2024-08-05_REVISION - M1986123 (4)

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Item 6. 5265 Ronald Reagan Blvd.,Suite 210 Ga"I owa Johnstown CO 80534 � 970.800.3300 GallowayUS.com Exhibit F to O'Brien Objections March 21, 2023 Ms. Caryn Nezat Larimer County Engineering Department 200 W Oak Street, Suite 3000 Fort Collins. CO 80521 RE: Floodplain Development Permit for Stroh Pit [Galloway Project No. CEX01] Caryn, With this letter, we are requesting approval of a floodplain development permit (FDP) (See Attachment A for the application form) for the placement of a temporary conveyor belt, underdrains, and a berm on the Stroh Pit site within the Big Thompson River (BTR) floodplain. The project site is located along the right (south) bank of BTR on the east side of 1-25. The main location of the site, berms, drains, and conveyor are located in the SW quadrant of Section 14 and NW quadrant of Section 23. So this includes parcels 8514000009, 8514000007, and 8523000004. The conveyor then extends up and into the SE quadrant of Section 15, which includes parcels 8515000003 and 8515000029. The conveyor belt will be located centrally in the site moving north through the West Pit and then west to the Material Processing Plant. The underdrains are proposed to run along the south, west and east Stroh Pit property boundary and outfall into the BTR channel in the north at two locations. The berm will run along the southern perimeter of the site and then north halfway up the western perimeter, which will be placed as part of the mining operation in the Stroh Pit site. A vicinity map displaying the approximate location of the site is provided in Attachment B. A map of the area to be disturbed by the proposed activities is provided as Attachment C. The preliminary FIS/FIRM information and models are utilized as the basis of this application. Proposed Activities in the Floodplain and Floodway The proposed activities will be located east of 1-25 and south of the BTR, and will be placed within the effective floodplain and floodway in Stroh Pit site. Only the conveyor belt and a portion of underdrains will be located within the effective floodway and the remaining improvements will be located in the floodway fringe. The area within the floodplain that will be disrupted by the proposed activities is approximately 5 acres. In orderto place the underdrains,trenches will be excavated.After the installation of the underdrain pipes, the trenches will be refilled to match the existing grade. Since the trenches for underdrains will be refilled to match the existing grade, no hydraulic modeling is needed for this work even with the work being located in the effective floodway. The placement of the temporary conveyor belt structure will, however, introduce obstructions within the effective floodway that was modeled to perform a no-rise analysis for the 1% annual chance (1-PAC) flood based on the effective model from the preliminary FIS (see Attachment D for excerpts from FIS & FIRM). The obstructions are located in cross sections 104063, 103233, 102588, and 101840. There are also excavations/cuts of existing surface under and along the conveyor alignment, measuring around 20-30 feet wide and half-foot deep. The resulting water surface elevations from the no-rise model are shown in comparison to the effective water surface elevations in the table below. Elevations from the effective model result from HEC-RAS, version 5.0.1, whereas water surface elevations from the No-Rise analysis come from HEC-RAS, version 6.3.1. Transition to a newer version of the program was done for efficiency and the analysis was performed in both version 5.0.1 and 6.3.1 to ensure that the results remained accurate. Water Surface Elevation WSEL Com arison of No-Rise to the Effective Analysis for 1-PAC Flood Effective WSEL No-Rise Analysis Difference in Cross Section • : : : : 106435 4845.99 4845.99 0.00 Stroh Pit FDP Memo-20230321.docx 352