Ordinance No. 20413 - Laserfiche WebLink

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occur as excavation taking place below existing ground surfaces. Within that mining methodology overburden will be removed and stockpiled in areas higher in elevation than base flood elevations and/or stockpiled at locations on company property that are below existing ground elevations. EGR concludes that the proposed mining methodology on the proposed expansion area will not impede flood flow, reduce flood storage volume within the flood plain or increase the velocity of water flowing across the proposed expansion area. EGR further concludes that the proposed mining methodology completely avoids all potential flood impacts. Opponents of the application argue that the existence and operation of the aquaclude (see subsequent discussion and findings regarding the aquaclude) will cause flooding on adjacent residential lands because groundwater will be prevented by the aquaclude from migrating from those lands to the mining site during heavy rain events. EGR has addressed that argument and has demonstrated, in its rebuttal materials provided during the planning commission joint public hearing and deliberation, that the movement of groundwater is not connected to surface water that constitutes flooding during such events. Nonetheless, in response to the opponents' argument, the applicant and EGR have provided a modified aquaclude construction that leaves the top elevation of the clay-filled aquaclude one foot below the measured elevation of the wetland that exists on the meandering scar, approximately six to eight feet below ground surface. While continuing to impede the rate of flow of groundwater from the adjacent property to the expansion area (and mining pit), the aquaclude would allow the passage of a limited amount of groundwater from the adjacent property, over the clay material and into the mining area during heavy rain events. That elevation will also ensure that the aquaclude has no negative impact on the water level of the wetland at different times during the year. We find that EGR has provided significant evidence to support it conclusion that the aquaclude will minimize potential conflicts resulting from the movement of groundwater from adjacent lands to the mining area and that its existence and operation will not produce flooding on adjacent lands. The Eugene Planning Commission voted 3-2 that there was not a conflict due to flooding. We find that approval of the application will not result in flooding conflicts with adjacent land uses. Groundwater EGR recommends that a low permeability barrier, groundwater dam or other flow restriction of the upper aquifer should be constructed, at the applicant's expense, as the excavation proceeds to the west. EGR concludes that the construction of a low permeability barrier could actually result in lower pumping of water, with significant benefit to the aquifer and area groundwater users. That low permeability barrier has been termed the "aquaclude" by EGR and the applicant. EGR recommends that excavation should begin on the east side of the expansion area and that the aquaclude should proceed ahead of the excavation to the west. Construction of the aquaclude should occur before excavation comes within 400 feet of its location. The aquaclude should be at least 12 feet wide at the bottom and slope upward through the upper aquifer from 1:5 to 2: 1 to the original ground surface. The aquaclude should be placed within the setback area to insure its separation from the excavation proper and to place its outer edge as close as possible to the surrounding off-site shallow aquifer. EGR also recommends that a buttress of native material be left between the aquaclude and the excavation area proper. EGR's ultimate conclusion is that placement of a low permeability barrier around most of the excavation should result in restoring groundwater levels around the excavation pit to near pre- development levels. DOGAMI has provided the applicant with its preliminary concurrence with EGR's ultimate conclusion. A copy of correspondence from the agency to the applicant is included with EGR's evaluation attached to the application. The correspondence states that "there may actually be a slight mounding of the ground water in the shallow aquifer within close proximity to the barrier thus eliminating the possibilities of drawdowns which could impact supply wells within the area." Exhibit A to Ordinance 20413 - 22