The quantity of earth to be moved in build- 

 ing the terraces and the dikes is approximately 

 proportionate to the maximum cut. Relatively 

 little earth needs to be moved to build narrow 

 benches on gentle slopes. For example, with 

 a 1-percent slope and a 14-foot bench, removal 

 of only 32 cubic yards per acre is required; on 

 the same slope, a 70-foot bench would require 

 removal of 160 cubic yards (table 3). On a 

 10-percent slope, a 14-foot bench would re- 

 quire 320 cubic yards, while a 70-foot bench 

 would require 1,603 cubic yards. 



Width of Dike 



To control water, each bench must have 

 a dike at its outer (front) edge, and one on 

 each end. Experimental results at Mandan 

 show that the best results in snow control are 

 achieved if the dikes are seeded permanently 

 to bromegrass and not harvested. Conse- 

 quently, the area occupied by the dikes and the 

 backslope at the front edge of each bench is 

 excluded from the productive area. To pro- 

 vide access to the benches for harvesting ma- 

 chinery, the end dikes should be built with 

 gentle slopes and seeded to alfalfa. 



In a field laid out with continuous benches, 

 the area occupied by dikes is a function of 

 the number of benches, and the area occupied 

 by the backslope below the dike is a function of 

 the depth of cut, which in turn is a function of 

 the slope. To calculate the productive area lost 

 to dikes and backslopes, it was assumed that 

 each dike would be 2.5 feet wide at the base 

 and the backslope ratio of horizontal to vertical 

 distance would be 1.5:1. Thus for 14-foot 

 benches without contributing areas, dikes and 

 backslopes would occupy 15.2 percent of the 

 area on a 1 -percent slope and 25.2 percent on 

 a 10-percent slope (table 4). For 70-foot 

 benches, dikes would occupy 4.9 percent of 

 the area on a 1-percent slope and 16.3 percent 

 of the area on a 10-percent slope. If benches 

 are built with contributing areas, the propor- 

 tion of land occupied by dikes is reduced by 

 a little less than half. 



Turning and Odd-Shaped Areas 



Since the dikes separating the benches 

 are built with a steep backslope, it is not prac- 

 tical to drive across them with most farm 



machinery. Each bench thus tends to become 

 a separate field for all tillage and harvest 

 operations. In designing a system of benches, 

 access to each bench must be provided. Prob- 

 ably the most practical way to assure easy 

 access would be to leave space for turning 

 along both sides of the field, as illustrated in 

 the diagrams of hypothetical fields (figs. 4 

 and 5). For each 100-acre field, 4.0 acres 

 were assumed to be occupied by turning areas. 



To achieve the most satisfactory pattern 

 of travel in harvest operations, it would be 

 ideal if the benches were rectangular and 

 parallel to one another. Such benches could be 

 built only on a field that was exactly uniform 

 in gradient and direction of slope, but areas 

 of such uniformity are seldom found. Changes 

 in the direction or degree of slope will in- 

 variably occur, resulting in adjustments in 

 alinement. When such adjustments are made, 

 small irregular or odd-shaped areas will 

 remain, unless a substantial amount of earth 

 is moved. Such areas are difficult to work with 

 either tillage or harvesting machines because 

 much extra turning is required. For this 

 analysis, it was assumed that these would be 

 harvested, and that the yields of hay would be 

 the same as on natural slopes. 



The number and size of odd-shaped areas 

 would probably increase as complexity of the 

 slope increased. Also, it is probable that the 

 steeper the slope the more difficult it would be 

 to maintain parallel alinement, so the amount 

 of odd area likely increases somewhat as the 

 slope increases. Actually, there is no way to 

 estimate these areas except on a case-by-case 

 basis, using the actual topography of a par- 

 ticular field. To illustrate the probable effects 

 of this hypothesis, it was arbitrarily assumed 

 that for benches without contributing areas 

 the area occupied by odd areas would increase 

 0.5 acre per 100 acres for each percentage 

 increase in slope. For benches with con- 

 tributing areas (1:1 ratio), the increase in 

 land used for odd-shaped areas was assumed 

 to increase by 0.3 acre per 100 acres for 

 each percentage increase in slope. In each 

 situation, a standard deduction of 3 acres 

 per 100 was made for roads, lanes, and 

 fences. 



