ACCESS NUMBER: 10-85243

STUDY TITLE: Observations of hydrography and currents in central Cook Inlet, Alaska during diurnal and semidiumal tidal cycles

REPORT TITLE: Observations of hydrography and currents in central Cook Inlet, Alaska during diurnal and semldlurnal tidal cycles

CONTRACT NUMBER: 1435-01-98-CA-30909, TO 85243

SPONSORING OCS REGION: Alaska

APPLICABLE PLANNING AREAS: Cook Inlet, Alaska

FISCAL YEAR OF PROJECT FUNDING: 2003

COMPLETION DATE OF REPORT: 2005

COSTS: FY 2002 -$1087, FY 2003 -$18,495, FY 2004 -$957, FY 2005 -$8265, FY 2006 - $4330.14.

CUMULATIVE PROJECT COST: $33,134.84

PROJECT MANAGER: Stephen R. Okkonen

AFFILIATION: University of Alaska Fairbanks

ADDRESS: Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska 99775-7220

PRINCIPAL INVESTIGATOR*: Stephen R.Okkonen

KEY WORDS: Tide np fronts, density-driven currents, Cook Inlet

BACKGROUND:

The Cook Inlet area is home to an extensive petrochemical Industry with both marine and land-based

components. This project investigates the temporal variability of the hydrographic structure and surface

currents across central Cook Inlet during semidiumal and diurnal tidal cycles. The results from this project

contribute to understanding how these physical features and processes might influence the behavior and

trajectory of spilled oil and, as such, are potentially useful for spill response planning and training. The

results also suggest that skillful numerical circulation models for Cook Inlet require high-resolution

bathymetry and buoyancy forcing in addition to tidal forcing.

OBJECTIVES:

The principal objectives for this project were to describe variability in the hydrographic structure and

surface currents in central Cook Inlet and to identify the dominant forces driving the observed variability.

DESCRIPTION:

Surface-to-boftorn measurements of temperature, salinity, and transmissivity, as well as measurements of

surface currents (vessel drift speeds) were acquired along an east-west section in central Cook Inlet,

Alaska during a 26-hour period on 9-10 August 2003. These measurements are used to describe the

variability of frontal features (tide rips), surface currents, and physical properties along this section during

semidiurnal and diurnal tidal cycles.
SIGNIFICANT CONCLUSIONS:

The association of strong frontal features with steep bathymetry directly follows from the functional

relationship between surface tidal current speeds and water depth. The positions and strengths of these

fronts vary with the semidiumal tide. The presence of freshwater gradients alters the phase and duration

of tidal currents across the section.

STUDY RESULTS:

The amplitudes of surface tidal currents are observed to be proportional to water depth, that is, faster

currents occur where the channel is deeper and slower currents occur where the channel is shallower. It

directly follows that laterally sheared currents occur over bathymetric slopes. Convergence zones (fronts)

are also observed to occur over bathymetric slopes. The positions and strengths of these fronts vary with

the semidiumal tide. The presence of freshwater promotes density-driven currents that alter the phase

and duration of tidal currents across the section. Where mean density-driven flow is northward (along the

eastern shore and near Kalgin Island), the onset of northward tidal flow (flood tide) occurs earlier and has

longer duration than the onset and duration of northward tidal flow where mean density-driven flow is

southward (in the shipping channel). Conversely, where mean density-driven flow is southward (in the

shipping channel), the onset of southward tidal flow (ebb tide) occurs earlier and has longer duration than

the onset and duration of southward tidal flow along the eastern shore and near Kalgin Island. These

observations suggest that Cook Inlet numerical spill trajectory models require high-resolution bathyrnetry

(0 100 m - l000m) and to need to be fully three-dimensional, incorporating buoyancy (temperature and

salinity) effects at seasonal to semidiumal time scales to improve their forecasting skill.

STUDY PRODUCTS:

Okkonen, S.R. 2005. Observations of hydrography in central Cook Inlet, Alaska during diurnal and

semidiumal tidal cycles. MMS Information Transfer Meeting, March 2005, Anchorage, Alaska.

Okkonen, SR. 2004 Observations of hydrography in central Cook Inlet, Alaska during diurnal and

semidiurnal tidal cycles. University of Alaska Coastal Marine Institute Annual Research Review,

February 2004, Fairbanks, Alaska.

Okkonen., S.R. 2005. Observations of hydrography and currents in central Cook Inlet, Alaska during

diurnal and semidiumal tidal cycles. Final Repori OCS Study MMS 2004-058, University of Alaska

Coastal Marine Institute, University of Alaska Fairbanks and USD01, MMS, Alaska OCS Region, 28

pp.