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20130331 RESUBMIT Yelm Creek Flood Study-Ed McCarthy 043020ED MCCARTHY, P.E., PS 9957 171 AVENUE SE Hydrology • Hydraulics • Engineering RENTON, WA 98059 (425) 271 -5734 April 30, 2014 Mr. Hal Grubb, P.E. Barghausen Consulting Engineers 1821572 nd Avenue South Kent, WA 98032 Re: Yelm Creek — Weber Property Flood Study — Hydraulic Analysis Tax Parcel No. 22719240300 Dear Mr. Grubb: This report documents the methods and results of a flood study I conducted to evaluate proposed grading activity on the Weber property along Yelm Creek. The subject property is 7.57 acres located between Crystal Springs Road SE and Rhoton Road SE in the City of Yelm (Figure 1). The subject site is located within a Zone AE Special Flood Hazard Area (SFHA) of Yelm Creek as published by FEMA's Flood Insurance Rate Map (FIRM). Grading on the site within the 100 -year floodplain of Yelm Creek is proposed. Current Site Flooding Conditions The subject site is located within a Zone AE flood hazard area with base flood elevations ranging from 333 to 335 feet NAVD 88 (Attachment A). The site includes FEMA cross sections J (River Station 10.0) and K (River Station 11.0). The main channel of Yelm Creek and its 1 -foot rise floodway passes through the western portion of the site. The eastern portion of the site is also within the floodplain. The central portion of the site, however, is above the mapped base flood elevation. Proposed Grading Activity Fill within the floodplain is proposed in the eastern portion of the site. A total flood storage volume of 3,650 cubic yards would be filled under the proposed grading plan to elevate the east portion of the site above the base flood elevation. As mitigation for flood storage volumes filled, a total compensatory flood storage volume of 3,650 cubic yards will be provided in the western portion of the site, adjacent to the right bank of the main channel of Yelm Creek. Mr. Hal Grubb Page 2 April 30, 2014 Modeling Approach The current site configuration and proposed site grading activity were modeled using HEC -RAS hydraulic software (Version 4.1.0). The base model used for in the project analysis is the same HEC -RAS model used by FEMA in the flood insurance study and mapping' for the basin. In assessing the site's hydraulics, the location of cross section K was slightly modified to pass through the proposed fill area in the eastern portion of the site (Figure 1). In addition, a new cross section was added to the hydraulic model between FEMA cross sections K and J. This new section, identified as cross section K1, passes through the proposed compensatory storage area located in the western portion of the site. The hydraulic model for existing conditions includes the site with it current grades, whereas the model for proposed conditions includes modifications to cross sections K and K1 to represent fill and compensatory storage grading activity. Elevation data for cross sections K and K1 were based on the project base map for existing conditions and on the proposed grading plan for proposed conditions. The hydraulic model was used to predict changes in flood elevations resulting from the proposed project. The modeling results were used to evaluate project effects on a comparative basis, using the existing conditions model results as a baseline. Modeling Results HEC -RAS modeling results for the reach of Yelm Creek including the project area are provided in Attachment B for both existing and proposed site conditions. Appendix B includes a tabulation of modeling results at selected cross section locations, flood profiles of the creek, and creek cross sections for existing and proposed conditions. The proposed fill is in an area of the floodplain with no flow velocity. The floodplain within the eastern portion of the site filled provides flood storage but not flood conveyance. On the other hand, the compensatory storage area, along the creek's main channel, would provide flood conveyance and storage. The HEC -RAS modeling results show that the predicted base flood elevations and energy grade elevations for proposed site conditions at River Stations 10.40 and sections ' Changes to the original FEMA model included adding a cross section at River Station 10.40 to characterize proposed fill and compensatory storage mitigation. Mr. Hal Grubb Page 3 April 30, 2014 upstream are equal to or slightly lower than those predicted for existing site conditions (Attachment B). The hydraulic modeling demonstrates that the flood conveyance capacity of the creek is maintained under proposed conditions and that zero -rise criteria for the propped project are met. Based on these modeling results, the proposed site grading will not alter flood flow paths or flood elevations. The project will not result in increased flood risk to adjacent properties. Sincerely, edwar C7 NC ca+'# 1 =/ 11 Edward J. McCarthy, Ph.D., P.E. Mc A O J f �QNAL Mr. Hal Grubb Page 4 April 30, 2014 References FEMA, October 16, 2012. FEMA FIRM Map Number 53067C0353E, Panel 0353E, Thurston County, Washington and Incorporated Areas. U.S. Army Corps of Engineers, Hydrologic Engineering Center. 2002. HEGRAS, River Analysis System User's Manual, Version 3.1. Davis, California. U.S. Army Corps of Engineers, 2008. HECRAS 4.0 Computer Software. Hydrologic Engineering Center, Davis, CA. 0 ti 0 U N U N U N N bIJ w Attachment A. 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MRE, E pg S s � y� �6F IV a lei Foal Trig E Nul @8 01 eqq H 5 8 : 5 � g N a5$E� a @C � �$ i.E $LL' `'a 5 =�°�zx o 3 8 � "� �'�' °� � ���m'� Ag �` °k �*Ee $EH® nb a 5 H a• e8F g� �� "3 -as as t59�a ay gs 5�$ S �� yg4 g� e� a� � SO Rhos a�A �sy '� ; -e €¢°�H $8gE Ed'S5� € E b �b_� �� _ ROM Hal h, Attachment B. HEC -RAS Model Results — Yelm Creek HEC -RAS Plan Description Existing Original FEMA FIS model with the following modifications: 1. Modified XS K to pass through proposed fill area on eastern portion of subject site. 2. Added XS K1, between XS K and XS J; cross section K1 passes through proposed flood storage mitigation area on western portion subject site. 3. Adjusted bridge deck and culvert lengths at RS 9.05 to correct run time error. 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