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HomeMy WebLinkAboutPlan Review Comments PRE 12-078 DEPARTMENT OF COMMUNITY AND ECONOMIC DEVELOPMENT M E M O R A N D U M   DATE: October 17, 2012     TO: Vanessa Dolbee, Planner     FROM: Jan Illian, Plan Review     SUBJECT: Boeing Cedar River Bridge Replacement 737 – Logan Ave N. PRE 12-078      NOTE: The applicant is cautioned that information contained in this summary is preliminary and non-binding and may be subject to modification and/or concurrence by official city decision-makers. Review comments may also need to be revised based on site planning and other design changes required by City staff or made by the applicant. I have completed a preliminary review for the above-referenced proposal. The following comments are based on the pre-application submittal made to the City of Renton by the applicant. Storm Drainage 1. Erosion control shall comply with the 2009 King County Surface Water Manual. 2. City Code requires compliance with the Critical Area Ordinance. 3. A drainage plan and drainage report will be required with the site plan application. The report shall comply with the 2009 King County Surface Water Manual and the 2009 City of Renton Amendments to the KCSWM, Chapters 1 and 2. All core and any special requirements shall be contained in the report. Based on the City’s flow control map, this site falls within the Flow Control Duration Standard, Existed Conditions. The drainage report will need to follow the area specific flow control requirements under Core Requirement #3. 4. The bridges and approach roads must pass the 100-year flow without creating hydraulic restrictions that case an increase in the floodplain elevation flooding. The design of the bridge and approach road shall demonstrate compliance with the floodplain zero-rise and compensatory storage requirements in the City’s Flood Hazard code. Please refer to section 4.3.3 of the City adopted 2009 King County Surface Water Design Manual for design standards for bridges. The zero rise hydraulic analysis of the new bridge shall be compared to the hydraulic analysis completed for the FEMA approved April 2006 Cedar River Flood Study, using the same river bed elevation (maximum allowable bed elevation) used in the Cedar River Flood Study, to demonstrate compliance with the zero rise requirement. The zero rise hydraulic analysis for the new bridge shall be provided to the City for review to verify compliance. 5. A zero rise hydraulic analysis is also required for the construction of a temporary bridge over the Cedar River that may be needed during the replacement of the North Boeing Bridge. The zero rise hydraulic analysis for the temporary bridge may use the use the existing survey elevation of the Cedar River channel bed elevation, rather than the maximum allowable bed elevation used in FEMA 2006 Cedar River Flood Study, provided that the temporary bridge will not remain in place for more than two (2) flood seasons. The zero rise hydraulic analysis for the temporary bridge shall be provided to the City for review to verify compliance. 6. The preferred minimum clearance between the 100-year flood elevation and the lowest elevation of the bottom low chord of the bridge is 3-ft. Due to the physical limitations to raising the bridge and making it accessible for airplane movements and conflicts with the Renton Municipal Airport runway it is recognized that 3-feet of clearance may not be feasible. The new bridge design must strive to provide as much clearance as possible (2-ft). The new bridge or installation of any temporary bridges shall not have their low chord elevation placed below the existing bridge low chord elevation and demonstrate compliance with the floodplain zero rise requirement. During a flood, large woody debris (LWD) is carried downstream that could cause problems at bridges and clearance maximizing clearance under the bridge is recommended. In addition, minimizing the number of support columns/piling in the floodway is recommended to meet the zero-rise floodway criteria and to minimize the potential for large woody debris from getting caught on support columns/piling under the bridge. 7. The April 2006 Cedar River Flood Study approved by FEMA established the 100-year FEMA floodplain elevation at the North Boeing Bridge to be elevation 24 (NAVD 88 Datum) or elevation 20.44 (NGVD 1929 Datum). The existing bridge low chord is at elevation 22.06 (NAVD 88 Datum) or elevation 18.5 (NGVD 1929 Datum). 8. The bridge replacement will need to comply with the FEMA National Flood Insurance Program requirements related to obstruction of the floodway and substantial improvement requirements. The existing bridge was constructed prior to the adoption of the FEMA floodplain maps for the Cedar River. The replacement of the existing bridge may exceed the “substantial improvement” threshold that would require the replacement of the North Boeing Bridge to be designed and constructed to current FEMA floodplain standards. 9. The replacement of the North Boeing Bridge will involve work in the FEMA 100-yr floodplain and will need to address the requirements associated with the National Marine Fisheries Service Biological Opinion regarding the FEMA National Flood Insurance Program impacts to endangered species as defined in the biological opinion applicable Reasonable and Prudent Alternatives requirements. 10. The North Boeing Bridge Replacement project will need to provide a Stormwater Technical Information Report demonstrating compliance all requirements with the City adopted 2009 King County Surface Water Design Manual and City amendments. 11. Any temporary piles or existing piles that are not removed completely from the river channel shall be cut of a minimum of 2-feet below the design dredge depth for Army Corps of Engineers Lower Cedar River Section 205 Flood Hazard Reduction Project. Based upon the Army Corps of Engineers construction plans for the dredging of the lower 1.25 miles of the Cedar River, that they did in 1998 as part of the Lower Cedar River Section 205 Flood Hazard Reduction Project, the design dredge depth extends down to elevation 10.56 (NAVD 88 Datum) for both upstream and downstream of the North Boeing bridge. Any piles or abutments that are not removed from the channel will need to be cut-off our pounded down to an elevation of 8.56 (NAVD 88 Datum) or below. 12. The new bridge design needs to allow for and accommodate access for future maintenance dredging of the Cedar River under the bridge.