Laserfiche WebLink
<br />Table 2.6-1 Maximum Allowable Overland (Sheet and Rill) Flow Lengths <br />Quality of Cover Slopes s 25% ~i Slopes > 25% <br /> Lengths (feet) <br /> Hay Field <br />Poor (0-30%) 250 I not encountered <br />Fair (30-70%) 275 not encountered <br />Good (70-100%) 300 not encountered <br /> Rangeland <br />Poor (0-30%) 150 125 <br />Fair (30-70%) 175 150 <br />Good (70-100%) 200 ~ 17~ <br /> Compacted Dirt Road and Gravel Areas <br /> ~ I00 not encountered <br />r,. <br />(- <br />i <br /> <br /> <br /> <br /> <br />1 <br />2.7 Curve Number (CN) <br />A curve number reflects the runoff potential of an area, and is a function of the type soil <br />(potential for infiltration) and the land use (the type of vegetation and litter). For instance. <br />a value of 100 represents an impervious area where I00% of the water would run off. ' <br />SEDCAD+ documentation generalizes % ground cover density into three categories of <br />poor, fair, and good, and uses mid-range CN values for the categories. To discriminate for <br />ground cover densities determined by the 1996 Vegetation Survey, CN values were ' <br />determined from Figure S-3 of the Natural Resources Conservation Service's (SCS) <br />Procedures For Determining Peak Flows Irt Colorado. (SEDCAD+ tables are derived from ' <br />the same source documents.) Figure 3 is entered with the total perennial vegetation (trees, <br />shrubs, subshrubs, grass and forbs) and litter cover to determine CN values. <br />The 87% ground density cover of the Alluvial Valley Floor Reference Area (1996 ' <br />Vegetation Survey) correlates with a CN value of 74 (Figure S-3, Dry Land Pasture or <br />Rangeland). ' <br />The 29% ground density cover of the Rangeland Reference Area (1996 Vegetation Survey) <br />correlates with a CN value of 8~ over Nunn Loam soils and 87 over Carracas Loam soils <br />(Figure S-3, Dry Land Pasture or Rangeland). ' <br />Page 6 of 34, 7 Oct 96 ' <br />