-ViN IKklCATUi.N S1:TTLKAU-:NT. ^2ij 



Ik'toro describing the furrow itself in detail, 1 must first describe the 

 two ieiubng features of the work to whicli the success of the whole is 

 largely due. namely, the drystone walling and the tilting escapes. Both 

 are clieap and effective methods of overcoming tiie two diliiculties of steep 

 krantz work and cross drainage which liave made st) many schemes 

 imjxissihlc on account of the great expense in overcoming them in any 

 other way. 



The attached sketch shows in section a typical example of drystone 

 walling. In Imilding these walls, three leadiiig points have to be kept 

 in view : — 



u/ ) During the early stages of the use of the wall, it has to act like 



a t'llter bed, arresting tlie passage of all line material and only 



allowing clear water to pass through. 

 ib) The tine material has to be arrested in such a way as to form 



eventually as permanent a water-tight channel as possible. 

 (,f) The structure must he such that crabs and rodents cannot 



easily cause a breacii. 



The most important preliminary operation is to clear the rock of all 

 Ime river silt, especially near the outer toe of the pitching. Nothing is 

 more certain to cause a breach than the neglect of this precaution. 

 Once a sn.iall stream starts in the silt, it soon enlarges itself till a hole 

 is formed sufficiently large for the internal gravel and packing to pass, 

 after which the collapse is rapid. The silt also forms an easy passage for 

 crabs, and they do not waste much time in finding it out and starting the 

 small destructive stream. 



The pitching is heavy and has to be exceptionally well done. The 

 gravel is obtained by blasting from the surface rock, which is decomposed 

 gneiss, and locally called ts gom. This provides an excellent gravel for 

 the purpose. It is clean and coarse, and the particles being of varied 

 size, the interstices are never very large. The liner gravel is reserved 

 for the last foot, and this is carried up to W'ilhm an inch of bottom con- 

 struction level. This point should be carefully noted. In most works, 

 where the silting process is used, it is customary to leave the channel 

 about a foot lower and wider than is necessary to carry full supply, this 

 extra area being subsequently tilled automatically by silt. The method at 

 Kakamas is, however, more suitable where the wall is entirely made of 

 such porous material. The silt cannot settle as a layer on the bottom, 

 but is carried by the water through the gravel until it settles in some 

 of the numerous pockets where the velocity of the water is checked. For 

 the first few weeks after the muddy water is let into the channel, water 

 oozes out gently in many places below the wall, where reeds and other 

 growths rapidly spring up. After a couple of months, however, this 

 growth lies dead and dry below the wall, owing to lack of moisture — 

 so rapidly and effectually does the structure become water-tight. The 

 water-tight area is not confined to the immediate bottom of the furrow, 

 but extends througliout the structure wherever the muddy water reached, 

 and it is exceedingly difficult for crabs or other animals to make a way 

 through it, and, c\ en if they succeed, they do not wreck the structure, 

 the process of hltering going on again lower down in the gravel until the 

 hole is automatically repaired. 



The tilting escapes are simple vertical wooden shutters 12ft. wide 

 by about 2ft. 3in. deep, pivoted on a horizontal axis, which is set at a 

 little less than one-third of the height from the bottom. They are 

 put in at bed level of the furrow wdiere the formation is hard, and, when 

 the water in the furrow rises to above full supply, they tilt automatically 

 and lie horizontal, the water escaping under and over them. It is 

 interesting to note that wdien Mr. Lutz first made these sluices on the 

 south furrow, he found the correct position >>i t!ie horiz( ntal axis l)v 

 experiment and not by calculation. 



There are thirty of these tilting escapes in the seventeen miles of 

 the north furrow, and half of these are in the first five miles. What a 



