REP03568 - Laserfiche WebLink

Home Browse Search

Harding Lawson Assodatea HELP is a comprehensive quasi-two-dimensional hydrologic model of water movement across and through repositories. The model accepts weather, soil and design data, then solves and tabulates surface storage, lateral subsurface drainage, unsaturated vertical drainage, and leakage through soil layers. The model enables the user to estimate runoff, evapotranspiration, percolation, and leakage. The program was developed by the COE for the U.S. Environmental Protection Agency (EPA), Risk Reduction Engineering Laboratory, Cincinnati, OH. HELP Version I was initially released in 1984, and has since become the national standard for landfill and repository design modeling. Climate data for the model was obtained from the NOAH National Weather Service station PAONIA ISW located in Paonia, Colorado, 15 miles west of Somerset (see Appendix A). Monthly averages for years 1957 through 1993 were used for the model. ' Exposed refuse was given the same permeability and texture of coarse gravel. The HELP model used an SCS runoff curve number of 60 for the exposed coal refuse. In addition, a level surface was used for the exposed refuse to limit surface runoff. The model was run fora 10-year period to allow steady state moisture conditions to establish and to simulate conditions over the operational lifespan of the LRP. The ' model consisted of three horizontal layers: ' 1) an 80-feet thick layer of new refuse, with hydraulic conductivity characteristics of coarse gravel; 2) an 80-feet thick layer of old refuse, with hydraulic characteristics of silty clay; and 3) an alluvial soils foundation. ' The model generated an estimated water storage (perched water table) maximum height of 5-feet in the upper, newer refuse after l0-years of simulation. This estimate is conservative as it assumes all stored ' water in the upper layer will accumulate at the old/new refuse contact, and does not account for surface runoff on the reclaimed portions of the LRP. The model also predicted a water mound of 22-feet perched atop the alluvial contact along the base of the lower, older refuse. This prediction compares reasonably well with the 27-feet perched mound estimate made when the LRP was originally permitted (Exhibit 51, July, 1981). The original estimate did not account for vertical percolation into the alluvial subsoils, making it larger and more conservative. Ground water level information from monitoring ' well GP-1 and piezometer GB-1 at the base of the LRP eastern flank show groundwater is not mounding within the lower older refuse material. This indicates that the underdrains and french drains are effectively removing the groundwater and alluvial soils are more hydraulically conductive than assumed. All models are presented in Appendix B. SLOPE STABILITY MODELING USING 5-FEET HIGH PERCHED WATER TABLE ' A 5-feet thick horizontal perched ground water table was incorporated into the original slope stability model used during the July, 1996 analysis. The model used the worst case upper refuse conditions where the entire mass consisted of 10% fines and had zero cohesion. The results produced are presented in Table 1. All slope stability calculations are presented in Appendix C. 1 1