Weather issues for travel through the Hinchinbrook Entrance to Prince William Sound will be on the agenda when the Prince William Sound Regional Citizens’ Advisory Council meets in Anchorage Jan. 24-25.
Among the reports to be heard by the PWSRCAC is one on timely reports on weather conditions at the Hinchinbrook Entrance and their effects on the feasibility of efficient and safe rescue operations by tanker escort vessels. Those weather condition, updates in part provided by the Seal Rocks buoy, help determine whether tanker escort vessels, which are powerful tug boats, would be able to tow tanker that lost power off to safety.
Tetra Tech Canada Inc. was retained by the council to conduct a study to better define weather related conditions at the Hinchinbrook Entrance and their effects on the feasibility of efficient and safe rescue operations by these tugs, by defining the frequency and duration of conditions under which the escort vessels could not operate. Those conditions are defined as closure conditions, during which the tankers are not allowed to pass through Hinchinbrook Entrance.
Closure conditions are defined as waves exceeding 15 feet in height or winds exceeding 45 knots. According to the report, the typical closure condition based on recordings at the Seal Rocks buoy occurred, on an annual basis, 10 to 26 times for wave exceedance, one to three times for wind exceedance, and one to three times for both wind and wave threshold.
Tetra Tech used both observational data and numerical model data to assess the reliability of the current method and to seek ways to improve it. Tetra Tech found that the Seal Rocks buoy typically over-reported wind speeds in comparison to nearby buoys, but that the Seal Rocks buoy under-reported wave conditions in comparison to nearly buoys at least for the large waves characterizing closure conditions.
These observations suggest that the Seal Rocks wave data should likely be scaled for use in assessing closure conditions, the report said. These observations also found that by computing the auto-correlation of the Seal Rocks wind and wave data, that once an assessment of wind and wave conditions is made there is a 30 percent probability that seven hours later, when a tanker leaving Valdez arrives at Hinchinbrook Entrance, conditions will have changed from closure to non-closure, or vice versa, the report said.
The full meeting agenda and documents are online at the council website, www.pwsrcac.org.
