Fort Walton Beach Council agenda — 02-26-13--regular — reading text

4eulpaW - Mid luewaaeueyr ialemwrolS 71Jaw4oelly O m ` Medinah Subdivision am Sturm Water Management Plan Yawum 1. 1 GENERAL The following is a storm water evaluation of the proposed development of a proposed six lot single family subdivision at the subject property in Fort Walton Beach, Florida. The parcel consists of approximately 1. 83 acres and is currently wooded and vacant, The property is located north of Beal Parkway between Linstew Drive and Briarwood Circle. See attached location map for further information. This storm water evaluation will be performed using the SCS Method. In accordance with City of Fort Walton Beach requirements the 1 - inch volume associated with the runoff from the developed areas will be retained on site and the retention system must be designed to recover within 72 hours. The post -developed runoff associated with a 25 -year 1 hour, 2 hour, 4 hour, 8 hour and 24 hour storm event will also be retained on site in accordance with jurisdictional requirements. 1. 2 PRE -DEVELOPED CONDITIONS For the purpose of the pre -development storm water evaluation, the parcel will be analyzed as one distinct drainage basin, which is approximately 1. 83 acres. No off -site run -off appears to impact the proposed site. Run -off from adjacent areas will route around the site or bypass through the site. The site is generally sloped from south to north, with elevations ranging from 25 to 19 feet. The storm water runoff generally flows to the northern portion of the site. Table 1. 2. 1 summarizes the area information of the pre -developed conditions for the design storm events. Table 1. 2. 1: PRE -DEVELOPMENT CONDITIONS- FORT WALTON BEACH, FL 25Yr - 2 Hr _ 25Yr - 4 Hr 25Yr - 8 Hr 25Yr - 24 Hr Storm Duration 25 Yr - 1 Hr 1. 83 Acres 1. 83 Acres 1. 83 Acres 1. 83 Acres_ Basin Area 1. 83 Acres 10 minutes 10 minutes 10 Minutes 10 Minutes_ Time of Conic. 10 minutes 60 60 60 60 60 Curve Number field uolslAlpgnS geulpaW : L61,Z) uolslAlpgnS geulpatq - ueld luawa6eueyy JaleMwuols auawgoellV e Medinah Subdivision am Storm Water Management Plan Y a 2.36 cis 3. 09 cis 1. 24 cis Flow Rate, Q 223 cis 2.36 cis 1. 3 POST- DEVELOPED CONDITIONS As with the pre -developed conditions shown above, the 1. 83 acre area will be analyzed as one distinct drainage basin. Table 1. 3. 1 summarizes the area information of the post -developed conditions. Table 1. 3. 1: CONDITIONS - FORT WALTON BEACH, FL 25Yr - 24Hr Storm Duration 25Yr - 1 Hr 25Yr - 2 Hr 25Yr 4 Fir 25Yr - 8 Hr. 1. 83. Acres Basin Area 1, 83 Acres 1. 83 Acres 1. 83 Acres 1.83 Acres 10 minutes 10 minutes 10 minutes 10 Minutes 10 Minutes Time-of-Conc. 73 73 73 73 73 Curve Number 4. 45 cis 4,55 cis 3. 43 cis 4.33 cis 1. 61 cis Flow Rate, Q 1. 4 STORM RETENTION CALCULATIONS The post -developed runoff for the site will be retained in an above ground retention system within the site boundaries. Seasonal high groundwater levels were obtained from the Geotechnical Report provided by Southern Earth Sciences, Inc. ( see attached). The estimated depth to the water table is estimated to be greater than 10.0 feet since no indication of seasonal high ground water was noted in the 10 foot deep boring, the maximum level of groundwater is estimated to be below an elevation of 8. 0. The lowest proposed pond bottom is located at a proposed elevation of 15. 0; therefore there is a minimum of two feet of separation between the proposed pond bottoms and the ground water level . A permeability rate of 7. 6 inches /hour was determined by permeability testing of the site material. A factor of safety of 2. 0 was applied to this rate and a design permeability of 15. 2 inches /hour was utilized in the evaluation of the retention system.  leld uolslAlpgns geu! POW : L61,Z) uolslAlpgns yeulpaW - ueld luawa6euew lalemwuols auewgoelly D d' Madinah Subdivision a a Storm Water Management Plan Y a The dimensions of the dry retention area are as follows. RETENTION POND onc ni n The attached ICPR model shows the peak flow rates and the routing of the 25 -year 1 hour, 2 hour, 4 hour, 8 hour and 24 hour storm events through the proposed retention area. The following peak stages and outflows were found for each event . a 17.08 0. 00 2. 23 25 YR - 1 HR 25 YR - 2 HR 17. 55 0. 97 2. 36 25 YR - 4 HR 17, 60 2.04 2. 36 17. 66 2. 53 3. 09 25 YR - 8 HR 17. 44 0. 95 1. 24 25 YR - 24 HR A maximum elevation of 17. 66 was calculated for the 25 year - critical storm event. An overflow weir ( elevation 17. 2) is located at the northern boundary of the retention area and is calculated to discharge 2. 53 cfs at the maximum stage of 17. 66. This rate is less than the pre -development discharge rate of 3.09 cfs therefore attenuation requirements are met. 1. 5 1" RETENTION REQUIREMENT In addition to meeting the attenuation requirements above, the retention area must also be sized to retain the first 1 inch of runoff from the developed area. The first 1" of runoff associated with the developed site is 6,643 cubic feet. The retention area is designed to retain 11, 766 cubic feet. The 1" retention requirement is met at an elevation of approximately 17. 2; therefore the retention area is adequately sized to retain the first 1" of runoff. 1. 6 72 HOUR RECOVERY REQUIREMENT  7eld uolslnlpgnS geulpapy : LW) uolslnlpgnS geulpayy - ueld luawa6euM Jatemu' JOIS : luawgoetty aa Medinah Subdmsron Storm Water Management Plan a a aUm Recovery is achieved through exfiltration /percolation. The proposed retention pond will recover in approximately 5. 9 hours for the 25 year —critical storm event. This timeframe exceeds City of Fort Walton Beach requirements.  Gai- : r i Lj tq 666: t i• `arc , ; - , , ` e -. ?. I -!' s r e _ CE Li y ' ifs i" NI e T i  leld uolslAlp4nS 4euipaiN : L61Z) uas1A! pQnS 4euipayN - ueld 1uaw36euepy lalemooJOIS : luaw43e11V a m L a Y A stage /Area NOdes V Stage / Volume aVN T Mime / Stage M anhole T Basin 1 Pie A B sin 1- Poat O8ae¢ Wetland ns Flow p Bes Vn¢ t CN U: Ba sin 1- Pie V: Basin . 1 - Post S S c. CN Y SCS 1m1 Z sBllH GA E: P¢ tc 1 W W¢Pip¢ ii W :' Je ii G Chann¢ I O Btop st nctoie Bx rag¢ R Rating Ccrve H Breach Pexclatlon T: BOOndaiy 1 '.. F Filter X Extil Trench Interconnected Channel and Pond Routing Model (ICPR) 02002 Streanline Technologies, Inc.  7eld uolalA! P9nS 4eulPaW c L6LZ) UOISIAIpgnS 4eu' PaW - Ueld ivawa6euuW lalefAtuloIS Iuaw9aegy cr r rn