TRANSACTIONS OF SECTION G. 703 



iriver channel thence downwards to Killaloe Bridge was more than 470 feet wide by 

 <>$ feet deep, with a surface inclination of 3£ feet per mile. From the bridge down- 

 wards there is a fall of 4 feet in one-quarter of a mile, as shown on the diagram 

 section. 



I ask, Was not that weir mound on that occasion a great obstruction P Evidently 

 it was. 



If that weir mound had not been there on that occasion, where would have been 

 the levels of the flood surface from its site upwards to Portumna and Meelick ? I 

 submit for your consideration that they would have been as I now point out on the 

 diagram. 



Let a line be drawn from the surface of the water just below the weir, upwards 

 to the broad deep water, and parallel to the surface of the flood river, as it was on 

 August 21. Had the weir not been there the water a few feet above the site of the 

 weir would have been a fraction of an inch higher than that line, and 800 feet up- 

 wards, which is above the head of the shoal, and in Lough Derg the surface of the 

 flood water would have been 6 inches above that line. 



The flood water would be running in a channel 6 feet deep instead of 8 feet deep, 

 and must run faster, and must have a greater inclination. The velocity in the 8 feet 

 ■deep channel was 281 feet ; in the 6 feet channel it would be 375 feet per minute. 



An increase of 6 inches in the surface fall from the broad deep water to the site 

 of the supposed removed weir mound would suffice to increase the velocity so as to 

 •carry off the same quantity of flood water in the same time with 2 feet less depth as 

 required. Of the 2 feet 2 inches that would be gained just above the site of the 

 supposed removed weir, 6 inches would be lost in generating the required greater 

 velocity for the shallowed channel, and 20 inches would be gained in Louo-h Derg ; 

 and by that lough the same would be gained at Portumna, that is, the flood water 

 would be 20 inches lower, while the meadows were covered only by 20 inches of flood 

 water, and that for merely five days. Therefore, if the weir mound had not been 

 at Killaloe in August, 1861, the low lands above it. at Portumna would not have 

 "been covered at all by flood water. They would have been merely saturated. 

 Instead of being flooded for twenty-seven days, the lands would have been merely 

 saturated for ten days, and thenceforward all the lands would have been quite dry 

 for twenty-eight days, and the crops might have been saved. 



If the Killaloe weir mound were removed and replaced by a movable regulating 

 weir, another important element would come into favourable action. The immense 

 capacity of Lough Derg as a storage reservoir, hitherto valueless, would be utilised. 

 The lake contains 30,000 acres = 1,306,800,000 square feet. The proper level for 

 steamboat navigation proposed and recommended by the Shannon Commissioners 

 and legalised by the Shannon Act is 2£ feet under the surface of the meadow land. 

 The capacity of that reservoir is 3,200 millions cubic feet. When the great rainfall 

 occurred on August 13, none of it was available. It had been filled up gradually 

 during the previous month by ordinary wet weather and small floods. It mi^ht 

 have been all available that time to store an immense quantity of flood water while 

 the rest would be flowing off gradually to the sea. The quantity of water in the 

 river during the previous month was under 400,000 cubic feet per minute. The 

 channel was fully capable of carrying off that quantity at the legal navigation level 

 and keeping the surface of the Shannon river and of Lough Derg down to the legal 

 2J feet under the surface of the low land. Had it been so, the rise of 21 inches 

 caused by the rainfall of August would have left that water surface, when highest 

 some inches under the low lands. 



Fifty wholly removable regulating weirs were constructed in (he Seine several 

 years ago. When quite closed up in summer they maintain the required depth of 

 water for an immense steamboat navigation. When wholly open in floods there is 

 no fall in the river surface. They occupy no part of the natural fall of the river 

 surface. A remarkable one of these has been in excellent action for several years at 

 a place called Port-a-F Anglais, above Paris, and above the junction of the Seine 

 and Marne. I saw it when all open. There was not a ripple on the river flowing 1 

 by. I saw it raised and lowered with ease and facility. I have here a letter from 

 M. Oambuzat, the chief engineer of the River Seine, in which he informs me that 

 all those wholly removable regulating weirs in the Seine are remarkably effective and 

 suitable for regulating: that great commercial river. 



