DRAINAGE 



279 



and depth in hand and team work ; in dredge construction 

 it decreases, and the excavation is not delayed by the 

 presence of water or sticky material. 



A 6-foot bottom is the minimum for a small-sized dredge, 

 and ditches with a bottom width of from 50 to 100 feet 

 have been constructed. The depth varies from 6 to 20 

 feet, the excessive depth being usually in a cut-off through 

 a ridge. In the construction of outlet ditches it is essential 

 to have the channel of a depth to furnish a good outlet for 

 all lateral drainage; in flat country 6 feet is the minimum, 

 and under many conditions of topography it should be 8 

 feet. Much of the lateral drainage will consist of large- 

 diameter tile, and at the time of designing the main chan- 

 nel the fall of the lateral ditches and their depth at the 



After University of Minnesota 



Fig. 229. Small floating dredge 



lowest point must be taken into consideration, and the 

 depth of channel determined which will give the required 

 drainage from the lowlands along either side and some 



