OPEN DITCHES 



115 



feet wide at the top (Fig. 48). It is used to remove surface water 

 during heavy rains and melting snows. Such a run may be con- 

 veniently used to protect depressions, and to prevent surface 

 water from entering a lower area. These drains should be kept 

 well sodded. As a rule a drainage system is not complete unless 

 surface-runs are provided to take care of the excess water during 

 flood flow. 



Open Ditches. Frequently small open ditches about 3 to 5 

 feet deep are employed, in place of surface-runs, to prevent flood 

 water from flowing on to low lands; the water is thus carried 

 around the low area or to one side instead. 



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FIG. 49. A small open ditch properly located. This serves as a makeshift outlet for the 

 tile system until the bed of the stream is lowered by a dredge. (Wisconsin Station.) 



A small open ditch about four feet deep and seven feet wide at 

 the top is sometimes dug through a low marsh kept wet by a slug- 

 gish stream, to serve as a temporary outlet for a system of tile 

 or underdrainage, until the stream is lowered and straightened by 

 a dredge (Fig. 49). 



Open ditches of considerable width and depth are employed 

 in large marshes where much water is to be removed and where 

 the fall is slight. Such ditches are of primary importance because 

 they form the outlets. They are dug by means of dredges. In 

 many marshes having winding and sluggish streams these ditches 

 are made by straightening and deepening the water-courses. 



On extensive marshes large open ditches, or canals, are con- 

 structed every mile or half-mile, with large tile at regular intervals 

 in between them. 



