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„~ The riverbank material along the north bank of the Big Thompson between the river and the proposed pit consists primarily of silt, sand, and gravel. Photographs were taken to document the characteristics ofthe riverbanks (see Figures 2 through 12). Along portions of the reach upstream of St. Louis Avenue, a berm exists on the top of the bank consisting of concrete rubble. Farther upstream (in the vicinity of where the river is follows anorth-south alignment), a berm consisting primarily of silty and sandy material has been pushed up on the bank. Observation of the riverbanks downstream of St. Louis Avenue indicate similar chazacteristics of silt, sand, and gravel banks. Relatively extensive riparian vegetation is growing along the riverbank consisting primarily of willow, cottonwood, and some Russian olive. The hydraulic analysis shows that from less than 1000 to over 10,000 cfs out of the 19,000 cfs peak flow is flowing over the north floodplain. Some of this overbank flow could obviously flow into the pits. A portion of the flow in the mainstem could also flow into the pits if a significant breach in the riverbank occurred. A significant portion of the total flow is likely to bypass any breach, however, due to the availability of conveyance capacity down the existing channel and on the south floodplain. The ma~cimum size of any of the phases of a pit are on the order of 6 million cubic feet. A thousand cfs of flow, either overbank or through a breach, would fill 3.6 million cubic feet of a pit in an hour. Thus, it is not possible for a breaching flow of any significant magnitude (i.e., >1000 cfs) to occur for more than about an hour or two. Once the pit fills, there is no significant gradient for water to flow into the pit. Water may flow over the pit once it is filled with water but, in this case, the flow does not have significant erosive power, being on the floodplain. Thus, while overbank flow may occur for a number of hours or possibly even a few days during a 100-yeaz flood, conditions of a high gradient flow into the gravel pit, as it is filling, is restricted to the relatively short time of an hour or two, even a relatively modest overbarilc flow rates compazed to the peak flow. Based on this evaluation, the information indicates that a 100-foot setback from the river to the pit should be sufficient to prevent significant damage or diversion of the main channel into the pit. In order to further limit flow and velocity from the river into the pit and to provide additional stability for the riverbank/embanlmrent between the river and the pit, it is recommended that riparian vegetation (primarily willows) be planted and maintained in any gaps that may currently exist on the riverbank side of the embankment. Additionally, another azea of willows should be planted and maintained along the inside edge of the embankment down into the pit. This azea should be approximately 10 to 20 feet wide and continuous along the inside edge neaz the top of the pit embankment. Such riparian vegetation reduces the magnitude and velocity of flow that can leave the main channel and flow out of the channel and onto the floodplain and into the pit. The toot structure also helps stabilize the soil thereby reducing erosion. Evaluation of the computed water surface elevation indicates that most of the overbank flow, as it flows over the floodplain, encounters the St. Louis Avenue embankment forcing most of the flow back into the channel (see Figure 13). As a result, the potential for erosion and breaching of the pits upstream of St. Louis Avenue into the pit downstream of St. Louis Avenue is negligible. This potential is minimized further by the 100-foot setbacks from the road easement to either pit and the 80 foot road easement itself.