WATER LEVEL ADJUSTMENTS FOR NOAA HYDROGRAPHIC SURVEYS IN COOK INLET: 11 YEARS OF STUDY, EXPERIMENTATION AND ADAPTION

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1 WATER LEVEL ADJUSTMENTS FOR NOAA HYDROGRAPHIC SURVEYS IN COOK INLET: 11 YEARS OF STUDY, EXPERIMENTATION AND ADAPTION Kathleen Mildon 1, Thomas S. Newman, PLS 1, Nathan C. Wardwell 2, Mike Zieserl 2 1 TerraSond Limited, Palmer, AK 2 JOA Surveys, LLC, Anchorage, AK Abstract In 2008 a combination of National Water Level Observation Network (NWLON) tide stations, tertiary tide stations, submersible pressure sensors, and hydrographic vessels were used to measure and zone waterlevels in Alaska s Cook Inlet. The inlet stretches for over 320 kilometers and has extreme tidal ranges in excess of 10 meters. The area also has variations in terrain and difficult logistical considerations making Cook Inlet a challenging environment to measure and estimate water levels. NOAA, OCS requires repeat multi beam surveys in this area due to high rates of bottom silting. Seabird submersible pressure sensors were deployed for 9 27 days at four different locations throughout the Upper Cook Inlet. The time series of water levels at each of the locations provide a set of spatially diverse samples that are used to refine Upper Cook Inlet s discrete tidal zoning scheme. Offshore and shore based water levels are compared in the NAD83 ellipsoid height reference frame. The latitude, longitude, and ellipsoid heights are derived from Post Processed Kinematic techniques observed with GPS, on two hydrographic survey vessels. NAD83 heights obtained during a GPS survey of historic National Ocean Service tidal benchmarks are used to derive the transformation parameters for water levels measured at the shore based tide stations. These methods used to measure and estimate water levels in Cook Inlet evolved from 11 years of contracted NOAA hydrographic surveys by TerraSond Limited (TSL) and JOA Surveys LLC (JOA). The goal during the 2008 field season was to apply the experience gained from surveys during the past decade to derive MLLW to NAD83 transformation parameters that can be used in future surveys. Geological Setting of Cook Inlet Cook Inlet is a large elongated body of water in Southcentral Alaska (Figure 1). It is oriented in a northeast/southwest direction and is approximately 320 kilometers long. Its width ranges from 128 kilometers wide where it opens up to the Pacific Ocean to 16 kilometers wide at the constriction between the East and West Forelands (Figure 1). It bifurcates at the end into two arms, Knik Arm and Turnagain Arm. Both of these arms are fed by glacial rivers. These and many other glacial fed rivers flowing into the arms deposit large amounts of silt in the inlet. This silt remains suspended for long periods of time before it settles to create both mudflats and shoals that are highly changeable. Most of the change is due to the fact that the inlet has the second largest tidal fluctuations in the world. The range of these fluctuations frequently exceeds 6 meters and has tidal velocities that can reach 8 knots. US Hydro

2 The tides and currents generally increase in size and strength as you move northeast toward the two arms. Figure 1 Geologic Map of Cook Inlet Region. Ice is found in the inlet for much of the winter. The northern half is ice covered by December. In some years, the ice cover extends farther south. Typically during the winter months, 10 centimeters of pancake ice forms over the surface of the Knik and Turnagain Arms and Upper Cook Inlet. During colder winters, heavier flows of ice reaching a thickness of 2 meters may form. Any ice that does form is consistently broken and transported by the strong tidal currents throughout the Inlet. Cook Inlet Vessel Traffic and Infrastructure Cook Inlet is used by a variety of vessels, ranging from commercial and recreational fishing vessels to bulk carriers for Liquefied Natural Gas to deep draft cargo ships and barges. There are also 15 oil and gas production platforms, predominately located in the upper half of the Inlet, that are supported by service vessels and are connected by pipelines crisscrossing the bottom of the inlet. Approximately 700 deep draft vessels call at Cook Inlet ports annually. These vessels are primarily destined for the Port of Anchorage and the two oil loading facilities at Nikiski and Drift River. (CIRCAC 2006). Note: Drift River facility is non operational at the time of writing due to the erupting volcano; Mt Redoubt. Unlike larger ports such as Boston Harbor and San Francisco Bay where the number of deep draft vessels creates a navigational challenge, Cook Inlet itself is one of the most challenging places to navigate due to sudden severe weather, strong tidal currents and large fast moving ice pans. In addition to these surface challenges, the seafloor is constantly transformed by large amounts of bed load US Hydro

3 transport and migrating shoals. TSL has seen 3 meter high sand waves move over 10 meters in less than a week. TSL has also seen large regions of sand wave activity flattened in a single storm. Cook Inlets shallow nature and these temporal changes are why Cook Inlet is designated as both Critical and a Resurvey area in NOAA s National Survey Plan. Similarly, it is the combination of these environmental challenges that create difficult and potentially dangerous conditions for carrying out hydrographic surveys and making water level measurements. The lack of NWLONs requires the installation of tertiary tide stations for tidal control. However, the extreme tidal range, coupled with a lack of manmade facilities means that the installation of bubbler gauges with orifice tubing results in lengths that exceed Only one of the seven tertiary tide sites used in recent years is accessible by road. The remaining sites require either small boats, bush planes with tundra tires for landing on beaches or short dirt air strips, or helicopters, which have proven to be the most economical method for deploying the anchors for the shore based tertiary stations. The difficult logistics complicate not only the installation and removal of tide stations, but also the regular checks that their operation requires. The use of aircraft, both fixed and rotary wing, quickly drives the cost for these gauges quite high. The same lack of shoreline infrastructure is reflected in the dearth of NOAA NWLON stations. In spite of the extreme tidal dynamics in this area, it has very few instruments and is poorly understood. In the entire area only three NWLON gauges operate. In the Continental US, an equivalent area with less dynamic tides would have several times as many NWLON sites. For example, in San Francisco Bay where the mean tidal range at station is 1.2 meters there are more than seven NWLONs and 60 historical stations listed on the CO OPS website, where as in Cook Inlet with its extreme tides there are only three NWLONs and around 50 historical stations. Similarly, along the 1344 kilometers of California s coastline, where the tide type is predominantly mixed, there are 19 NWLONS, whereas along the 10,240 km of Alaska s coastline where the three tide types can be found, there are 26 NWLONS, eight of which have been installed in the past five years. Not only does the lack of tidal control complicate survey operations around Cook Inlet and the entire state, but vertical datum relationships in general are poorly defined. Recently, through the efforts of NOAA s Gravity D project, new airborne gravity data has been collected that will eventually be used to develop a more accurate Geoid model. Until then, there is no accurate Geoid model available. Evolving methods of Controlling for Tides in Cook Inlet TSL and JOA have been working through these challenges for many years. Both firms were founded in Southcentral Alaska with Cook Inlet as their backyard. These two companies have been working together, and independently, on solutions for NOAA contract hydrographic surveys in the area for 11 years, and longer for other clients. Through the years a number of approaches have been tried with varying degrees of success. Standard discreet zoning with six minute zones has been supplied by NOAA for use on all the contract surveys performed in Cook Inlet. The only survey that successfully used that approach was the very first survey conducted by TSL (then Terra Surveys) in 1998 (Figure 2 below). That survey was small in size and was located next to one of the three NWLONS in the Inlet, station at Nikiski. The six minute zoning US Hydro

4 scheme created substantial error in the data and steps at the zone edges, but was acceptable under the specifications in effect at the time. In 1998 and 1999 John Oswald and TSL assisted RACAL Survey in their work between Nikiski and Fire Island. The six minute zones were used only as a starting basis for general shape and geometry and the zoning was cut into two minute zones and improved upon with an additional five tertiary gauges. The zoning was verified with a recon survey conducted with single beam soundings and Real Time Kinematic (RTK) positioning. The tighter zones and five extra tertiary gauges provided a workable solution. A single remaining Cook Inlet sheet was competed in 2000, with the same support, by RACAL with one tertiary gauge. In 1999 TSL was performing a charting survey for NOAA south of the Nikiski NWLON and had a single tertiary gauge available at the southern end of the project. TSL s approach was to use a linear interpolation of the water levels at both gauges. This technique had been developed by TSL for support of dredging surveys near Knik Arm Shoal. The interpolation was based on time and sounding position relative to a straight line between the two gauges. It required minimal software development and was crude, but was also very effective. Figure 2 Chart with 1998 Charting Survey Area. In 2001 TSL was tasked with surveying an area north of the previous surveys that completed the approaches to Anchorage. This survey took place in the highest tides encountered during any of the Cook Inlet contracts. Using two new tertiary sites, the original two minute zoning scheme developed by JOA for the project area was further modified by JOA and TSL to provide a successful zoning solution. The final zoning method consisted of averaging the zoned water levels from both the primary and secondary gauges. This solution worked quite well and was ultimately used to reduce all the bathymetry. This technique was also successful during a 2004 re survey by TSL in roughly the same area. Between 1995 and 2008, TSL has performed several hundred surveys for other clients. The techniques used evolved over time and have both contributed to and drawn from our approaches to control for tides during these NOAA charting surveys. For example, an increasing number of non charting surveys were surveyed using RTK GPS for both positioning and water level. This technique provided a very consistent solution on small projects where the relationship between MLLW and the ellipsoid is understood, and gave us the confidence to apply an ellipsoid based solution to a much larger area. US Hydro

5 2008 Field Season In 2008 TSL was tasked with a resurvey of a large portion of the Cook Inlet covered previously by TSL and RACAL in addition to coverage in areas not touched by the other surveys. Rather than set the seven tertiary stations required previously to support this effort TSL and JOA proposed a plan using two tertiary tide stations, two bottom mounted pressure sensors moved frequently to confirm zoning, and Smooth Best Estimate of Trajectory (SBET) positioning using Applanix POS/MV Motion systems and Post Processed Kinematic (PPK) positioning. The plan was to use SBET PPK water level heights and waterlevel from the bottom mounted pressure sensors to determine the NAD83 to MLLW transformation parameters at site specific locations in Cook Inlet. The intention was also to use these transformation parameters to validate a NAD83 to MLLW difference surface. The difference surface was to be derived by controlling for tides using conventional tidal zoning and PPK water levels. Due to complications with the control tide station (Anchorage NWLON ) final tidal zoning for the Inlet was delayed, which has held up the development of a difference surface. Thus, what will be presented in the remainder of this paper are some of the results from the roving bottom mounted pressure sensor approach. Figure 3 Overview map of the survey area in Alaska s Upper Cook Inlet. The 2008 field work was carried out using the R/V Mt. Augustine, which is a 10.2 meter survey launch and the R/V Mt. Mitchell, which is a 70 meter vessel. An Applanix POS/MV was installed on each vessel. The POS/MV is a two antennae GPS aided inertial system capable of recording raw GPS and inertial data for combined post processing to produce a Post Processed Kinematic (PPK) Smoothed Best Estimate of US Hydro

6 Trajectory (SBET) position. The observations at the L1 phase center of the primary antennas were translated to the vessel reference frame using the XYZ lever arm offsets. Dynamic draft was accounted for using a settlement and squat model that was developed using Real Time Kinematic (RTK) techniques. Static draft corrections were derived from draft measurements at the beginning and end of the survey period. Static and dynamic draft corrections were then applied directly to the SBET files. The SBET files were derived using PPK techniques with data from two base stations. A base station was installed by TSL at Tyonek, AK, which is near the southern portion of the survey area. The Continuously Operating System (CORS) station ANC1 at the Ted Stevens Anchorage International Airport, which is near the northern portion of the survey area, was the other base station. The Tyonek base station recorded at a rate of 1 Hz while ANC1 records at a rate of 0.2 Hz. In processing, the data from ANC1 was interpolated to 1 Hz. The roving seabird effort deployed two Sea Bird SBE26+ (seabird) pressure sensors at four different locations throughout the survey area (Figure 3). Each of the seabirds has a Digiquartz temperature compensated sensor rated to 100 psi. These sensors have an initial accuracy of meters and have a resolution of 0.2 millimeters for one minute integration (Sea Bird, 2004). Because the SBE26+ is fully submerged below the ocean surface it measures a combination of atmospheric and water pressure. The measured pressure is a function of the local gravity and water density. In the conversion from pressure to depth, atmospheric pressure was accounted for by using the barometric pressure measured by the National Weather Service (NWS) at weather station This NWS weather station is located at Point Woronzof just south of Anchorage, AK. Gravity was accounted for using local gravity predicted by the National Geodetic Survey s Geodetic Toolkit. Surface water density measurements were made at Point Possession (Figure 3). The average water density was used to convert the pressure readings to depth. One of the seabirds was deployed at three different locations: Middle Ground, South of Susitna River and Point Mackenzie (Figure 3). The other seabird was deployed at South Moose Point Shoal (Figure 3). A third seabird gauge deployed was buried by silt and was not recovered, causing the loss of both the instrument and the recorded data. At each of the locations the seabird was deployed for at least nine days. The seabird deployed offshore of the mouth of the Susitna River provided the longest time series of water level measurements, 27.5 days (Figure 3). The data measured by each seabird was reduced to a 19 year MLLW equivalent using the Tide by Tide method of simultaneous observations (CO OPS, 2003). This method consists of comparing simultaneous high and low water levels at a control station that has an accepted 19 year MLLW datum to high and low water levels at the subordinate station. The control station used for this project is the NWLON at the Port of Anchorage. This NWLON is also part of NOAA s Physical Oceanographic Real Time System (PORTS). The site specific NAD83 to MLLW transformation parameters for the locations that the seabirds were deployed are based on the average difference between the water level heights measured by the R/V Mt. Augustine and the water level depths measured by the seabirds. Prior to differencing the two sets of water levels PPK water level heights measured by the R/V Mt. Augustine were filtered in three steps. The first step consisted of several parts. The mean of the all the observations within the 30 second US Hydro

7 boxcar window was computed. Then all the points more than two standard deviations from the mean were removed. Lastly, the mean of the remaining points in the window was recomputed and time stamped with the center of the 30 second window. The second step consisted of removing PPK observations with a formal standard deviation larger than 5 centimeter. The second step consisted of eliminating observations that were observed while the survey launch was traveling faster than 6 meters per second over ground. The data were filtered using the second parameter because it was apparent that at speeds over ground faster than 6 meters per second the PPK water level height increased by approximately 25 centimeters (Figure 4). Once the PPK water levels were filtered and the water levels from the seabirds were corrected to a 19 year MLLW equivalent the PPK water levels within 5 kilometers of each seabird were compared to the seabird water levels. The seabird time series were interpolated from six minutes to the same 30 second interval of the decimated PPK time series. The differences between the PPK water level heights and the seabird data at each of the locations are shown in the four histograms in Figure 5. Figure 6 shows the geographic location of the PPK water level heights used to develop the histograms in Figure 5. Based on Figure 5 and Figure 6 it is apparent that R/V Mt. Augustine did most of its surveying about 4 kilometers from the seabird deployed at Moose Point. Thus, it is suspected that differences between the two types of water level measurements are biased by spatial variability in the tidal characteristics. Similarly, the Susitna and Middle Ground plots in Figure 6 show that the majority of the PPK data within 5 kilometers of the seabird deployed at those locations were collected to one side of where the seabirds were deployed. The Point Mackenzie plot in Figure 6 shows that the survey launch surveyed an area directly over the seabird deployed at this location. The launch also surveyed an area to the east of the seabird. The remaining analysis will focus on Figure 4 Example of PPK water levels observed using the R/V Mt. Augustine. The blue dots are observations with a formal PosPac assigned standard deviation larger than 5 centimeters. The red dots are observations obtained while the survey launch was traveling faster than 6 meters per second over ground. The green dots are some of the observations that were used in this analysis. the Point Mackenzie data because the data at this location provides the best geographic relationship between the PPK and seabird water levels US Hydro

8 Figure 5 This figure shows histograms of the PPK toseabird comparisons at the four locations that the two seabirds were deployed. Figure 6 Geographic location of the PPK water level heights within 5 kilometers of the four seabird deployments. The coordinates in these plots are not projected. The red triangle in each plot represents the location that a seabird was deployed. Figure 7 is a map of all the PPK data collected within 5 kilometers of the seabird deployed at Point Mackenzie. In this figure the PPK data are colored based on area and time. For example, the red dots represent a period of time during August 7 th that the launch was transiting near the seabird at a speed less than 6 meters per second. The blue dots represent the period of time that the launch was surveying directly over the seabird. The yellow dots represent the period of time that the launch finished surveying over the seabird and transited to an area east of the seabird. The cyan dots represent a period of time on August 13 th that the survey launch was transiting within 5 kilometers of the seabird. The two survey areas (blue dots and green dots) provide an excellent opportunity to validate the accuracy of the water levels measured by the survey launch and to investigate the spatial variability water levels measured over the two areas. US Hydro

9 Figure 7 Geographic location of the PPK water level measurements obtained by the R/V Mt. Augustine that are within 5 kilometers of the location a SBE 26+ was deployed. Figure 8 shows the distribution of all the differences between PPK water levels within 5 kilometers of the seabird deployed at Point Mackenzie and the seabird water levels. The bins are colored to correspond with the six sets of observations described previously. This distribution shows a bimodal structure with one peak near 2.7 meters and another peak near 3.0 meters. The differences derived using only the observations when the launch was within 1300 kilometers of the seabird (blue and red dots in Figure 8 and blue and red bins in Figure 8) are grouped together. The differences derived from the observations obtained while the launch was surveying east of the seabird are grouped together. This group of differences is offset from the mean of the distribution of the blue bins by about 30 centimeters. The cause of these two peaks is clearly shown in Figure 9, which is a time series of the difference between the NAD83 and MLLW water level heights that are within 5 kilometers of the seabird deployed at Point Mackenzie. Between 18:41 and 18:55 on August 9th the difference changes from about 2.7 meters to 3.0 meters. Thus, it is clear that while the vessel is surveying near or directly over the SBE 26+ the average difference between NAD83 and MLLW is 2.7 meters. Once the launch begins surveying east of the seabird the tidal characteristics change resulting in a change in the estimated NAD83 to MLLW separation. US Hydro

10 Figure 8 Histogram showing the distribution of the NAD83 to MLLW separation derived by the Point Mackenzie SBE 26+ data and the R/V Mt. Augustine data within 5 kilometers of the seabird. Figure 9 Difference between the water level measurements that were obtained by the R/V Mt. Augustine and the Point Mackenzie seabird. The seabird measurements were also compared to data from the Anchorage NWLON that was zoned using time and range correctors from the preliminary zoning provided at the beginning of the field season. The correctors are 12 minutes and 0.960, respectively. The distribution of the difference between the zoned Anchorage data and the seabird data is shown in Figure 10. The mean difference is 0.06 meters with a standard deviation of 0.07 meters. Figure 10 Distribution of the differences between data zoned from Anchorage NWLON to Point Mackenzie. Conclusions There are several valuable pieces of information that can be obtained from the above comparisons. First, water levels measured by the survey launch using PPK while it was surveying within 1300 meters of the seabird are within 0.07 meters of the seabird water levels at the 95% confidence level. Secondly, the roving seabird deployments provided valuable checks for the PPK water level heights measured by the survey launch. Lastly, the comparison of the roving seabird data to the PPK water level heights helped to identify the distance over which the tidal characteristics change within a survey area. US Hydro

11 Based on the available data, the water level measurements obtained using the survey launch agree to the seabird data to within 7 centimeters at the 95% confidence level, whereas the zoned data agrees to within 14 centimeters at the 95% confidence level. Chapter 4 of the April 2008 NOS Hydrographic Surveys Specifications and Deliverables (NOAA, 2008) state in section (Error Budget Considerations) that The measurement error, including dynamic effects, should not exceed 0.10 meters at the 95% confidence level. This suggests that based on the analysis of the data collected during the 2008 field season, if data from Anchorage were to be used to reduce the hydrographic soundings, the data would be out of spec. However, the water level measurements obtained by the R/V Mt. Augustine using PPK and SBET while it was surveying provide water level measurements that are within the measurement specification. Recommendations Problems with the Anchorage NWLON have delayed completion of the project. In spite of this set back the data indicates that using PPK for water levels is more accurate and repeatable than tides zoned from water level stations. This is consistent with our findings for Real Time Kinematic GPS (RTK). Also, the use of bottom gauges is proven to be an accurate and cost effective method of providing water level measurements in areas requiring additional gauges. Cook Inlet is designated by NOS as a Critical Resurvey area. We will recommend that the next survey uses PPK derived heights for controlling for water level. The addition of bottom gauges will also be recommended to provide both a cost effective back up to PPK and also an opportunity to further refine the vertical relationship between NAD83 and MLLW. Finally, the results of the 2008 airborne gravity survey of Cook Inlet should be incorporated into the NAD83 to MLLW model to improve and extend the model generated from this effort. References: NOAA, 2008, NOS hydrographic surveys specifications and deliverables April 2008, US Department of Commerce National Oceanic and Atmospheric Administration, Silver Spring, MD, p150. Sea Bird, 2004, SBE 26plus SEAGAUGE wave and tide recorder user manual version 003, Sea Bird Electronics Inc., Bellevue, Washington p180. CO OPS, 2003, Computational techniques for tidal datums handbook: NOAA Special Publication NOS CO OPS 2, NOAA National Ocean Service, Center for Operational oceanographic Products and Services, Silver Spring, MD, p113. CIRCAC, 2006, Cook Inlet Vessel Traffic Study, Cape International Inc and Nuka Research and Planning LLC, p 49. US Hydro