noaa’s center for operational oceanographic products and services 1 establishing tide control in...

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES

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Establishing Tide Control in an Area with Insufficient Observational Water Level Data:

A Cast Study of the Kuskokwim River, AK

David Wolcott

CO-OPS Hydrographic Planning Team

04/27/2011


Overview

• The Project Area• Data availability

– Lack of Control– Lack of Observed Water Level Measurements

• Initial Gauge Assessment• Subordinate Installations and Observations

– Overall Characteristics– River Effects

• Implications for Zoning• Discussion

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The Project Area

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The Kuskokwim River flows from interior Alaska and empties into the Kuskokwim Bay in southwestern Alaska. A 2010 hydrographic survey explored the lower reaches of the river.


Data Availability - Historical

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STATION High Water Interval HWI (hrs)

Low Water Interval LWI (hrs)

Great Diurnal Range of Tide GT (m)

SERIES (Length and Date)

9465831Quinhagak

8.75 3.0 3.78 5 Highs1914

9465944 Warehouse Creek

9.2 3.76 3.88 4 Highs / Lows1914

9466007 Kuskokwim Creek

10.0 4.6 3.88 4 Highs/ Lows1914

9466057 Popokamute

10.37 5.07 3.35 6 Highs / Lows1915

9466098 Eek Island

10.64 5.45 3.38 2 Highs / Lows1915

9466477 Bethel

1.31 9.89 1.23 3 months July – Sep 1970

After examining the survey area, a quick query of data availability revealed a lack of time series data and very limited historical datum information. Six stations fell within the general survey area but the most recent data was collected at Bethel in 1970. The rest of the limited observations were made in

1914 and 1915.


Data Availability - Control

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350 miles 265 m

iles

The Kuskokwim River and Kuskokwim Bay fall within an National Water Level Observation Network (NWLON) Gap, which means that there is not adequate control coverage. The closest NWLON stations are located at Port Moller and Village Cove, located roughly 265 miles and 350 miles away from the survey area, respectively.


Data Availability - Control

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Above – Cophase lines of the M2 constituent showing the location of a semidiurnal amphidromic point in the

northern part of Bristol Bay

Below - The amphidromic point forces the secondary tides to be flipped about the time axis as the tides propagate across Bristol Bay. As a result, while the tide signals at Port Moller and Quinhagak have similar ranges and phases, no amount of range/ phase offsets could be applied to Port Moller to replicate the tide signal in the Kuskokwim Bay.


Assessment• With extremely limited tidal information

available in the river and no control, the only option was to recommend the installation of water level measurement systems directly within the survey area.

• Tide control was to be determined using discrete tidal zoning and application of the data from these stations.

• Datum computations were to be limited to First Reduction method (direct average)

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Subordinate Installations

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Five stations were installed at Quinhagak, Popokamute, near Helmick Point, Lomavik Slough, and Bethel to support the survey. Quinhagak and Bethel were installed in May, 2010 and ran for more than four months. Popokamute, Helmick, and Lomavik were installed in June and

ran for two months.


Subordinate InstallationsLimitations

• Quinhagak – shallow sloping river banks and mud flats, no low waters measured, but installation team used two bottom-mounted Sea-Bird® to pick up on full range of tide, leveled to shore.

• Helmick Point – silting issues, flattened lower low waters, data processors used spline fill to compensate.

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Observations - GeneralGeneral Tidal Characteristics

• Characterized by mixed semi-diurnal signal becoming mixed diurnal approaching Bethel

• Range of tide decreases from 3.7m at Quinhagak to 1.1m at Bethel

• Increasing effects of river bathymetry is evident in the higher harmonic constituents

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Observations - Datums

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STATION HWI LWI MN GT

9465831 Quinhagak

7.75 1.47 2.751 3.637

9466057 Popokamute

8.98 3.68 2.502 3.352

9466328Lomavik

11.75 6.89 1.720 2.415

9466477Bethel

1.576 8.905 0.738 1.117

Datum Computations for four of the five subordinate locations*.

* Note: At the time of writing, the datum at Helmick Point is pending acceptance. Several of the lower low water values were inferred using a mathematical interpolation. The water level data has been accepted.


Observations – Time Series

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Concurrent data series from four of the five subordinate stations. Diminishing range of tide, tide type transition, and frictional effects apparent as the tide propagates up the river. Notice the substantial transition between Lomavik and Bethel.


Observations – River Effects

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STATION (K1+O1) / (M2+S2) M4 / M2 M6 / M2 2M2o – M4

o

9465831 Quinhagak

0.840 0.013 0.025 152.1o

9466057 Popokamute

0.922 0.058 0.032 85.7o

9466328 Lomavik Slough

1.143 0.091 0.030 95.2o

9466477 Bethel

1.511 0.188 0.013 84.2o

Harmonic analysis using a 29-day Fourier Transform shows the effects of river bathymetry and friction on the influence of the higher harmonics. Shown are amplitude ratios of principle diurnal and semidiurnal constituents and phase offsets relative to the M2 constituent.


Observations – Harmonic Constituents

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STATION M2 (h) M2 ° K1 (h) K1° O1 (h) O1°

9465831 Quinhagak

1.229 212.8 0.649 119.3 0.444 83.4

9466057 Popokamute

1.078 261.1 0.599 147.9 0.449 111.2

9466328 Lomavik

0.706 348.1 0.466 199.0 0.391 160.6

9466477 Bethel

0.320 61.4 0.268 244.6 0.258 203.0

Amplitudes and phases of the principle constituents in the Kuskokwim River. (Amplitudes in meters, phases relative to Greenwich Mean Time)


Differences – Old / New

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STATION Old HWI(hrs)

NewHWI(hrs)

DiffHWI (hrs)

OldLWI(hrs)

NewLWI(hrs)

DiffLWI(hrs)

Old GT(m)

NewGT(m)

DiffGT(m)

9465831 QUINHAGAK

8.75 7.75 -1.0 3.0 1.47 +1.53 3.78 3.64 +0.14

9466057POPOKAMUTE

10.37 8.98 -1.39 5.07 3.68 +1.39 3.35 3.35 0.00

9466328LOMAVIK

--- 11.75* --- --- 6.89* --- --- 2.42* ---

9466477BETHEL

1.31 1.57 -0.27 9.89 8.91 +0.98 1.23 1.12 +0.11

The differences in the datum information prior to the 2010 installations compared to the datum information indicate a change in tide type. Notice that the HWI values all decrease, the LWI values all increase, and the GT values all increase. While the range differences seem minimal, the phase offsets are significant considering the range in tide. With a range of tide of 3.6m, the LWI difference at Quinhagak of +1.53hrs could result in a realized vertical offset of 0.89m (2.95 ft). While this should be addressed during the final zoning, it indicates variability in tides that needs to be considered when planning for tide control.


Implications for Zoning Updating cotidal lines with new datum information is routine

practice when subordinate stations are installed and can have a great impact on the vertical offsets of the bathymetry. Ultimately, however, a transition in tide type poses a problem for which zoning cannot account. A tidal zone references one station’s data for tide reduction and ultimately there will be a point where the zoning switches from one station to another. Identifying the location of major tidal transitions is very important for choosing the most appropriate control station and can only be done with historical knowledge of the tides at multiple points in the survey area. However, boundaries will still exist. The only way to smoothly capture tide transition and reduce the vertical offsets is to install subordinates at several locations. Data collected for this survey is extremely valuable for addressing these issues for future projects.

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Discussion• Plots of concurrent time series data shows an acute tide

type transition between Lomavik and Bethel. This type of transition can pose a challenge when applying final zoning.

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Discussion• Correctors for an imaginary zone located somewhere

between the two stations were computed assuming a perfect average of the datum values to be applied to the two zones.

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Discussion• When the correctors were applied to three days of water

level data from the corresponding subordinate locations, the maximum differences were nearly a meter.

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The zoned curveusing data from Lomavik is in Blue.

The zoned curveusing data from Bethel is in Red.

The differences are In Green.

Units: Meters

1.0 meter


Discussion• Accounting for this type of transition is difficult

without additional data collection between these two locations. Historical data can help identify where transitions occur, but cannot account for the offsets when changing control stations. Use of more continuous tide reduction schemes, like Tidal Constituent Residual Interpolation (TCARI) or hydrodynamic models could improve the accuracy of tide reduction, but would still be dependent upon adequate data availability.

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Questions

??

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