The Tides in the Neighbourhood of Portsmouth.” by BERNARDODRISCOLL TOJVNSHESD, Assoc
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selected TOTVNSHEND ON THE TIDES NEAR PORTSNOUTH. 273 Papers.] SECT.11.-OTHER SELECTED PAPERS. (Pupel- No. 2235.) ‘‘ The Tides in the Neighbourhood of Portsmouth.” By BERNARDODRISCOLL TOJVNSHESD, Assoc. M. Inst. C.E. (Ab8hCt.) A SERIES of tidal-current observations, by means of floats, was made by the Author, in July andDecember, 1882, with the object of ascertaining the probable course which sewage matter, issuing from an outfall at the mouth of Langstone harbour, would take withthe ebb-tide. Theseobservations led theAuthor to in- vestigatethe set of thetides in Langstone and Portsmouth harbours, and their neighbourhood, and to collect diagrams giving the recorded heights of high-water at Portsmouth harbour for a whole year (1885-86), together with t.he height of the barometer, and the direction andforce of the wind, taken twice daily during the same period (Plate 2, Figs. 1 to 12). Langstone harbour, situated between Port,sea andRayling islands, is a large lagoon, with a narrow deep entrance channel about 3 milelong, and l50 yards wide (Plate 2, Fig. 13). The ebb-tide flows intwo main streams from Langstone harbour. Theeastern, or Hayling Island, strea,m runs continuously due southstraight out of the harbour. It acquiresaconsiderable velocity within fifteen minutes of the turn of the tide, and retains its velocitymuch longer andfarther out than the other main stream. The western, or Cumberland Fort, stream soon inclines’ westward with reduced velocity, and runs parallel with the shore till, on coming nearly opposite the Eastney barracks, it inclines to the south-west, and turning gradually eastwards, it assumes, when due east of the Horse Fort, a south-easterly direction near low- water, making for the Owers shoal off Selsea Bill. The early flood-’ tide comes into Langstone harbour from the west, flowing through the Solent. Afterwards its mainvolume, with its greatest speed of 23 knots, comes from the south, from the main flood-current passing to the south of the Isle of Wight. Portsmouth harbour, situated to the west of Langstone harbour, and separated from it by Portsea Island, Plate 2, Fig. 13, has an [THE INST. C.E. VOL. XCJ-IT.] T Downloaded by [ University of Liverpool] on [16/09/16]. Copyright © ICE Publishing, all rights reserved. 274 TOWNSREND ON THE TLDES NEAR PORTSMOUTH. [Sclccted area of about 4,400 acres at high-water; and the general width of the channel inside the harbour is 430 yards, with a depth of from 8 to 3 fathoms. The tide flows into Portsmouth harbour for about seven hours, and ebbs during five hours. For the first five hours of flood, the tide enters Portsmouth harbourfrom the stream coming from the west past the Needles and up the Solent, part of the flow coming direct round Gilkicker Point, and part from a portion of the current running direct to Spithead, and then turning northwards. For the last two hours of flood, the flow comes from the east ; and rounding the Spit, and impinging against the Haslar breakwater, a portion of this current runs into the harbour, ancl completes the filling of the harbour. This westward flow during the latter part of the flood is due to the action of the main tidal current, running up thechannel to the south of the Isle of Wight, which, with its large volume, completes the filling of Portsmouth, Langstone, and Chichester harbours, which the earlier direct tidal stream,restricted by the narrow passage off Hurst Castle, and divertedinto Southampton Water, is inadequate to accomplish. The first ebb out of Portsmouth harbour sets towards the south- west,and joins the westward current referred to above, which continues in the same direction for about two and a half hours after high-water. At about half ebb, the stream from Portsmouth harbourturns more tothe south, and near low-water joinsthe final ebb from the west and Southampton Water, passing round the eastern end of the Isle of Wight. The ordinary velocities of the tidal currents at Portsmouth range from l& knot to 5fr knots on the ebb, and from 1 knot to 44 knots on the flood. The harbours of Langstone and Poytsmouth are connected by Hilsea Channel, or Portcreek, which bounds Portsea Island on the north. At the beginning of the flood, the tide runs from Langstone harbour into Portsmouth harbour, but is soon stopped by the rise of tide in Portsmouth harbour; and the water remains stagnant at the western end of the creek till high-water. After high-water the current againflows into Portsmouth harbourtill two anda half hours ebb, when the water at the western end of the creek again becomes stationary, but flows into Langstone harbour at the other end. During the lastof the ebb, the water runs botheast and west in the creek; but the pointof separation of the currents is nearest to Langstone harbour. Accordingly water flows out of Langstone harbourinto Portsmouth harbour, but never vice versd. This circumstance appears to be due to the better filling of Langstone harbour by the flood-tide, therebyraising its high-water level above that of Portsmouth harbour. Though the flow into Ports- Downloaded by [ University of Liverpool] on [16/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] TOWNSHEND ON THE TIDES NEAR PORTSMOUTH. 275 mouth harbour through Portcreek, during the early part of the ebb, occurs toosoon to be of much assistance in increasing the scour of the ebb in the main channel, afterthe mudbanks are bared, yet by augmenting thevolume of backwater in Portsmouth harbour,which is not fed by any river, it is of valuetowards the maintenanceof the harbour; and consequently the reclamation of Langstone harbour would be inexpedient. The floats, used for observing the course of the ebb-tide from the mouth of Langstone harbour, were made of wood, conical in form, 9 inches long, 4 inches in diameter at top, and 13 inch at bottom, painted, numbered, and weighted with 1 lb. of lead attached by a cord to the ring at the bottom of the float (Plate 2, Fig. 14). The float, with its cord and weight, was about 24 feet long. Circular disks, 15 inches in diameter, and 1%inch thick, were also em- ployed as floats, but proved useless on account of the interference of thewind with their motion. In eachobservation, the floats were watched from a steam-launch and two four-oared gigs ; and the progress of each float was recorded at intervals by bearings taken to objects on shore, and subsequently laid down on a chart. As an example of the system, the course of the centrefloat (No. 151) of a set of three, placed in the water off the Portsmouth sewage outfall, on the 6th of December, 1882, at 1 h. 11 m. after high- water, is indicated in the chart (Plate 2, Fig. 13). The various positions at which bearings were taken, and the times at which these positions were reached by the float are shown; and at 7 h. 11 m. after high-water, when the observation was concluded, the float was 200 yards due northof the Dean Tail buoy. Twelve diagrams show the observed heights of high-water, day and night, at Portsmouth harbour, for the year, from the 1st of October, 1883, to the 30th of September, 1886, as recorded by Sir William Thomson’s self-registering tide-gauge, situated on the master attendant’s jetty (Plate 2, Figs. 1 to 12). The dotted line on the diagrams shows the calculated heights of tide for the same period; and thediscrepancy betweenthe calculated and therecorded heights is attributedto the use of better instruments thanformerly, affording more reliable results, as well as to the various influewes affecting the rise of tide. The general recorded results bear out the rules laid down in the tide-tables, especially with respect to the reduction of the tidal rise with a north-east or easterly wind, and a high barometer. The upper line in each diagram shows the diurnal barometric readings, which have been drawn inverted so as to correspond in the diagram with their influence on the tide. The lower line represents the force of the wind reckoned by the T2 Downloaded by [ University of Liverpool] on [16/09/16]. Copyright © ICE Publishing, all rights reserved. 276 TOWNSREND ON THE TIDES NEAR PORTSMOCTH. [Seloctcd ordinary nautical scale (Plate 2, see Table, note B) ; and in addi- tion to the record of the direction of the wind, shading has been introduced to distinguish ata glance westerly from easterly winds. TheAuthor, in conclusion, desires to testifyto the valuable assistance received from Mr. Harding, the late Queen’s Assistant Harbour-Master at Portsmouth, with his extensive knowledge of the English Channel, in the preparationof this Paper. The Paper is accompanied by a number of tracings from which Plate 2 has been engraved. Downloaded by [ University of Liverpool] on [16/09/16]. Copyright © ICE Publishing, all rights reserved. PLATE. 2 J?q:13. GHART OF PORTSMOUTH HARBOUR AND ENVIRONS. DownloadedB.TOWNSHEND, by [ University of DEL?.Liverpool] on [16/09/16]. 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