68 On the Improvement of the 



between this bridge and Melbourne the volume of the Yarra 

 was considerably augmented by numerous small streams 

 and creeks flowing into it, adding, at least, from 800 to 1000 

 feet additional of sectional area of flood water. 



Since the date of my paper I have noticed that Mr. Gordon 

 in one of his reports estimates that an additional flood 

 channel, of about 4000 feet sectional area, in addition to the 

 present river, is required for the passage of flood waters 

 below Melbourne to the Bay — making a total of about 8000 

 feet of area; but in the face of all the facts known of the 

 great volume of the waterflow through Prince's Bridge, I do 

 not think such sectional area equal to the work to be done. 



The discharge in heavy floods through Prince's Bridge 

 and the Dry Arch is a pitch or fall of water rather than a 

 flow, whilst over the St. Kilda-road causeway the water 

 rushed as over a weir head, the velocity in each case being 

 necessarily very great. 



In Flinders-street the water stood upwards of ten feet 

 deep, and spread in a sheet southwards to the foot of Emerald 

 Hill, and although extending over so large a surface, it 

 flowed with considerable velocity even when the flat was 

 comparatively unobstructed ; but now, with solid embanked 

 causeways and extensive piles of buildings covering the low 

 ground, the waters of any future flood will of necessity be 

 confined in narrower bounds, and rise to a greater height, 

 in order to escape to the Bay. 



For the above reasons I do not think the width given for 

 the proposed new channel (1000 feet) excessive for the 

 outflowing water when the above conditions are fully 

 considered ; but although 1000 feet width be adopted for a 

 flood channel, it is unnecessary for the present to excavate 

 the full breadth and depth for that purpose only, as the 

 work may, be deferred until the space is required for dock 

 extension, or the materials wanted for reclamation of new 

 land. 



For carrying away flood waters, the channel, if taken out 

 to 1000 feet wide, and to the depth of ordinary high- water 

 mark, and the ship channel taken out for its full depth of 20 

 feet at low water, and 400 feet wide at the top, would give 

 a sectional area of about 10,000 feet, the mean velocity and 

 area of which would be more nearly approaching the required 

 capacity for discharging the excess of the waters requiring 

 passage, without unduly impeding the free flow and conse- 

 quent backing-up of the flood waters which a nan^ower 

 channel would cause. 



