DRP-4-02
June 1990
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Application Criteria for the Automated
Real-Time Tidal Elevation System (ARTTES)
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This technical note describes application criteria for the Automated Real-
lime lidal Elevation System (AK1 l kb; ibcu xur uusnutc vcniuu water xevei
control during survey and dredging operations. The criteria are formulated to
permit those involved in offshore dredging operations to determine whether an
ARTTES system may be of potential aid in planned or ongoing dredging
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Background
ARTTES systems are presently being operated in support of two large-
scale channel deepening projects, one by the Jacksonville District at Saint Marys
River entrance, the other by the Charleston District at the entrance channel to
Charleston Harbor. Because the systems are site specific, questions have arisen
in the field as to where and under what economic conditions these systems
might be suitable for other dredging projects. While each dredging project
ultimately must be assessed on an individual basis, some general criteria can be
used to eliminate projects for which the systems are ill suited or unnecessary.
The criteria discussed herein are minimal in the sense that meeting ail of them
indicates only that a project is a good candidate, not necessarily that a system is
feasible or required.
Additional Information
Contact the author, Mr. Andrew W. Garcia, (601) 634-3555, or the
manager of the Dredging Research Program, Mr. E. Clark McNair,
(601) 634-2070.
US Army Engineer Waterways Experiment Station
3909 Halls Ferry Road, Vicksburg, MS 39180-6199
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Introduction
A long-standing problem associated With tmei uuaituic
operations has been accounting for tidal effects in hydrographic surveys and m
the dredging process itself to assure that design channel depth has been
reached and to aid determination of the quantity of material moved in order to
compute payment. The shift from uj .rumy cuips ui cugmcxia uwncu anu
operated dredging equipment to contract dredging operations brought with it
the need for more accurate specification of channel cross sections and dimen¬
sions. However, more accurate channel S|icLiiiuuiuns aic icirVam 111 y 11
means exist to ensure the specifications are met. In addition, requirements to
dredge to greater depths and greater distances unsuuic iuivc cui^uajiifcu me
monetary consequences of inaccurate channel measurements. Because credible
water level data typically are scarce or absent at offshore locations, disputes
between the Corps of Engineers and contractors have arisen as to the amount
of material dredged and the payment due.
An obvious means of acquiring offshore tide data is to install a platform
or tower or series of platforms or towers, each with a transmitting tide gage,
along the channel to be dredged. While conceptually straightforward, this
scheme typically has a very high initial cost and tends to be very maintenance
intensive. The high maintenance costs are due primarily to the requirements to
maintain a vertically stable platform in areas with hcav)
provide a reliable, independent power source
and transmitter.
vith heavy marine traffic and to
for operation of each sensor suite
At the request of the Jacksonville District, an alternative system was
developed to support deepening of the entrance channel to the US Navy
Trident Submarine Base at Kings Bay, GA. The design project depth of 46 ft
relative to Mean Low Water requires an entrance channel extending approxi¬
mately 13 miles seaward of the ends of the jetties. Because the system is used
by contractors as well as District survey vessels, the system was designed from
the onset to be used by relatively unskilled persons with little "hands on" inter¬
vention and investment in user equipment. The system was designated the
Automated Real-Time Tidal Elevation System and took the acronym ARTTES.
The ARTTES allows virtually unlimited numbers of users to obtain
instantaneous tidal elevation data over a designated area. It is based on a
predictor-corrector method and consists of a high precision water level sensor
linked to a VHP transmitter which continuously broadcasts the water level as
measured at some location in the designated area. Users have a VHP receiver
linked to a lap-type or desktop computer. Resident on the computer is
communication/computation software which predicts the tide level at a user-
specified location based on data previously acquired within the designated
uired within the designated
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received via the radio transmi
itter and is then displayed to the operator.
Technical Note DRP-M2 (June 1990)
system permits highly repeatable elevation control during offshore surveys.
Moreover, it can allow dredge operators to make online adjustments for
changes in water level, thus nuruiiiiiiiiLg me item iu uvciuim^c. i iguie i is o
schematic diagram outlining operation of the system. A detailed description of
,’ater level, thus minimizin 2 the need to overdredgs. Figure 1 is a
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RECEIVING STATION
Figure 1, Automated Real-Time Tidal Elevation System
Application Criteria
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The criteria for determining the applicability of an ARTTES fall into two
general categories—technical and economic. The technical criteria include both
objective and subjective aspects. The economic criteria include aspects which
may be unknown at the time a decision is required, making some assumptions
necessary.
Torlitiiral Pritoria
The ARTTES is applicable only to nearshore, open-ocean areas, that is,
seaward of the mouth of a river or jetty system up to approximately 20 miles
offshore. Dredging and survey operations requiring tide data within the
reaches of jetties or into estuaries and rivers are better served using other tech¬
niques or systems. The limit of 20 miles is due to the present type of radio
Technical Nate DRP-t-02 (June 1990)
3
transmitter being used. A more powerful transmitter could be used to extend
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etry of the candidate area should be reasonably well represented
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cally in the onshore direction. The channel area should be free of deep marine
canyons and very shallow shoals, especially any that are exposed during only
part of the normal tide cycle. Current bathymetric or navigational charts of the
vicinity are usually adequate to determine whether the area is suitable for
ARTI ES application from a bathymetric standpoint.
At least 80 percent of the annual water variance at the candidate site
must be due to the astronomic forced tide. National Ocean Service (NOS) tide
data for the general vicinity can be used to determine whether this criterion is
met. Most locations along the Atlantic and Gulf of Mexico and many along tne
Pacific coastlines of the continental United States meet this criterion.
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There should be significant differences in the range and/or phase of the
tide along the extent of the channel. Differences of 15 to 20 percent in range
are common in shallow coastal areas where dredging often occurs. Where the
bide range is small (for example, 2 ft), differences may be only a few tenths of a
foot, perhaps small enough to be of no significant concern. Where the tide
range is greater (for example, 6 ft), differences may exceed 1 ft and have to be
considered if accurate survey and dredging are to be conducted.
Economic Cutcna
Based upon recent contracts awarded by Charleston, Galveston, and
Jacksonville Districts, typical costs for offshore dredging range from about
$1.5G/cu yd to $3.50/cu yd. use of an ARTTES reduces the vertical root mean
square uncertainty in offshore survey and dredging operation from abut 1.5 to
0.5 ft. For a typical entrance channel width of 800 ft, use of the system reduces
the amount or dredging required to assure a given channel depth by
156,000 cu yd per statute mile or channel. Assuming an average dredging cost
of $2.00/cu yd, this translates to saving about $312,000 per statute mile of
channel.
The present initial cost of an ARTTES is $250,000 to $300,000, depending
upon site-specific characteristics. Therefore, for a deepening project, wherein
the channel will be dredged along the entire length, use of a system can
produce net savings for channels more than 1 mile long.
For maintenance dredging operations, identification or savings by use of
a system is more complicated. Typical maintenance cost for an ARTTES is
about $50,000 per year. Assuming a 10-year system life, initial cost plus
4
,i xw e TY»p*4-02 (June 1990)
maintenance is a total of about $700,000 or $70,000 per year. Again, assuming a
$2.00/cu yd dredging cost, the system would have to eliminate 35,000 cu yd of
annual maintenance dredging to be economically justifiable. Since the system
reduces the amount of dredging required to achieve design depth by about
0.3 cu yd for each square yard, the annual dredging project would have to
cover a minimum of 115,000 sq yd. For a typical channel width of 800 ft, this
is equivalent to about 1,300 lin ft of channel.
Secondary, more intangible sources of cost savings can result from more
accurate identification of areas requiring dredging, better estimates of quantities
to be dredged to assure design depth, and reduced risk of contractor claims
resulting from disputes over quantities.
Conclusions
Maintaining a real-time tide gage in a channel is, in principle, a simple
matter. However, experience indicates that there is considerable difficulty in
maintaining such a gage and assuring a stable vertical datum in an area with
normal navigational traffic and commercial fishing interests. The problem is
compounded by the requirement to have some means of cross checking the
data in order to alert users to system problems other than complete failure.
Users who are tempted to use something "simple and inexpensive" are cau¬
tioned that the offshore dredging environment is one in which there is strict
application of Murphy's Law. Those in the user community who need to
establish offshore tide control, regardless of whether an ARTTES system is the
best solution, are urged to contact the author of this Technical Note or the
Program Manager of the Dredging Research Program.
Reference
Lillycrop, W. Howell, G. L., Garcia, A. W., Grogg, W. E., and Andrew, M. E.
1988. "An Automated Real Time Tide Elevation System," Proceedings , US Army
Corps of Engineers Surveying Conference, Savannah, GA.
Technical Note DRP-4-02 (June 1990)
5
INTERNET DOCUMENT INFORMATION FORM
A . Report Title: Application Criteria for the Automated Real-Time Tidal
Elevation System (ARTTES)
B. DATE Report Downloaded From the Internet: 07/06/99
C. Report's Point of Contact: (Name, Organization, Address, Office
Symbol, & Ph #): Dredging Operations Technical Support
Attn: Dr. Engler (601) 634-3624
3909 Halls Ferry Road
Vicksburg, MS 39180-6133
D. Currently Applicable Classification Level: Unclassified
E. Distribution Statement A: Approved for Public Release
F. The foregoing information was compiled and provided by:
DTIC-OCA, Initials:_ VM _Preparation Date 07/06/99
The foregoing information should exactly correspond to the Title, Report Number, and the Date
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