2008-09-29_PERMIT FILE - C1981019A (6)

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RULE 2 PERMITS Sediment Yield and Loss Rates Suspended sediment data is presented here separately from the water quality data for other constituents of concern. Suspended sediment concentrations are a function of many variables including precipitation patterns, soil types, vegetative cover, surface areas disturbed, slopes and drainage density. Suspended sediments are important from land productivity and aquatic habitat perspectives. Sediment yield estimates have been calculated by each investigation and are briefly summarized here. The EMRIA Report estimated sediment yield based on a method developed by the Pacific Southwest Interagency Committee in 1968 and assumed that the entire mine lease area was made up of seven homogeneous basins. Extrapolations of the per basin estimates were extended to include the entire drainage area. No data were collected to substantiate the model at that time. However, the EMRIA report concluded that the sediment yields during and after reclamation would not be excessive. VTN analyzed water quality data for suspended sediments for the period November, 1974 through July, 1975 for Goodspring Creek. Mean daily sediment concentrations ranged from a low of 58 mg/1 in November to a high of 310 mg/1 in May. These data reflect the climatic conditions with low values during low flow periods and increasing values occurring with spring runoff. Table 2.04.7-21 is a presentation of this data. VTN also analyzed suspended sediment data for Wilson Creek below the confluence with Taylor Creek. These data were collected for the period December, 1974 to September, 1975. Table 2.04.7-22 presents this information. Mean daily values are higher than those for Goodspring Creek but still reflect the seasonal effects described previously.. Since that report it became apparent that the Wilson Creek drainage was more susceptible to erosion than Goodspring Creek. Bank erosion on Wilson Creek during the excessively high runoff in 1984 is more appropriately defined as mass wasting. It has been estimated that in excess of one million cubic yards of unconsolidated material was down cut, slumped, and transported by Wilson Creek during spring 1984. An analysis of TSS during June, 1984 showed a concentration of 40,000 mg/1 in Wilson Creek. The spring runoff period is especially important in analyzing suspended sediment data. Both drainages showed tremendous increases in concentrations on May 27, 1975. These high values are probably a result of a storm event occurring within or just prior to the sampling period. Nevertheless, the values were not exceedingly high or even approaching values obtained in urban runoff studies. Data (see Exhibit 7A) collected by the USGS on Wilson Creek for the period October, 1979 through September, 1980 also show the tremendous seasonal variability. These data are presented in Table 2.04.7- 23. Exact locations for both sample periods, the previous VTN and the current USGS studies, were assumed to be comparable. The USGS data encompass an entire year. Again the seasonal fluctuations are apparent and storm event related concentrations increase dramatically over the ambient periods. Storm events not only move surface particles through overland flow, but also create bottom scour which is reflected in the increases. Also, the 1980 data was collected during an exceptionally wet year compared to 1974-75. This is the prime cause for mean values being higher. The Colowyo Coal Company Permit Application contains information regarding sediment yield from the mine site proper to Streeter Gulch. These estimates of erosion using a rainfall factor of 30.0 were an average of 3.17 tons/acre/year broken down into component parts of undisturbed and disturbed areas. The total acres contributing drainage to Streeter Gulch were 1367 with a maximum of 181 acres being disturbed at any one time. Exhibit 7 contains this information. Striffler and Rhodes (1981) conducted field tests at eleven surface coal mines in Colorado to determine erodibility and- infiltration capacity. of reclaimed land and overburden piles at each mine. Their report "Hydrologic and Erosional Characteristics of Regraded Surface Coal Mined Land in Colorado" is briefly Rule 2 Permits 2.04.7-23 Revision Date: 6/23/08 Revision No.: MR-91