Observed bed elevation changes in the data may arise as a result of any of several causes:

Transcription

1 10 July 2014 TECHNICAL MEMORANDUM: INTRODUCTION NEWARK BAY STUDY AREA, NEW JERSEY BATHYMETRIC COMPARISON AND ANALYSIS Periodic single- and multi-beam sonar bathymetric surveys within Newark Bay (Bay) in New Jersey have been recently completed to support navigation missions (e.g., by the U.S. Army Corps of Engineers, USACE), system understanding (e.g., by the University of Delaware, UDel) and the ongoing remedial investigation/feasibility study of the Newark Bay Study Area (e.g., for the U.S. Environmental Protection Agency). Representative surveys are compared in this document to better understand temporal system changes and morphology (erosion, deposition, and altered bathymetry). Evaluation of the changes provides both qualitative and quantitative data; however, caution must be exercised when deriving quantitative information from measured changes, and/or attempting to determine long-term change rates in the Bay. Apparent bathymetric changes are sometimes the results of instrumentation variations and/or analysis techniques, and may not be representative of actual sediment accumulation or deposition. Observed bed elevation changes in the data may arise as a result of any of several causes: 1) Use of dissimilar survey equipment, vessels and techniques between surveys (e.g., GPS and/or sonar system performance differences) 2) Human-induced errors (i.e., blunders) 3) Mathematical analysis technique differences (e.g., interpolation, TIN modeling) 4) Anthropogenic activities (e.g., dredging, vessel activity) that have taken place between surveys (i.e., human-induced changes to the environment) 5) Meteorological impacts that may have induced changes in the system between surveys (including extreme episodic events) 6) Natural erosion, transport and deposition within the system (e.g., ambient/natural eventcaused changes to the environment) Each potential cause of uncertainty requires consideration when quantifying bathymetric change between surveys and applying confidence intervals to the computed change. This is important when attempting to define long-term bathymetric change rates of a system, particularly if the data record is not sufficiently long 1. Anthropogenic activities occurring between surveys may have unknown direct or indirect future impacts on the local and regional sediment dynamics. Further, episodic extreme meteorological events (e.g., high flow events and/or infrequent storm surge events) may cause short-term changes in system bathymetry that act to bias the long-term computed bathymetric change rates. Therefore, to fully characterize a site s morphologic patterns, comprehensive bathymetric site evaluation should also include knowledge of the site s history to supplement the data analysis. 1 This is a relative statement: As with any dataset, the longer the record, the more confidence in the statistical interpretation. But the necessary length of a dataset is difficult to define and requires professional judgment on behalf of the analyst. 1

2 With a long enough record of bathymetric surveys, and comprehension of the major anthropogenic and natural events that may have affected sediment dynamics, a more refined understanding of the change rates can be attained. A discussion of historical changes, events, and developments to the Bay, however, is not completed within this document. Rather, this memo solely examines data from several recent bathymetric surveys and quantifies the apparent bathymetric changes that have occurred between these surveys. Representative figures presented include spatial data-coverage maps, bathymetric change maps, and histograms that statistically compare surveys. Table 1 lists the bathymetric surveys that were used for analysis. Only surveys completed in 2005, 2008, and 2012/2013 were evaluated as they were the only full Newark Bay surveys available (i.e., where the entire Bay was surveyed at approximately the same time period). Additional surveys are publicly available, at a minimum, for the years between 1996 to present (e.g., USACE surveys) for smaller, focused areas of the Bay; however, they were not evaluated within this document. Table 2 describes the bathymetric comparisons that were completed in the below investigation. Date /2008-8/ /2012-2/2013 Table 1. List of Newark Bay surveys utilized within. Surveyed Survey Firm / Type Notes Area(s) Organization Entire Single-beam survey of Newark OSI Newark Bay 2 Bay (coarse spacing) Single-beam survey of Newark Entire UDel Bay (higher resolution than 2005 Newark Bay survey) Single-beam (SB) Single-beam Multi-beam (MB) / Singlebeam Entire Newark Bay OSI Table 2. Newark Bay survey comparisons completed within. Comparison Survey Evaluations 2013 SB to MB point comparison SB point to 2013 DEM (Point Comparison) SB point to 2013 DEM 2 (Surface Comparison) SURVEY EQUIPMENT AND CONSIDERATIONS 2005 DEM to 2008 DEM 2008 DEM to 2013 DEM 2005 DEM to 2013 DEM Multi-beam and single-beam survey of Newark Bay (highest resolution) The 2005 survey was completed with an Odom MKIII dual-frequency single-beam echo-sounder (Tierra, 2007). The dual-frequency transducer simultaneously transmitted at 200 khz and 24 khz frequencies, which allowed for potential simultaneous measurement of the sediment surface and sub-bottom layer(s) 5. The surveyors used real-time kinematic (RTK) GPS positioning control with a targeted horizontal positioning accuracy of 1.0 (0.3 meters). A GPS base station was setup on shore-based, established USACE survey monuments and transmitted position corrections to the ship-based GPS receiver. Vertical positioning (i.e., the measurement of the water surface elevation 2 Ocean Surveys, Inc. 3 Approximate dates provided by C. Sommerfield via personal communication, May, Digital Elevation Model (DEM) khz (or higher) is a commonly used frequency for measuring water depths to the surface of the bottom sediment. Lower frequencies are often used to measure sub-bottom layers or higher-density layers(s) beneath lower density (e.g. fluid mud) layers. 2

3 at the time of surveying) was obtained through deployment of a tidal gauge at Port Elizabeth. The vertical positioning uncertainty was subject to the accuracy of the tide gauge data and survey distance from the tide station 6. The 2008 survey was completed with two different mobilization efforts, in February and August, 2008 (C. Sommerfield, personal communication, May, 2014). The equipment used included a Knudsen 320 B/P dual-frequency (200 khz/28 khz) single-beam echo-sounder and Leica MX400 GPS Navigator differential GPS (DGPS) (Sommerfield and Chant, 2010). Vertical positioning was provided by the NOAA Bergen Point West tidal station data. The stated horizontal positioning uncertainty was approximately 16.4 (5.0 meters) (Sommerfield and Chant, 2010). The vertical positioning uncertainty, again, was subjected to the tide gauge accuracy and the survey distance from the tide gauge. During hydrographic surveys, motion compensation units are often used to measure the dynamic heave (vertical motion e.g., from waves), pitch and roll (and sometimes the heading) of the survey vessel and sonar so that wave activity, the proper sonar orientation angles, and slant distances (ranges) are applied to the raw sounding data to properly compute depth. It is not presently known if a motion compensation unit was used for the 2005 survey; a motion compensation unit was not used during the 2008 surveys (C. Sommerfield, personal communication, May, 2014). Without a motion compensator (or depending upon the accuracy of the motion compensator used) horizontal and vertical uncertainties may be slightly larger than those stated above 7. In 2013, a Reson Seabat 8125 swath multi-beam sonar echo-sounder was used to measure the Newark Bay bathymetry (Tierra, 2013). The echo-sounder was paired with a Trimble RTK GPS and an Applanix POS MV high-accuracy motion compensation unit to account for pitch, roll, heading, and heave motion throughout the survey. Horizontal and vertical positioning was provided by the RTK GPS, resulting in horizontal and vertical positioning accuracies of approximately ( meters). Performance and accuracy of any echo-sounder is strongly influenced by the beam angle (α) formed by the echo-sounder's transducer. An echo-sounder can be thought of, in a very basic sense, as ensonifying a footprint on the bottom, whose size is a function of the transmitted beam angle and local water depth. The larger the beam angle, or the deeper the water depth, the larger the bottom footprint area measured. In general, acoustic signals are emitted from the transducer and return after reflecting off dense objects (e.g., sediment surfaces, rocks). The distance traveled by the acoustic signal is determined from the product of the total travel time (round-trip) of the acoustic signal and the sound speed in water. The first acoustic pings that propagate back to the transducer are often 8 considered to be representative of the recorded depth beneath the echo-sounder since they are typically the strongest (most intense) returns. In most mild-sloped bottom cases, this is not generally a concern; 6 Water surface elevations (e.g., tides) are dynamic and transient. Actual surface elevations vary with distance from the measurement gauge; the elevation at the gauge is not always in agreement with the elevation at the survey location. 7 The depth errors induced by not using a motion compensator are directly dependent upon the type of vessel used, the amount of motion the vessel is subjected to during the survey, the local water depths, and the amount of surface wave activity on the day of surveying. 8 Echo-sounders incorporate varying algorithms for computing the bottom depth from the acoustic signals. More sophisticated equipment (e.g., MB sonars) may use complex methods of calculating the distance to the sediment surface. 3

4 however, on steeper slopes, the first returned signal may not truly represent the actual water depth beneath the transducer, artificially biasing the sounding data (see Figure 1). Figure 1. Example of sonar beam coverage on seafloor for one representative beam (not to scale). Both single- and multi-beam sonars are subject to the same potential errors in computing depth. However, the beam angle of a multi-beam echo-sounder is often much less than that of a singlebeam setup. Single-beam echo-sounders frequently have beam angles of , whereas highresolution multi-beam echo-sounder beam angles are often 1.0 or less 9. Therefore, the respective bottom footprint area of a multi-beam sonar is smaller than that of a single-beam sonar and the data usually not as susceptible to measurement errors over uneven seabeds 10. When comparing single-beam and multi-beam surveys, and attempting to quantify bathymetric change, it is important to consider the systematic differences between the two technologies. Singlebeam survey systems often have more total propagated uncertainty (TPU), which is a metric that describes the total equipment-specific and human-induced errors. This is because of the larger beam angles, typically-utilized coarser positioning techniques (e.g., stand-alone GPS or DGPS compared to higher-accuracy RTK GPS) and, sometimes, less precise motion sensors (when utilized) often used with single-beam systems. 9 The beam angles of the 2005 and 2008 single-beam echo-sounder transducers are not presently known. The beam angle from the 2012/2013 single-beam survey was approximately 3. The beam angles from the 2012/2013 multi-beam survey were approximately 0.5 x A multi-beam echo-sounder may be subjected to significant measurement errors in deep water depths where the ensonified bottom footprint area is large. 4

5 BATHYMETRIC SURVEYS AND COVERAGE In 2005, Ocean Surveys, Inc. (OSI) collected single-beam bathymetry within Newark Bay for Tierra Solutions, Inc (Tierra, 2007). The survey extended throughout Newark Bay, into the Passaic and Hackensack River mouths to the north, and Arthur Kill and Kill van Kull to the south. The survey transects were coarsely spaced (as much as 1300 [400 meters] between parallel transect lines) and did not comprehensively encompass Newark Bay in its entirety (no navigation channel soundings were measured near the Kill van Kull). Further, the survey comprised mostly cross-channel transects, but did not include comprehensive cross-check lines to tie the resulting survey surface together. The data were collected for comparison to the pre-defined geomorphic regions of Newark Bay and to assist the field crew in positioning select coring locations (Tierra, 2005; Tierra, 2007). Transects through the southeastern Bay navigation channel (near the Kill van Kull) were omitted from this survey in anticipation of upcoming USACE survey activities in that region. A coverage map of the individual soundings from the 2005 data is shown in Figure 2. In 2008 the University of Delaware collected finer-resolution single-beam bathymetry of Newark Bay to identify areas of long-term sediment accumulation and compare to seasonal deposition patterns (Sommerfield and Chant, 2010). Similarly to the 2005 OSI survey, the 2008 survey extended throughout Newark Bay, into the Passaic and Hackensack River mouths to the north, and Arthur Kill and Kill van Kull to the south. The data comprised more numerous transects and crosscheck lines (when compared with the 2005 survey) at approximately 330 (100 meter) spacing between parallel track-lines. In February 2008, soundings were collected from a small grid of fine resolution transects east of the Port Newark and Port Elizabeth terminals. Subsequently, in July and August 2008, a full Newark Bay survey was completed at a coarser transect resolution than the February 2008 survey (C. Sommerfield, personal communication, May, 2014). The 2008 single-beam coverage map and finer resolution sounding areas are shown in Figure 3. Between December 2012 and February 2013, OSI completed comprehensive multi-beam and single-beam surveys in Newark Bay (Tierra, 2013). Like the previous full-bay surveys, the 2012/2013 survey extents included the Passaic and Hackensack River mouths to the north and the Arthur Kill and Kill van Kull to the south. The multi-beam portion of the survey encompassed all bottom surface elevations in the survey area up to -8 to -6 National Geodetic Vertical Datum of 1929 (NGVD29), approximately. The single-beam portion of the survey overlapped with the multibeam survey at these elevation contours (to ensure measurement of a continuous elevation surface) and extended toward the shoreline with the intent to survey the seafloor up to an elevation of -2 NGVD29. Herein, the 2012/2013 survey is referred to as the 2013 survey, for brevity. To ensure manageability of the large 2013 survey dataset, OSI sorted the high-resolution multibeam data to a 2 x 2 uniformly gridded surface prior to submittal 11. The particular sorting manner (i.e., how the representative cell value was chosen) used by OSI on the 2013 multi-beam data is not presently known. The 2013 single-beam data were collected by OSI at 100 (30 meter) parallel transect spacing, approximately. OSI sorted the single-beam data to 2 along-track-line spacing prior to submittal. The exact sorting manner is unknown. 11 Sorting a dataset decimates the dense soundings such that only one point within each 2 grid cell (e.g., the average value) is selected as representative of that cell. 5

6 The 2013 single-beam (SB) coverage maps are shown in Figure 4 and Figure 5. The 2013 singleand multi-beam (MB) overlapping coverage is shown in Figure 6. The combined 2013 SB and MB DEM surface is shown in Figure 7. When the combined SB/MB 2013 DEM interpolated surface was generated, where overlaps occurred, the multi-beam data were retained and the single-beam data were excluded. Figure 2. OSI 2005 Newark Bay single-beam survey sounding locations and coverage extent (black dots). 6

7 Finer-resolution Feb soundings Coarser-resolution July/Aug soundings Figure 3. UDel 2008 Newark Bay single-beam survey sounding locations and coverage extent (black dots). 7

8 Figure 4. OSI 2013 Newark Bay single-beam survey sounding locations and coverage extent (North Bay view). 8

9 Figure 5. OSI 2013 Newark Bay single-beam survey sounding locations and coverage extent (South Bay view). 9

10 Figure 6. OSI 2013 Newark Bay single-(gray shading) and multi-beam (colored) survey coverage extent (Full Bay view). This image illustrates the amount of overlapping single- and multi-beam points available for comparison. 10

11 Figure 7. OSI 2013 combined Newark Bay single- and multi-beam survey surface DEM (Full Bay view). 11

12 DATUM CONVERSION The horizontal coordinates of the 2013 single- and multi-beam survey data were referenced to the New Jersey State Plane Coordinate System, NAD-83, with units in U.S. Survey Feet. The vertical datum utilized was NGVD29, also in U.S. Survey Feet. The 2005 and 2008 survey data that were provided for the analysis were in alternative coordinate systems and units; therefore, to facilitate direct comparison of the bathymetric records, the 2005 and 2008 data files required conversion to the equivalent coordinate system as the 2013 survey. The conversions were subsequently verified in the following section. Since each of the 2005, 2008, and 2013 data appeared to be in horizontal agreement, no horizontal adjustments were made to any of the datasets. Vertical datum conversions were required, however, and conversion factors were derived from local NOAA 12 tidal benchmark data (Bergen Point West Reach, Sta ; and The Battery, Sta ); and, redundantly, through NOAA s VDatum program 13. The following vertical conversions were made to the as-received datasets: The 2005 single-beam survey data from Newark Bay were initially provided as depths 14 (positive z values) relative to the North American Vertical Datum of 1988 (NAVD88). Units were assumed to be U.S. Survey Feet 15. o The conversion required that the depths first be converted to elevations (converted to negative numbers) and then shifted from the NAVD88 to the NGVD29 vertical datum. o The conversion used was NGVD29 = (-1*NAVD88) The 2008 single-beam survey data from Newark Bay were initially provided as depths relative to Mean Lower Low Water (MLLW). Units were meters. o The conversion required that the depths first be converted to U.S. Survey Feet, then to elevations, and then shifted from MLLW to NGVD29. o The conversion was NGVD29 = -1*(MLLW* ) For the 2013 single- and multi-beam surveys no datum shift was required as both were submitted in NGVD29 elevations (US. Survey Feet units). INDIVIDUAL POINT COMPARISONS Survey data were compared at survey-to-survey overlapping areas to assess vertical agreement between surveys, to verify that survey data were aligned in a common datum, and so that bathymetric changes between surveys could be confidently evaluated. Individual single-beam soundings were converted to raster grid cells since individual point measurement locations rarely (if ever) coincide between different survey data. Rather than interpolating single-beam data to encompass full bay-wide DEMs, 2 x 2 grid cells were created only in proximity to the locations at which individual point data existed; the purpose being to compare surveys only where data existed, not at interpolated surface regions in between data points Depths are positive distances below a specified datum (e.g., the water level). Elevations are orthometric heights relative to a vertical datum and can be positive or negative. 15 Not all datasets used in this analysis were obtained directly from the surveyors; therefore, some pertinent information regarding data units and coordinate systems was, at times, deduced. 12

13 The interpolation algorithm used to create the gridded raster cells (ArcGIS [ESRI] Point to Raster Conversion) created an initial matrix of cells based on the total spatial extent of the dataset. Therefore, raster cell locations were pre-determined by the input dataset and not necessarily centered on the individual point data locations. Individual raster cells whose cell extents encompassed point data were assigned the mean value of the point elevations that existed within those cells. Raster cells which did not encompass point data were left empty (i.e., no data). This algorithm often resulted in a staggered raster grid dataset as shown below. Examples from the 2005, 2008, and 2013 surveys are shown in Figure 8 - Figure 10. Figure 8. Conversion of 2005 single-beam point data (dots) to single-beam 2 x 2 raster data (gray cells). 13

14 Figure 9. Conversion of 2008 single-beam point data (dots) to single-beam 2 x 2 raster data (gray cells). Figure 10. Conversion of 2013 single-beam point data (dots) to single-beam 2 x 2 raster data (gray cells). 14

15 Comparison 1a 2013 Multi-beam to Single-beam Point Comparison Before creating a combined dataset of the 2013 single-beam and multi-beam data, it was necessary to assess the vertical (i.e., elevation) agreement between the two datasets. The SB and MB data raster grid cells were differenced via simple subtraction of the SB data from the MB data where data from both survey methodologies overlapped. The bathymetric difference results were plotted in a histogram (Figure 11) where 95% of the values (of approximately 335,000 total points) were between +/- 0.5 (indicating vertical agreement between the datasets). Less than 0.02% of the difference values fell outside the plotting range in Figure 11 (+/-10 ), and likely signify larger, physical Newark Bay system changes (e.g., dredging between surveys), sonar system performance differences (e.g., single-beam and multi-beam idiosyncrasies) and/or analytical biases (e.g., due to interpolation or differencing methods). Basic analysis statistics are listed in Table 3. Negative values indicate the multi-beam data were of lower elevation (deeper) than the single-beam data. In general, the mean bias and median differences are less than +/-0.05, which is within sonar system uncertainty bounds. The overall standard deviation was less than 0.4. The statistics in Table 3 and the large percentage of differences that are within +/- 0.5 indicate 2013 MB and SB datasets are on a common vertical datum. Therefore, the datasets were combined to create an accurate, Bay-wide 2013 DEM (see Figure 7). For the remaining analysis, this combined DEM will be representative of the 2013 survey. MB data were Deeper SB data were Deeper 95% within +/- 0.5 Figure 11. Histogram of the bathymetric differences between the 2013 SB and MB data at overlap areas. 15

16 Table 3. Statistics from the 2013 SB to MB point overlap comparison. Statistic Value Mean Median 0.02 Standard Deviation 0.38 Minimum Maximum Comparison 1b 2005 Point to 2013 DEM Comparison In order to validate that the 2005 survey had been converted to a common datum with the other datasets, a similar procedure as described above was completed. Though the 2005 and 2013 data were collected several years apart, if a large bias was observed in the difference statistics, it might indicate that the survey data being used were not on the same vertical datum. The 2005 SB data raster grid cells were subtracted from the 2013 DEM via simple subtraction of the 2005 data from the 2013 data, and where data from both surveys overlapped. The difference histogram (Figure 12) indicated 57% of the bathymetric difference values (of ~171,000 compared points) were between +/- 0.5 ; and 67% were between +/ This is considered to be in vertical datum agreement since the timeframe between surveys was approximately eight (8) years. Between surveys, it is likely that anthropogenic and natural events occurred that caused numerous bathymetric changes larger than +/-0.5. Less than 4.5% of the difference values were larger than +/-10, and were likely, again, a result of physical Newark Bay system changes, sonar system performance differences, and/or analytical biases. The percentage of extreme bathymetric differences is larger in this comparison than the previous; yet, these differences have occurred over a longer timeframe and are expected due to anthropogenic influences (e.g., dredging) or other development of the Bay. Analysis statistics are listed in Table 4. The mean difference indicates that the 2013 DEM is 0.91 lower in elevation than the 2005 data. However, the mean value is skewed by large negative values (i.e., where the 2013 surface is significantly deeper than the 2005 data from, for example, harbor deepening activities). A more representative statistic to evaluate is the median difference which still indicates that the 2013 DEM was deeper (-0.10 ) than the 2005 data, but by a lesser overall magnitude. It is a more representative number to use because it is not influenced as greatly by extreme values. Some degree of depth variations are expected as any hydrodynamic system inherently changes over an 8-year period; yet based on these results, the 2005 and 2013 survey datasets being used in this comparison are on a common vertical datum. 16

17 Figure 12. Histogram of the bathymetric differences between the 2005 singlebeam data and 2013 DEM at overlap areas. Table 4. Statistics from the 2005 SB point to 2013 DEM overlap comparison. Statistic Value Mean Median Standard Deviation 4.52 Minimum Maximum Comparison 1c 2008 Point to 2013 DEM Comparison The 2008 single-beam survey required similar validation as the 2005 survey. The 2008 data raster grid cells were subtracted from the 2013 DEM. The purpose of this comparison, too, was to ensure vertical datum consistency. The difference results are presented in the histogram shown in Figure 13, where 52% of the difference values (of ~123,000 compared points) were between +/- 0.5 feet. Approximately 71% of 17

18 the difference points were between +/- 1.0 feet. The histogram shape and statistics are very similar to those observed in the 2005 comparison: i.e., a large peak near 0.0 with a few larger values greater than +/ There are fewer anomalous extreme high and low values than in the 2005 to 2013 comparison (less than 2.0% of the difference values were larger than +/ ), and are likely a result of physical Newark Bay system changes, sonar system performance differences and/or analytical biases. These differences have occurred over a shorter timeframe than the previous comparison (approximately 5 years), yet still long enough to be influenced by natural and anthropogenic changes. The overall mean difference indicates the 2013 DEM is 0.51 lower in elevation than the 2008 data (Table 5) due to large negative values that bias the averaging statistic. The median difference value suggests system deepening of 0.14 during this timeframe. Based on these results, the 2008 and 2013 datasets used in this comparison are considered to be on a common vertical datum; therefore, after the adjustments described above, all three survey datasets (2005, 2008, and 2013) have approximately the same vertical datum and can be directly compared in the remaining analyses. Figure 13. Histogram of the bathymetric differences between the 2008 singlebeam data and 2013 DEM at overlap areas. 18

19 Table 5. Statistics from 2008 SB point to 2013 DEM overlap comparison. Statistic Value Mean Median 0.14 Standard Deviation 3.22 Minimum Maximum SURFACE TO SURFACE COMPARISONS In an attempt to quantify site-wide bathymetric changes to Newark Bay, a triangular irregular network (TIN) model was created from each point dataset. The TIN models were then linearly interpolated to uniformly-spaced, gridded raster surfaces, allowing simple, surface to surface differencing of the DEMs. Comparisons made by differencing interpolated survey data require cautious interpretation, though. Interpolated data accuracy is dependent upon accuracy of the originally measured data points, the spatial resolution of the original points, and the interpolation method applied. Any horizontal and vertical positioning and sounding errors made at the time of surveying will propagate through to the interpolated surface; and interpolation of any bathymetric record (especially single-beam) may not incorporate small-scale elevation variations (e.g., scour holes) if the bottom features were not directly measured during survey operations. Furthermore, the surveys analyzed within this document were conducted with non-coincident survey track-lines (i.e., single-beam transects from different years were not spatially coincident), so the actual measured data being interpolated varied for each survey. Surface to surface comparisons are valuable tools to ascertain large-scale and long-term changes (e.g., large-scale deposition and/or erosion), but any small-scale and short-term variations (e.g., scour hole evolution, small scale erosion/deposition) require additional verification and are not discussed further within this document. Data Accuracies and Uncertainties Hydrographic surveying methodologies have recently become highly advanced but they are not as precise as those incorporated in land surveying. Hydrographic surveying comprises open-ended (one-way) traverse practices and typically utilizes acoustic methods for estimating range (e.g., distance, depth). Land surveying traditionally comprises closed loop traverses and more accurate distance computing methods (e.g., laser-ranging and sight). This distinction is important to understand when attempting to define hydrographic survey accuracies, especially when comparing hydrographic survey datasets. The USACE Hydrographic Survey Manual (EM ) describes minimum performance standards and tests required to be met in order to assure quality control on hydrographic survey projects. They are statistically determined criteria that incorporate the likely uncertainty associated with a variety of hydrographic surveying techniques. Similarly to other published and widely used hydrographic 19

20 survey performance criteria (e.g., The International Hydrographic Organization, IHO, Special Publication 44), EM provides a means for stating approximate accuracies of survey data and the potential confidence interval ranges of the data. During any hydrographic survey, there are many manners in which uncertainties may be incorporated into a dataset, and the uncertainties can propagate through to subsequent analyses. The sum of all these uncertainties is the TPU of survey data or a survey system, and is a function of some or all of the following list of variables: 1) Echo-sounder system variations a. Use of single-beam vs. multi-beam b. Beam width(s) and system performance specifications of the sonar(s) used c. Use of proper sonar draft (distance of the transducer below water level) d. System-specific idiosyncrasies (proprietary methods in which the acoustic signals are processed internally within the sonar) 2) Positioning methods (horizontal and vertical) a. Type of GPS used (stand-alone, DGPS, RTK GPS) b. Accuracy and number of tide gauge(s) and/or water level logger(s) used (locally or regionally installed pressure sensors) c. Vessel static and dynamic draft changes (vessel drafts change if weight is redistributed and if underway) 3) Motion compensation unit a. Heave accuracies (to remove surface wave / boat wake activity from the depth records) b. Pitch, roll and yaw accuracies c. Accuracy in variable weather and wave conditions 4) Sound speed in the water column a. Accuracy of sound speed sensor used b. Frequency of sound speed water column profile casts (especially in areas influenced by rapidly changing water densities such as tidally influenced estuaries) 5) Reflectiveness/consistency of the bottom material a. Denser materials (e.g., rock, sand) tend to reflect stronger acoustic signals with lower uncertainties b. Less dense materials (loosely consolidated silt, clay, mud) may attenuate the signal or allow partial penetration of the acoustic signal, increasing the uncertainty in the bottom depth 6) QA/QC procedures during surveying activities a. Accuracy of the equipment physical and angular offsets measured on the vessel b. Sonar system performance testing (on-site verification of sounding data) c. Manufacturer-specific calibration protocols 7) Data manipulations a. Accuracy of unit conversions (meters, U.S. survey feet, international feet) 20

21 b. Accuracy of coordinate system conversions (Latitude/Longitude to projected coordinate systems) c. Effects of data sorting / decimating d. Effectiveness of data processing techniques Measurement uncertainty can result from a large number of systematic- and human-induced factors. Though there are mathematical methods to computing the TPU, it is not quantified in this report as not all of the equipment, procedural, and environmental variables from the survey dates are presently known. Evaluation Criteria Based on the above considerations of equipment used and procedural variances, a set of criteria to identify morphologic areas was defined for the purposes of evaluating long-term morphologic patterns within Newark Bay. These criteria, listed in the bullets below, were used throughout this analysis for consistency: Bottom elevation decrease was identified by those bathymetric differences larger (more negative) than o A decrease of bed elevation indicates erosion or removal of sediment by other means that occurred between survey dates. Bottom elevation increase was defined by those bathymetric differences larger (more positive) than o An increase of bed elevation indicates deposition or addition of sediment by other means that occurred between survey dates. Bathymetric differences within +/- 0.5 were considered uncertain, as this range was potentially within the error bounds of procedural, systematic and/or analytic uncertainty. 16 Newark Bay Regional Feature Locations To facilitate simple classification of regional sedimentation patterns, key system features were identified and are labeled in Figure 14 and Figure 15. The locations are referenced consistently throughout the bathymetric comparisons. The -12 NGVD29 elevation contour is shown in black and is used as a representative elevation to divide deeper regions (e.g., the navigation channels) from shallower, sub-tidal regions. 16 The TPU of the 2005 and 2008 survey data and resultant bathymetric differences with the 2013 DEM was potentially greater than +/-0.5. The +/- 0.5 uncertain criteria is being suggested, at least initially, in order to quantify long-term bathymetric change; and, it can be refined, as appropriate, in future efforts. 21

22 Passaic River Kearny Flats Hackensack River Northeast Subtidal Flats Port Newark Channel Channel Shoal I-78 Bridge Crossing Confined Disposal Facility (CDF) Port Elizabeth Channel Northern Newark Bay Navigation Channel Eastern Subtidal Flats Figure 14. Newark Bay regional feature definition map (North Bay). 22

23 Port Elizabeth North Channel Port Elizabeth South Channel Eastern Subtidal Flats (cont.) Southwest Subtidal Flats Southern Newark Bay Navigation Channel Arthur Kill Shooter s Island Kill van Kull Figure 15. Newark Bay regional feature definition map (South Bay). Procedure and Considerations Newark Bay DEM surfaces were differenced to visualize the bottom morphology that has occurred between survey years. The results indicated variable morphologic patterns exist throughout the area of study. The comparisons are beneficial for qualitative and quantitative use, and are shown in Figure 16/Figure 17, Figure 19/Figure 20, and Figure 22/Figure 23 for the , , and comparisons, respectively. Cooler (blue) colors indicate a bed elevation decrease (sediment loss), whereas warmer (red) colors indicate a bed elevation increase (sediment accumulation). The color scheme is graduated between the +/- 2 elevations since a majority of the bathymetric difference values are within this range. 23

24 The white coloring denotes the -0.5 to +0.5 difference values, respectively. This is the range defined previously as being of uncertain change, where the calculated bathymetric differences are small enough to make indistinguishable with procedural, systematic and/or analytic errors. Dark blue and brown coloring indicates areas that have lost or gained more than 2.0 of sediment between survey dates, respectively. These areas require additional consideration, though, as the change may be natural or unnatural. For example, areas indicating a loss of sediment of more than 2.0 may have been dredged between survey events, and should not be used to quantify long-term change rates in that specific area. Areas that have gained more than 2.0 of sediment, on the other hand, may also indicate sediment deposited via anthropogenic (e.g., dredge disposal) means. Comparison 2a 2005 DEM to 2008 DEM Comparison Evaluating the bathymetric change that occurred in the North Bay first (Figure 16), the following general morphologic observations were made for the time period between 2005 and 2008: Kearny Flats: o The sub-tidal flats generally experienced uncertain change during this time period. o Near the navigation channels of the Passaic and Hackensack River mouths, the morphologic change patterns were more variable, evident by the varied color contouring in the figure (+/- 2.0 change along channel slopes). Passaic and Hackensack River Mouths: o A majority of the surveyed areas of the river mouths and lower reach of the rivers accumulated sediment up to, and exceeding, 2.0 in some locations. o Other portions of the river reaches upstream of the mouths comprised uncertain change. Northeast Sub-tidal Flats: o At the most northern end of the Northeast Sub-tidal Flats, near the Hackensack River Mouth, the area decreased in elevation, up to a loss of 1.5 of sediment. o South of this area, the morphologic change was uncertain (at approximately the same latitude as the river mouths). o The remaining portion of the flats largely increased in elevation (up to of accumulated sediment in some locations), though some variability is evident. I-78 Bridge Crossing: o In the immediate north and south vicinities of the interstate bridge crossing, sediment accumulation is observed during this time period (up to , approximately). Eastern Sub-tidal Flats: o In the region between the I-78 bridge crossing to the north and the Port Newark channel latitude, the morphologic change of the sub-tidal flats was mostly uncertain with a localized decrease in elevation in some areas, up to 1.0 or more, approximately. o Between the Port Newark channel latitude and the Port Elizabeth North channel latitude, the morphologic change becomes more uncertain (+/- 0.5 ). Port Newark and Elizabeth Channels: o The morphologic change observed in the Port channels during this timeframe is difficult to evaluate accurately due to the coarse spacing of the single-beam surveys being compared, and the likelihood that dredging and/or ship propeller scour has occurred in the channels during this time period. 24

25 o The bathymetric difference indicates changes up to, and exceeding +/- 2.0, but the data are sparse within the channels, leading to high uncertainty about the accuracy of these difference values. Confined Disposal Facility (CDF) and Channel Shoal: o In the CDF, sediment has accumulated in excess of 2.0, likely due to disposal of dredged material. o Around the perimeter of the CDF, still on the channel shoal, the change is mostly uncertain with some areas indicating a decrease in elevation. o Along the perimeter of the shoal, within the navigable waterway, the bottom elevation has increased in excess of 2.0. Western Sub-tidal Flats: o The difference map indicates a variable pattern of morphologic change on these flats, with elevation increases up to 2.0 and elevation decreases in excess of 2.0. o The morphologic change observed here is potentially a result of the coarsely-spaced single-beam data from this time period at this location. Northern Newark Bay Navigation Channel: o Within the navigation channel, the morphologic pattern during this time period is variable, with elevation increases and decreases up to, and exceeding 2.0 in different locations. o Some of this variability may be due to the coarsely-spaced single-beam data and interpolation routines applied. o Other variability may be authentic and may be a sign of the dynamic variability within the navigation channel (e.g., due to the ambient hydrodynamics and/or vessel traffic). Evaluating the South Bay bathymetric change (Figure 17), the following patterns were identifiable for the period 2005 to 2008: Southeastern Sub-tidal Flats: o The morphologic pattern is varied in the area between the Port Elizabeth North channel latitude and the southern end of Bayonne, NJ. o At the northern end of this region, the change is largely uncertain. o Further south, there was mostly an elevation decrease during this time period. o At the most southern end of this reach, a mix of extreme elevations changes is visible (larger than +/- 2.0 in some locations). Kill van Kull, Shooter s Island, and Arthur Kill: o A majority of this stretch of the Bay has undergone elevation decreases of more than 2.0, likely to do dredging associated with channel deepening projects. o Around Shooter s Island, the change is varied and uncertain as a result of coarse data resolution in this area. o In a few locations, the toes of the navigation channel slopes appear to have increased in elevation (up to 2.0 in some locations). Southwest Sub-tidal Flats: o The morphologic change across this region is largely uncertain with some broad elevation decrease patches visible (up to 1.0 elevation decrease) and small patches of more extreme elevation increases or decreases evident (larger than +/- 1.0 ). 25

26 o The channel from the historic Central Railroad of New Jersey train bridge crossing appears to have decreased in elevation during this time period. Southern Newark Bay Navigation Channel: o The two rectangular areas of large elevation decreases (more than 2.0 ) adjacent to the Port Elizabeth terminal are a result of dredging events occurring during this time and not erosive loss of sediments. o The eastern navigation channel toes have largely increased in elevation during this timeframe (2.0 or more). o The Port Elizabeth South Channel and the western channel toes along the Southwest Sub-tidal Flats area have also increased in elevation 2.0 or more. o All other navigation channel areas have experienced mostly uncertain and/or variable change. 26

27 Mostly uncertain change & Localized elevation increases/decreases Elevation increased at river mouths Uncertain change / Localized elevation changes Uncertain change Elevation increased Variable localized change (Nav. Ch.) Elevation increase patterns Elevation decrease patterns Elevation increased (CDF) Uncertain change Variable localized change (Nav. Ch.) Figure 16. Bathymetric difference map comparing the 2005 surface to 2008 surface (North Bay). 27

28 General elevation decreased (dredging) Uncertain change (Nav. Ch.) Uncertain change Historic train bridge crossing Uncertain change Elevation decrease patterns Elevation increased (Nav. Ch. Toes) General elevation decreased (dredging) Channel toe elevation increased Figure 17. Bathymetric difference map comparing the 2005 surface to 2008 surface (South Bay). The bathymetric surface difference results were also plotted in a histogram (Figure 18), where 43% of the difference values were between +/- 0.5 and 59% were between +/- 1.0 (out of ~6,200,000 compared points). Approximately 4.25% of the difference values were greater than +/- 10.0, and are likely a result of systematic change, performance and analytic variations, as previously mentioned. The analysis statistics are listed in Table 6. In a statistical sense, the mean and median values indicate that the 2008 surface is slightly deeper in elevation than the 2005 surface (mean value of and median value of ). The standard deviation was approximately 4.43, indicating a relatively wide spread in the bathymetric change values. 28

29 Figure 18. Histogram of bathymetric differences between the 2005 and 2008 surfaces at overlap areas. Table 6. Statistics from 2005 to 2008 surface comparison. Statistic Value Mean Median Standard Deviation 4.43 Minimum Maximum Comparison 2b 2008 DEM to 2013 DEM Comparison Subsequently, the 2008 and 2013 full DEM surfaces were compared in the same way to derive understanding of the general system changes between these respective years. Evaluating the North Bay bathymetric change first (Figure 19), the following general morphologic observations were made: Kearny Flats: o A majority of the flats comprised uncertain change (+/-0.5 ). 29

30 o Some small patches of elevation increases (more than 2.0 ) are visible around the perimeter of the flats. o Near the Hackensack River navigation channel, the morphologic pattern becomes more variable, with mixed elevation increases and decrease. Passaic and Hackensack River Mouths: o The bathymetric difference during this timeframe at the Passaic River mouth was variable. o Upstream from the Passaic River mouth, the bottom elevation largely increased (accumulated sediment up to, and exceeding, 2.0 ). o Along the western shoreline of the Passaic River, elevations decreased, an indication of erosion and/or anthropogenic removal of sediments. o At the Hackensack River mouth, a large decrease in elevation was exhibited (more than 2.0 in some locations). o Upstream from the Hackensack River mouth, the morphologic change was mostly uncertain and exhibited a pattern of elevation decreases during this time period. Northeast Sub-tidal Flats: o The sub-tidal flats in this region comprised mostly uncertain change during this time period. o Mixed patches of larger elevation increases and decreases (up to 1.0 bottom elevation gain/loss) were visible. I-78 Bridge Crossing: o In the immediate north and south vicinities of the interstate bridge crossing, the change appears mostly to be uncertain, with some small localized patches of elevation increases and decreases up to 1.0 or more. Eastern Sub-tidal Flats: o During this time period, the majority of the sun-tidal flats comprised uncertain change. o Further south on the flats, near the Port Elizabeth channel latitude, there was a pattern of increasing elevation visible. Port Newark and Elizabeth Channels: o The morphologic change observed in the Port channels during this timeframe is difficult to evaluate accurately due to the coarse spacing of the single-beam surveys being compared, and the likelihood that dredging and/or ship propeller scour has occurred in the channels during this time period. o The bathymetric difference indicates changes up to, and exceeding +/- 2.0, but the data are sparse within the channels, leading to high uncertainty about the accuracy of these difference values. o Port Elizabeth channel has likely been dredged during this time period. CDF and Channel Shoal: o In the CDF, sediment has accumulated in excess of 2.0, likely due to disposal of dredged material. o Around the perimeter of the CDF, on the shoal, the change is uncertain during this time period. o Along the perimeter of the shoal, within the navigable waterway, the bottom elevation has decreased in excess of 2.0. This is possibly a result of anthropogenic activity (e.g., dredging, ship traffic). Western Sub-tidal Flats: o The morphologic change observed here is mostly uncertain, with some patches of elevation increases in excess of

31 Northern Newark Bay Navigation Channel: o Within the navigation channel, the morphologic pattern during this time period is highly variable, with localized elevation increases and decreases up to, and exceeding 2.0 in certain locations. o As above, some of this variability may be due to the coarsely-spaced single-beam data and interpolation routines applied. o Other variability may be authentic and may be a sign of the dynamic variability within the navigation channel (e.g., due to the ambient hydrodynamics and/or vessel traffic). Evaluating the South Bay bathymetric change (Figure 20), the following patterns were identifiable for the period 2008 to 2013: Southeastern Sub-tidal Flats: o The morphologic pattern indicates mostly uncertain change and elevation increases during this time period; with sediment gains up to and exceeding 2.0 in some locations. Kill van Kull, Shooter s Island, and Arthur Kill: o Arthur Kill appears to have been dredged between these survey years. o Around Shooter s Island, the change remains varied, but uncertain, as a result of coarse data resolution in this area. o Within the Kill van Kull area surveyed, the morphologic change is mostly uncertain. Southwest Sub-tidal Flats: o The area comprised mostly uncertain morphologic change with several localized areas increasing in elevation (up to 1.0 or more). Southern Newark Bay Navigation Channel: o Many of the portions of the navigation channel that were not dredged prior to 2008 were dredged by o The two rectangular areas near the Port Elizabeth Terminals that were previously dredged were not dredged during this timeframe, and exhibited signs of both elevation increases and decreases. o The southern end of the Newark Bay navigation channel, near the intersection with the Kills, remained un-dredged, and of uncertain change. The results were plotted in a histogram (Figure 21) where 46% of the difference values were between +/- 0.5 and 62% were between +/- 1.0 (out of ~6,220,000 compared points). Approximately 5.05% of the difference results were greater than +/ The analysis statistics are listed in Table 7. Overall, the mean indicates that the 2013 surface is slightly deeper in elevation than the 2008 surface (mean value of 0.72 ); however, a median value of 0.09 indicates they are statistically at the same elevations (within sonar system error bounds). The overall standard deviation was

32 Elevation increased Mostly uncertain change Variable increasing/ decreasing pattern Decreased elevation Uncertain change Uncertain change / localized elevation increases Decreased elevation patches Localized elevation increases Variable localized change (Nav. Ch.) Increased elevation (CDF) Uncertain change Uncertain change Increased elevation pattern Figure 19. Bathymetric difference map comparing the 2008 surface to 2013 surface (North Bay). 32

33 General deepening (dredging) Uncertain change Localized elevation increases Increased elevation patterns Increased elevation (Nav. Ch. Toes) General deepening (dredging) Uncertain change Figure 20. Bathymetric difference map comparing the 2008 surface to 2013 surface (South Bay). 33

34 Figure 21. Histogram of bathymetric differences between the 2013 and 2008 surfaces at overlap areas. Table 7. Statistics from the 2008 to 2013 surface comparison. Statistic Value Mean Median 0.09 Standard Deviation 4.69 Minimum Maximum

35 Comparison 2c 2005 DEM to 2013 DEM Comparison To assess the net sediment bed changes occurring within the system between 2005 and 2013, the corresponding DEM surfaces were compared in the same manner as the previous examples. Evaluating the North Bay bathymetric change first (Figure 22), the following general morphologic observations were made: Kearny Flats: o A majority of the flats exhibited net uncertain change during this time period, with some localized patches of bottom elevation increases and decreases (up to, and exceeding 1.0 ). Passaic and Hackensack River Mouths: o The Passaic River mouth and lower reach increased in elevation in excess of 2.0 during this period. o Only the western shoreline along the Passaic River mouth decreased in elevation; it is unclear, though if the reason was natural erosion or due to anthropogenic activities. o The net morphologic pattern in the Hackensack River mouth and lower reach was variable during this period, varying between +/- 2.0, or more, in localized patches. Northeast Sub-tidal Flats: o The sub-tidal flats largely comprised uncertain change and variable morphologic patterns. o Near the Hackensack River, the change was more variable, with localized elevation increases and decreases. o Further south, toward the I-78 bridge crossing, the bottom elevation generally increased (up to, and exceeding 1.0 ). I-78 Bridge Crossing: o In the immediate north and south vicinities of the interstate bridge crossing, patterns of localized elevation increases up to 1.0 were prevalent during this time period. Eastern Sub-tidal Flats: o South of the I-78 bridge crossing was a localized area of decreased elevation during this time (up to and exceeding 2.0 ). o Further south, the morphologic change was mostly uncertain with some elevation increases (up to 1.0 ) toward the Port Elizabeth latitude. Port Newark and Elizabeth Channels: o The net morphologic change observed in the Port channels during this timeframe was, again, difficult to evaluate accurately due to the coarse spacing of the singlebeam survey being used in the comparison, and the likelihood that dredging and/or ship propeller scour has occurred in the channels during this time period. o The bathymetric difference indicates changes up to, and exceeding +/- 2.0, but the data are sparse within the channels, leading to high uncertainty about the accuracy of these difference values. CDF and Channel Shoal: o In the CDF, sediment accumulated in excess of 2.0, due to disposal of dredged material. o Around the perimeter of the CDF, on the shoal, the change is uncertain during this time period, with some areas decreasing in elevation overall (at the eastern reach of the shoal). 35

36 o Along the north and northwest perimeter of the shoal, within the navigable waterways, the bottom elevation increased in excess of 2.0. o Around the remaining shoal perimeter, the results were variable and localized. Western Sub-tidal Flats: o The morphologic change observed here is mostly uncertain and variable, with localized elevation increases and decreases observed. Northern Newark Bay Navigation Channel: o Within the navigation channel, the morphologic pattern during this time period was highly variable, with elevation increases and decreases up to, and exceeding 2.0 in different locations. o As above, some of this variability may be due to the coarsely-spaced single-beam data and interpolation routines applied. o Other variability may be authentic and may be a sign of the dynamic variability within the navigation channel (e.g., due to the ambient hydrodynamics and/or vessel traffic). Evaluating the South Bay bathymetric change (Figure 23), the following patterns were identifiable for the period 2005 to 2013: Southeastern Sub-tidal Flats: o The morphologic pattern indicates mostly uncertain change in the central axis of these sub-tidal flats. o There were patterns of net elevation increases nearer to the navigation channel and at the southern end of the flats (up to, and exceeding 2.0 ). o There were patterns of net elevation decreases nearer to the Bayonne, NJ, shoreline (up to, and exceeding 2.0 ). o Net elevation decreases at the shoreline may be authentic or they may be a consequence of coarse bathymetric spacing and interpolation algorithms. Kill van Kull, Shooter s Island, and Arthur Kill: o The reach from Kill van Kull through Arthur Kill has decreased in elevation during this time period as a result of channel deepening efforts (i.e., dredging). o Around Shooter s Island, the change was varied overall, with localized increases and decreases in elevation exceeding 2.0 in some areas. Southwest Sub-tidal Flats: o The area comprised mostly uncertain morphologic change with localized elevation increases and decreases (up to, and exceeding 1.0 ). Southern Newark Bay Navigation Channel: o Much of the navigation channel between Kill van Kull and the Port Elizabeth channel decreased in elevation due to channel deepening efforts. o At a small region near the intersection with the Kill van Kull, bottom elevation changes remained relatively uncertain. Rock blasting and removal activities were underway at this area at the time of the 2013 survey, as a part of the channel deepening efforts. o One exception is the net elevation increase (exceeding 2.0 ) that occurred along the eastern toe of the navigation channel at the intersection with Kill van Kull. From the bathymetric difference histogram (Figure 24), 42% of the difference values were between +/- 0.5 and 55% were between +/- 1.0 (out of ~7,170,000 compared points). Approximately 36

37 5.35% of the difference results were greater than +/ The analysis statistics are listed in Table 8. Overall, the mean and median indicate that the 2013 surface is slightly deeper in elevation than the 2005 surface (mean value of 1.29 and median value of ). The overall standard deviation was Uncertain change Increased elevation Variable localized change Increased elevation Increased elevation (toes) Uncertain change Uncertain change Increased elevation patterns Increased elevation (CDF) Decreased elevation pattern Uncertain change Uncertain change Figure 22. Bathymetric difference map comparing the 2005 surface to 2013 surface (North Bay). 37

38 General deepening (dredging) Uncertain Change Uncertain change Increased elevation (offshore) Decreased elevation (shoreline) Increased Elevation Uncertain Change General deepening (dredging) Figure 23. Bathymetric difference map comparing the 2005 surface to 2013 surface (South Bay). 38

39 Figure 24. Histogram of bathymetric differences between the 2005 and 2013 surfaces at overlap areas. Table 8. Statistics from 2005 to 2013 surface comparison. Statistic Value Mean Median Standard Deviation 4.82 Minimum Maximum

40 Focused Surface to Surface Comparisons (-12 NGVD29 and Above) Deriving quantitative conclusions about general morphology is difficult when comparing surface to surface data, or point data, over large regions such as over the entire Newark Bay. The reason is that between surveys, anthropogenic activities, such as dredging, may cause large elevation changes in the data that may artificially bias the statistical analysis. In the previous sections, the full DEM bathymetric difference surfaces were evaluated as a baseline assessment. However, because of the large elevation changes observed at some locations (e.g., in the navigation channels) accurate quantification of morphologic change rates is not possible on a site-wide basis. To quantify the natural sediment erosion and deposition patterns, areas that have been potentially subjected to anthropogenic change between surveys must be omitted from the analysis, and a more focused approach implemented. As one example of a focused assessment of the natural morphologic change occurring in the Bay, general morphology patterns on only the sub-tidal flats were investigated since anthropogenic impacts are likely smaller in these regions than in the navigation channels. To better evaluate the changes that have taken place on the sub-tidal flats between the 2005, 2008, and 2013 surveys, only data from bottom elevations greater (shallower) than -12 NGVD29 were incorporated from each survey and differenced. The -12 NGVD29 elevation was chosen as a representative elevation that exists near the tops of the navigation channel slopes and encompasses regions that appear to have experienced little to no anthropogenic influence between surveys. Any similar elevation could have been used. The same comparisons were made between surveys as in the sections above: for the years 2005 to 2008, 2008 to 2013, and 2005 to The site-wide maps for each are shown for reference in Figure 25 to Figure 27. The legends are the same as in the previous section, but all areas at a lower elevation than -12 NGVD29 have been removed for clarity. Histograms of the bathymetric differences were created and are shown in Figure 28 to Figure 30. Histogram bin sizes are smaller than previously shown (0.25 ) to show additional fine-scale bathymetric change detail. The black dashed lines indicate zero change, and provide a visual indication of histogram skewness. All three comparisons are centered on a value of 0 nearly symmetrically, and have fewer extreme values (+/-5.0 ) than the full surface comparisons from above. The percentage of bathymetric difference values within specific elevation difference ranges are shown in Table 9: at least 66% are within +/- 0.5 ; and at least 85% of the differences are within +/ Table 10 lists statistics computed from all three difference comparisons. When viewed in time from 2005 to 2008 to 2013, the mean and median statistical values change from negative (blue coloring in Table 10) to slightly positive (red coloring in Table 10). Overall, between 2005 and 2013 the mean and median values in the same regions are nearly zero (gray coloring in Table 10), indicating, statistically, no net sediment accumulation or erosion in these areas. In a practical sense, the regions of Newark Bay at elevation -12 NGVD29 and higher seem to be largely unchanging, temporally. The mean values of all comparisons are within +/-0.3, which is potentially within sonar system error bounds and potentially subjected to uncertainties introduced by interpolation algorithms. The median values are all within +/- 0.2 ; standard deviations are all less than 1.0. Therefore, this analysis concludes that the areas of Newark Bay at higher elevations than -12 NGVD29 are unchanging, or are changing at a scale that is indeterminable by present-day standard acoustic sensing technologies. 40

41 Figure 25. Bathymetric difference map comparing the 2005 surface to 2008 surface at elevations higher than -12 NGVD29 (Full Bay). 41

42 Figure 26. Bathymetric difference map comparing the 2008 surface to 2013 surface at elevations higher than -12 NGVD29 (Full Bay). 42

43 Figure 27. Bathymetric difference map comparing the 2005 surface to 2013 surface at elevations higher than -12 NGVD29 (Full Bay). 43