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Fort Morgan Farms,LLC Delta Reservoir—West Cell Preliminary and East Cell Conceptual Design April 16,2021 Report Jan14. ACE developed the effective hydrology for the CLPR using HEC-HMS Version 3.5, the USACE's Hydrologic Modeling System computer model and land cover information, rainfall data, and topographic data from 2017 whereas the hydrology for the SPR 1 D hydraulic analysis was determined by the 2015 URS post-flood hydrology report "South Platte River Watershed Phase 2 Hydrologic Evaluation, Post September 2013 Flood Event" prepared for the Colorado Department of Transportation (CDOT) and Colorado Water Conservation Board (CWCB) based on gage analysis and natural attenuation calculations for design points along the SPR. The CLPR reach contained within the project area experiences a peak flow of 10,200 cfs while the SPR peak flows downstream of the confluence with the CLPR and immediately upstream of the project area are 41,828 cfs and 40,805 cfs, respectively, during the 1-PAC storm event. Unsteady Non-Uniform Flow The hydrologic data used in the unsteady 2D model was obtained from stream gauge data for the CLPR and SPR. The stream gauge for the CLPR identified as CLAGRECO is located near Kersey, directly upstream of the project site while the stream gauge for the South Platte is located just downstream of the project area and is designated as PLAKERCO. Hourly streamflow hydrographs of the largest modern storm event occurring in 1999 on the South Platte River Basin were collected for each river. The 1999 flood hydrograph from the CLAGRECO stream gauge was used to reproduce the CLPR hydrography whereas the 1999 flood hydrograph from the PLAKERCO stream gauge data was used to reproduce the South Platte River hydrograph upstream of its confluence with the CLPR. Each stream gauge hydrograph was then scaled to the magnitude of the 1-PAC peak discharges to replicate the 1-PAC flood events on both rivers. Hydraulic Analysis One-Dimensional OD) Steady Hydraulic Model The hydraulic analysis simulated one dimensional (1 D) steady, uniform flow through the project area using HEC-RAS version 5.0.7. The model of the Lower CLPR Risk MAP Study performed by AECOM in January 2019 and the model of the SPR CHAMP Study performed by AECOM Denver in September 2017 served as the baseline models for this project. The Risk MAP Study used LiDAR-based topography (DEM and contours) with 2-foot contour accuracy, produced by Ayres Associates based on an October 2012 flight supplemented by in-channel and bridge structure geometry data (King Surveyors Inc. (KSI), 2013) and incorporated the 2017 Letter of Map Revision (LOMR), including the recently constructed WCR 47 bridge located immediately upstream of the project area. The survey data used in the SPR CHAMP Study include the high-resolution post-flood 2013 and 2014 LiDAR data supplemented by channel and structure survey data (AECOM, 2017). The existing or pre-project conditions models were developed from the Risk MAP and CHAMP models by altering the model geometries within the project area with ground survey data collected by American West (AW) in 2019 and/or KSI in 2020. The existing models are described in detail in the following section. The post-project conditions models were developed from the existing conditions models by altering the model geometries with the proposed design or grading. Comparison of modeling results for existing and proposed conditions are included in Table 1 and Table 2 in Appendix 2. Cross-sections from the 1 D FM F_TechMemo-20210416.docx Galloway&Company,Inc. Page 2 of 7