146 ENGINEERING FOR LAND DRAINAGE. 



that these excesses come at a season of the year when a 

 short flooding \^ill injure growing crops, yet these facts 

 should all be taken into account when we provide a 

 drainage system. There are two difficulties which are 

 encountered in providing for floods with tile drains. 

 The first is that there are many soils that will not per- 

 mit the water to pass through them rapidly enough 

 so that the water will not accumulate upon the surface 

 when the rainfall is sudden and large in quantity. The 

 second is that the tile which should be used to carry it 

 all off must be of double the ordinary capacity, which 

 involves a great expense. 



In general land drainage it will be found wise to pro- 

 vide for the excessive rainfall by keeping well-con- 

 nected depressions or broad shallow open ditches along 

 the course of natural drainage, so that the flood water 

 will pass off over the surface and thus relieve the un- 

 der drains of the excess. In level lands, where there 

 is the most necessity for this provision, no harm will 

 come from surface washing, for as soon as the excess 

 passes off over the surface the remainder of the drain- 

 age passes off through the soil. There is no loss of 

 land, since in most cases these broad depressions or 

 ditches can be cultivated. 



Where the fall is very considerable, the capacity of 

 the underdrains will be greatly increased by reason of 

 the grade that may be obtained and hence there will 

 be less necessity for surface drainage, yet this part of 

 the work should never be lost sight of. 



There are, however, valleys or depressions like large 

 ponds or swamps for which it is impracticable to main- 

 tain any surface drainage. For such places we must 



