2011-12-16_REVISION - M1977572 (71)

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V. PROPOSED CONDITION HYDRAULIC ANALYSIS A hydraulic analysis of the proposed conditions was conducted in order to ensure that 100 -year water surface elevations are at or below the established corrected effective condition 100 -year water surface profile. Hydraulic analysis of additional flood events was also conducted in order to support the design of the proposed Berm A spillway. 5.1 Proposed Condition Floodplain Analysis A proposed condition HEC -RAS model was developed by modifying the existing condition cross section to incorporate the proposed site grading. It should be noted that all cross section locations are coincident with the existing condition analysis. A comparison of 100 -year water surface elevations for the corrected effective, existing, and proposed condition is provided in Table 5.1. A graphical comparison of 100 -year water surface elevations is provided in Figure 5.1 and Figure 5.2. The comparison shows that the proposed conditions water surface elevations are at or in some cases lower than the corrected effective condition. Results of the proposed condition analysis meet 100 -year floodplain criteria based upon the best available information. In addition, proposed condition 100 -year water surface elevations do not adversely affect adjacent properties. 5.2 Berm 'A' Spillway Hydraulic Analysis An additional proposed condition hydraulic model was developed to analyze low flow conditions associated with the Berm A spillway. The purpose of the model is to determine the discharge and frequency at which the Berm A spillway will begin to pass flow. Results of the analysis will also be used to support the design of the proposed Berm A erosion protection. 5.2.1 Annual Flow Duration Analysis The proposed Berm A spillway will begin spilling when a total discharge of approximately 865 cfs is in the Arkansas River. The wet, average, and dry annual flow duration curves presented in Section 2.2 were used to estimate how many days per year flow may overtop and pass through the proposed Berm A spillway. Historically, between 1949 and 2009 a daily average discharge of 865 cfs was exceeded for 79 days during water year 1987 (wet year), for a total of 2 days during 1949 (average year), and 0 days in 2003 (dry year), see Figure 5.3. The average number of days per year 865 cfs was exceeded during the entire period between 1949 and 2009 was determined to be 6 days (or 1.7% of the time). COVALCOO1_Phase 11 Report_082211.docx 5.1 ANdERSON CONSULTING ENGINEERS, INC.