and Proposed Works for its Improvement. 113 



causing a littoral drift northerly and westerly past Sandridge 

 towards the river's mouth. 



Such being the state of things prior to 1853, I propose to 

 examine what lias been done since, and show the conse- 

 quences which theory leads us to expect, and what practice 

 has actually realised. 



The construction of the stone pier and timber jetty at 

 Point Gellibrand and other timber jetties for the railway 

 in the vicinity, should, in accordance with theory, obstruct 

 and divert the current previously existing along the shore at 

 these points, and cause deposit, whilst the stone walls at the 

 mouth of the river must absolutely turn the stream into the 

 bay in the direction of Sandridge, and in doing so destroy 

 its current, and cause precipitation of all solid matters held 

 in suspension, and this precipitation will take place much 

 more rapidly on entering saline than in fresh waters. 

 Such are the teachings of theory, and they may be illustrated 

 by examples from practice. 



The rivers entering the sea on our south coast have 

 all bars, and the rivers Latrobe and Mitchell are excellent 

 examples of bars in slightly brackish water, formed under 

 precisely similar conditions to those existent in Hobson's 

 Bay, whether natural or artificial, but more especially the 

 latter. 



Of the influence of open timber piling in checking currents 

 and causing deposit, I beg to instance two or three which 

 have fallen more immediately within the range of my own 

 experience and observation. At Caernarvon, in North Wales, 

 the River Sieont enters the Menai Straits, and at its mouth 

 a stone pier was built out to accommodate the shipping ; but 

 this being at right angles to the tidal current, became silted 

 up, and was extended further out with open timber piling ; 

 but this also silted up, and about 1846 and '47, a further 

 extension of timber pier was run out also at right angles to 

 the stream, finishing with an L end parallel to the current, 

 but before I left the district in 1850, the direct extension 

 had silted up, whilst the L end being exposed to the direct 

 scour remained clear, showing how very influential open 

 timber work is in checking and diverting current, and 

 causing deposit. 



The next instance is an extension of the Beaumaris Pier, 

 also in the Menai Straits, which being at right angles to the 

 current, diverted it, and also caused deposit. 



The next instance is the old pier at Leith, in the Firth of 



I 



