Dredging Surveys and Volume Computations Port of Anchorage, Alaska

This 3-year project was originally specified as a single beam and lead-line survey that was to be conducted more than 40 times in a 6-month period. Surveys were scheduled every other day at times throughout the project. The project area was approximately 1.5 miles long and 0.5 mile wide with depths referenced to Mean Lower Low Water (MLLW) ranging from -70 feet to +10 feet. The project area was subject to extreme tides of up to 40 feet and currents that regularly exceeded 4 knots.

We chose to perform the surveys with a multibeam sonar system rather than single beam. Our experience with multibeam survey technology provides a more efficient approach to completing these surveys. Multibeam technology allows the same area to be surveyed in less than half the time that is required for single beam. Also, it was not necessary to collect lead-line soundings along the dock face due to the wider swath coverage we were were able to collect with the multibeam sonar system.

We used Real-Time Kinematic (RTK) GPS 3D positioning referenced to MLLW, and the survey was repeatable using RTK or NOAA tides for vertical reference. Daily redundant measurements over a known object verified the accuracy throughout the surveys.

Our survey techniques allowed for a prompt, complete and accurate turnaround of the survey data. We performed 60 multibeam surveys in 2004 and 50 multibeam surveys in 2005.

In 2006, the Alaska District reduced the number of survey lines required, but increased the number of surveys. Due to the change in the project specifications, we chose to perform single beam surveys. A CodaOctopus F185 motion platform that provides heading, heave pitch and roll, as well as RTK GPS 3D positioning was used so that the data could be measured as accurately as possible. In 2006, 76 single beam surveys were completed.

Both single and multibeam surveys were completed and delivered in 1-2 days. Data reduction included applying all necessary correctors, computing dredge and infill volumes, and drafting AutoCAD drawings of the survey.

Back to top