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Fort Morgan Farms, LLC Delta Reservoir—West Cell Preliminary and East Cell Conceptual Design April 16,2021 hydraulic models referenced in Table 1 and Table 2 are presented in plan-view in Figure 3 and Figure 4 in Appendix 1. Existing(Pre-Project) Conditions 1 D Hydraulic Models The pre-project conditions hydraulic models incorporate ground survey collected by AW in 2019 and/or KSI in 2020 into the digital elevation models (DEMs) utilized by the Risk MAP and CHAMP Studies. The revised existing area modifies a region of approximately 2 square miles directly east of WCR 47, south of the CLPR and north of the SPR. As with subsequent models, the remaining surface covering the project area was preserved from the Risk MAP and CHAMP Study DEMs. These models served as the basis for evaluation and comparison for the proposed design. The existing hydraulic model for the CLPR begins at upstream cross-section 10681 (indicating feet distance from the confluence with the South Platte River) from the Risk MAP Study and ends at cross-section 1235. When compared with the baseline model from the Risk MAP Study, the existing model decreased the flood elevations from both the 1-PAC floodplain and floodway directly downstream and within the project area. However, it slightly increases the flood elevations approximately 1900 feet upstream of the project site. The existing hydraulic model for the SPR begins at upstream cross-section 1403473 from the CHAMP Study and ends at cross-section 1394739, immediately downstream of the confluence with the CLPR. When compared with the baseline model from the CHAMP Study, the existing model decreased the flood elevations from both the 1-PAC floodplain and floodway within the project area. Proposed(Post-Project) 1 D Hydraulic Models Post-project conditions were modeled for the proposed design described below in a 1 D hydraulic analysis for comparison with the Risk MAP and CHAMP, and Existing Conditions models of the CLPR and SPR. The post-project condition model of the proposed design modifies the Existing Conditions hydraulic models by modifying the geometry of cross- sections that traverse each respective storage cell design. When compared with the existing (or corrected effective) models, the proposed (or post-project) model decreased the flood elevations from both the 1-PAC floodplain and floodway within the project area. Two-Dimensional (2D) Unsteady Hydraulic Model Additional modeling for the proposed design was performed using HEC-RAS version 5.0.7 2D unsteady hydraulic modeling and the stream gauge hydrographs described above. The 2D hydraulic model simulates flows through a computational mesh with approximately 40 feet x 40 feet cell spacing and a total of 22300 cells with break lines and refinement regions placed as needed to capture spatial heterogeneity. The 2D mesh begins upstream of Weld County Road 47 on the CLPR and extends generally Southeast towards the ridgeline and continues eastward downstream of the confluence as shown in Figure 5 in Appendix 1. The 1-PAC chance flows from the CLPR and SPR are conveyed through the computational mesh of the hydraulic model. The 2D hydraulic model uses a 6 second computational time step over a simulation period of approximately 12 days. FM F_TechMemo-20210416.docx Galloway&Company,Inc. Page 3 of 7