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Coulson Excavating Company, Inc. - Bonser Gravel Pit Exhibit D -Mining Plan appropriate mitigation techniques will be implemented to ensure that the soil and groundwater are do- contaminated promptly and efficiently. Pit Setback and Maximum Cell Depth A setback of at least 100 ft will be rnaintained from the Big Thompson River as measured from the pit boundary to the near edge of the river. The pit boundary will be clearly delineated with flags. Within the 100 ft buffer zone, vehicular traffic, equipment, and any mining related activities will be forbidden. In addition, the entve permit area will not exceed a vertical depth of approximately 32 ft, with mining conducted at a 1:1 slope (see Figure 1, Exhibit D). On and Off-Site Protection From the 100-Year Flood Floodwaters aze expected to flow in an east northeasterly d'vection, with a velocity of 4 ft/sec fora 1 chance flood event (Department of the Army, Omaha District, Corps of Engineers, 1977 Big Thompson River Flood Plain Information). The 1977 Army Corps of Engineers study calculated 1 % chance floodwater elevations to reach 4857 ft, 4855.5 ft, 4852 ft, and 4848 ft, from the western boundary of the proposed excavation area to the eastern boundary, respectively (see Exhibit C). Because the proposed gavel pit will effectively lower flood surface elevations and dissipate stream energy if floodwaters enter the pit during a flood, its presence will not increase the risk of damage to existing structures. Moreover, little risk exists of the gavel pit capturing the river after floodwaters recede. This can be assumed based on channel constrictions at the I-25 Bridge, where the 100-year flood plain is constrained to no more than 40 ft from the main channel, and all floodwaters are forced to move through this corridor. This constraint should effectivety cause floodwaters to become a placid backwater azea in the vicinity of the proposed project, and lower the probability of stream bank modification. In addition to channel constrictions, the combination of overburden stockpiles and a high density of riparian vegetation and ponds north and south of the Big Thompson River, will contribute to the dissipation of stream energy, and effectively contribute to the maintenance of the original stream banks following a 100-year flood. In the event that any channel modification should occur within property boundaries, Coulson Excavating Company, Inc will ro- excavate the modified azea to its original conditions. During mining, overburden stockpiles will be partially or completely located in the 100-year floodplain, and the asphalt batch plant will be partially located in the 100-year flood plain. Stockpiles are assumed to be relatively stable in the event of a 1% chance flood event. This assumption is based on the fact that an extensive root system will be in place due to the vegetation established on the berms; as well as the fact that the berms will be considerably compacted due to the heavy machinery that travels on them. Additionally, because the most recent 1 % chance event was roughly 15 years ago, the probability of another 100-year flood occurrence during the span of operations is extremely low.