2008-02-04_GENERAL DOCUMENTS - M1989056

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Mr. Rex Burns Page Two May 1, 1998 The potential for channel cut-off was examined according to the following procedure: • We developed a rating curve for the quantity of flow in the gravel pit area (the right overbank). • We computed the sediment transport rate for a range of discharge values (given the soil parameters for the potential cut-off area). • We computed the volume of sediment transported during the 100-year flood hydrograph. We also examined the 1983 flood hydrograph to address the impact of a long duration of relatively high flows. Our analysis showed that flow through the gravel pit would not occur under the flowrates that occurred in 1983. This is supported by witnesses who note that flood water did not flow across the property in 1983. • Using the computed transport volume, we estimated the extent of erosion that could be expected to form during the 100-year flood. Hydraulics We first determined the quantity of flood flows in the right overbank area (location of the proposed gravel pit). To accomplish this, we supplemented the HEC-2 data file for the effective flood insurance study with a cross section placed at the north end of the gravel pit area. This cross section, 170900, was developed by using the channel and left overbank data from the next downstream cross section (adjusted vertically based on the channel slope) and mapping . data for the right overbank. As it happened, the right overbank of the cross section was located on top of the berm along the south side of the Fossil Creek Reservoir outfall ditch. Because the berm was not consistent in height nor construction, and because it does not meet FEMA requirements for levee protection, we assumed that the berm did not exist and used the ground elevations along the toe of the berm to define the right overbank. The results of the HEC-2 modeling indicate that flow begins to spill out of the main channel into the right overbank when channel flow reaches approximately 5,000 cfs. Significant quantities of flow do not enter the right overbank from the main channel until channel flows reach approximately 8,000 cfs. To complicate the issue, there is a contributor of flood flows also enter the gravel pit area from the west. This water originates from the right overbank of the Poudre River upstream of Interstate 25. The separated flood waters flow south along the west side of I-25 for approximately a mile before turning east and returning to the main channel. The return flow path is located at the north end of the proposed gravel pit. Therefore, the flow that spills out of the channel far upstream also contributes to the quantity of flood flows passing through the proposed gravel pit. We have included the separated flood flows in our overbank flow computations. Table 1 shows a rating curve for the right overbank flow versus the total flood flow at the proposed gravel pit location: BURNS-4A.LTR 34-0648.00