7049 - Laserfiche WebLink

Home Browse Search

, -" \"ill be steeper. ihese hillslopes probcbl\',.,ill have greater runoff and ero- sion rates than the original Dndisturbe~ hi l:sl~pes. As reclamation proceeds and vegetation becomes reestabl ished, the erosion rate sho~ld gradually decrease to amounts comparable to adj3cent u~disturbed hi llslopes. An estimate of the change In sediment y=eld due to surface mining may be computed by the PSIAC method. As oescr;beJ ~(eviously, this method rates the character of a 'datershed in nine cateQo;-ies ta~le 3). An estimated, se'dinent yield is detern'ined by cOflpaiing the nw'er:ccl score of the '/!qt~:r:shed b.::ing evaluated wi th the numerical score_of - 'l:atelS'neds vJi th measured sedinent yields. The PS~-i\C method I,S apcropriate for estimating the jncreased sediment yielddue-the<s~rface mininq, beca:lse so~ne of the:tategories listed in table 3, such as soil' and _ve~tatior! co\/er, vJi 11 change. Other categories, such as surfa~e-geology and cl imate, wi II not change. ~_....~' Approiimately 6.0 mi I I ion tons (5.4 mi I lion t) of coal were mined in the Yampa River basin during 1976. The ar'ount or coal mined has increased significantly since 1962, and there is sc~e uncertainty as to how rapidly coal mining and utilization \'JiJr~..'exDand in the basin in the near future. Three alternative levels of coal pro~ucticn through 1990 have been assumed (Udis and others, 1977; Steele and others, 1978). The three estimates aSSume that, in 1990, 80 percent of the coal production \'Iill come from surface mines and 20 percent from underground mines. It is projected that surface-mined coal will increase to 8.0 million tons 0.3 nillion t) per year as a S]o\'l- grovJthestimate, 16million tons (14.5n:illion t) per year as a moderate- growth' estimate, and 24 mi I lion tons (21.8 mi Ilion t) per year as a rapid- growth estimate. By using a coal-yield ratio of 20,000 tons per acre (44,840 t/ha) of land mined, these projected levels of coal production can be expressed in terms of land Grea disturbed per year. I t is further assumed that the disturbed land wi 11 be partly reclaimed within 5 years and complete- ly reclaimed in 10 years. On the basis of th~se projections and assu~ptions, the area of land affected each year by mining as wel I as partly reclaimed can be calculated for each of the three levels of production for 1990. For purposes of this analysis, it wi 11 be assumed that al I increased surface min- ing will occur in the Yampa River subbasin (U.S. Department of the Interior, 1976). Estimates of sediment yield fro"1 recently mined and partly reclaimed land were computed using the PSIAC [:letho:!. First, a numerical rating ,'Jas computed for the unmined area, and t~e c51culated sediment yield compared \oJith the measured sediment yield. The n'..;mc>,i:::al rating then was revised so that the calculated sediment yield 2greej wit~ the measured sedi~ent yiel~. Not all of the factors rated by the PSIAC method will change due to surface mining. Bedrock geology and cl i~ate, of course, wi 11 rerain unchanged by surface mining. Other factors, such as runoff and topography, will most I ikely change slightly; whereas, soil type, grou~d cover, and land use wi 11 be radically changed. The numerical rating of these factors must be adjusted accordingly. The PSIAC method i~dicates annual sediment yields of 4,000 tons/mi2 (1,400 t/km2) from unreclaired surface-mined land and 2,000 tons/miL (700 t/km2) for partly reclaiQed land. 27