604- PROTECTION AGAINST SWA MI'S. 



1. Vertical Drainage. 



Vertical drainage is carried out by piercing an impermeable 

 stratum and thus allowing the water to descend into a lower 

 permeable stratum and be thus drained off. One or more 

 borings of sufficient breadth should be made through the 

 impermeable stratum at its lowest point, as shown in Fig. 269. 



As an instance of the effects of vertical drainage, the 

 planting of part of the Bagshot sand district, near Bagshot, 

 by Schlich (1890-1900), may be cited. The land was mostly 

 flat, with a pan 6 to 18 inches below the surface. It formed 

 a swamp .during winter, and was often very dry in summer. 

 The pan was bored through at the planting holes with a 

 pickaxe, and Scots and Weymouth pines planted. This 

 communication between the upper and lower strata of the 

 soil led to a complete drainage of the surface, and a fine crop 

 of pine trees is now growing there. 



2. Surface Drainage, 

 a. By Open Ditches. 



i. MODK OF LAYING OUT A SYSTEM OF DRAINS. 



The mode of laying out a system of drains depends on the 

 nature of the locality. On level ground and in valleys, a 

 complete network of drains is laid out after the land has been 

 carefully levelled, and consists of main drains, leaders and 

 feeders. 



The main drain should run along the lowest part of the 

 area to be drained, necessary excavations being made to give 

 it a uniform gradient between 0*5 and 1 per cent., and to 

 conduct it into the nearest watercourse. 



The leaders must carry off the water in the shortest direction 

 from the feeders to the main drain, while the feeders have to 

 collect water from the soil and conduct it to the leaders, the 

 direction of which depends on the gradient of the ground. 

 When the gradient is moderate, the main drains and leaders 

 should run along lines of greatest fall ; if it is too steep, their 

 length must be increased by causing them to wind or bend so 

 that the drainage water may not wash away the bed or sides 

 of the drains, and cause the formation of ravines. The best 



