Navigable Area 2023 F00875 Alaska Bristol Bay Nushagak Opportunistic Survey 80000 2023-06-15 2023-09-22 2023-01-30 OPR-R340-KR-23 Terrasond Andrew Orthmann MBES Teledyne RESON SeaBat T50-R MBES Backscatter Teledyne RESON SeaBat T50-R Pacific Hydrographic Branch meters Any revisions to the Descriptive Report (DR) applied during office processing are shown in red italic text. The DR is maintained as a field unit product, therefore all information and recommendations within this report are considered preliminary unless otherwise noted. The final disposition of survey data is represented in the NOAA nautical chart products. All pertinent records for this survey are archived at the National Centers for Environmental Information (NCEI) and can be retrieved via https://www.ncei.noaa.gov/. Products created during office processing were generated in WGS84 UTM 04N, MLLW. All references to other horizontal or vertical datums in this report are applicable to the processed hydrographic data provided by the field unit. World Geodetic System (WGS) 1984 Mean Lower Low Water Projected UTM 4 2024-01-18 CC0-1.0 (NOAA Contractors) CC0-1.0 https://creativecommons.org/publicdomain/zero/1.0/ https://creativecommons.org/publicdomain/zero/1.0/legalcode These data were produced under contract with NOAA and any potential copyright was assigned to NOAA. NOAA waives any potential copyright and related rights in these data worldwide through the Creative Commons Zero 1.0 Universal Public Domain Dedication (CC0). 2024-01-16 Chief, Pacific Hydrographic Branch, NOAA OCS Andrew Orthmann Charting Program Manager TerraSond PHB The purpose of this survey was to attain opportunistic data during transit, acquiring depth data in the navigational channels of Nushagak Bay and Nushagak River. The area was transited whenever visiting Dillingham, Alaska, which was the overall project's port of call. Rotations were approximately every two weeks, and data was acquired whenever conditions allowed, with a planned line spacing of 240 m through the navigational channels. Up to 700 LNM were specified in the Project Instructions; about 492 were actually acquired as conditions often proved difficult to acquire data safely in this area during the periods the vessels were passing through, especially during the Bristol Bay salmon fishing season in late June and July, and very strong currents on ebb tides due to river flow and the high tidal range of the area (up to 5 m). An $AREAS polygon was also assigned in the Project Instructions in the vicinity of Ekuk. The Project Instructions called for assigned feature investigations in the $AREAS polygon and additional investigation of the shoal at the pinch point in the northern part of Hip Channel. From Ekuk north to Dillingham there is only one primary channel in the Nushagak River, and the surveyed corridor there was expanded whenever possible. South of Ekuk there are multiple channels; progress was made surveying corridors through three channels, with the choice of channel to survey primarily made based on where the survey vessels were transiting to or from elsewhere in the project. Crosslines and Junctions were waived. However, Junctions were analyzed where this survey overlapped the other surveys in OPR-R340-KR-23. Refer to the DRs for those surveys (H13715 and H13720) for specific results. At least 99.5% of grid cells agreed within allowable TVU by depth, demonstrating very good overall agreement between this and the other surveys. The final depth information for this survey was submitted as a 4 m resolution CARIS BASE surface (CSAR format) which best represents the seafloor at the time of the 2023 survey. The surface was created from fully processed data with all final corrections applied. The surface was created using NOAA CUBE parameters and resolutions in conformance with the 2022 HSSD. The surface was finalized with a 0 to 80 m depth limit, "standard deviation" selected as the final uncertainty source, and designated soundings applied (if present). Horizontal projection was selected as WGS84 / UTM zone 4N. A non-finalized version of the CSAR surface is also included with the survey deliverables for reference. This does not have the "_Final" designation in the filename. Multibeam Acoustic Backscatter (MBAB) surfaces, produced with QPS Fledermaus Geocoder Toolbox (FMGT), are also provided. MBAB data for both vessels, acquired using 400 kHz, is combined in the mosaic. A 200 kHz mosaic is also provided applicable to when the survey vessels utilized that frequency. A Final Feature File (FFF) in S-57 format provides the results of feature investigations and bottom sample results. NOTE: A 1 m resolution surface is also provided to demonstrate that Complete Coverage was achieved over feature investigation locations. To create this, a 1 m resolution surface was first created over the general area of the feature investigations. Next, this version was finalized in CARIS HIPS. Lastly, the finalized version was clipped using a Coverage Area polygon (cvrage object) so as to exclude area not surveyed to Complete Coverage standards from the 1 m surface. Note that finalized (but non-clipped) surface is included with the survey deliverables with "_F" designation in its filename. The HOB file used for clipping is included with the survey deliverables in the "Surfaces_Mosaics" directory. All products, as well as CARIS HIPS projects and raw data, were provided to PHB with this DR Memo. Final correction to MLLW was completed with ERS methodology using the NOAA-provided WGS84 to MLLW separation file "OPR-R340-KR-23_AK_ERTDM_2023_WGS84(G2139)-MLLW_". All survey systems and methods utilized during this survey were as described in the document OPR-R340-KR-23_DAPR. Some additional survey specific details are provided below. "Busts", or vertical mismatches between overlapping survey line data, are relatively frequent in the data set and are due to two primary environmental issues: 1) The area is exceptionally changeable. Significant bottom change occurred during the course of the survey. Data collected weeks to months apart regularly mismatches, but in some cases data collected as little as 24 hours apart shows change by as much as a meter. Sandwaves, indicative of sediment transport, are ubiquitous on the seafloor here. In one extreme example--near Clarks Point at the charted wreck assigned feature investigation--change was as much as 3 meters vertically between lines collected early in the project and lines collected for the investigation late in the project. An example of this is shown below. Note that busts from bottom change usually were not edited to "choose" a bottom. One exception is at the charted wreck feature investigation, where data unnecessary for the investigation was rejected due to the extreme vertical offset. 2) The sonars on the vessels would sometimes track incorrect or two bottoms, presumably due to a soft "fluff" layer higher in the water column. This was usually in the deeper part of some of the channels. It was more prevalent in the area around Ekuk and Clarks Point, though it was observed periodically throughout the area, getting less frequent to the south. The likely cause is finer grain sludge or mud concentrating in the deeper parts of the channels but becoming diffused as it moves towards the bay and spreads out over a larger area. The deeper layer was assumed to be the actual seafloor because the deeper layer would often show sandwaves while the shoaler layer would be uniform and featureless. When this was observed on the survey vessels the sonars were tuned in attempts to track the deeper layer (through power, gain, and frequency changes), sometimes unsuccessfully. However, even when the deeper layer was able to be tracked, the return would be very noisy with intermittent soundings on the deeper layer, and from only a relatively few nadir or near-nadir beams. In data processing, soundings tracking the shoaler layer were rejected where the two layers were clear, and sparse returns on the deeper layer accepted. This resulted in some lines having a swath reduced to little more than nadir, as well as along-track gaps where the sonars were simply unable to track good seafloor. A similar issue was experienced in H13714 in upper Kvichak Bay in the Kvichak and Egegik Rivers. An example of the issue is shown below. Data Processing Exceptions: Applanix Smart Base (ASB) GNSS post-processing (instead of the project-standard PPRTX) was used on some lines to improve vertical agreement. These were as follows. Arctic Seal, JD167, line 0355, all segments ASV, JD246/248, lines 2439 through 2501 The following lines did not have Delayed Heave available. Realtime heave was used instead. Arctic Seal, JD222, line 1232 Note that an exemption was obtained to provide horizontal positions for this survey in WGS84 instead of NAD83(2011), see Correspondence for details. All data were reviewed for DTONs and none were identified in this survey. The currently charted channels are still in the same general locations, but have changed in depth and width to a degree that existing charts should be updated with the results of this survey. The changes are numerous with no overall trends; where some areas have deepened, others have shoaled. The shoal at the pinch point in the northern part of Hip Channel was of particular interest in the Work Instructions. The area was developed with splits, and Complete Coverage investigations were conducted there on two features. The area is still navigable but the shoalest soundings have moved due to sandwaves shifting since the 2012 survey that the chart is based on. The area is shown in an image below. Another area of interest was the Clarks Point to Ekuk area. The channel here is still in relatively the same area with similar charted depths, but there is overall deepening in the channel there. The area is shown in an image below. This survey should supercede charted data where they overlap. Regular re-survey of the area from Dillingham to Bristol Bay is recommended due to the highly changeable nature of the seafloor here. Overview of coverage achieved in F00875. SupportFiles\F00875_Coverage.png Example from CARIS HIPS subset of extreme bottom change on this survey, from sandwave movement. Almost 3 m of vertical change occurred during a 38 day period, between JD209 and JD247. However, even in a 1 day period, between JD246 and JD247, there was up to 1 m of vertical change. SupportFiles\F00875_Example_BottomChange.png Example from CARIS HIPS subset of bottom tracking two layers, an issue in many of the channels in the area. The shoaler "fluff" layer would often be tracked while few returns were received on the actual seafloor, the deeper layer. SupportFiles\F00875_Example_DoubleBottom.png This survey overlaid on the affected ENCs near the "pinch point" in upper Hip Channel. Soundings from this survey are in blue. All soundings in meters. SupportFiles\F00875_PinchPoint.png This survey overlaid on the affected ENCs in the Clarks Point to Ekuk area. Soundings from this survey are in blue. All soundings in meters. SupportFiles\F00875_ClarksPoint-Ekuk.png The hydrographer met the specifications of the HSSD when limiting depths to 0m and deeper, however, based on Hydro Team Lead recommendations for other surveys (H13725 and H13726 are examples) in this project, negative depths (depths above MLLW) were included in the office surface re-creation. This survey does meet charting specifications and is adequate to supersede prior data. The survey was completed in line with the 2022 HSSD specifications, in accordance with the Work Instructions, along with 14 other surveys completed under OPR-R340-KR-23. It was an opportunistic survey but received similar QC as all the other surveys completed under the project, which were traditional hydrographic surveys. Especially given the age of the existing data (newest from 2012), and highly changeable nature of the seafloor, this survey should supersede prior data where they overlap.