12 June 2008 Harbor Safety Committee Slide 1 Corps of Engineers® San Francisco Engineer District “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 1130-2-xxxx (under review)
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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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12 June 2008 Harbor Safety Committee Slide 1
Corps of Engineers®
San Francisco Engineer District
“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 1130-2-xxxx (under review)
12 June 2008 Harbor Safety Committee Slide 2
Corps of Engineers®
San Francisco Engineer District
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?
*
12 June 2008 Harbor Safety Committee Slide 3
Corps of Engineers®
San Francisco Engineer District
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
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.
12 June 2008 Harbor Safety Committee Slide 6
Corps of Engineers®
San Francisco Engineer District
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 1130-2-xxxx specifies using Average Cell Depth (minimum depth in a cell shall never be used)
12 June 2008 Harbor Safety Committee Slide 7
Corps of Engineers®
San Francisco Engineer District
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.
Squat Test Conditionsvs 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
0.1 ft
0.1 ft
0.25 ft
0.1 ft
0.1 ft
0.1 ft
0.3 ft
Error BudgetMeasurement System Accuracy
Velocity Calibration Accuracy
Sounder Resolution
Draft/Index Accuracy
Tide/Stage Correction Accuracy
Platform Stability Error
Vessel Velocity Error
Bottom Reflectivity/Sensitivity
12 June 2008 Harbor Safety Committee Slide 13
Corps of Engineers®
San Francisco Engineer District
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