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Item A: Public Hearing onMetro Plan Amendment (Delta Sand and Gravel)
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Item A: Public Hearing onMetro Plan Amendment (Delta Sand and Gravel)
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6/9/2010 12:58:15 PM
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12/7/2006 11:34:08 AM
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City Council
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Agenda Item Summary
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12/12/2006
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<br /> <br />EGR & Associates, Inc. <br /> <br />25358 Prairie Road <br />Eugene, Oregon 97402 <br />(541) 688-8322 <br />Fax (541)688-8087 <br /> <br />October 30, 2006 <br /> <br />~~c ~KJ~j~ <br /> <br />Date \ GJ - 0 ~ <br />Exhibit No. ~~ q <br /> <br />Steve Cornacchia <br />Hershner Hunter, LLP <br />180E11thAve <br />Eugene, Or 97401 <br /> <br />RE: Modification to Recommended Construction of the Groundwater Aquiclude <br /> <br />Dear Mr. Cornacchia: <br /> <br />As described orally in testimony and shown in drawings submitted to the joint planning commissions <br />of Lane County and City of Eugene, the "aquaclude" can be modified to address residents' concerns <br />'regarding the design and operation of the aquaclude (herewith spelled with an "a" to distinguish this <br />manmade structure from a naturally occurring poor permeability layer or "aquiclude"). Specifically, <br />concerns regarding both de-watering the nearby wetlands or causing excess water accumulation in <br />those wetlands. Some residents were even concerned about flooding from the aquaclude construction <br />but this is not hydraulically possible. Flooding comes from surface water accumulation and overland <br />flow. Groundwater cannot cause surface water flooding because the transmissivity of the soil is such <br />that water in sufficient quantities to cause surface inundation cannot be passed through the soil (it <br />sinlply runs off too fast). When flooding occurs, it always originates from surface water sources such <br />as the Willamette River or its tributaries. <br /> <br />To alleviate these fears and to more closely mimic pre-excavation conditions, EGR recommends that <br />the design of the aquacludebe modified so that the highest point of the aquaclude construction (a <br />silt/clay soil filled trench) be completed to an elevation 1 (one) foot below the mapped wetland <br />elevation in the drainage channel west of the excavation site. That elevation is nominally 377.6 feet <br />above mean sea level using the NA VD 88 datum. Therefore, we recommend that the top of the <br />aquaclude be constructed to an elevation of 376.6 feet (NA VD 88). This is roughly 6 to 8 feet below <br />the natural grade in the vicinity of the proposed resource area. See the attached sketch. <br /> <br />Constructing to this elevation will help retain water in the wetland during normal periods, but will also <br />allow groundwater discharge into the excavation during higher water table periods during and after <br />heavy rains. The aquaiclude would be topped with permeable materials above th~ top elevation of the <br />clay soils fill from which the aquaclude is constructed. <br /> <br />If you have any further questions or need for clarification please feel free to contact us regarding this <br />design parameter. The drawing from our earlier subn1ittal regarding this design element is included <br />here for visual clarification. <br /> <br />Sincerely, <br />/-~ <br />/ / <br />(I+L ~ <br /> <br />\ <br />Ralph Christensen G-870 <br />Senior Geologist <br />
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