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<br />o <br />i'? . <br />C\l <br />N' <br /> <br />,-' <br />--' <br />,-, <br />"-'"' <br /> <br />In other words, the inflow assigned to the narrow elements used to represent the dike was less than the <br />inflow assigned to the wider adjoining elements. Of the 1.0 ft3 Is of total inflow specified along the <br />boundary, only 0.2 ft3/s was assumed to be captured by the detention basin in the simulation; this <br />corresponds to the lowest estimated value of inflow and was calculated earlier in the report. The actual <br />inflow rate is not critical to the analysis when solute transport is not considered, The side boundaries. <br />parallel to ground-water flow and representing streamlines, were specified as no-flow boundaries, To avoid <br />potential negative effects on the flow system caused by these boundaries during simulation, they were placed <br />far enough (5,000 ft) from the elements representing the dike to ensure that they would not influence <br />simulation results in the vicinity of the detention basin. <br /> <br />," <br /> <br />Results of the Model Simulations <br /> <br />The first simulation was designed to verify that the simulated hydraulic gradient was similar to the <br />observed gradient. A constant hydraulic conductivity of 30 flld was assigned to all elements, including <br />those used in the following simulation to represent the dike and slwry wall. The simulated and observed <br />hydraulic gradients proved to be similar (figs. I2B and 7). Both the actual and simulated water levels reveal <br />a constant gradient with a total drop in hydraulic head of about 25 ft from the upgradient to the <br />downgradient extent of the detention basin. The steady-state simulated heads were used as the initial <br />conditions for subsequent simulations incorporating the dike and slwry wall. <br /> <br />For the second simulation, no ground water was allowed to penetrate the slwry wall or over-top the <br />dike. This was accomplished by setting the hydraulic conductivities of the elements representing the dike <br />and slurry wall to 0.0 ft/d. By making the dike and slwry wall impermeable to flow, all of the upgradient <br />inflow was diverted around the detention basin after the water levels within the basin had risen to the levels <br />at the upgradient end of the dike (fig, 13A). Figure 13B shows the simulated flow field as a result of the <br />impermeable dike and slurry wall. The usefulness of this simulation is not limited to its application here <br />with an impermeable dike, Ita confining layer of low hydrologic conductivity is assumed to occupy the <br />top part of the aquifer, the resulting water levels and corresponding flow field would be similar to those for <br />an impermeable dike. but perhaps not as pronounced. A silty clay of low hydrologic conductivity does. in <br />fact, occupy the uppermost part of the aquifer (fig,S) and may cause an increase in water levels behind the <br />dike and slwry wall, resulting in a diversion of sollie flow around the dike and perimeter. <br /> <br />The third simulation differed from the second in that it permitted water to over-top the dike wben the <br />water body within the detention basin was more than 6 ft deep adjacent to the downgradient leg of the dike <br />(this depth coincides with the top of the dike along its downgradient leg), To accomplish this, the computer <br />program was modified (see appendix A) by incorporating a conductance term into the specified-pressure <br />(head) boundary at the top of the dike, The conductance represents the relative resistance to flow and the <br />pressure boundary was selected to equal the altitude of the top of the dike. A conductance value of 1,0 <br />represents no resistance to flow, whereas a value of 0.0 represents no-flow conditions. This modification <br />allowed water levels to rise behind the dike structure until the head within the detention basin reached the <br />height of the dike (the specified value of the pressure-boundary condition), The program then changed the <br />conductance from 0.0 to 1.0 to allow water to flow over the top of the dike (figs, 14A and B). Simulation <br />results indicate that flow was not inhibited from entering the detention basin as long as water was permitted <br />to over-top the dike, In addition. ground water was not deflected around the detention basin in the <br />simulation when over-topping was permitted. Only immediately behind the dike were water levels higher <br />than during pre-slwry wall conditions. This scenario probably is the most accurate representation of how <br />the dike and proposed slurry wall would work. <br /> <br />These three simulations do not address the effects of the migration and distribution of salts. <br />heterogeneities and anisotropy of aquifer properties. surface-water inflow, or seasonal variations in <br />evapotranspiration. These effects are addressed in the following sections that describe cross-sectional <br />models of the Whitney area. <br /> <br />-31- <br />