Sentence:  It has already been explained that the ebb tide in the Hooghly is a much more powerful scouring agent than the flood tide, that the tide therefore should be used for <WSD>scouring</WSD> a channel in preference to the flood whenever it is practicable to use it, hence it is to the body of ebbing water that <WSD>scours</WSD> out the fine channel from Akelmeg to Crossing Creek that attention will be principally directed, and not to the flood that <WSD>scours</WSD> the channel near the Roopnarain entrance. 
Meaning of the word marked with <WSD> tags:  To remove debris and dirt by purging; to sweep along or off (by a current of water).