10 BULLETIN 114, U. S. DEPARTMENT OF AGRICULTURE. 



SIZES OF DITCHES. 



The computations of the ditch capacities have been based on the 

 Chezy formula for flow in open channels, V = CVRS, in which C was 

 determined by Kutter's formula, using 0.030 as the value of the 

 roughness coefficient " n. ;; Except at the lower end of Boggy Swamp 

 and Black Swamp, however, the controlling feature in determining 

 the sizes of the ditches has been the method of construction rather 

 than the required capacity of the waterway. 



Experience has shown that in heavily timbered lands such as these 

 the best type of machine to use is the floating dipper dredge; but 

 to float a machine of this kind frequently requires a larger ditch 

 than would be necessary to take care of the maximum run-off. In 

 lieu of the dredge the only practicable methods of construction will 

 be hand labor or explosives; by the former method a ditch of just 

 the required size could be constructed, but at a price per cubic yard 

 very much greater than for dredge work, so a ditch roughly two 

 and one half times the required size, provided it is no smaller than 

 the minimum dredge ditch, could be constructed by the dredge for 

 the same cost as by hand labor and in a much shorter time. The 

 construction of ditches with explosives is comparatively new, and 

 has met with varying degrees of success. It might be well to experi- 

 ment with this method on some of the smaller ditches, which are of 

 a size best adapted to its use. 



The smallest ditch that can be constructed with a dipper dredge 

 of sufficient power to remove the stumps and sunken timbers is one 

 with a 14-foot bottom width, 6-foot depth, and side slopes \ hori- 

 zontal to 1 vertical. Such a ditch would have a top width of 20 

 feet. However, to afford proper drainage the depths of the dredged 

 ditches should in this case be not less than 7 feet, and therefore 

 wherever in this report the phrase "minimum dredge ditch" is 

 used it refers to a ditch with 14-foot bottom width, 7-foot depth, and 

 \ to 1 side slopes. Ditches of this character are proposed for all of 

 the main tributary branches except Hurricane Branch, Scott Swamp 

 No. 1, Ballifore Branch, Rose Hill Branch, Titi Branch, Causeys 

 Branch, Long Branch, and Lyas Bay, which streams are too short to 

 warrant putting in a dredge. On some of the tributaries the upper 

 mile of each ditch should be excavated by hand, as such channels will 

 have sufficient capacity and will cost considerably less than minimum 

 dredge ditches. The ditches to be constructed by hand labor, with 

 one exception, are recommended to have 3-foot bottom widths, 

 5-foot depths, and J to 1 side slopes; such a ditch has an 8-foot top 

 width. This kind of ditch will be referred to in this report as a 

 "handmade ditch," as all of the ditches not constructed with dredges 

 are recommended to be of this size except the upper end of Boggy 

 Swamp, which should be 6 feet deep. The excavation for a minimum 



