688 report— 1878. 



or arterial drainage works in Ireland were becoming fewer in number, until last 

 year there was only one application received by the Commissioners of Public 

 Works, although previous to 1863 the Drainage Commission (Ireland) relieved 

 266,736 statute acres from floods at a cost of £2,390,612 under the Act 5 and 6 

 Vict., and since 1863 down to July 1878 over 71,000 statute acres were drained by a 

 further expenditure of £389,000 by Drainage Boards formed under the Act 26 and 

 27 Vict., c. 88 ; the total number of drainage districts amounting to 157, of which 

 37 were carried out under the last-named Act. 



The author stated that it frequently happened that when a number of lowland 

 proprietors promoted a scheme for the improvement of their larger, and consequently 

 more costly sections, they generally tried to tax the upland proprietors for some of 

 the additional outlay, and the latter then resist the attempt ; while if the upland 

 proprietors try to improve their smaller and less costly rivers, they are opposed by 

 the lowland proprietors, who contend that such works, by the drainage of the up- 

 lands, would increase their lowland floods. This kind of opposition renders it very 

 difficult to obtain assent to the execution of works from the owners of two-thirds 

 of the injured land. 



The author stated bis belief that such objections should not be allowed to have 

 weight, because, from experiments made by him on flood discharges before and 

 after the execution of extensive river or arterial drainage works, he found that such 

 works had not the effect of increasing the river floods either above or below the 

 termination of the works. The reasons given for this result are as follows. 



It has been proved that the area of land actually covered by flood water (with 

 scarcely an exception) seldom exceeds seven per cent, of the entire catchment- 

 basin, or area of land unwatered by rivers, and in 120 districts completed by the Irish 

 Drainage Commissioners the flooded land did not amount to seven per cent, of the 

 total area of the catchment-basins, and that thi3 seven per cent, of flooded land was 

 generally to be found along the river banks or shores of lakes affected by the rivers. 



It also happens that this three, four, or seven per cent., as the case may be, is 

 always more or less saturated with water, particularly in winter, and that when so 

 saturated any additional flood waters coming on it will rise and flow over its surface. 



It is believed by many that the waters covering a river valley in flood times 

 are stationary. This might be so if the river valley had no fall or inclination in the 

 direction of its length ; but without some such fall the flood waters of former times 

 could not have scooped out, as it were, a river, bowever insufficient and imperfect 

 it may have been ; therefore it is a fact that, unless in very exceptional cases, the floods 

 in river valleys continue to move towards the natural outfall for each valley. 



It is also known that every flooded river valley has its fixed maximum flood 

 marks, beyond wbicb the floods never rise, and if, when the floods reach this level, 

 rain should still continue to fall, then the whole of the floods due to this additional 

 rain must reach the river valley, passing away as quickly as they arrive, or the 

 flood levels would rise in the valley. 



If, then, the whole maximum flood due to continuous rain passes from an upper 

 to a lower division of a district, in the natural flooded state of a district before 

 arterial drainage works are carried out, then it would be impossible that the deepen- 

 ing or enlargement of so many miles of a river could increase the floods or rainfall 

 either above or below the improved section of the river. 



It should be remembered that, before the execution of such works, just in pro- 

 portion as the rainfall ceases, so would the floods subside, were it not that towards 

 the termination of the rainfall they are largely increased in volume by the escape 

 of the flood-waters that previously overflowed the river banks. 



It may be said, if the effect of arterial drainage works is to prevent the accumu- 

 lation of large sheets of flood waters in a river valley, then the floods must be in- 

 creased by the passing away of these waters. 



This the author believes to be a mistake. For up to the present time he has ex- 

 pended on main and tributary river works (sanctioned by Government)of an aggregate 

 length of 150 miles, nearly half of the whole amount expended on such works in 

 Irefand since 1863. As all these works proved successful, both from a scientific and 

 financial point of view, he was afforded opportunities of observing the flood dis- 

 charges before and after the execution of such works as the Inny River "Works, 





