02-19-2002 Planning Packet - Laserfiche WebLink

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been designed so that an adjacent wetland will be created, not a side channel to the drainage ditch. Side slopes within the mitigation site will be a minimum 5:1 along the southern property line and a minimum of 10:1 where it lies adjacent to the existing ditch. The bottom elevations of the wetland will be left rough to provide a diverse bottom contour and to decrease possible channelization by moving water. No construction shall take place if the site is inundated or has potential floodwater in the immediate future. Hydrology in the mitigation area will be primarily from natural flooding of the drainage ditch. Hydrologic modeling of the existing drainage ditch has indicated that it is a very' flashy system with rapid rises and falls in water elevation and discharge (Appendix F). The elevation of the mitigation site has been designed so that when water levels rise one foot above base flow in the ditch, water will be allowed to enter the mitigacion site. This water will be held in the mitigation area by a second berm. Hydrology is expected to mimic a small floodplain area with short term, seasonal inundation. It is anticipated that the mitigation site will appear to be dry for most of the growing season. This is similai to the hydrology in the impacted wetland. Hydrology will also enter the mitigation site as surface runoff from the south, but most of the hydrology will be from the adjacent stream. Ground water is not expected to be a significant hydrology source. Soils in the wetland mitigation area are silty clay loam or clay loam. This is consistent with the soils on the remainder of the site. During construction, all soils within the mitigation site will be removed and taken off-site. A one-foot subcut will be dug and replaced with clean upland topsoil. The replacement soil must also be a silty clay loam or clav loam and must not contain buckthorn or other known invasive 0 species. This soil will be finished to final grade and loosened if excessive compaction has occurred. Using the same soil type as the impacted wetland will further reproduc; similar conditions. Heavy soils, such as silty clay loam or day loam will also reduce infiltration rates and increase retention folloving flooding events. To prevent erosion, the entire site will be disk anchored with clean weed-free straw immediately after final grading, loosening, and seeding. Vegetation Prior to construction, existing buckthorn in the area w ill be cleared and grubbed. In areas that will not be graded, the buckthorn will be cleared and have stumps treated with a 2% solution of glyphosphatc. This is especially important in the buffer area north of the ditch. This area will be planted with native vegetation and will require the iwfinnehaha Crsek Watershed District Permit Application City ol Orono, Minnesota A-LONGL0204 00 Page 9