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

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)




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?

*




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




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 1130-2-xxxx Typical Dredging Section (soft bottom maintenance dredging)




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.




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)




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




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 1130-2-xxxx (Summary)




“ASSESSMENT AND REPORTING OF ACOUSTIC CLEARANCE SURVEYS

IN DEEP-DRAFT NAVIGATION PROJECTS”

The End

Final Version of EC 1130-2-xxxx – July 2008

QUESTIONS?

Possible Effects of Draft EC 1130-2-xxxx (under review)




Harbor Safety Committee Briefing (12 June 2008)

STOPBack Up Slides Follow






Current Data Collection System

DGPS for horizontal position

Tide Elevations for vertical

Heave-Pitch Compensator

Without CompensatorWeakest Link

Suboptimal Performance




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 Benchmarkvs tidal surface gradient errors

Effects of sea Roll, Pitch, and Heave

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




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

: 0.1 ft

: 0.25 ft

: 0.3 ft




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 1130-2-xxxx)

Documents

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