SAND-WAVES IN TIDAL CURRENTS 305 



streaiDL below the bridge. The ebb current in the 

 main channel ,under the west bank, flowing swiftly 

 on a steep bed, was not stopped by it, but, on the 

 contrary, turned it aside, and the flood waters pass- 

 ing under the bridge filled up the blind channel 

 on the ,east of the Waveridge Sand. Everything 

 went quietly until the critical moment when the 

 incoming tide, having filled the blind channel to 

 the brim', overtopped the -Waveridge Sand. Then 

 suddenly a broad sheet of water swept laterally 

 over from the blind channel to the main channel, 

 covering the shoal so quickly that the air in the 

 interstices of the dry sand was trapped, and could 

 only escape py flinging up jets of water which 

 spouted with great violence from all over the shoal. 

 Thus the flow commences as a swift current from 

 side to side across the shoal . At a later stage when 

 there is deep water the flood tide is free to travel 

 longitudinally over the shoal. As the tide at 

 ordinary springs ^rises 27 feet at Sharpness, just 

 below the bridge, the depth on the Waveridge 

 Sand at high water must be somewhere about 

 20 feet. During the early stages of the ebb I 

 watched the water flowing quietly over the shoal, 

 but later, when there was less water upon it, the 

 ebbing current rushed violently across stream' over 

 the shoal with m^uch noise, the surface of the water 



