Sentence:  The general manner of working this sluice is to take in every alternate tide during the high tides in dry weather, and to let them off for <WSD>scours</WSD> for the harbour. In wet weather, or during neap tides, no <WSD>scours</WSD> are kept back, nor is it of any consequence to the harbour if no tides are suffered to pass up through the sluice at such times. 
Meaning of the word marked with <WSD> tags:  The removal of sediment caused by swiftly moving water.