486 EIVEK EMBANKMENTS. 



"Wlien a river is embanked at a given point, and consequently, 

 the water of its floods, which wonld otherwise spread over a wide 

 surface, is confined within narrow hmits, the velocity of the cur- 

 rent and its transporting power are augmented, and its burden of 

 sand and gravel is deposited at some lower point, where the ra- 

 pidity of its flow is checked by a dam or other artificial obstruc- 

 tion, by a diminution in the inclination of the bed, by a wider 

 channel, or finally by a lacustrine or marine basin which receives 

 its waters. Wherever it lets fall solid material, its channel is 

 raised in consequence, and the declivity of the whole bed between 

 the head of the embankment and the slack of the stream is re- 

 duced. Hence the cm-rent, at first accelerated by confinement, 

 is afterwards checked by the mechanical resistance of the matter 

 deposited and by the diminished inclination of its channel, and 

 then begins again to let fall the earth it holds in suspension, and 

 to raise its bed at the point where its overflow had been before 

 prevented by embankment.* The bank must now be raised in 



* In proportion as the dikes are improved, and breaches and the escape of 

 the water through them are less frequent, the height of the annual inunda- 

 tions is increased. Some towns on the banks of the Po, and of course within 

 the system of parallel embankments, were formerly secure from flood by the 

 height of the artificial mounds on which they were built ; but they have re- 

 cently been obliged to construct ring-dikes for their protection. 



Lombardini lays down the following general statement of the effects of river 

 embankments : 



" The immediate effect of embanking a river is generally an increase in the 

 height of its floods, but, at the same time, a depression of its bed, by reason of 

 the increased force, and consequently excavating action, of the current. 



"It is true that coarser material may hence be carried further, and at the 

 same time deposit itself on a reduced slope. 



"The embankment of the upper branches of a river increases the volume, 

 and therefore the height of the floods in the lower course, in consequence of 

 the more rapid discharge of its afiiuents into it. 



" When, in consequence of the flow of a river channel through an alluvial 

 soil not yet regulated, or, in other words, which has not acquired its normal 

 inclination, the course of the river has not become established, it is natural 

 that its bed should rise more rapidly after its embankment 



" The embankment of the lower course of a river, near its discharge into 

 the sea, causes the elevation of the bed of the next reach above, both because 

 the swelling of the c\irrent, in consequence of its lateral confinement, occa- 

 sions eddies, and of course deposits, and because the prolongation of the 

 course of the stream, or the advance of its delta into the sea, is accelerated." 

 —Die cangiamenti cui soggiacqm Vidraulica condizione del Po, etc., pp. 41, 43. 



Del Noce states that in the leveUings for the proposed Leopolda railway, he 



