Lake_Youngs_Bathymety_Hydrographic_Survey_Report_2013.pdf

Lake Youngs Bathymetry Multibeam Hydrographic Survey Report Contract No. R00-77-01-02, Work Assignment No. 4

November 2013

Owner:

Seattle Public Utilities P.O. Box 34018 Seattle, WA 98124-4018

Prepared by:

David Evans and Associates, Inc.

Marine Services Division 2801 SE Columbia Way, Suite 130 Vancouver, WA 98661

(360) 314-3200

Lake Youngs Bathymetry – Multibeam Hydrographic Survey Report Contract No. R77-00-01-02, Work Assignment No. 4 November 2013

David Evans and Associates, Inc., Marine Services Division Page i

TABLE OF CONTENTS

Page

Acronyms and Abbreviations ...................................................................................................... ii

1.0 INTRODUCTION

2.0 DATUMS AND PROJECT CONTROL

3.0 BATHYMETRIC SURVEY METHODOLOGY

3.1 SURVEY COVERAGE

3.2 SURVEY VESSEL AND CREW

3.3 POSITIONING AND NAVIGATION

3.4 WATER SURFACE OBSERVATIONS

3.5 MULTIBEAM DATA ACQUISITION

4.0 EQUIPMENT CALIBRATION

4.1 PATCH TEST

4.2 SOUND SPEED

5.0 DATA PROCESSING

5.1 MULTIBEAM DATA PROCESSING

5.2 DATA EXPORT

6.0 RESULTS

6.1 LAKE YOUNGS INLET DISCHARGE STRUCTURE

6.2 VEGETATION

7.0 DELIVERABLES

List of Figures

Figure 1: Survey Coverage Figure 2: Survey vessel William R. Broughton at the Lake Youngs boat ramp Figure 3: Lake Youngs Inlet Discharge Structure Figure 4: Inlet Discharge Structure full resolution point cloud 3-D perspective Figure 5: Inlet Discharge Structure full resolution point cloud plan view Figure 6: Gray scale hillshade indicating potential vegetation north of inlet discharge Figure 7: Grey scale hillshade indicating potential vegetation along northwest shore Figure 8: Grey scale hillshade indicating potential vegetation north side of west bay

List of Tables

Table 1: Control Monuments and Coordinates Table 2: Control Point Checks

Lake Youngs Bathymetry - Multibeam Hydrographic Survey Report

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Acronyms and Abbreviations

ASCII American Standard Code for Information Interchange CORS Continuously Operating Reference Station DEA David Evans and Associates, Inc.

GLONASS Global Navigation Satellite System (Russian) GNSS Global Navigation Satellite System (GPS + GLONASS) GPS Global Positioning System (U.S.)

HIPS Hydrographic Information Processing System HSX Hypack multibeam data format Hz Hertz INS Inertial Navigation System kHz kilo Hertz NAD83 North American Datum of 1983 NAVD88 North American Vertical Datum of 1988 NSPS National Society of Professional Surveyors PPK Post-Processed Kinematic PLS Professional Land Surveyor RTK Real-Time Kinematic SPCS State Plane Coordinate System SSP Sound Speed Profile S/V Survey Vessel THSOA The Hydrographic Society of America USACE U.S. Army Corps of Engineers U.S. United States

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1.0 INTRODUCTION

In November 2013 David Evans and Associates, Inc. (DEA) conducted a precision multibeam bathymetric survey of Lake Youngs, WA for the City of Seattle. The purpose of the survey was to collect high resolution multibeam data to map the reservoir for volume computation. Survey operations were conducted on November 6, 2013 through November 10, 2013. This report describes the control used for the survey, data acquisition methodology, data processing procedures, results obtained and a list of specific deliverables.

2.0 DATUMS AND PROJECT CONTROL

Conducting a survey on an established coordinate system referenced by monuments, enables the survey to be reproduced at a later date with repeatable results. Control for this survey was provided by the City of Seattle. The horizontal datum is the North American Datum of 1983 (NAD83) with the adjustment of Continuing Operating Reference System 1996, epoch 2002.00 (CORS96, Epoch 2002) projected to the State Plane Coordinate System (SPCS), Washington North Zone with units in U.S. Feet. The vertical datum for this survey was the North American Vertical Datum of 1988 (NAVD88), using GEOID 2009.

Primary positioning and vertical control for this survey was provided by a Real-Time Kinematic (RTK) Global Navigation Satellite System (GNSS). An RTK GNSS base station was deployed on control point pillar “PED-C3” with position and elevation provided by the City of Seattle (Table 1). GNSS capable (GPS+GLONASS) receivers owned by DEA were used at the base to generate carrier phase correctors these primary and secondary position systems aboard the survey vessel. Use of GNSS incorporates both the U.S. Global Positioning System (GPS) and the Russian GLONASS system which improves positioning system accuracy in areas where signals may be obstructed.

Table 1: Control Monuments and Coordinates

Monument Use Monument Designation

NAD83 (CORS96), epoch

2002.00 SPCS WA-N Zone NAVD88

North US Ft. East US Ft. Elev. US Ft.

RTK GNSS Base PED-C3 156518.257 1323241.613 507.757

Check Point PED-C2 156370.849 1323856.378 508.496

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To verify the acquisition software geodetic parameters and RTK GNSS base station setup were correct, the vessel navigation systems and a rover GNSS were used to measure the positions of a known survey control monument. The vessel RTK GNSS antenna and rover antenna were mounted to pillar “PED-C2” and a position check was conducted to document positional accuracy relative to provided control. Table 2 presents the differences from published coordinates to measured northing, easting and elevation.

Table 2: Control Point Checks

Date GNSS Device

Monument Designation

Difference (ft.)

Northing Easting Elevation 11/6/2013 Vessel PED-C2 0.01 0.02 0.04

11/10/2013 Rover PED-C2 0.00 0.00 0.05

3.0 BATHYMETRIC SURVEY METHODOLOGY

3.1 Survey Coverage

The intent was to survey Lake Youngs over the full extent. Actual survey coverage was limited to where the vessel could safely navigate and where the sonar was not impeded by structures, obstructions, aeration in the water column and aquatic vegetation. Figure 1 shows the final survey coverage as a hillshade image generated from a 3-foot grid color coded by depth.

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Figure 1: Survey Coverage

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3.2 Survey Vessel and Crew

The vessel used for the survey was a customized 24-foot survey vessel (S/V) William R.

Broughton (Figure 2) owned and operated by DEA. The vessel is equipped with an integrated navigation and data acquisition system and a custom mount for the multibeam sonar head.

Hydrographic survey operations were conducted under the direction of a Washington licensed Professional Land Surveyor (PLS) and a Certified Hydrographer certified by the National Society of Professional Surveyors (NSPS) and The Hydrographic Society of America (THSOA).

The hydrographic survey crew consisted of a senior hydrographer and vessel operator from DEA. The crew has conducted numerous multibeam and side scan sonar surveys and has had extensive training in hydrographic surveys. The vessel operator has a U.S. Coast Guard 50-ton inland masters license.

Figure 2: Survey vessel William R. Broughton at the Lake Youngs boat ramp

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3.3 Positioning and Navigation

During multibeam survey operations, positions were acquired with an Applanix POS/MV combined inertial and RTK-GNSS system. The POS/MV system integrates two GNSS receivers with a motion reference unit. Motion information (heading, roll, pitch, and heave) combined with precise position and height data are provided by the POS/MV to compute X, Y, Z data for each multibeam sonar beam. The system can provide accurate inertial navigation through GNSS outages for up to 30 seconds which enabled the survey vessel to run near shore without jeopardizing positioning integrity from satellite signal loss or multipath. The baseline length from the survey vessel to the base station was less than 2 kilometers and provided centimeter-level accuracy for horizontal and vertical positioning measurements during acquisition.

A Trimble SPS851 RTK GNSS secondary positioning system was installed on the survey vessel to provide redundant measurements. This system was also used for positioning system checks.

Position data was used in real-time to provide navigation information to the vessel operator and was time tagged and logged with multibeam and other ancillary data. The actual survey tracks are displayed with multibeam swath coverage in real-time on a monitor located at the helm to aid in a systematic survey of the area.

3.4 Water Surface Observations

All bathymetric data was time tagged and recorded relative to the water surface. Accurate water surface observations in the vicinity of the survey are required to account for water level changes and reference the soundings to the project vertical datum, NAVD88, using GEOID 2009. Water surface measurements were obtained using each navigation system: the Applanix POS/MV and the Trimble SPS851 RTK GNSS receiver. The water surface elevation at the time of survey was in the range of 501.2 feet and 501.5 feet NAVD88.

3.5 Multibeam Data Acquisition

A Reson SeaBat 7101 multibeam bathymetric sonar operating at a frequency of 240 kilo Hertz (kHz) was used to acquire soundings. The system was configured to record 141 soundings in a single sonar ping over a 120-degree swath. Additionally, DEA’s Reson SeaBat 7101 includes options such as a stick projector for enhanced shallow water performance and the ability to output backscatter sonar imagery. The system was configured in an equi-angular mode with an effective beam spacing of approximately 0.9 degrees.

Multibeam data were collected by running lines both parallel and perpendicular to the lakeshore.

Sonar swaths were recorded at a rate of 20 Hz as the vessel transited along the survey track lines.

In the deeper areas of the project, multibeam data were clipped at 45 degrees (90-degree total swath width) during processing to improve data quality. Running with a 90-degree swath (45 degrees to port and starboard), the system provided two times the water depth coverage in a single pass. The total swath width of full coverage mapping in a single pass varied with the water depth. Swath filters were increased to 55 degrees port and starboard in the shallow areas to increase coverage and increase line spacing. This increased the coverage to approximately three times the water depth.

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Along the shorelines, the sonar swath was rotated toward shore to capture as much data as possible. The Reson 7101 system has the ability to steer and compress the swath to any angle up to 210 degrees. For shoreline passes, the swath was oriented such that the outer beams were horizontal (90 degrees from nadir) while only 30 degrees of data was retained on the opposite side.

Hypack/Hysweep version 2013 was used to provide real-time navigation and to log time-tagged sensor data for later post processing. This software allowed the swath bathymetric data to be displayed as a painted color image on the navigation screen giving the hydrographer immediate indications of data quality and coverage.

The survey vessel was navigated along the outer edge of the previous swath where possible.

Soundings were obtained by overlapping previous passes, over the lakebed and from multiple passes. This technique also provides multiple measurements on features from differing angles.

4.0 EQUIPMENT CALIBRATION

4.1 Patch Test

Prior to data acquisition, a patch test calibration was conducted to determine the alignment bias of the multibeam transducer with the positioning and motion data, and to verify individual sensor timing bias. The calibration procedure consisted of a series of lines run in a specific pattern, which were used in pairs to analyze roll, pitch, and heading alignment bias angles, relative to the sonar swath as well as latency (time delays) in the time tagging of the sensor data.

4.2 Sound Speed

Detailed measurements of the sound speed profile (SSP) through the water column are crucial in multibeam surveys. Changes in the speed of sound in water not only affect acoustic distance measurements, but can also cause refraction or bending of the acoustic path as it passes through layers in the water column with different velocities. An AML Smart X probe was used to measure the SSP in the survey area several times over the course of each day in order to record any changes in water column properties. The Smart X instrument measures sound speed, temperature and depth to create an accurate sound speed profile of the water column.

5.0 DATA PROCESSING

5.1 Multibeam Data Processing

Post-processing of multibeam data was conducted utilizing Caris Hydrographic Information Processing System (HIPS) multibeam processing software (version 7.1, 64-bit, Service Pack 1, hot fix 1). Each survey line was converted from the native Hypack/HySweep format HSX file to a HIPS data file. All relevant data correctors such as vessel attitude, water levels, sound speed profiles, alignment angles and horizontal and vertical offsets were applied to the HIPS data files prior to data analysis and editing.

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Raw GNSS and Inertial Navigation System (INS) data collected from the Applanix POS/MV and the raw observables logged at the RTK-GNSS base station at survey control point “PED-C3” were post-processed in POSPac MMS version 6.2. This software uses advanced Post-Processed Kinematic (PPK) and inertial blending algorithms to provide optimal integration of inertial and GNSS measurements. In post-processing, POSPac was used to compute a forward and reverse navigation and elevation track which are merged to obtain an accurate and smooth result for navigation and delayed heave.

The post-processed INS results were imported into Caris HIPS then position and attitude sensor data were reviewed and edited for any anomalies. Sounding points were reviewed and edited for data flyers in the HIPS subset editor. Sounding data, including sonar beams reflecting from sediment in the water column or noise due to aeration in the water column, were carefully reviewed before being flagged as rejected. In each case, data was not eliminated and can be re-accepted in the future if required.

5.2 Data Export

Two grid resolutions were created using the HIPS processing software. The 3-foot grid surface was exported to an American Standard Code for Information Interchange (ASCII) easting, northing and elevation (XYZ) file and a color hillshade. The 1-foot grid surface was exported to a gray scale hillshade. Delineation of vegetation would be best achieved by using the provided gray scale hillshade image derived from a higher resolution 1-foot grid.

All data and imagery are referenced to the NAD83 (CORS96, Epoch 2002), SPCS, Washington North Zone projection with units in U.S. Feet. The vertical datum for elevations is NAVD88 using GEOID 2009.

6.0 RESULTS

This bathymetric survey was performed in compliance with U.S. Army Corps of Engineers (USACE) EM 1110-2-1003, “Hydrographic Surveying”, dated January 1, 2002 with multibeam updates dated April 1, 2004 in accordance with requirements for “Navigation & Dredging Support Surveys over a soft bottom”. All work has been supervised and final plans approved by a DEA Washington licensed PLS and NSPS/THSOA Certified Hydrographer.

6.1 Lake Youngs Inlet Discharge Structure

Additional survey passes were acquired around the Lake Youngs Inlet Discharge Structure (Figure 3). The Inlet Discharge Structure is located along the east bank of the reservoir and was active during the time of the survey, causing aeration in the water column that blocked multibeam sonar signals. Figures 4 and 5 show the full resolution data of the stilling basin and the detail of the structure obtained from the multibeam sonar.

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Figure 3: Lake Youngs Inlet Discharge Structure

Figure 4: Inlet Discharge Structure full resolution point cloud 3-D perspective

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Figure 5: Inlet Discharge Structure full resolution point cloud plan view

6.2 Vegetation

Significant areas of vegetation were encountered which intensified data processing efforts to map the true lake bottom. Vegetation primarily resides above an elevation of 476 feet (depths less than 25 feet at the time of survey) and ranged in height from 0.5 to 1.0 foot. In some areas the vegetation was dense enough to obscure any bottom returns and removal would result in large data gaps. After discussion with the City of Seattle, it was mutually decided to not remove the sonar returns on vegetation. An inverse distance weighted mean of accepted bottom returns over a 3-foot grid were used to map the lake floor. In areas of vegetation, the resultant bottom elevations have a potential depth error of approximately 0.5 feet higher than the actual lake floor.

A gray scale hillshade image was generated using a 1-foot grid resolution to enhance visual delineation of vegetation. Areas of vegetation are indicated by the area outlined in red in figures 6, 7, and 8. Figure 6 shows a detailed outline of the apparent vegetation, while Figures 7 and 8 encompass areas of interpreted spurious vegetation in rectangular regions.

A detailed analysis of the extents of the interpreted vegetation was outside of the scope of this task order. A georeferenced gray scale image is provided in the final deliverables, to be used by others at the City of Seattle to objectively delineate the potential areas of apparent vegetation.

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Additional processing techniques could be applied to this dataset under a new task order to delineate the areas of potential vegetation. Some limited work was done using a roughness analysis in ArcGIS to evaluate these areas, which showed promising results. Additionally, time-series backscatter data collected by the multibeam system could also be processed to map vegetation. Once the georeferenced extents of the apparent vegetation are established, quantitative assessments of areas could be made. Another benefit from this survey is to use data as a baseline to monitor change when compared to subsequent surveys.

Figure 6: Gray scale hillshade indicating potential vegetation north of inlet discharge.

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Figure 7: Grey scale hillshade indicating potential vegetation along northwest shore

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Figure 8: Grey scale hillshade indicating potential vegetation north side of west bay

7.0 DELIVERABLES

Final deliverables were limited to processed data and consisted of the following:

• Letter report documenting survey methodology, calibration procedures, data processing, quality assurance procedures and description of deliverables;

• Digital map including 1-foot contours and sun-illuminated hillshade imagery (AutoCAD Civil 3D file format); Geo-Referenced sun-illuminated hillshade imagery in GeoTIFF format;

• Inverse distance weighted processed data at 3 foot grid spacing presented in ASCII comma delimited Easting, Northing, Elevation (X,Y,Z) format;

• Geo-referenced sun-illuminated hillshade imagery in GeoTIFF format. Color hillshade of 1 and 3 foot; 1 foot grey scale hillshade;

• FGDC compliant metadata file for each XYZ dataset; and

• One set of hard copy maps at a scale of 1”=100’ of the 1 foot contours with sun-illuminated hillshade imagery on standard 24”x36” sheets (10 sheets), stamped and signed by a Washington licensed PLS.

All digital deliverables were provided on two optical media (DVD-ROM’s).