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Mr. Roger Schmidt <br /> September 2, 2016 <br /> Page 14 <br /> CONCLUSIONS <br /> Based on the documents reviewed, interviews conducted and observations from field visits to the <br /> subject area, the following conclusions were made by WWE with a reasonable degree of <br /> engineering probability regarding the erosion in the F Street agricultural field, the breaches of <br /> sections of pipeline owned by the BIC and the identified causes: <br /> • The runoff events that caused damage in 2013 and 2014, with flow rates of 4080 cfs and <br /> 4500 cfs, respectively, were greater than the flow published in the 2016 FIS for a 10-year <br /> runoff event (approximately 3,400 cfs just downstream of 8th Avenue), but substantially <br /> less than the FIS flow rate for a 100-year event of 10,700 cfs. <br /> • Regarding floodplain mapping, the FEMA FIRM, dated September 28, 1982, which was <br /> in effect at the time the Al Pit was constructed, shows that the F Street agricultural field, <br /> and the area where the Boyd-Freeman Ditch was relocated along the west side of the Al <br /> Pit, is in "Zone AO," which indicates that the area is subject to shallow flooding during a <br /> 100-year flood event. The Iverson/Martin Marietta Pit is also within Zone AO. The <br /> updated FEMA FIRM, effective on January 20, 2016, also shows large portions of the F <br /> Street agricultural field, the relocated Boyd-Freeman Ditch and the Iverson/Martin <br /> Marietta Pit to be within the 100-year floodplain. The original location of the Boyd- <br /> Freeman Ditch in the area where the Al Pit now exists, prior to the ditch being relocated, <br /> is largely outside(south)of the updated FEMA 100-year floodplain boundary. <br /> • The lower velocities in the flows across the field in the pre-mining condition (e.g., <br /> approximately 1.5 feet per second for 300 cfs) would not be expected to erode the silt, <br /> sands and lean clay materials at the site identified in a geotechnical analysis conducted by <br /> Olsson Associates, particularly when the river's flood flows are expected to be <br /> transporting colloidal silts. In contrast, the higher velocities in the post-mining, or <br /> headcut-forming, condition (e.g., roughly 5 feet per second at 300 cfs) could be expected, <br /> given a reasonable degree of engineering certainty, to exceed the permissible velocities <br /> for those materials (Chow, 1959) and thereby cause erosion, leading to formation of a <br /> headcut. <br /> • The erosional feature on the F Street agricultural field was formed when water <br /> overflowing the east bank of the Cache La Poudre River crossed the field and breached <br /> the BIC pipeline and its embankment. The breach allowed water to flow down into the <br /> excavation of the Aggregate Industries (AI) gravel pit, which resulted in increased flow <br /> velocities through the breach that caused the formation of an erosional headcut across the <br /> field that progressed from east to west. <br /> • If the Al gravel pit was not present to cause the accelerated flow velocities through the <br /> breach, the headcut would not have formed, even with overland flow conditions across <br /> the field such as those that occurred in 2014. Without the Al Pit, flood flows across the F <br />