NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 1 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

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 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 2

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES The Project Area 3 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.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Data Availability - Historical 4 STATIONHigh Water Interval HWI (hrs) Low Water Interval LWI (hrs) Great Diurnal Range of Tide GT (m) SERIES (Length and Date) Quinhagak Highs Warehouse Creek Highs / Lows Kuskokwim Creek Highs/ Lows Popokamute Highs / Lows Eek Island Highs / Lows Bethel 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 The rest of the limited observations were made in 1914 and 1915.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Data Availability - Control miles 265 miles 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.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Data Availability - Control 6 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.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 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) 7

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Subordinate Installations 8 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.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Subordinate Installations Limitations 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. 9

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Observations - General General 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 10

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Observations - Datums 11 STATIONHWILWIMNGT Quinhagak Popokamute Lomavik Bethel 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.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Observations – Time Series 12 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.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Observations – River Effects 13 STATION(K 1 +O 1 ) / (M 2 +S 2 ) M 4 / M 2 M 6 / M 2 2M 2 o – M 4 o Quinhagak o Popokamute o Lomavik Slough o Bethel o 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 M 2 constituent.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Observations – Harmonic Constituents 14 STATIONM 2 (h)M 2 °K 1 (h)K1°K1°O 1 (h)O1°O1° Quinhagak Popokamute Lomavik Bethel Amplitudes and phases of the principle constituents in the Kuskokwim River. (Amplitudes in meters, phases relative to Greenwich Mean Time)

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Differences – Old / New 15 STATIONOld HWI (hrs) New HWI (hrs) Diff HWI (hrs) Old LWI (hrs) New LWI (hrs) Diff LWI (hrs) Old GT (m) New GT (m) Diff GT (m) QUINHAGAK POPOKAMUTE LOMAVIK * * * BETHEL 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.

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 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. 16

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 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. 17

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 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. 18

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 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. 19 The zoned curve using data from Lomavik is in Blue. The zoned curve using data from Bethel is in Red. The differences are In Green. Units: Meters 1.0 meter

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES 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. 20

NOAA’s CENTER for OPERATIONAL OCEANOGRAPHIC PRODUCTS and SERVICES Questions ?? 21