LOCAL ENGINEERING WORKS. Ill 



liours and upwards the tide level in the Avon is higher 

 than the Floating Harbour, and sluicing therefrom is neces- 

 sarily suspended. One foot in depth over the area of the 

 Floating Harbour represents about 3j million cube feet, 

 and if flood water, at the rate of six million cube feet per 

 hour, were poured into the Floating Harbour for three hours 

 or more, whilst the sluices were closed, there would occur 

 a rise in the level of nearly six feet, which would be objec- 

 tionable, both as to its effect on the shipping, and the probable 

 flooding of cellars near the Harbour. If, however, a flood of 

 eight million cube feet should occur, and it should reach the 

 Harbour, the variation of level would be still greater. It is 

 true that the coincidence of the period of greatest flood dis- 

 charge with that of high spring tides in the Avon is an 

 event that might rarely occur, but the possibility of its 

 occurrence cannot be disregarded in considering any scheme 

 for the prevention of floods. A very valuable addition to 

 the means for regulating the capacit}^ of the Floating 

 Harbour to receive and discharge flood water from the 

 Frome, is provided by the new sluices at Cumberland Basin, 

 constructed by the Docks Committee at a cost of about 

 £13,000. The length of the proposed Black-Rock Culvert is 

 a little over three miles. About two-thirds of its length 

 would be through red sandstone and marl, with a short 

 length of millstone grit. This part is intended to be lined 

 with brickwork, except where harder rock may render brick 

 lining unnecessary. The remaining third would pass through 

 mountain limestone, and would not be brick-lined, but the 

 surface of the excavated rock would be rendered compara- 

 tively smooth by filling hollow places with cement concrete. 

 The discharge from the culvert w^ould vary between 5| 

 million cube feet per hour at the period of highest 

 equinoctial tides, and 8f to 9 million cube feet per hour at 



