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HomeMy WebLinkAbout522 W. MAGNOLIA ST. CARRIAGE HOUSE - PDP - PDP130013 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT1 City of Fort Collins Stormwater Utility 700 Wood St. Fort Collins, Colorado 80522 April 11, 2013 RE: Drainage Letter for 522 W. Magnolia Fort Collins, Colorado Dear Staff: This letter is provided for the proposed carriage house to be constructed at 522 W. Magnolia. The carriage house is to be located within the backyard of the existing lot, just to the north of the existing home. A recycled asphalt drive will also be added and will serve as access from the existing alley and will provide parking. The proposed carriage house will be constructed in conjunction with an addition to be constructed along the north and east sides of the existing home. The additional impervious area proposed with this project has been calculated as 2374 Square Feet, and considers all proposed improvements including the carriage house, the addition, additional concrete walks and drives. Please see the attached Existing and Proposed Condition Impervious Area Exhibits. Table 1 - Calculation of Impervious Area Existing Condition Total Impervious Area (sq.ft.) 2942 Proposed Condition Total Impervious Area (sq.ft.) 5316 Additional Proposed Impervious Area (sq.ft.) 2374 The proposed project is located within the 100-year high risk flood fringe in the Old Town Basin, and shall satisfy the standards of Chapter 10 of City Code. The 100-year flood elevation has been provided by the City of Fort Collins Stormwater Utility (Please see the attached Floodplain Risk Map). The Regulatory Flood Protection Elevation (RFPE) is 18-inches above the flood elevation. The flood elevation per the attached 2 Floodplain Risk Map is 5001.10. Thus, the RFPE for the proposed carriage house is calculated as: RFPE = 5001.10 + 1.50 = 5002.60 All duct work, heating ventilation, air conditioning systems, hot water heater, electrical must also be placed at or above the Regulatory Flood Protection Elevation. The carriage house is to have crawl space construction. Flood venting with an open area of 1 square inch per 1 square foot of building footprint must be provided. The attached Grading and Erosion Control Plan shows LID features to be constructed on the south and north sides of the carriage house. The LID concept for this site has been discussed with City Stormwater Staff. Per the direction of Staff the LID feature should capture storm runoff and cause infiltration of stormwater. The majority of rooftop drainage and drainage of areas immediate to the carriage house will be conveyed to and captured in the LID feature. If you have any questions as you review this report, please feel free to contact me at your earliest convenience. Sincerely, FRONT RANGE STORMWATER & FLOODPLAIN CONSULTING Aaron Cvar, P.E. 522 W. Magnolia Street A B = = 4394 80 ft ft Y = X BFE =5001.at 522 8 ft W. Magnolia St. C = 4215 ft Z = 500.2 ft (Y Y-= Z) Z + / ((B-B-C) C) * = ((X-X-Z) Z) / / ((A-A-C) C) Solve for BFE at 522 W. Magnolia: Y = 5001.1 ft NGVD29 XS#:4215 5000.2ft NGVD 29 XS#:4394 5001.8ft NGVD 29 XS#:4045 5002.6ft NGVD 29 W MAGNOLIA ST S WHITCOMB ST .2013 Printed: 03/19/ All are floodplain approximate. boundaries This Agency information (FEMA) Flood is based Insurance on the Federal Rate Map Emergency (FIRM) and Management the City of Fort that the Collins referenced Master Drainageway property will or Plans. will not This be free letter from does flooding not imply or damage. City Designated A property Floodplain not in the may Special be damaged Flood Hazard by a flood Area greater or in a than not shown that predicted on the map. on the This map map or from does a not local create drainage liability problem on the part that results of the City, from or reliance any officer on this or employee information. thereof, for any damage City Flood Risk Map Moderate Risk May - Areas include:of FEMA 500-year floodplain (FEMA Zone X-shaded). - average Areas of depths FEMA or of City less 100-than year 1 foot. floodplain (sheet flow) with Low Risk - Areas protected by levees from the 100-year flood. Areas floodplains. outside Local of FEMA drainage and problems City mapped may 100-still year exist. and 500-year City - Areas Flood of FEMA Fringe 100-- May year Include: floodplain (FEMA Zones A, AE, AO, and AH) - flow Areas areas of City with 100-average year floodplain depths of including 1-3 feet. ponding areas and sheet There is a 1% annual chance that these areas will be flooded. High Risk City fastest Floodway velocities. - Area of 100-year floodplain with greatest depths and 0 12.5 25 50 75 100 Feet 0 north 20' EXHIBIT 1 EXISTING IMPERVIOUS AREA EXHIBIT 522 W. MAGNOLIA SCALE: 1" = 20' EXISTING CONDITION TOTAL IMPERVIOUS AREA = 2942 SQ. FT. DW 0 north 20' EXHIBIT 2 PROPOSED IMPERVIOUS AREA EXHIBIT 522 W. MAGNOLIA SCALE: 1" = 20' PROPOSED CONDITION TOTAL IMPERVIOUS AREA = 5316 SQ. FT. CALCULATTION OF ADDITIONAL IMPERVIOUS AREA: 5316 SQ.FT. - 2942 SQ.FT. = 2374 SQ.FT.