KiNAHAN — Tidal Currents and Wind-waves. 



447 



until the tide begins to flow out of the estuary, -whicli in some cases 

 ("when the estuary "widens considerably inside) may be hours after the 

 true turn of the tidal "vrave. If, ho"weYer, the bank ( h) is lo"o^, after 

 it is covered (say at quarter or half-tide), the tide "n'ill flo"sv over it in 

 the dii'ection of the arro"n" (/), and there "will be no " counter-tide" 

 in the direction of (c). The "counter-tide" (c) has a considerable 

 drifting power ; its effects, ho"weyer, are much modified by the after 

 current {e). From this it will be seen that, if the bank (h) is only at 

 times submerged, the channel {e) between it and the shore will be shal- 

 lower than if the shoal is covered during the greater portion of the 

 flow of the tidal wave. Other driftages take place in connexion with 

 such a bank ; they are, however, usually so slight as not to affect the 

 general question, besides that in a great measui'e they modify one 

 another. 



It has been already mentioned that the unaided outgoing tidal 

 current appears to have little or no drifting power, not even when flow- 

 ing through a narrow, where we find the seaweed-covered stones are 

 rarely moved. This, however, may be more apparent than real ; for if 

 the bottom of a narrow is covered with shingle or coarse gravel, sea- 

 weed may grow on these, and prevent them being distiu^bed ; while, if 

 the bottom is small gravel or sand, a portion is carried oif by the out- 

 going tide. It seems, however, to have very little eficect on mud, and 

 in the Irish estuaries the detritus brought in, or forced back, by the 

 tide is usually in excess of that carried out. This is very apparent in 

 the tidal flats and banks, where the tributaries of the main stream 

 nearly always flow into it up-stream, as shown in fig. 4, where l, h are 

 the tidal muddy flats, the arrow A indicates the dii'ection of flow tides, 



Fig. 4. 



and the arrow C the direction of ebb tides — a being the stream when 

 the tide is out, into which the tributaries {c, c, c, c) enter with the 

 direction of the incoming tide. Even in some places the bed of a 

 large river will be banked up by materials brought up by the tide and 

 the stream forced out of its proper course : for example, the Slaney, 

 Co. Wexford, where, at the point of Park, a mud bank has accu- 

 mulated, and forced the bed of the river northward. Such tidal 



