DRAINAGE DISTRICTS. 201 



work should be determined by the methods outlined in 

 previous chapters. The topography should be found 

 in detail sufficiently to furnish data for the necessary 

 computations for defining the dimensions that will be 

 required for the main outlet and for such branches as 

 may be needed to provide for the drainage of individual 

 lands. The provision of outlets for the property of 

 owners located at some distance from the main channel 

 or drain will incidentally require the construction of 

 drains across farms which will thereby be much more 

 completely drained at the general expense than other 

 tracts for which a mere outlet is constructed to the 

 property line. In such cases there is a difference in 

 the amount of benefit conferred by the work which 

 must be considered in apportioning the cost to the sev- 

 eral owners. 



The system may consist of open ditches supple- 

 mented by large tile drains, or of either separately, ac- 

 cording to the area to be drained and the requirements 

 of the land which will be affected. Where it is desira- 

 ble to use large tiles for mains, and to lay them in the 

 line of natural drainage courses, it is always wise to 

 provide overflow ditches which will relieve the surface 

 at times of unusually heavy rainfall and at the same 

 time retain the benefits which are peculiar to under- 

 drains. (Fig. 40.) These surface drains should be shal- 

 low and broad and so graded that while all water may 

 be removed from the surface by the open ditch, the 

 flood water will pass off in sufficient quantities to per- 

 mit the tile drain to complete the work perfectly. The 

 surface drains may be called into action only occasion- 

 ally, but at certain times save crops worth many times 



