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Home My Public Portal AboutSWMR Letter 08-01-19.pdf ADDRESS PHONE WEB 4 Cedar View Court, Savannah, GA 31410 (912) 662.2914 ballmaritime.com August 1, 2019 Mr. George Shaw Planning & Zoning Manager City of Tybee Island P.O. Box 2749 Tybee Island, GA 31328 re: PN 19-1000 – TIMA Expansion: Stormwater Management Plan Dear George: This submittal has been assembled based on our understanding of the project, the requirements for stormwater management discussed with Mr. Downer Davis, PE, and the previous responses from the Planning Commission. In order to meet the requirements for approval, it is our understanding that the proposed site must not increase the impervious coverage from the pre-developed site and the site must detain the post development stormwater run-off below water is released from the site currently. Please refer to the submitted plans for information complimentary to this narrative. The engineering drawings for the project will reflect the actual construction activities proposed below (Attachment D). Pre-development and Post Development Considerations The project site is considered to be the property boundaries on the north and east, the north edge of the existing gym, and the edge of pavement along Lovell Avenue. This is approximately 0.25-acres. The pre-development impervious coverage on the site is 0.21-acres. The post development conditions will include additional impervious area to accommodate the garbage dumpster pad, but impervious area will be reduced in the existing parking areas along Lovell Avenue to balance out the net results. The table below reflects this change. The modular building does not count as an increase in impervious area due to it being elevated and the footprint being used as a gravel infiltration best management practice. Unpaved parking is currently provided along Lovell Avenue and will remain that way for this project. TABLE 1 – DRAINAGE BASIN COMPARISON Existing Area (SF) Proposed Area (SF) Impervious Impervious Concrete Pad 24 Concrete Pad 12 Crush & Run 1,391 Crush & Run 502 Dumpster Pad 300 Concrete Sidewalk 217 Pervious Pervious Open Space 9,271 Gravel Building Pad 4,352 Open Space 4,783 Metal ADA Ramp 520 Total 10,686 10,686 Percent Impervious 13% 10% PN 19-1000: TIMA Expansion Page 2 BMG/GS ADDRESS PHONE WEB 4 Cedar View Court, Savannah, GA 31410 (912) 662.2914 ballmaritime.com Based on a topographic survey performed for the site in July 2019 by Gammon Surveying (Attachment A), there is no run- off through the site from the adjacent properties. The run-off generated from the impervious surface of the modular building rooftop in a 25-yr 24 -hr storm event will be pitched to eastern side into a 6” gutter. The run-off will be conveyed downward through four 4”x4” downspouts to the ground level into energy dissipators. The run-off will then drain from the east to the west over the re-graded site, traveling through the gravel base installed beneath the footprint of the building. It is anticipated that no drainage will leave the site under these design conditions. The volume of run-off generated from the roof top will be approximately 114-cubic feet using a 25-year 24-hour storm event with an intensity of 0.33-inches (from GSMM Volume 2 for Savannah). A gutter sizing spreadsheet was used to confirm the minimum number of downspouts and gutter size required (Attachment B). This will be used for the design of the modular building. The volume of storage provided by the 6” gravel base beneath the building is approximately 870.4- cubic feet using a porosity factor of 0.4 for gravel. Since the soils are Group A (Attachment C – Web Soil Survey), it assumed that the infiltration rate is 6”/hour or 2,176-cubic feet per hour. This means the infiltration practice will drain in approximately 30-minutes or less. The site will effectively accommodate the 25-year 24-hour storm event onsite without contributing to any conditions downstream of the site. We look forward to any questions you may have after reviewing this submittal. Please contact our office if you have any questions, concerns, or desire additional information. Regards, Jason P. Ball, PE, D.PE, D.NE President Ball Maritime Group, LLC Attachments: Attachment A – Survey w/ Topography Attachment B – Gutter Sizing Tool Attachment C – Web Soil Survey Attachment D – Site Plans 1010999 9 888 84°m ° + ° ° 9°m °+°°9° m ° = ° ° 9° m ° = ° ° 9° m ° = ° ° 9°m °+°°9° m ° = ° ° 4°m °=°°LSF# 1280THOMAS G. GAMMONLICENSE NO. 3005GA. REG. LAND SURVEYOR7/20/2019FORTH IN O.C.G.A. SECTION 15-6-67.PARCELS OF LAND AND DOES NOT SUBDIVIDE OR CREATE A NEW PARCEL OR MAKE ANY CHANGES TO ANY REAL PROPERTY BOUNDARIES. THE RECORDING INFORMATION OF THE DOCUMENTS, MAPS, PLATS, OR OTHER INSTRUMENTS WHICH CREATED THE PARCEL OR PARCELS ARE STATED HEREON. RECORDATION OF THIS PLAT DOES NOT IMPLY APPROVAL OF ANY LOCAL JURISDICTION, AVAILABILITY OF PERMITS, COMPLIANCE WITH LOCAL REGULATIONS OR REQUIREMENTS, OR SUITABILITY FOR ANY USE OR PURPOSE OF THE LAND. FURTHERMORE, THE UNDERSIGNED LAND SURVEYOR CERTIFIES, TO THE BEST OF HIS KNOWLEDGE AND BELIEF, THAT THIS PLAT COMPLIES WITH THE MINIMUM TECHNICAL STANDARDS FOR PROPERTY SURVEYS IN GEORGIA AS SET FORTH IN THE RULES AND REGULATIONS OF THE GEORGIA BOARD OF REGISTRATION FOR PROFESSIONAL ENGINEERS AND LAND SURVEYORS AND AS SET THIS PLAT IS A RETRACEMENT OF AN EXISTING PARCEL OR NOTES:1. FIELD EQUIPMENT USED: GEOMAX ZOOM 80 5" TOTALSTATION AND CHAMPION TKO RECEIVER2. BEARINGS AND COORDINATES SHOWN ON THIS SURVEY AREBASED ON GEORGIA STATE PLANE COORDINATE SYSTEM(NAD83), EAST ZONE.3.ELEVATIONS SHOWN ON THIS DRAWING ARE BASED ONNAVD88, UNLESS OTHERWISE NOTED. CONTOUR INTERVAL ISONE FOOT.4.THE PROPERTY IDENTIFICATION NUMBER (PIN) OF THEPROPERTY DEPICTED HEREON IS 40005 19001.5.UNDERGROUND UTILITIES NOT SHOWN MAY BEENCOUNTERED.THE UTILITY PROTECTION CENTER SHOULD BE CONTACTEDPRIOR TO BEGINNING ANY CONSTRUCTION.6.ACCORDING TO F.I.R.M. MAP NO. 13051CO, PANEL 0213G,EFFECTIVE 08/162018, IT IS MY OPINION THAT THE SUBJECTAREA SHOWN ON THIS SURVEY, FALLS WITHIN A FLOODHAZARD AREA IN ZONE AE 97.TREES ARE LOCATED IN INCHES THE SIZE OF THE SYMBOLDOES NOT INDICATE THE SIZE OF THE DRIP LINE AND/ORCANOPY LINE.CONTOUR LINE12WATER VALVECONTROL POINTSANITARY MANHOLEPOWER POLEIRON REBAR SETGUY WIREFIRE HYDRANTIRON REBAR FOUNDN:731617.53E:1065843.21N:731455.92E:1065787.29 K-Style_Gutter - 1 of 1 Project: Prepared By: Date: ABC =Inputs XYZ = Results 1 Frequency = 25 years Amount, P = 0.33 inches 2 Compute the capacity of the selected gutter size. tw = 6.0 in bw = 4.600 in d = 4.600 in Qg = 0.35 cfs, capacity of gutter fb = 0.5 in Ag = 21.42 sq in, cross-sectional area of gutter wp = 12.99 in, wetted perimeter of gutter r = 1.65 in, Ag/wp, hydraulic radius 3 Compute the capacity of the selected downspout. W = 4.000 in Qd = 0.010457 x Ad x h^0.5 L = 4.000 in Qd = 0.34 Ad = 16.00 sq in, cross sectional area of downspout h = 4.10 in, head (generally the depth of the gutter minus 0.5 in) 4 5 Determine roof area that can be served per downspout. Q = 0.34 Ar = 3,696 sq-ft of roof area that can be served by this single downspout with P =0.33 inches/ 5 min. 6 Determine the number of downspouts necessary to control the water from the entire roof area. length = 64 ft width = 68 ft Atotal = 4,352 sf 1.18 so use 2.00 Total downspout(s) capacity = 0.68 cfs =304 GPM Linking to a commercial website is NOT a USDA endorsement Example @ www.raingutterstore.com/ - others available and April 12, 2011 (2) As an alternative for the Pacific Islands, multiply the amount for a 15-minute duration storm by 0.6 for a close approximation of the amount for a 5-minute storm. http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_hi.html www.egutter.com/RAIN-GUTTERS-GUTTER-SUPPLY cfs, capacity of one downspout Gutter dimensions: cfs, capacity of single downspout system (smaller of Qg and Qd) The area of a roof that can be served by a gutter and downspout system is controlled by either capacity of the gutter (channel flow) or by the capacity of the downspout (orifice flow), as shown in 2 and 3 above. In a multiple downspout system, the number of downspouts is determiend by dividing the entire roof area draining into the gutter, Atotal, by the roof area that can be served by one downspout, Ar, as follows: Comparing the above calculated capacities, Qg and Qd, Total roof area to be served by this gutter system, Atotal = roof length x roof width. CPS 558, Roof Runoff Structure - Gutter and Downspout Capacities (1) (1) Roof Runoff Management (AWMFH, Chapter 10, pgs. 10-1 thru 10-3) Downspout capacity controls Qg = 0.01184 x Ag x r^0.67 Roof Dimensions Number of downspouts, Nd = Atotal / Ar = Downspout dimensions: Determine the rainfall intensity for a 5-minute duration storm for a given return period(2). TIMA Modular Building BMG 31-Jul-19 Gutters-v2012_-07May2014_14600.xlsx 0 K-Style_Gutter 8/1/2019 Bryan and Chatham Counties, Georgia Cuc—Chipley-Urban land complex Map Unit Setting National map unit symbol: 46g9 Elevation: 20 to 450 feet Mean annual precipitation: 44 to 52 inches Mean annual air temperature: 64 to 70 degrees F Frost-free period: 230 to 290 days Farmland classification: Not prime farmland Map Unit Composition Chipley and similar soils: 60 percent Urban land: 35 percent Minor components: 5 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Chipley Setting Landform: Flats Landform position (three-dimensional): Rise Down-slope shape: Linear Across-slope shape: Convex Parent material: Marine deposits Typical profile H1 - 0 to 6 inches: fine sand H2 - 6 to 77 inches: fine sand Properties and qualities Slope: 0 to 5 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Moderately well drained Capacity of the most limiting layer to transmit water (Ksat): High to very high (5.95 to 19.98 in/hr) Depth to water table: About 24 to 36 inches Frequency of flooding: None Frequency of ponding: None Available water storage in profile: Low (about 3.7 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 3s Hydrologic Soil Group: A Hydric soil rating: No Minor Components Osier Percent of map unit: 5 percent Map Unit Description: Chipley-Urban land complex---Bryan and Chatham Counties, Georgia TIMA Modular Building Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/1/2019 Page 1 of 2 Landform: Depressions, swales, flats Landform position (three-dimensional): Dip Down-slope shape: Concave, linear Across-slope shape: Concave, linear Hydric soil rating: Yes Data Source Information Soil Survey Area: Bryan and Chatham Counties, Georgia Survey Area Data: Version 13, Sep 13, 2018 Map Unit Description: Chipley-Urban land complex---Bryan and Chatham Counties, Georgia TIMA Modular Building Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/1/2019 Page 2 of 2