Intercepting Underflow 



459 



In actual practice, it will usually be found that no 

 single system, such as has been represented, can be 

 used alone, but rather a combination of them in 

 various ways growing out of the irregularity of slopes 

 and surface conditions. 



INTERCEPTING THE UNDERFLOW FROM HILLSIDES 



Cases are not infrequent where seepage from the 

 high lands surrounding a flat area approaches so close 

 to the surface at the foot of the rising ground that a 

 single line of underdrains placed here at a good 



Fig. 147. Structural conditions producing swamp lands by underflow, and 

 methods of intercepting the underflow. 



depth will so completely intercept the underflow as to 

 make little other draining needed. The structural 

 conditions which render underdrainage in such cases 

 needful, the method of accomplishing it, and the 

 underlying principle, are represented in Fig. 147. 



In this case the comparatively impervious rock 

 bottom of the valley holds up the water and forces 



