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HomeMy WebLinkAbout99070 Nelson Middle School March 15. 1999 Job No ----- Storm Drainage Letter Report NELSON MIDDLE SCHOOL for Renton School District No. 403 1220 4"' Street Renton, WA 98055 Prepared by: Brian G. Harron, P.E. e Job No: 99070 c Q March 15, 1999 ciTvoF R ENTO sN** MAR 1 7 1999 s')NAL BUILDING DIVISIONr. c. sir pP Nelson Middle School March 15, 1999 _ Job No. 99070 March 15, 1999 Renton City Hall 1055 South Grady Way Renton, WA 98055 RE: Nelson Middle School Modernization Dear Gentleman: On November 25, 1998 a pre-application meeting was held with DDES (A98V01 1 1 ) for the sister school to Nelson - Dimmit Middle School. At the pre-application meeting the County stipulated that certain drainage aspects be reviewed as part of the new roof installation. The following was specifically agreed to and we are assuming that the city will take a similar approach with Nelson Middle School: 1 . According to Jeff Bunnell, King County Drainage & Site Review Engineer, the Dimmit Middle School would be exempt from storm drainage review since no additional impervious areas will be added. No additional impervious areas will be add to the Nelson Middle School. If there is an increase in roof area and/or an increase in impervious area associated with vehicular traffic, then a comprehensive storm drainage analysis would be required. This project did not increase the existing impervious roof areas to any significant extent. 2. The county stated that any new storm drainage lines that are required for the purpose of tying in the new downspouts (required when the existing flat roof is covered with a new pitched roof) would not initiate the requirements of the stormwater manual. The county conditioned this statement with the instruction that all new storm lines continue to connect to the existing on-site facilities. Any storm drainage systems that require a new connection to the existing public storm facilities in the street would "kick-in" a storm drainage review. This project did not create any new connection to the existing storm drainage system in the public Right of Way. 3. The county directed that any replacement of the existing private storm drainage lines be a minimum 8 inches. Our hydraulic analysis did determine that some of the existing storm sewers were undersized and would need to be replaced. In all cases the replacement sewer was 8-inch diameter or larger. When the replaced sewer was determined to be a downspout connection line then the 8-inch minimum was deemed not pertinent. Nelson Middle School March 15 1999 Job No. 99070 4. The county requested that we complete a hydraulic analysis of the existing on-site system (in particular the storm pipes within the building) to determine if the re- directed runoff from the new pitched roof will surcharge the existing storm sewers. Due to the small impervious areas associated with a given downspout location, the county agreed that the Rational Method would be an appropriate method of analysis. Accomplishing this hydraulic analysis is the main component of this letter report. Analysis: The Rational Method was used to analysis the existing roof area and their associated downspouts. A 100-year design was used for both the existing and developed conditions. Certain existing downspout discharge lines were deemed to be at or over their design capacity and as such were recommended for replacement. The proposed roof line and their associated downspouts, as developed by the architect, were review for capacity and in some cases pipe diameters were increased, additional downspout connections from the interior court yards were added, and in the most sever case the downspouts were relocated to different discharge lines. Reviewing the original 1960 and 1964 construction plans determined the existing downspout locations and their inter-connections. The architect and the engineer jointly determined the new locations. Appendix "A" provides a detailed analysis of the roof area(s), the existing/proposed downspout locations and the downspouts hydraulic carrying capacity. Based upon a review by the Rational Method the engineer has determined that the existing and new downspout sewer system will hydraulically carry the 100-year storm event. I have tried to be complete in this letter report; however, If the reviewer has any specific questions then please do not hesitate to contact this office. Sincerely, CONCEPT ENGINEERING, INC. G. A, WASI��a rian G. Harron, P.E. BGH:bh `v�ONAL 3. Nelson Middle School March 15 1999 Job No. 99070 APPENDIX A Roof Drainage Calculations F , �4 IN ks I8'MR Cot ERA& o--MIMM x_. 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NELSON MIDDLE SCHOOL Worksheet for Circular Channel Project Description Project File untitled.fm2 Worksheet NELSON MIDDLE SCHOOL Flow Element Circular Channel Method Manning's Formula Solve For Discharge Input Data Mannings Coefficient 0.013 Channel Slope 0.020000 ft/ft Depth 0.83 ft Diameter 10.00 in Results Discharge 3.10 cfs Flow Area 0.55 ftZ Wetted Perimeter 2.62 ft Top Width 0.00 ft Critical Depth 0.76 ft Percent Full 100.00 Critical Slope 0.017456 ft/ft Velocity 5.68 ft/s Velocity Head 0.50 ft Specific Energy FULL ft Froude Number FULL Maximum Discharge 3.33 cfs Full Flow Capacity 3.10 cfs Full Flow Slope 0.020000 ft/ft 03/01/99 FlowMaster v5.11 02:32:44 PM Haestad Methods,Inc. 37 Brookside Road Waterbury,CT 06708 (203)755-1666 Page 1 of 1 NELSON MIDDLE SCHOOL Worksheet for Circular Channel Project Description Project File untitled.fm2 Worksheet NELSON MIDDLE SCHOOL Flow Element Circular Channel Method Manning's Formula Solve For Discharge Input Data Mannings Coefficient 0.013 Channel Slope 0.010000 ft/ft Depth 0.67 ft Diameter 8.00 in Results Discharge 1.21 cfs Flow Area 0.35 ft2 Wetted Perimeter 2.09 ft Top Width 0.00 ft Critical Depth 0.52 ft Percent Full 100.00 Critical Slope 0.010992 ft/ft Velocity 3.46 ft/s Velocity Head 0.19 ft Specific Energy FULL ft Froude Number FULL Maximum Discharge 1.30 cfs Full Flow Capacity 1.21 cfs Full Flow Slope 0.010000 ft/ft 03/01/99 FlowMaster v5.11 02:32:05 PM Haestad Methods,Inc. 37 Brookside Road Waterbury,CT 06708 (203)755-1666 Page 1 of 1 NELSON MIDDLE SCHOOL Worksheet for Circular Channel Project Description Project File untitled.fm2 Worksheet NELSON MIDDLE SCHOOL Flow Element Circular Channel Method Manning's Formula Solve For Discharge Input Data Mannings Coefficient 0.013 Channel Slope 0.020000 ft/ft Depth 0.67 ft Diameter 8.00 in Results Discharge 1.71 cfs Flow Area 0.35 ftZ Wetted Perimeter 2.09 ft Top Width 0.00 ft Critical Depth 0.60 ft Percent Full 100.00 Critical Slope 0.017602 ft/ft Velocity 4.90 ft/s Velocity Head 0.37 ft Specific Energy FULL ft Froude Number FULL Maximum Discharge 1.84 cfs Full Flow Capacity 1.71 cfs Full Flow Slope 0.020000 ft/ft 03/01/99 FlowMaster v5.11 02:31:44 PM Haestad Methods,Inc. 37 Brookside Road Waterbury,CT 06708 (203)755-1666 Page 1 of 1 NELSON MIDDLE SCHOOL Worksheet for Circular Channel Project Description Project File untitled.fm2 Worksheet NELSON MIDDLE SCHOOL Flow Element Circular Channel Method Manning's Formula Solve For Discharge Input Data Mannings Coefficient 0.013 Channel Slope 0.020000 ft/ft Depth 0.33 ft Diameter 4.00 in Results Discharge 0.27 cfs Flow Area 0.09 ft2 Wetted Perimeter 1.05 ft Top Width 0.00 ft Critical Depth 0.29 ft Percent Full 100.00 Critical Slope 0.018410 ft/ft Velocity 3.08 ft/s Velocity Head 0.15 ft Specific Energy FULL ft Froude Number FULL Maximum Discharge 0.29 cfs Full Flow Capacity 0.27 cfs Full Flow Slope 0.020000 ft/ft 03/01/99 FlowMaster v5.11 02:31:23 PM Haestad Methods,Inc. 37 Brookside Road Waterbury,CT 06708 (203)755-1666 Page 1 of 1 NELSON MIDDLE SCHOOL Worksheet for Circular Channel Project Description Project File untitled.fm2 Worksheet NELSON MIDDLE SCHOOL Flow Element Circular Channel Method Manning's Formula Solve For Discharge Input Data Mannings Coefficient 0.013 Channel Slope 0.020000 ft/ft Depth 0.50 ft Diameter 6.00 in Results Discharge 0.79 cfs Flow Area 0.20 ftz Wetted Perimeter 1.57 ft Top Width 0.00 ft Critical Depth 0.44 ft Percent Full 100.00 Critical Slope 0.017869 ft/ft Velocity 4.04 ft/s Velocity Head 0.25 ft Specific Energy FULL ft Froude Number FULL Maximum Discharge 0.85 cfs Full Flow Capacity 0.79 cfs Full Flow Slope 0.020000 ft/ft 03/01/99 FlowMaster v5.11 02:31:04 PM Haestad Methods,Inc. 37 Brookside Road Waterbury,CT 06708 (203)755-1666 Page 1 of 1 NELSON MIDDLE SCHOOL Worksheet for Circular Channel Project Description Project File untitled.fm2 Worksheet NELSON MIDDLE SCHOOL Flow Element Circular Channel Method Manning's Formula Solve For Discharge Input Data Mannings Coefficient 0.013 Channel Slope 0.010000 ft/ft Depth 0.50 ft Diameter 6.00 in Results Discharge 0.56 cfs Flow Area 0.20 ft2 Wetted Perimeter 1.57 ft Top Width 0.00 ft Critical Depth 0.38 ft Percent Full 100.00 Critical Slope 0.011553 ft/ft Velocity 2.86 ft/s Velocity Head 0.13 ft Specific Energy FULL ft Froude Number FULL Maximum Discharge 0.60 cfs Full Flow Capacity 0.56 cfs Full Flow Slope 0.010000 ft/ft 03/01/99 FlowMaster v5.11 02:30:40 PM Haestad Methods, Inc. 37 Brookside Road Waterbury,CT 06708 (203)755-1666 Page 1 of 1