84 Kkpokt S. A.A. Advancemkxt of Sciknck. 



tliem ha^'e at some time in their liistory suffered more or less clestruction- 

 from floods. 



The weak points are usually either — 



(a) Too small provision of overflow for floods, or 



(b) Defective arrangement of outlet. 



The former is an almost universal defect in farmers' dams, and the 

 outlet, if in the form of a pipe through the dam — which is usual — is 

 often badl}' constructed, no proper precautions being taken to prevent 

 seepage along the outside of the pipe. 



Rpsprroir Sitei<. — The country abounds in good sites for large 

 storage reservoirs, many of the poorts already referred to, through 

 which the rivers pass, offering excellent sites. In the majority of 

 cases, ho\ve\er, sufficient is not yet known regarding the quantity of 

 water available, and until some reliable knowledge of the rainfall and 

 run-off is obtained the expenditure of large sums of monej- would be 

 better deferred. Where the site for a reservoir is a poort with a lai'ge 

 catchment above it, the ideal work would generally be a masonry' dam 

 over the top of which flood-water would be passed. 



The narrowness of the valle3's and their steep sides at the poorts 

 does not usually allow oi the provision of an overflow of sufficient 

 width for an earthen dam at reasonable cost. 



It is rather a curious circumstance, however, that at many of 

 these poorts there is a natural spillway — often at some distance away 

 from the dam site — which can be utilised if its level is such as to give 

 a suitable capacity to the reservoir. These natural spillways apparently 

 mark the course of the river at some earlier geological period, and are 

 often at about 50 to 60 ft. above the river bed, Avhich is generall}'^ a 

 ccin^enient height for an earthen dam. 



As to materials, good clay is almost unknown, so that a puddle 

 core can seldom be attempted, but earth can generally with judicious 

 selection be obtained which will give a satisfactory dam of medium 

 height. Hock is generally obtainable in abundance suitable for con- 

 crete or masonry. Thus the masonry dam would appear to be an 

 enticing proposition. Unfortunately difficulty of transpoi't often 

 makes the cost of Portland cement so high ars to be almost prohibi- 

 tive. Some da}' this may be remedied by better transport facilities or 

 by the cheap production of good local limes, though experiments made in 

 the latter direction have not hitherto pro\ed very encouraging, chiefly, 

 no doubt, owing to high price of fuel for the burning of the lime. 



C'o!<t of Irri<iafion. — The cost of pumping 2 ft. of water on to the 

 ground may be anything from £,\ to £10 per acre according to height 

 of lift and total area irrigated. For areas above 20 acres and lifts of 

 frou), say, 30 to 150 ft. the cost pei- acre-foot per annum of pumping 

 will be given approximateh' by the formula^ — 



£, = \^ + -01 H, 



where £ is the cost in pounds sterling per acre-foot per annum, A is 

 the total area to be irrigated, and H is the height pumped in feet. 



