PROTECTION AGAINST FIRES AND MOVING SAND 37 



These ditches must be at least 60 centimetres (24 inches) deep, 

 and the sand of the lower wall must be packed or pounded 

 to prevent erosion by the winds. The theory of spacing the 

 ditches is officially described as follows: 



"The horizontal spacing of the two ditches will be such that 

 the amount of water falling between the ditch above and the 

 ditch below can be entirely reservoired in the lower ditch; 

 for example, for a fall of 25 millimetres (0.97 inch), taking 

 the maximum fall for Tozeur, it would be necessary that the 

 distance L between the two ditches should be given by L -r- 

 0.025 = S (S being the cross section of the plowed furrow). 

 If the ditch has a depth of 60 centimetres (24 inches) and a 



width of 60 centimetres its section is S = ^-^ = 0.18 



2 



or L = - - = 7 metres (23 feet) more or less. 

 0.025 



"It thence results that all the water will be retained on the 

 surface of the soil, until* it sinks into the sand in the bottom 

 of the ditches, bringing quantities of seed which, placed under 

 the most favorable conditions, ought to germinate the more 

 readily. It must be carefully seen to that the alignment of 

 the ditches should be exactly horizontal; if not, the water 

 collects at one point on the ditch, overruns the bank, and 

 the damage is worse than if nothing had been done." 



This is the theory of the operation; in actual practice the 

 ditches soon become partially filled with sand and have to be 

 deepened to prevent the water from overflowing. Nor does 

 the seeding take place naturally as often as might be wished. 



"In the case of ravines which are (already) eroded, it is 

 necessary at the start to partially fill them up, usually until 

 the side walls are vertical; profiting by the facility with which 

 the soil can be worked, it is necessary to cave these walls 

 into the ravine ... in order to obtain a new profile which 

 is well rounded . . . where one can establish several cor- 

 dons of brush to diminish the velocity of the water. In the 

 bottom of this ravine, in part corrected, dams should be 

 built of palm tree trunks, . . . near together, each built 

 of two palm trees split horizontally . . . ; when these 

 dams, which may easily be destroyed, appear insufficient, 

 there is nothing to do but to increase them " 



