell shall never be used)

7 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 7 Required Channel Depth Allowable Over Depth Estimated Survey Tolerance ~ Published Soundings Survey Tolerance >0.5 ft => Round to Nearest 1.0 ft Survey Tolerance ±0.2 ft to ±0.5 ft => Round to Nearest 0.5 ft Criteria for Rounding Depths Furnished to the Public * Nearest 0.5 ft is the highest “accuracy” that can be published. * Better Equipment & Procedures Yields Smaller Survey Tolerance

8 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 8 1 – Criteria for Channel Acceptance A – Estimated Survey Tolerance (EST >= 0.2 ft)‏ B – High Spots (<= EST) Are Acceptable 2 – Criteria for Multi-beam Cell Depth (use average depth)‏ 3 – Criteria for Rounding Published Depths (± 0.5 ft or ± 1.0 ft)‏ Recognizing the steep learning curves for all things new: 4 – First priority = reduce EST of Government surveys 5 – Next priority = improve survey posting timeline The Engineering Circular is evolving – Comments Are Still Being Accepted Taking Uncertainties into Account Possible Effects of Draft EC xxxx (Summary)‏

9 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 9 “ASSESSMENT AND REPORTING OF ACOUSTIC CLEARANCE SURVEYS IN DEEP-DRAFT NAVIGATION PROJECTS” The End Final Version of EC xxxx – July 2008 QUESTIONS? Possible Effects of Draft EC xxxx (under review)‏

10 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 10 Harbor Safety Committee Briefing (12 June 2008) STOP Back Up Slides Follow Corps of Engineers ® San Francisco Engineer District

11 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 11 Current Data Collection System DGPS for horizontal position Tide Elevations for vertical Heave-Pitch Compensator Without Compensator Weakest Link Suboptimal Performance

12 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 12 No matter how repeatable our surveys are, we are still just estimating the channel depth. Constant “draft” Correction vs Loading Changes during the Survey Old Sounders in Deep Channels vs New in Shallow Stationary Bar Checking vs Dynamic Surveying 0.1 ft Precision ≠ 0.1 ft Accuracy On-shore Reference Benchmark vs tidal surface gradient errors Effects of sea Roll, Pitch, and Heave Squat Test Conditions vs Operating Velocity and Loading Echo sounding returns are dependent on the frequency of the acoustic pulse, receiver sensitivity settings, and distinct density changes in the subsurface material. Inherent Survey Uncertainties ± 0.9 ft = RMS (95%)‏ Quantitative estimate of acoustic depth measurement accuracy 0.1 ft 0.25 ft 0.1 ft 0.3 ft Error Budget Measurement System Accuracy Velocity Calibration Accuracy Sounder Resolution Draft/Index Accuracy Tide/Stage Correction Accuracy Platform Stability Error Vessel Velocity Error Bottom Reflectivity/Sensitivity

13 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 13 POS MV 320 GPS-Aided Inertial Navigation System functionality of a GPS receiver, gyrocompass & conventional motion sensor POS (Position and Orientation System) Computer System –controls the IMU and GPS receivers –computes velocity, roll, pitch and true heading –outputs data in the correct format to interface with Hypack Inertial Measurement Unit (IMU)‏ –contains 3 gyroscopes and 3 accelerometers GPS Sub-system –two antennas plus two receiver cards embedded in POS Computer –computes position to 0.02m with optional RTK New Positioning System‏ Heave-Pitch-Roll Compensation Critical for Multibeam Horizontal & Vertical Position Position ± 0.07 foot BETTER: - Draft/Index Accuracy - Vessel Velocity Error - Tide/Stage Correction Accuracy - Platform Stability Error Error Budget : 0.1 ft : 0.25 ft : 0.3 ft

14 Corps of Engineers ® San Francisco Engineer District 12 June 2008 Harbor Safety Committee Slide 14 Standards and Specifications for Acoustic Surveys Contract Specifications Quantity Computation Procedures Selection of Controlling Depths Use of Multi-beam Survey Systems RTK Position and Elevation Control Response Times for Dissemination of Survey Data The devil is in the details ! Other Aspects Covered (Draft EC xxxx)‏


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