272 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



lock at a depth of about 8 feet 7 inches below the outer sills 

 during the period of low water, would have a tendency to suck 

 the mud out of the channel. 1 This latter opinion appears to 

 have been confirmed, at any rate so far as regards the entrance 

 to the fairway channel. The channel itself up to the lock 

 entrance is maintained and deepened by the scouring action of 

 about three lockfuls of water per month 2 discharged at low 

 water. This would amount approximately to 3' 50 million cubic 

 feet per month. The deposit in the same time, if left undisturbed, 

 would amount to about 15 inches. 3 



Tilbury Dock Basin. 4 In the Tilbury Dock Basin, which has 

 an area of 17 acres, with a depth of 26 feet at low-water spring 

 tides, the daily deposit is about 1J to 2 inches. The system 

 adopted for getting rid of this accumulation of mud is a combi- 

 nation of harrows and high-pressure water-jets, towed from the 

 quarters of a small tug during ebb tide. The water-jets work 

 at a pressure of about 80 Ibs. on the square inch at the pumps, 

 with an effective pressure at the bottom of the dock of about 

 60 Ibs. per square inch. 



Barry Dock Entrance. 5 For clearing the mud from gate plat- 

 forms and entrance sills, the ordinary practice is to construct 

 side culverts in the entrance walls, with openings into the back 

 of the gate recesses. At the Barry Docks, the engineers have 

 adopted a different course, considering the cutting away of the 

 bottom of a wall just at the part where extreme pressure has 

 to be encountered an objectionable course, and one that should 

 be avoided, unless exceptional conditions necessitate such a 

 method of construction. Accordingly all the sluices are placed 

 in the gates, or in a large culvert on the west side of the 

 entrance and passage, and at a higher level than the sills. The 

 sluices in the gates are especially intended for removing any 

 accumulation of mud that may take place on the sill and in the 

 entrance channel leading to the dock, and for this purpose a 

 sluice area of 200 square feet is provided. 



Thames Embankment Landing-places. 6 In order to prevent 

 any accumulation of mud in the recesses containing the landing- 

 stages, arches were constructed under the embankment, into 

 which the tide is admitted and retained at high tide, by means 

 of penstocks. On these penstock valves being raised at low tide, 



1 M.P.I.C.E.,lv. p. 91. 3 Ibid. * Hid., p. 7. * Ibid, vol. c. p. 65. 



6 Ibid., ci. p. 167. fl Ibid., vol. xxviii. p. 227; vol. liv. p. 11. 



