Hooper, D. J.

Langhorne, D. N.

English

University of New Brunswick: Centre for Digital Scholarship Journals

AN EXAMINATION OF THE STABILITY OF THE PORT OF LONDON AUTHORITY ESTUARY DECCA HI-FIX CHAINb y  D. N .  L a n g h o r n e , BA, M S c ,  and D. J. H o o p e r , ARICS, Master Mariner.The Port of London Authority Estuary Decca Hi-Fix Chain was set up in 1965 as an aid to hydrographic surveying and dredging operations. The hyperbolic chain layout was designed to provide the best general cover to that part of the estuary significant to navigation (figure 1). In 1967 the chain configuration was altered by moving the Slave II station from Clacton to Orfordness. The main parameters of the present (1974) chain layout are given in table 1. The chain alteration was made in order to improve the Hi-Fix cover of the northern part of the estuary in association with the decision to route large tankers into the Thames via Long Sand Head and Black Deep Channel, instead of through the more restricted EdinburghT a b l e  IThe Port of London Authority Estuary Hi-Fix ChainCoordinates in metres inNational Grid of GreatBritainFrequency 1899.5 kHzMaster Transmitter position (North 638346.99 E 171612.98 NForeland)Slave I Transmitter (Allhallows) 583772.97 E 178436.27 NSlave II Transmitter (Orfordness) 644922.20 E 248823.60 NBase line lengths : Pattern I 54990.59 metresPattern II 77465.42 metresBaseline : 78.876020 metres (assuming a speed o f propagation o flane widths 299650 km/sec).Number of lanes : Pattern I 697.178Pattern II 982.116Channels. The complex navigational problems of the Thames Estuary are discussed by W h i t e  (1972).In order to ascertain the operational accuracy of the Hi-Fix chain a series of monitoring studies have been carried out in conjunction with the Port of London Authority (table 2). For these studies Hi-Fix receivers were set up at various fixed sites on shore around the estuary. Owing to the lack of suitable structures with the necessary power supplies and access, it was not possible to monitor the Hi-Fix pattern away from the shore. For most of the studies Hi-Fix receivers were interfaced with twin track analog recorders. These proved to be only partially suitable in that they required frequent attention and the amplitude o f shift that could be recorded was restricted to a maximum of ±  15/100ths of a lane. In later studies on two occasions a Decca data logger was used; this recorded Hi-Fix pattern data at ten second intervals, regardless of the amplitude of shift. A preliminary report by H o o p e r  (1971) gave details of the analysis of the earlier recorded data.T a b l e  11Data used for Hi-Fix Monitor Studies1970 1971 1972 1973January S(2) 04, S3February 04, S3March S(2), W(2)April NF(2)MayJuneJuly t A l, 0 ( 1), S(l) 04, S2August A( 1), 0 ( 1),S2, W 1 04, S2 + 3, L2September- S<2), M(2) S(2),M(2) 04, S2, L2October 04, S2, W2 04, S(2), M(2), W(2)November 04, S2, W2 04, S2, M2December S2, W(2) 04, S2, M(2), W(2)Location Type o f  dataA = Allhallows 1 = Single track analogue recorder0  =  Orfordness 2 = Twin >» »» »»S = Sheerness 3 = Decca data loggerM = Margate 4 = Rain gaugeW = Walton ( ) Poor data, or of limited periodL = Landguard PtNF = North ForelandAnalysis of the recorded data shows that short and long period devia­tions o f the Hi-Fix patterns occur. The form er are defined as having a period of less than ten minutes, whilst the latter may continue for several hours.SHORT PERIOD DEVIATIONSThe following three types of short period disturbance causing pattern deviations can be readily identified from the recorded data :(a) A continous small amplitude “ noise” type fluctuation occurs. This is related to propagation conditions and the general state of the component electronics (Figures 2 and 3).(b) Re-radiation produced by a large vessel passing close to the monitor site can cause localised pattern disturbance. Figure 2 shows the pattern fluctuation resulting from two tugs and a large dredger passing within half a mile o f the monitor station. No relatable fluctuation was recorded at the monitor station at Walton.PATTERN I3NVT V dO SHiQadQNnHPATTERNZi—/00 zo0 30 10 O 10 30 8« 5 30 10 O 10 30 5<(*( {s0 0 9 010 0 10 30 S 5 30 10 0 10 3000S0> 30 10 0 10 30 S'éf4!qte io o io soi ff -oo»o - ?0 30 10 O 10 30 S< O 10 30 51ooso3 30 IO O- 10 30 S 5 3 0 s -  J A - O  10 30'zr.i/:HZ<0020 o.;it Orfordness 02.50.(c) Intermittent short period deviations (less than 10 seconds) causing “spikes” on analogue records (figure 3) occur on some occasions during the period of darkness, These are thought to be caused by mistriggering at either the slave transmitters or the monitor receiver. The spikes are predominantly in the negative direction and may be coincidental on both patterns or restricted to either one. Figure 3 shows that the amplitude of the deviation is normally about 8/100ths of a lane, but can increase to 20/100ths during rainfall.LONG PERIOD DEVIATIONS (PATTERN SHIFT)In order to minimise the effects of changes in propagation conditions for a Hi-F ix chain it is important that land transmission patterns are kept to a minimum. This however has not been possible at the monitor sites used, as in all cases they are subject to effects caused by land paths from one or more chain transmitters. Because of this the recorded fluctuations are probably greater than those which occur in the Outer Estuary. B r a d l e y  (1970) concludes that banks exposed at low water do not affect Hi-Fix accuracy in that they remain damp and maintain a conductivity similar to that o f seawater.Throughout the investigation no correlation was found between pattern shift and tidal range, as was described by B r a d l e y  (1970). This was not unexpected as the monitor sites are all at some distance from the chain transmitters, and the change in ratio of length of land path to water path does not alter significantly with tidal range.a) Pattern IPattern I remains relatively constant throughout the period of obser­vations. Typically, analysis of mean hourly readings obtained at Sheerness between the 3rd and 31st August 1973 gives a standard deviation of 1.23/1 OOths of a lane about the mean value. Due to the different trans­mission paths only poor agreement existed between simultaneous monitor recordings at different stations, though the magnitude of the deviation remained similar.b) Pattern IIThe genera] stability of Pattern II is similar to that of Pattern I, except when positive pattern shift occurs which can be associated with rainfall at the Pattern II skive station (fig. 4).The Pattern II slave station is sited at Orfordness and stands on a large expanse o f shingle ol' some 15 m thickness ( C a r r , 19f>7). The earth mat forF ig. 4. —  The co in c iden ce  o f  pattern  shift w ith  ra in fa ll .F i g . 5. —  Three phases o f  pattern sh i f t  associated w ith  ra in fa ll .the transmitter aerial is buried and anchored in the surface shingle. Both the water table and the water content within the shingle bank fluctuate with precipitation and tidal height, thus altering the effective earthing conditions at different levels.It is considered that most instances o f long period pattern shift in the positive direction are related to earthing conditions at the slave II trans­mitter, which in turn are related to rainfall at the slave station. No correla­tion can be detected between pattern shift and tidal height, whilst no data is available on the changes in water content in the bank during the periods of monitoring. No equivalent shift occurs on Pattern I which suggests that the pattern disturbance is related to the conditions at the Pattern II slave transmitter and not to the propagation conditions in the Estuary.With the onset of rainfall at Orfordness positive pattern shift occurs on Pattern II. W ithin between fifteen minutes to three hours, depending upon the intensity o f rainfall, a maximum shift is attained. This does not increase with continuing rainfall, suggesting that a limiting condition is reached. Records from the Decca data logger which was used in January and February 1973 show that the full extent of the pattern shift reachedbetween 12 and 15/100lhs of a lane at Sheerness. After rainfall ceases the pattern shift reverts to normal over a period of up to twenty hours. The recovery period probably depends upon the drying conditions, and sub­sequent rainfall during this period again offsets the pattern (fig. 5). The initiation of pattern shift does not always coincide with the onset of rainfall. This may be partially explained by the distance of 3 km between the slave station and the site of the rain gauge, in that the onset of rain is not necessarily at the same time at the two locations. In the case of the data presented in figure 5, the occurrence of snow may have delayed the response of the rain gauge.The comparison o f monitor records obtained at Sheerness with those obtained at Landguard Point shows that although time coincident, the ampli­tude of pattern shift associated with rainfall is greater at Landguard Point. This indicates that the en tire shift is not produced oniy by the local effect at the slave II transmitter. In addition secondary factors, such as different propagation velocities over different transmission paths, probably occur.As a result of information gained in the monitor observations carried out in January and February 1973 an attempt was made to improve the earth mat at the Orfordness slave station. Metal rods were driven 7m into the shingle and connected to the outer ends of the wires forming the earth mat. These modifications have resulted in some improvement in that the monitor records obtained during July and September 1973 at Sheerness show that pattern shifts associated with rainfall are reduced to 10 to 12/100ths of a lane.CONCLUSIONSUnless the long period fluctuations can be eradicated, it seems necessary that at least one permanent recording monitor station should be maintained and the relevance of its data established throughout the Estuary. This objective will be maintained in future studies in which it is also hoped to obtain and incorporate monitored data from offshore sites.ACKNOWLEDGMENTSThe authors wish to acknowledge and thank the Port of London Auth­ority for their co-operation and assistance with this investigation, and in particular Cdr J. C. E. W h i t e , who assisted with the preparation of this paper. Thanks is also due to Miss P. B o w d i t c h  and Mr C. N, P u c k f .t t  of the Institute of Oceanographic Sciences who helped with the collection and analysis of data.REFERENCESB r a d l e y ,  E.M, (1970) : Practical experience with Hi-Fix. Chart. Surv., 102 (8), pp. 369-378.C a r r ,  A.P. & B a k e r ,  R.E. (1968) : Orford, Suffolk : Evidence for the evolution of the area during the Quaternary. Trans. Inst. Rr, Geogr., 45, pp. 107-123.H o o p e r ,  D.J. (1971) : Decca H i - F ix  : an analysis of chain performance from monitor records (Preliminary Report). Unit of Coastal Sedimentation, Taunton.W h i t e ,  J.C.E. (1972) : Hydrographic and Tidal Information for Deep Draught Ships in a Tidal Estuary. Int. Hydrog. Rev., 49 (2), pp. 33-46.(Manuscript subm itted  in English).CARIBBEAN SURVEYA lover of the sea, President Franklin D. R o o s e v e l t  often recuperated from his strenuous duties by taking fishing trips. Summoning an aide, he would say “ Call the Hydrographic Office and find out where the fishing is good ” . During the 1930s, funds for hydrographic survey work were scarcer than usual and Hydro was especially anxious to survey areas rarely entered by merchant or Navy ships, but considered strategically important. For several years, therefore, an inspired gentleman at Hydro would reply that the fishing was particularly good in the Caribbean.While the President happily fished from the deck of a cruiser in areas remote from shipping traffic, fatho­meters obtained the soundings Hydro needed to complete its bottom contour charts of the Caribbean.(R eprin ted  from  Proceedings  b y  p e r m is s io n ;  C opyright ©  1973, U.S. Naval Institute. C on tr ib u tor  : P a o lo  E. C o l e t t a ) .

An Examination of the Stability of the Port of London Authority Estuary Decca Hi-Fix Chain

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An Examination of the Stability of the Port of London Authority Estuary Decca Hi-Fix Chain

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