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The magnitude and extent of the groundwater mounding conditions is also contingent upon the <br />aquifer's transmissivity (aquifer permeability multiplied by aquifer thickness), the specific yield of <br />the aquifer materials, and the duration of the high water levels in the pond. In general, thicker <br />aquifers with higher permeability will experience less mounding than thinner aquifers of lower <br />permeability. Perched aquifers (i.e. groundwater zones less than about 10 feet that overlie extensive <br />clay layers) typically experience the greatest amount of mounding. <br />Overview of Variance Evaluation Method <br />All of the combinations of settings, pond configurations, aquifer parameters, and distances from <br />ponds cannot be anticipated before hand in coming up with a method to quickly evaluate whether or <br />not a variance to the minimum floor elevation ordinance should be considered. However, by making <br />some generalities, the most commonly encountered situations can be evaluated. This is the approach <br />taken here. <br />A groundwater flow model of a "typical" pond and aquifer setting was developed. Aquifer <br />parameters and pond elevations were varied and the resulting water table mounding conditions were <br />simulated. The following conditions were evaluated: <br />1. Pond elevation increases of 2 feet, 4 feet, and 6 feet above normal or dry conditions <br />2. Depth to the water table (before flooding) of 3 feet (to represent conditions of 3 feet or less) <br />and 10 feet (to represent conditions where the depth to the water table is greater than 3 feet). <br />The purpose of simulating these two conditions is that with shallow water tables, the rate of <br />infiltration is substantially reduced as the groundwater mound rises into the pond. For deeper <br />aquifer conditions, the pond bottom is always above the water table and the depth to the <br />water table has no bearing on the seepage rate. <br />3. Three aquifer conditions: clay or perched aquifers (transmissivities of 7 ft2/day and specific <br />yield values of 0.1); silt aquifers (transmissivity of 70 ft2/day and specific yield values of 0.2) <br />and sand and gravel aquifers (transmissivities of 2000 ft2/day and specific yield values of <br />0.2). <br />4. Pond bottom sediment thickness of 1 feet and bottom sediment hydraulic conductivity of 1 <br />ft/day. <br />5. Instantaneous occurrence of a flood condition in the pond, which lasts for25 days, followed <br />by instantaneous reduction to normal conditions. The purpose of using this condition is that <br />