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Full text of "DTIC ADA365405: Application Criteria for the Automated Real-Time Tidal Elevation System (ARTTES)"

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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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DTIC QUALITY INSPECTED 4 


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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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UP-TOP 

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RECEIVING STATION 


Figure 1, Automated Real-Time Tidal Elevation System 


Application Criteria 

* i 

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 


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,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 
the accompanying report document. If there are mismatches, or other questions, contact the 
above OCA Representative for resolution.