05/09/2007 P&Z Packet - Laserfiche WebLink

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The large Anoka County Ditch wetland contains areas of peat extending to over 14' below the surface. Soil borings adjacent to the one taken in the ACD #25 ( #2 and #3a) wetland show groundwater elevations to be below observed groundwater elevations in ACD #25. Those borings do not contain peat which indicate those upland areas are not directly feeding the ACD #25 wetland, and that ACD #25 wetland characteristics are mainly as a result of the peat deposits - abi -lity -te -hold water: -Given the ACD #25-wetland- type {Type If), the wetland -can function without baseflow and by definition, can have occasional dry periods. The groundwater tables for all other areas not located in peat appear to be dictated by clay layer locations and possibly artesian pressure from lower groundwater aquifers. Given the watershed, which is relatively flat and serves large floodplain areas, groundwater and surface water could easily enter the ACD 25 watershed area due to backflows from downstream watersheds, or lateral groundwater movement from upstream surface watersheds. ANTICIPATED NWL OF PONDS The anticipated NWL or Ordinary High Water Level for each of the ponds are shown in the attached sketch. The elevations are estimated based on soil boring readings. Mottling of soil above the groundwater elevations do exist for Borings 1 and 2 which indicate the NWL could be higher than what is currently shown. We expect the NWL of Pond C will remain relatively consistent given the presence of sandy soil above the NWL. Pond B has more clay concentration, however we feel the soil will allow effective transmission of water as to keep NWLJOHW at or near the 881.5 elevation because there is sufficient larger particle size material. DEWATERING Ponds A and B dewatering outlets will be placed at a location upstream of the main ACD #25 channel within the outer wetland area so that direct discharge into the main ACD #25 channel will not occur. Pond C dewatering outlet will be to the southern wetland located adjacent to the pond. FLOODPLAIN ALTERATION Pond A will provide an additional 1.73 Ac -ft of floodplain storage. Pond B will create an additional 14.25 Ac -ft of floodplain storage. The additional storage is taken from the pond overflow to the 892.1 flood elevation. Pond C should not add additional storage due to the permanent inundation of the surface from the bottom to the 100 -year flood elevation. Excavation of material will not be filled in existing floodplain_ Attachments: Excavation Pond Sketch Soil Borings •