1997-09-15_REVISION - M1981302 (9)

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Rationale for TR-06 Approval-Western Mobile Page 5 Deepe Farm Pit, Permit No. M-81-302 September 15, 1997 proposal is to increase the freeboard by raising the levee in certain locations. The Division thus concludes that although the levee has not been certified and cannot be relied upon from a Flood Insurance Rate Mapping standpoint, as a practical matter, the levee may very well be expected to hold during a one percent annual chance flood even in its current unimproved configuration. As such, the changes to the levee proposed in TR-06 would not result in an adverse impact to the hydrologic balance that has prevailed since the time the levee was originally constructed, which was prior to the issuance of the Deepe Farm Pit reclamation permit. C. The topography of the left overbank of South Boulder Creek, and the presence of the Highway 36 embankment astride the stream valley, will cause flood water resulting from the one percent annual chance flood to spill over the highway into neighborhoods to the north. The magnitude of the spill into the neighborhoods is substantially lessened by the presence of the Deepe Farm Pit perimeter levee, which directs a greater percentage of the flood waters under the highway bridge. With this basis, it is the Division's finding that the Deepe Farm Pit perimeter levee will not damage adjoining property or create an adverse impact to the prevailing hydrologic balance. In fact, it appears that the levee benefits the neighborhoods north of the Denver-Boulder Turnpike by lessening the magnitude of flooding that may occur in these areas during the one percent annual chance flood. 3. On July 18, 1997, the operator submitted a Geotechnical Investigation of the Levee at the Deepe Farm Pit prepared by CTL/Thompson, Inc. consulting engineers. This document includes basic construction guidelines for installation of the proposed levee raise which the Division considers to be binding permit conditions (see the "Division's Decision" section below). This document also adequately addressed most of the Division's concerns with the stability of the Deepe Farm Pit perimeter levee during a one percent annual chance flood. Upon review of the Geotechnical Investigation, the Division raised a concern with the soil shear strength values input to the slope stability analyses contained therein. The slope stability evaluations in the Geotechnical Investigation used assumed values for shear strength that were based on the gradation of the soil, rather than the measured shear strength values for the retained fraction of a screened sample of the levee soils. To address the Division's concern, the slope stability analyses were rerun using the more conservative measured soil shear strengths. These more conservative analyses yielded a minimum safety factor of 2.5 for the creek side slope during an earthquake with an assumed peak lateral acceleration of 0.5 ft/sec/sec. The safety factor for the creek side slope during peak flows from the one percent annual chance flood was determined to be 4.5. Any safety factor greater than unity is indicative of a stable slope, and for critical