LAND DRAINAGE 635 



The cost of maintenance of a system of open ditches is 

 heavy, because of erosion, the accumulation of silt, and 

 the growth of weeds, all of which make frequent repairs 

 necessary. 



Underdrains when properly constructed are more 

 permanent than open ditches and cost less for mainte- 

 nance. They do not interfere with surface operations. 

 The better grade gives them a relatively larger carrying 

 capacity than open ditches have, and their greater depth 

 below the surface permits much higher efficiency in the 

 removal of excess moisture from the root zone. 



535. Construction of small open ditches. — Small 

 field ditches may be used in the field to remove small 

 accumulations of surface water. They usually consist of 

 a furrow run in the lowest parts and made with a large 

 single shovel plow, with a turning plow, or with a two- 

 way plow having moldboards to turn the soil on either 

 side. Another modification in the construction of open 

 ditches, which is frequently combined with the foregoing, 

 is the use of " dead furrows." The land is plowed in 

 narrow beds two or three rods in width, with a deep 

 " dead " furrow between each which drains off some of 

 the surplus water from the higher parts of the intervening 

 area. A further modification is sometimes used in plant- 

 ing cultivated spring crops on wet land. Ridges are 

 thrown up along each row and the seed is planted on these 

 ridges. The intervening trench affords some drainage. 



536. Construction of large open ditches. — Where 

 larger volumes of water must be removed, a larger channel 

 is necessary, its size being determined by the area to be 

 drained, the grade of the ditch, its length, its straight ness, 

 and the smoothness of the sides and bottom. The ideal 

 shape for the ditch for the largest carrying capacity is a 



