University of Nebraska - LincolnDigitalCommons@University of Nebraska - Lincoln

US Army Research U.S. Department of Defense

1992

An Intrusive Fluid Mud Surveying SystemAllen TeeterU. S. Army Engineer Waterways Experiment Station Hydraulics Laboratory

Glynn BanksU. S. Army Engineer Waterways Experiment Station Hydraulics Laboratory

Michael AlexanderU. S. Army Engineer Waterways Experiment Station Hydraulics Laboratory

Andrew SalkieldSIRAD Inc.

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Teeter, Allen; Banks, Glynn; Alexander, Michael; and Salkield, Andrew, "An Intrusive Fluid Mud Surveying System" (1992). US ArmyResearch. 94.http://digitalcommons.unl.edu/usarmyresearch/94

AN INTRUSIVE FLUID MUD SURVEYING SYSTEM

Allen Teeter1 Member, ASCE, Glynn Banks2, Michael Alexander3 and Andrew Salkield4

Abstract

A towable sled was developed at the Hydraulics Laboratory of the waterways Experiment station in con­junction with SIRAD, Inc. S to survey navigation channels with "fluff" or fluid mUd. Fluid mud layers on the bottom of navigation channels can result in a false bottom record when surveyed with conventional acoustic fathometers. The WES gage was designed to tow along channel bottoms at a specific gravity of about 1.15-1.20 glcc and coincide with nautical depth. To monitor this towed density value, a nuclear density gage was installed in the original prototype. Field trials have been completed at the Gulfport, MS, Calcasieu, LA, and Sabine Pass, TX navigation channels. Data are presented that show the sled tracks a fairly constant densi ty level at a given site, and that its depth is situated between the high and low frequency acoustic depths in channels with a significant layer of fluid mud.

Introduction

Defining firm bottom in navigation channels where

lResearch Oceanographer, 2Research Hydraulic Engineer, 3Research Hydraulic Engineer, U. S. Army Engineer waterways Experiment station Hydraulics Laboratory, 3909 Halls Ferry Rd., Vicksburg, MS 39180-6199. 4pres ident, SIRAD INC. 2353 N.W. Glisan ST., Portland, OR 97210. SSediment Instrumentation Research and Development, Incorporated.

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Published in Hydraulic Engineering: Saving a Threatened Resource—In Search of Solutions: Proceedings of the Hydraulic Engineering sessions at Water Forum ’92. Baltimore, Maryland, August 2–6, 1992. Published by American Society of Civil Engineers.

FLUID MUD SURVEYING 1013

fine-grained cohesive sediments form fluid mud layers can be difficult and generally inaccurate using acoustic sounding methods. Where fluid mud exists, a bottom record collected with a typical high frequency fathome­ter might show a depth shallower than the draft of ves­sels using the channel. This can only occur in the presence of a fluid mud layer, all or part of which can be considered navigable. Lower frequencies can be used in an attempt to identify a deeper bed level, but that level is still not necessarily connected with navigabil­ity. A lead line can be used to verify firm bottom with acoustic surveys, but this method is slow and impracti­cal for routine surveys over many kilometers of channel.

The Dredging Research Program of the U. S. Army Corps of Engineers is addressing material properties. The WES sled was developed under that program as a "towable lead line" to rapidly determine nautical depth in channels with fluid mUd. The sled was designed to follow a depth at which sediment material has an appre­ciable strength and resistance, anticipated to be at densi ties between about 1. 15 and 1.20 gl cc. The gage (Figure 1) was designed with a volume versus mass that results in a specific grav­ity of about 1. 3 glcc with adjustable ballast. Pre­viously devel­oped density survey methods in Europe had adopted a 1. 15 to 1.2 glcc density layer as nautical ( K i r by, Parker, and van Oostrum 1980; and De VI ieger and De Cloedt, 1990). To monitor the densi ty plane that the sled

Figure 1. Nautical Depth Gage

tows along, a nuclear transmission density record is collected from the prototype sled. In order to evaluate the sled survey profiles against conventional survey techniques, a high and low frequency acoustic fathometer is operated simultaneously with the sled survey. Also, field trials include sediment sampling along selected sled survey profiles. These samples allow a direct

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evaluation of rheological, density, and grain size prop­erties above and below the sled profile plane.

The Sled Survey System

The prototype sled is operated from a 47-foot sur­vey vessel using a hydroelectric SeaMac winch (Fig­ure 2). Information collected with the sled includes depth, den-sity, sled angle, bridle angle, and cable tension. This informa­tion is trans­mi tted through the tow cable and logged in real-time on a Hew let t e Packard 9000 series com-puter. Posi-tioning data are collected u sin g a Mot 0 r 0 I a Falcon IV Mini-Ranger in conjunction with an IBM Figure 2. Sled Winching System computer and the Small Boat Survey System software (McCleave and Hart, 1989). Following a survey, data files are con­verted and processed on the IBM computer. Both acoustic depth values (200 kHz and 24 kHz) are output from the Odom Echotrac fathometer to the HP computer. Acoustic depths are also a part of the data files converted and processed on the IBM computer.

Field Operation

An obvious potential problem with the sled system would be jamming the sled on underwater obstructions or debris. After multiple surveys at 3 field sites, this has not been a problem. On one occasion the sled was stuck in a channel bank during a survey cross-section. The overload protective slip-clutch on the winch acti­vated automatically, and the sled was freed as the sur­vey boat pilot reversed the towing direction. No damage occurred to the system.

To date, two surveys each have been performed at

FLUID MUD SURVEYING 1015

the Gul fport , MS , and Calcas ieu River, LA, entrance channels. The most recent field trial took place at Sabine Pass, TX. These surveys constitute a total of over 50 kilometers on the various channel bottoms, less initial trials and tests that were not stored. The sled design and construction has proved seaworthy, and field trials have demonstrated no need for modifications.

Data Analysis

Recent field trials at Calcasieu Pass, Louisiana, demonstrated the problems associated with conventional pre- and post-dredging surveys in fluid mud channels. A series of profile lines were established over an 1,800 meter-long stretch of the Calcasieu Bar Channel containing fluid mud. This profile grid was surveyed in June of 1991, prior to scheduled dredging, and again in late November, 1991, immediately following dredging.

A sample of centerline profile data is shown in Figure 3. Both pre- and post-dredging 24 kHz frequency

5

200 kHz Pre-dredging Profile

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E .si Q. 15 G)

C

20

o

24 kHz Pre-dredging Profile

24 kHz Post-dredging Profile

450 900

Profile Length, meters

1350 1800

Figure 3. Acoustic Profiles Compared to Channel Grade

fathometer depth signals penetrated the fluid mud layers to -45 ft (-13.7 m) mean low gulf (mIg), well below the dredging prism. The pre-dredging 200 kHz frequency sur­vey was about -36 ft (-11 m) and 2-4 ft (approximately 1 m) shallower than the sled profile survey. The post­dredging high frequency surveys were also 2-4 ft (approximately 1 m) shallower than the sled profiles. Figure 3 shows that even the post-dredged high frequency profiles do not show that the channel depth of -42 ft (-12.8 m) mIg was achieved. The acoustic data set shown

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here reveals that low frequencies may not indicate necessary maintenance material and that high frequencies cannot always determine whether sufficient maintenance material has been removed, nor can they estimate reason­able maintenance volumes in fluid mud channels.

Maintenance along the profiled section of Calcasieu Channel includes the provision for 1-2 ft (0.3-0.6 m) of advance maintenance using a rental-type dredging contract. Figure 4 shows the sled survey along the same profile line presented in Figure 3. Based on the sled profile data, the channel was deepened from -38 ft (11.6 m) mIg to -43 ft (13.1 m) mIg. This depth information was collected along a consistent density plane of 1.18 1.20 glcc for both pre- and post­dredging sled surveys.

5

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.s£ Q. G) 15 Q

20

Pre-dredging Sled Profile

Post-dredging Sled Profile

o 450 900 1350 1800

Profile Length, meters

Figure 4. Pre- and Post-dredging Sled Survey Profiles

Ship Effects

During the pre-dredging survey in June of 1991 a ship passed through the survey grid approximately along the channel centerline. The ship captain reported his draft to be -37 ft (-11.3 m). Less than 1/2 hour after this vessel passed, the centerline was surveyed with the sled system, and the high frequency acoustic signal returned a "split" 200 kHz trace. The ship's passage had disturbed the light material on the channel bottom forming a fluidized layer of material approximately 2 ft (0.6 m) thick. Later surveys showed that it had re­consolidated to within a few inches (7-8 em) about 6 hours later. The high frequency profiles outside of the passing vessel's path were -36 ft (-11 m) mIg. The

FLUID MUD SURVEYING 1017

ship effects had "fluffed" the upper layers of fluid mud to -35 ft (-10.7 m) mIg. The vessel passed through the survey reach near high tide, which afforded an addi­tional 2.5 ft (0.8 m) of clearance at the time, but this example indicates the behavior of fluid muds and the need for continued research in the area of navigability.

Conclusions

An intrusive survey system has been developed and tested in areas of fluid mud. Experience has shown that the sled is a practical and comprehensive solution to the problem of fluid mud channel surveying. The only reasonable pre- and post-dredging profiles among the sample surveys were obtained with the sled.

Relating the sled depth to physical properties of the material at the test sites is in progress. Indica­tions are that density alone is not a complete indicator of nautical depth. As expected, site-specific fluid mud properties such as shear strength, combined with ship's effects and material density, all contribute to fluid mud navigability. Development of the towable lead line nautical depth system will continue as these contribut­ing parameters are evaluated.

Acknowledgements

The tests described and the resulting data presented herein, unless otherwise noted, were obtained from research conducted under the Dredging Research Program of the united states Army Corps of Engineers by the waterways Experiment station. citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. Permission was granted by the Chief of Engineers to publish this information.

References

De Vlieger, H. and De Cloedt, J. "Navitracker: A Giant step Forward Economics of Maintenance Dredging," No. 35, pp 2-18.

1987 (Dec) • in Tactics and Terra et Aqua.

Kirby, R., Parker, W.R., and van Oostrum, W.H.A. 1980. "Definition of the Seabed in Navigation Routes Through Mud Areas," International Hydrographic Review. Monaco, LVII (1), January 1980.

McCleave, B.W., and Hart, E.D. 1989. "Small-Boat Sur­vey Systems," Technical Report HL-89-21, U.S. Army waterways Experiment station, Vicksburg, MS.