Appendix_C_Bathymetric_Survey_Report.pdf

Wheatley Reservoir ASI Summary of Findings Prepared for Langan 1/14/2025

Summary of Findings – Bathymetry and Sub- Bottom Profiling Survey

Wheatley Reservoir, Charles County, Maryland

Overview of Survey Area

Prepared for Langan

January 14, 2025

Survey Overview

A geophysical remote sensing survey was conducted covering the specified project areas of the Wheatley Reservoir in Charles County, Maryland. The intent of the survey was to map the bathymetry and determine sediment thickness.

The following surveys were completed:

1. Dual Frequency Single Beam Bathymetry (Dec 9-10, 2024)

2. Sub-Bottom Profiling (Dec 11, 2024)

The survey was completed using real-time kinematic (RTK) global positioning to provide centimeter location precision.

Survey Positioning and Control

Survey Positioning

Vessel positioning was provided by a Trimble SPS-855 satellite compass global navigation satellite system (GNSS) RTK system. RTK corrections were supplied through KeyNET real-time network

(RTN) networked transport of RTCM via internet protocol (NTRIP) service. This service provides the RTK corrections through an internet connection to the primary computer and GNSS receiver.

Pre and post survey, the RTK system was checked against monument HV9730-SMC 121, The antenna offset of the Trimble SPS-855 GPS satellite compass/RTK GNSS unit was adjusted as needed to ensure agreement with the control point (See Appendix A) and a local project control point was set and occupied daily.

Survey Datum

All survey control, positioning, and data products related to the survey were referenced to:

Horizontal Datum

• Datum: North American Datum of 1983 (NAD 83)

• Projection: Maryland State Plane

• Units: US Survey Feet

Vertical Datum

• Datum: North American Vertical Datum of 1988 (NAVD 88)

• Units: US Survey Feet

Survey Methods

General Methodology

The systems were checked and vertically calibrated using both a local benchmark and a local project control point. Dual-frequency single beam sonar was used for its sounding accuracy in shallow areas and the ability to see through soft sediments to a hard bottom with a separate low frequency reading. Sub-bottom sonar was later used for its ability to penetrate soft sediments for sub-surface material differentiation.

Single Beam Bathymetry Survey

Equipment

A TELEDYNE Odom Echotrac CVM Dual Frequency Echosounder paired with a Trimble SPS-855

GPS were used. The systems consist of the following equipment:

• Odom transducer on a pole mount

• Odom topside control unit

• Trimble SPS-855 GNSS units with integrated RTK

• Notebook computer running Hypack 2024

Sensor Positioning

The sensor was mounted on the bow of the vessel with the sensor fixed approximately one foot under the water. The RTK antenna was mounted on a pole directly over the transducer to avoid lateral offsets. The vertical offset and sensor draft was accounted for in recording and post processing.

Field Survey Methods

Survey speeds were between 2-3 knots. For the single beam system, lead line soundings were taken to confirm transducer mounting and system calibration and accuracy. The sound velocity in water was recorded by a SonTek CastAway-CTD. The reference measurement of RTK antenna to transducer is fixed at all times ensuring adherence to NAVD 88 elevations. Survey lines were collected perpendicular to the shore at a nominal 50-foot spacing.

Data Processing

All raw bathymetric data were logged and processed in Hypack 2024. Water level data and heave correction were applied to the raw data based on the RTK vertical measurements. Data were reviewed for any potential issues, outliers, or data drop-outs, with erroneous data points removed. To maintain data quality, the all sounding profiles are manually reviewed. The final data was exported as an ASCII XYZ file, DXF bathymetric contours (Figure 1) PDF map sheets were created for soundings and contours as NAVD 88 elevation, and plots were made with depths from the observed lake level at the time of the survey.

Challenges

Park Management shared with our team that the water level at the time of survey was about two feet shallower than normal, as recent conditions had been very dry. The north end and lagoons were quite shallow and the boat was pushed as far into the shorelines as the water would allow.

Dual-frequency sonar results at first appeared less than optimal. Due to the high degree of sedimentation on the north end of the reservoir (and to the southwest) it was anticipated that a high degree of sedimentation would be identified even in the deeper portion of the south. But there was not much differentiation between the high and low frequencies. The low frequency channel of a dual frequency sonar is used so as to penetrate into sediments. However, some bottom types can severely attenuate the transmitted pulse. This is often related to the material type and thickness. The pulse will only go so far before attenuating or dispersing without returning defined sediment layers. It was suspected that the Wheatley sediment may have been of such composition leading to inconclusive results.

ASI determined to a return to the site was necessary to deploy a sensor specifically designed to optimize the gathering of data about sediment layers . The sub-bottom profiler functions at a much lower frequency than the lower channel of the dual-frequency transducer, and can put a great deal more energy into the sediment. Ground-truthing with a manual sediment probe to collect sediment thickness information would also be beneficial, especially in shallow areas where the sub-bottom profiler can’t achieve quality data. (The sub-bottom profiler requires a certain amount of depth to function. The low-water conditions of the reservoir limited coverage to the far north of the reservoir.)

Sub-Bottom Profiling Survey Equipment

A SyQwest Stratabox sub-bottom profiler was used to collect and record the sub-bottom data during the survey. The system consists of the following equipment:

• Low frequency transducer

• Stratabox topside control unit

• Trimble SPS-855 GNSS unit with integrated RTK

• Notebook computer running Hypack 2024 for navigation

Sensor Positioning

The sub-bottom profiler transducer was mounted on the bow of the survey vessel, with the GPS antenna mounted directly above the sub-bottom unit. The sensor was deployed at a depth of approximately 1.5 feet to minimize interference with the vessel and surface water. The navigational data was logged at 5Hz intervals by the Stratabox digital recording system and electronically paired with the sub-bottom data for the geo-referencing of all data collected.

Field Survey Methods

Transect lines were run on approximately 50 ft station intervals parallel to shore. Additional lines were run for coverage and if an area of note was seen. A sediment probe was deployed in different regions as a preliminary ground-truthing method.

Data Processing

The sub-bottom data was processed in Chesapeake Technology’s SonarWiz 7. Sub-bottom profilers such as the Stratabox use acoustic methods to generate high-resolution (on the order of 0.5-1 ft) cross-sectional images of the marine sub-bottom to depths of up to 100 feet beneath the seafloor in optimal conditions. The transmitted sound pulses travel through the water column and sediment layers and are reflected when changes in acoustic impedance (equivalent to a material’s sonic velocity times its density) are encountered. Reflector layers, indicating changes in sediment layer type or thickness were identified. These layers appeared relatively thin in the larger, lower portion of the reservoir than anticipated given the heavier sedimentation to the north.

Challenges

Acoustic impedance changes commonly occur at boundaries between materials (e.g., interfaces between water and sediments, sediments and gas, different types of sediments, and sediments and buried objects). The reflected sound pulses travel back to the profiler where their amplitudes, as a function of travel-time, are digitally recorded. Because they do generate secondary reflections in the data it can be difficult to record useful data in shallow depths.

The sub-bottom unit has difficulty producing good results in less than two meters of water. We bested the published limits in many areas, though useful information becomes more difficult to discern in processing. A few feet more water would have made a difference in the lake’s shallow areas. To the north the sediment layer clearly gets deeper, as the water gets shallower to the point of being right on the edge of the sensors ability to operate.

Combined Findings

In the single beam bathymetry, the project area was covered to project specifications and the bathymetry of the lake to the extents the instrument could reach was mapped. Although the dual-frequency sonar did not function as well as expected, both the supplementary manual probing and sub-bottom profiler yielded further interesting results in regards to determining sediment volume.

The manual sediment probe is a small diameter pipe with a wide collar on it. As the pipe is pushed through the material to a point of refusal, the bottom of the collar slides to remain on the initial surface and remains in place so the depth of sediment can be measured once retrieved. In the far north it went quite deep, but for most of the reservoir the probe went in a foot or two and stopped. Several locations had a hard or crunchy feel to one or more layers and the sediment was perhaps not as deep as expected. This manual probing method appeared to corroborate the results of the sub-bottom sonar.

There was a lot of sediment in shallower regions such as the two eastern lagoons and the north of the lake. Not much sediment was found near the center and southern sections of the lake approaching the spillway. Park management informed our team that there are occasions when there is a great deal of water flow through the reservoir. It may be that the dual frequency and sub-bottom profiler’s inconclusive findings to the south were because there wasn’t much unconsolidated sediment but a very thin layer.

For less ambiguous results, the only definitive way we have of characterizing the sediment layers is to perform core sampling. Perhaps the sediment is as thin a layer as it appears over much of the reservoir. How consolidated the sediment is can only be determined by ASI through coring.

However, there are trees growing to the southwest in what was reported as open water years ago and it is known that the north end is filling. Sediment core sampling would yield definitive physical data about the nature of the reservoir’s sedimentation.

Of note, the team found interesting that an elevation as recorded in Federal digital terrain data outside the spillway’s outlet on the opposite side of the dam, was similar to the deepest portion of the reservoir. The soft sediments expected at the bottom of the reservoir could be flowing out with the water through the spillway. The engineering team may want to examine if soft sediments are draining from the southern portion of the reservoir. This is speculative in nature and should be field verified.

Figures

Figure 1. Height-Shaded Bathymetry Image

Figure 2. Sub-Bottom sonar return with sediment layers (Red)

Appendix A

Survey Benchmark Control Point Information

Wheatley Reservoir, Charles County, MD

HV0674 NGS Data Sheet

PROGRAM = datasheet95, VERSION = 8.12.5.19

Starting Datasheet Retrieval...

1 National Geodetic Survey, Retrieval Date = OCTOBER 7, 2024 14:40:51 EDT

HV0674 ***********************************************************************

HV0674 DESIGNATION - R 40

HV0674 PID - HV0674

HV0674 STATE/COUNTY- MD/ST MARYS

HV0674 COUNTRY - US

HV0674 USGS QUAD - CHARLOTTE HALL (2016)

HV0674

HV0674 *CURRENT SURVEY CONTROL

HV0674 ______________________________________________________________________

HV0674* NAD 83(2011) POSITION- 38 27 30.60487(N) 076 45 38.99670(W) ADJUSTED

HV0674* NAD 83(2011) ELLIP HT- 16.760 (meters) (06/27/12) ADJUSTED

HV0674* NAD 83(2011) EPOCH - 2010.00

HV0674* NAVD 88 ORTHO HEIGHT - 50.354 (meters) 165.20 (feet) ADJUSTED

HV0674 ______________________________________________________________________

HV0674 GEOID HEIGHT - -33.614 (meters) GEOID18

HV0674 NAD 83(2011) X - 1,145,299.088 (meters) COMP

HV0674 NAD 83(2011) Y - -4,868,036.473 (meters) COMP

HV0674 NAD 83(2011) Z - 3,945,433.632 (meters) COMP

HV0674 LAPLACE CORR - -2.64 (seconds) DEFLEC18

HV0674 DYNAMIC HEIGHT - 50.323 (meters) 165.10 (feet) COMP

HV0674 MODELED GRAVITY - 980,016.6 (mgal) NAVD 88

HV0674

HV0674 VERT ORDER - FIRST CLASS II

HV0674

HV0674 Network accuracy estimates per FGDC Geospatial Positioning Accuracy

HV0674 Standards:

HV0674 FGDC (95% conf, cm) Standard deviation (cm) CorrNE

HV0674 Horiz Ellip SD_N SD_E SD_h (unitless)

HV0674 -------------------------------------------------------------------

HV0674 NETWORK 2.00 4.43 0.94 0.62 2.26 -0.14316605

HV0674 -------------------------------------------------------------------

HV0674 Click here for local accuracies and other accuracy information.

HV0674

HV0674.The horizontal coordinates were established by GPS observations

HV0674.and adjusted by the National Geodetic Survey in June 2012.

HV0674

HV0674.NAD 83(2011) refers to NAD 83 coordinates where the reference frame has

HV0674.been affixed to the stable North American tectonic plate. See

HV0674.NA2011 for more information.

HV0674

HV0674.The horizontal coordinates are valid at the epoch date displayed above

HV0674.which is a decimal equivalence of Year/Month/Day.

HV0674

HV0674.The orthometric height was determined by differential leveling and

HV0674.adjusted by the NATIONAL GEODETIC SURVEY

HV0674.in June 1991.

HV0674

HV0674.Significant digits in the geoid height do not necessarily reflect accuracy.

HV0674.GEOID18 height accuracy estimate available here.

HV0674

HV0674.Click photographs - Photos may exist for this station.

HV0674

HV0674.The X, Y, and Z were computed from the position and the ellipsoidal ht.

HV0674

HV0674.The Laplace correction was computed from DEFLEC18 derived deflections.

HV0674.The ellipsoidal height was determined by GPS observations

HV0674.and is referenced to NAD 83.

HV0674

HV0674.The dynamic height is computed by dividing the NAVD 88

HV0674.geopotential number by the normal gravity value computed on the

HV0674.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45

HV0674.degrees latitude (g = 980.6199 gals.).

HV0674

HV0674.The modeled gravity was interpolated from observed gravity values.

HV0674

HV0674. The following values were computed from the NAD 83(2011) position.

HV0674

HV0674; North East Units Scale Factor Converg.

HV0674;SPC MD - 87,924.210 420,874.694 MT 0.99997621 +0 09 00.4

HV0674;SPC MD - 288,464.68 1,380,819.73 sFT 0.99997621 +0 09 00.4

HV0674;UTM 18 - 4,258,157.224 346,365.571 MT 0.99989068 -1 05 43.3

HV0674

HV0674! - Elev Factor x Scale Factor = Combined Factor

HV0674!SPC MD - 0.99999737 x 0.99997621 = 0.99997358

HV0674!UTM 18 - 0.99999737 x 0.99989068 = 0.99988805

HV0674

HV0674_U.S. NATIONAL GRID SPATIAL ADDRESS: 18SUH4636558157(NAD 83)

HV0674

HV0674 SUPERSEDED SURVEY CONTROL

HV0674

HV0674 NAD 83(2007)- 38 27 30.60572(N) 076 45 38.99625(W) AD(2002.00) 0

HV0674 ELLIP H (02/10/07) 16.769 (m) GP(2002.00)

HV0674 ELLIP H (08/30/02) 16.770 (m) GP( ) 4 2

HV0674 NAD 83(1991)- 38 27 30.60515(N) 076 45 38.99616(W) AD( ) 1

HV0674 ELLIP H (05/28/98) 16.846 (m) GP( ) 3 1

HV0674 NAD 83(1991)- 38 27 30.60509(N) 076 45 38.99597(W) AD( ) 1

HV0674 ELLIP H (11/16/93) 16.831 (m) GP( ) 3 1

HV0674 NAVD 88 50.35 (m) 165.2 (f) LEVELING 3

HV0674 NGVD 29 (11/26/84) 50.620 (m) 166.08 (f) ADJUSTED 1 2

HV0674

HV0674.Superseded values are not recommended for survey control.

HV0674

HV0674.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.

HV0674.See file dsdata.pdf to determine how the superseded data were derived.

HV0674

HV0674_MARKER: DB = BENCH MARK DISK

HV0674_SETTING: 32 = SET IN A RETAINING WALL OR CONCRETE LEDGE

HV0674_SP_SET: HEADWALL

HV0674_STAMPING: R 40 1941

HV0674_MARK LOGO: CGS

HV0674_MAGNETIC: O = OTHER; SEE DESCRIPTION

HV0674_STABILITY: C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO

HV0674+STABILITY: SURFACE MOTION

HV0674_SATELLITE: THE SITE LOCATION WAS REPORTED AS SUITABLE FOR

HV0674+SATELLITE: SATELLITE OBSERVATIONS - August 08, 2018

HV0674

HV0674 HISTORY - Date Condition Report By

HV0674 HISTORY - 1941 MONUMENTED CGS

HV0674 HISTORY - 1942 GOOD CGS

HV0674 HISTORY - 19921117 GOOD MCCRON

HV0674 HISTORY - 19970709 GOOD GPI

HV0674 HISTORY - 20000824 POOR GEOMET

HV0674 HISTORY - 20020915 GOOD WR

HV0674 HISTORY - 20050311 GOOD NGS

HV0674 HISTORY - 2017 GOOD NGS

HV0674 HISTORY - 20180808 GOOD NGS

HV0674 STATION DESCRIPTION

HV0674'DESCRIBED BY COAST AND GEODETIC SURVEY 1941

HV0674'2.1 MI NW FROM MECHANICSVILLE.

HV0674'ABOUT 2.1 MILES NORTHWEST ALONG STATE HIGHWAY 5 FROM THE POST

HV0674'OFFICE AT MECHANICSVILLE, SAINT MARYS COUNTY, AT THE INSIDE OF

HV0674'A SOUTH-TO-NORTHWEST CURVE, ABOUT 500 FEET SOUTHEAST OF A LARGE

HV0674'2-STORY FARMHOUSE WEST OF THE HIGHWAY, 99 FEET SOUTH OF A

HV0674'3-WAY CORNER FENCE POST, 4 FEET EAST OF THE RIGHT-OF-WAY FENCE,

HV0674'AT A 12-INCH CONCRETE CULVERT, IN THE TOP OF THE NORTH CORNER OF

HV0674'THE WEST HEAD WALL, AND 16 FEET WEST OF THE CENTER LINE OF THE

HV0674'HIGHWAY. A STANDARD DISK, STAMPED R 40 1941. NOTE-- 1.6 MI NW

HV0674'ALONG HIGHWAY 5 FROM THE POST OFFICE AT MECHANICSVILLE, INSTEAD

HV0674'OF 2.1 MI. NOTE-- 288 FT SE OF A LARGE 2 STORY FARM HOUSE W OF

HV0674'THE HIGHWAY, INSTEAD OF 500 FT. NOTE-- 93 FT S OF A 3 WAY

HV0674'CORNER FENCE POST, INSTEAD OF 99 FT. NOTE-- 15.2 FT W OF THE

HV0674'CENTERLINE OF THE HIGHWAY, INSTEAD OF 16 FT.

HV0674

HV0674 STATION RECOVERY (1942)

HV0674

HV0674'RECOVERY NOTE BY COAST AND GEODETIC SURVEY 1942

HV0674'RECOVERED IN GOOD CONDITION.

HV0674

HV0674 STATION RECOVERY (1992)

HV0674

HV0674'RECOVERY NOTE BY J R MCCRONE JR INCORPORATED 1992

HV0674'RECOVERED IN GOOD CONDITION.

HV0674

HV0674 STATION RECOVERY (1997)

HV0674

HV0674'RECOVERY NOTE BY GREENMAN PEDERSEN INCORPORATED 1997 (JC)

HV0674'RECOVERED AS DESCRIBED.

HV0674

HV0674 STATION RECOVERY (2000)

HV0674

HV0674'RECOVERY NOTE BY GEOMETRICS GPS INCORPORATED 2000 (TL)

HV0674'ROAD WIDENED, MARK POSSIBLY DESTROYED.

HV0674

HV0674 STATION RECOVERY (2002)

HV0674

HV0674'RECOVERY NOTE BY WHITMAN, REQUARDT & ASSOCIATES 2002 (MAB)

HV0674'IT SHOULD BE NOTED THAT THE MILEAGE WORKS OUT WITH THE INTERSECTION OF

HV0674'MECHANISVILLE RD AND OLD RTE. 5, NOW CALLED OLD VILLAGE RD. THE POST

HV0674'OFFICE DESCRIBED IS NOW AN ANTIQUE STORE. MARK IS SUITABLE FOR GPS AT

HV0674'THIS DATE HOWEVER A LARGE TREE IS STARTING TO OVERHANG THE MARK.

HV0674

HV0674 STATION RECOVERY (2005)

HV0674

HV0674'RECOVERY NOTE BY NATIONAL GEODETIC SURVEY 2005 (CB)

HV0674'RECOVERED IN GOOD CONDITION.

HV0674

HV0674 STATION RECOVERY (2017)

HV0674

HV0674'RECOVERY NOTE BY NATIONAL GEODETIC SURVEY 2017 (LHB)

HV0674'RECOVERED AS DESCRIBED. THE MARK IS ON A ROAD WITH NO OUTLET.

HV0674

HV0674 STATION RECOVERY (2018)

HV0674

HV0674'RECOVERY NOTE BY NATIONAL GEODETIC SURVEY 2018 (RWA)

HV0674'RECOVERED IN GOOD CONDITION.

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Photos from Shared OPUS Solution Website

HV9730-SMC 121