1)    Title Page

i)      Tidal Currents in Glacier Bay, Alaska

 

ii)    Byron Kilbourne (University of Tennessee, Visiting)

iii)   bkilbour@u.washington.edu

iv)   206-226-6230

 

v)    13 February, 2008

 

2)    Project Summary

i)      Objective

(a)   Study tidal current at the mouth and inside Glacier Bay

ii)    Methods

(a)   Take current readings using ship mounted ADCP

(b)  Measure water properties using shipÕs CTD

(c)   Log the measurements using VMdata acquisition software

(d)  Apply analytical and numerical methods to dataset to look for correlations

iii)   Predicted Outcomes

(a)   Creation of a current dataset over several tidal cycles

(b)  Analysis of this data is expected to illuminate the source water entering the bay

 

3)    Introduction

i)      Problem

(a)   Surface currents in Glacier Bay are well studied, deeper currents are more difficult to measure.  It is not well understood exactly where seawater enters the bay.  As strong tides move water across the entrance sill, the water becomes well mixed.  This makes it difficult to determine the local source of seawater entering the bay.  There is very little data on the net transport of deepwater due to tidal currents in the Glacier Bay region.

ii)    Implications

(a)   Glacier Bay is well known for its high biological productivity.  This ecosystem relies on both terrestrial and marine sources.  Understanding how currents flow into and out of the bay will help determine specifically which nutrients are provided from the ocean, how much oxygen is transported in the deepwater inflow, and how tidal energy sustains the ecosystem through mixing across sills.

iii)   Objective

(a)   The focus of this project is to gather subsurface current data with corresponding water property information.  This data will be analyzed to look for cyclic patterns in tidal currents, and to provide information on nutrient transport for ecological studies.

 

4)    Proposed Research

i)      Goals

(a)   The dynamics in Glacier Bay are highly seasonal.  The data from this cruise must be considered in the context of the seasonal and tidal stage in which it was collected.  With this caveat, the primary goal of this project is to identify the source/s of deepwater flowing into Glacier Bay and to place reasonable limits on fluxes of nutrients, oxygen, and carbon trough the mouth of the bay.

(b)  This cruise will build upon the work of a previous ADCP transect of Glacier Bay (ref. g).  This effort will improve upon the previous work in two significant ways.

(i)    The ADCP and CTD casts will be taken over the same cruise, in the previous work ADCP readings were correlated to CTD casts from a separate cruise under similar conditions.

(ii)  This work will focus on the currents near the mouth of the bay, which will help determine the transport flux through the mouth and also provide some insight into the circulation patterns in Icy Strait, outside the bay.

ii)    Methods

(a)   The ship mounted acoustic Doppler current profiler (ADCP) will be used to make current measurements.  ShipÕs CTD will be used to measure oxygen, chlorophyll, temperature, salinity, and turbidity of the water mass associated with the current measurement.

1.     ADCP 

i.      The R/V Thompson uses an RD Instruments 75Khz ADCP mounted in the keel of the ship.  The ship drafts just over 6m fully loaded.  The ADCP signal needs some depth below to spread before it can give a useful signal.  The combination of these factors limits the upper current measurement to a depth of approximately 20m.  There is a similar area at the bottom where the signal cannot be interpreted.  This limits the ADCPÕs usefulness in waters less than 30m deep.  The data is grouped into data bins, 4m depth averaged cells.  The ADCP can record up to 128 cells at once, so it will be possible to maintain this resolution throughout the bay.  The ADCP can ÒseeÓ down to 1000m, although it is not anticipated that we will encounter these depths in Glacier Bay.

2.     CTD

i.      The shipÕs CTD will be used at the mouth of the bay during ebb and flood tides, and at all other practical stations, to measure temperature, salinity, oxygen, chlorophyll, turbidity, and nutrients.  Water samples will be taken for the purpose of calibration of electronic CTD measurements.

iii)   Proposed Route

(a)   The route that would suit my project best, assuming the ship enters the bay from the east, is as follows.  Enter the bay from the east at and wait for maximum tidal currents near the mouth, then proceed to the western edge of the mouth.  Again wait for a maximum tidal currents, then proceed into the bay.  A transect of the bay including one or both of Tarr and Muir inlet would be desirable.  Included is a figure (fig.1) illustrating this path.  To obtain at least two measurements for each location it would be desirable to retrace this path on leaving the bay, again waiting for maximum tidal currents at the mouth.

Figure 1

-Estimated locations of maximum tidal flux, with a rough outline of projected cruise route (black arrows)

 

 

iv)   Data Analysis

(a)   After the cruise the data from ADCP readings and CTD casts will be compiled into one dataset.  A current vector timeseries will be constructed from the data.  This timeseries, when examined in the context of the tidal phase, will be the backbone of the analysis.  The expected results will show internal waves propagating from the entrance sill, stronger tidal inflow on the eastern edge of the mouth, and stronger tidal outflow at the western edge of the mouth.

 

5)    Budget                  

i)      The budget for this cruise is split into two parts.  There is a ÒrealÓ $600 allotment for each studentÕs project.  The large cost of the ship time and scientific equipment will be covered by the University of Washington, but will still be represented in the budget.  A summary of anticipated cost is shown in table 1.

 

Table 1

Individual $600 Budget

 

 

 

Consumables for Oxygen Titrations

 

100.00

Subtotal

 

 

 

100.00

Estimate of provided Services

 

 

12 Days Aboard the R/V Thomas G. Thompson

264000.00

Operational Budget

 

 

 

264100.00

Overhead (56% of Total Budget)

 

336127.27

Total Requested

 

 

600227.27

 

6)    References

 

a) Etherington, L., Hooge, P., Hooge, E., Hill, D. 2007. Oceanography of Glacier Bay Alaska: Implications for Biological Patterns in a Glacial Fjord Estuary. Estuaries and Coasts. 30: 927-944.

 

b)Freeland, H., Farmer, D. 1980. Circulation and Energetics of a Deep, Strongly Stratified Inlet. Can. J. Fish. Aquat. Sci. 37: 1398-1410

 

c) Mickett, J., Cregg, M., Seim., 2004. Direct Measurements of diapycnal mixing in a fjord reach – Puget SoundÕs Main Basin. Estuarine, Coastal and Shelf Science. 59: 539-558.

 

d)Janzen, C., Simpson, J., Inall, M., Cottier, F. 2005. Across-sill circulation near a tidal mixing front in a broad fjord. Continental Shelf Research. 25: 1805-1824

 

e) Johnsson, M., Green, J., Stigebrandt, A. 2007. Baroclinic wave drag from two closely spaced sills a narrow fjord as inferred from basin water mixing. Journal of Geophysical Research. 112: 1-11

 

f)     Matthews, J. 1981. The Seasonal Circulation of the Glacier Bay, Alaska Fjord System. Estuarine, Coastal, and Shelf Science. 12: 679-700

 

g)    Cokelet, E., Jenkins, A., Etherington, L. 2007. A Transect of Glacier Bay ocean currents measured by acoustic Doppler current profiler (ADCP), in Piatt, J.F., and Gender, S.M.,eds, Proceedings of the Fourth Glacier Bay Science Symposium, October 26-28, 2004: U.S. Geological Survey Scientific Investigations Report 2007-5047, p 80-83