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HomeMy WebLinkAboutStormwater Draiange Calculations Mill Lane Farm Use 9-11-20251 101 down cape engineering, inc, CIVIL MAINM5 & LANP 5UMY95 939 MAIN 5f / U9 6A YARMOUTHPOkf, MA 02615 (508) 362-4541 FAX (508) 362-9880 )k", DRAINAGE CALCULATIONS: For Agricultural Use at # 8 8+ 110 Mill Lane Yarmouthport, MA 0N OF Mgss�cy �o DANIEL o JJA-,'t C?V!L y o NO.465D2 Q, DATE: 9-11-2025 ������,STe��°��`` ` Ss�ONAL ENG CLIENT: Oliver and Natasha Brahmst DCE File#25-094 Prepared for: Farm at 88/100 Mill Lane Yarmouthport- Oliver and Natasha Brahmst 9-11-25 DRAINAGE SUMMARY DESIGN EVENT: 25 YEAR STORM EVENT The drainage systems specified for farm use at 88/100 Mill Lane in Yarmouthport have been designed to comply with the Low Impact Drainage guidance of the local Yarmouth Stormwater regulations and the State Stormwater Management Guidelines. The proposed drainage system utilizes gutters and downspouts to drywells for the small roof areas proposed, and low impact grass lined swales for the existing grass pastureland. The design includes a berm with grass lined swale to infiltrate the design storm from the pasturelands as shown on the Site Plans dated revised 9-11-2025. DA1 is the barn and drives and the main pasture area on the entire east side of the site, the upper pasture is diverted with a grass swale to the east and the existing irrigated grass pasture then flows down toward the fence and is captured by a 12" high berm and swale as indicated on the updated Site Plans. The limited runoff from the fenced garden area will be treated by the additional ten foot native buffer to the farm pond. Clear water from roof runoff is allowed to be directly infiltrated per the stormwater guidelines, so this is accomplished with a large drywell between the structures. A Rational Method calculation is attached showing the drywell is wholly adequate. For the main pasture, in creating a full 12" high berm between the lowpoint of the swale and the top of the berm for the 860 linear feet along the eastern wetland, and figuring on a 3" per hour infiltration rate through the grass and the pervious grass lined berm, the design is shown to be capable of infiltrating the full rainfall design event of a 25 year return frequency Type III Storm, which is shown by the hydraulic calculations to be fully infiltrated and treated onsite. The drainage area and a HydroCAD run for the pasture drainage area DA1 shown on the attached plan are included. The HydroCAD output for the proposed conditions shows that the highest water level reached during the storm is one inch below the top of the berm, so it is shown the 860' long berm/swale system can fully infiltrate a 25 year storm onsite. The HydroCAD drainage calculations and drainage area sketch plan are attached for reference. Based on the attached drainage calculations the proposed drainage system is adequate for the site, and represents a significant improvement over existing conditions. Prepared by: ��(N OF,yAs9cy ~ �o DANIELA. GN 1 _ �1/(Itt� o O,1ALA Cv!L Daniel A. Ojala PLS, PE date No, 46502 Down Cape Engineering, Inc. � SSjoNAL 88/100 Mill Lane Roof Drywell DRAINAGE CALCULATIONS DRAINAGE AREA: PROPOSED ROOFS DRAINAGE AREA COMPOSITE CURVE NUMBER CALCULATIONS: AREA OF BUILDINGS: AREA OF LAWNS AND WOODS: TOTAL DRAINAGE AREA: CURVE NUMBER PVT/BLDGS: 0.95 CURVE NUMBER LAWN/WOODS: 0.35 down cape engineering, inc. DATE: 9-11-2025 DOWN CAPE ENGINEERING 530 SF 0.01 AC. 0 SF 0.00 AC. 530 SF 0.01 AC. COMPOSITE CURVE NUMBER: 0.95 (PVT AREA * 0.95 + GRASS- 0.35)/ TOTAL AREA) RATIONAL METHOD DRAINAGE SIZING: Q=CIA, SF REQ= Q*448.8 GPM/CFS (1/0.5) GPM/SF DRAINAGE AREA: 530 SF CURVE NUMBER: 0.95 RATIO RAINFALL INTENSITY: 5.1 IN./HR (SEE NOMOGRAPH) GPM/SF RATIO: 0.3 GPM/SF (<10MIN/IN PERC RATE) LEACHING AREA REQUIRED: 88 SF USE 1 LEACHING PIT 4'X6'DIA WITH 2'STONE AROUND TOTAL SQUARE FOOTAGE OF LEACHING AREA PROVIDED = 204 SF >88 SF O.K. RATIONAL 25-094 Roof Drywells.xls Page 1 of 1 is Pasture Are Siva 3 leading to - DA1 Grass Lined Swale Storage and Infiltration Model Subcat> Reach on Link C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Prepared by Down Cape Engineering Inc HydroCADO 10.20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 2 Rainfall Events Listing (selected events) Event# Event Storm Type Name Curve Mode Duration B/B Depth AMC (hours) (inches) 1 Barn Cty 25 Yr Type III 24-hr Default 24.00 1 5.92 2 C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Prepared by Down Cape Engineering Inc HydroCADO 10 20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 3 Area Listing (all nodes) Area CN Description (sq-ft) (subcatchment-numbers) 3,618 98 Barn/driveways (1S) 77,371 61 Pasture/grassland/range, Good, HSG B (1S) 80,989 63 TOTAL AREA C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Prepared by Down Cape Engineering Inc HydroCAM 10 20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 4 Soil Listing (all nodes) Area (sq-ft) Soil Group Subcatchment Numbers 0 HSG A 77,371 HSG B 1S 0 HSG C 0 HSG D 3,618 Other is 80,989 TOTAL AREA C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Prepared by Down Cape Engineering Inc HydroCADO 10 20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 5 Ground Covers (all nodes) HSG-A HSG-B HSG-C HSG-D Other Total Ground (sq-ft) (sq-ft) (sq-ft) (sq-ft) (sq-ft) (sq-ft) Cover 0 0 0 0 3,618 3,618 Barn/driveways 0 77,371 0 0 0 77,371 Pasture/grasslan d/range, Good 0 77,371 0 0 3,618 80,989 TOTAL AREA C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Prepared by Down Cape Engineering Inc HydroCADO 10 20 7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 6 Notes Listing (all nodes) Line# Node Notes Number 1 is Time of Concentration 2 1 P Storage/Infiltration Model of Grass Lined Swale C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Type 111 24-hr Barn Cty 25 Yr Rainfall=5.92" Prepared by Down Cape Engineering Inc HydroCADO 10 20-7a s/n 13540 C 2025 HydroCAD Software Solutions LLC Page 7 Time span=1.00-24.00 hrs, dt=0.01 hrs, 2301 points Runoff by SCS TR-20 method, UH=SCS, Weighted-CN Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment1 S: Pasture Area leading to Runoff Area=80,989 sf 4.47% Impervious Runoff Depth>2.12" Tc=5.0 min CN=63 Runoff=4.63 cfs 14,296 cf Pond 1 P: Grass Lined Swale Storage and Peak Elev=10.95' Storage=5,121 cf Inflow=4.63 cfs 14,296 cf Outflow=0.73 cfs 14,029 cf Total Runoff Area = 80,989 sf Runoff Volume = 14,296 cf Average Runoff Depth = 2.12" 95.53% Pervious = 77,371 sf 4.47% Impervious = 3,618 sf C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Type III 24-hr Barn Cty 25 Yr Rainfall=5.92" Prepared by Down Cape Engineering Inc HydroCADO 10 20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 8 Summary for Subcatchment 1 S: Pasture Area leading to Swale- DA1 Time of Concentration Runoff = 4.63 cfs @ 12.08 hrs, Volume= 14,296 cf, Depth> 2.12" Routed to Pond 1 P : Grass Lined Swale Storage and Infiltration Model Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 1.00-24.00 hrs, dt= 0.01 hrs Type III 24-hr Barn Cty 25 Yr Rainfall=5.92" Area (sf) CN Description 77,371 61 Pasture/grassland/range, Good, HSG B 3,618 98 Barn/driveways 80,989 63 Weighted Average 77,371 95.53% Pervious Area 3,618 4.47% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Time Concentration Subcatchment 1S: Pasture Area leading to Swale- DA1 Hydrograph ■ Runoff 1 2 3 4 5 6 7 8 9 1U 11 I 10 14 10 10 11 10 a au ai « Time (hours) C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Type 111 24-hr Barn Cty 25 Yr Rainfall=5.92" Prepared by Down Cape Engineering Inc HydroCADO 10.20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 9 Summary for Pond 1 P: Grass Lined Swale Storage and Infiltration Model Storage/Infiltration Model of Grass Lined Swale Inflow Area = 80,989 sf, 4.47% Impervious, Inflow Depth > 2.12" for Barn Cty 25 Yr event Inflow = 4.63 cfs @ 12.08 hrs, Volume= 14,296 cf Outflow = 0.73 cfs @ 12.63 hrs, Volume= 14,029 cf, Atten= 84%, Lag= 33.1 min Primary = 0.73 cfs @ 12.63 hrs, Volume= 14,029 cf Routing by Stor-Ind method, Time Span= 1.00-24.00 hrs, dt= 0.01 hrs / 2 Peak Elev= 10.95' @ 12.63 hrs Storage= 5,121 cf Plug -Flow detention time= 88.7 min calculated for 14,029 cf (98% of inflow) Center -of -Mass det. time= 78.2 min ( 932.4 - 854.3 ) Volume Invert Avail.Storage Storage Description #1 10.00, 5,590 cf Infiltration System Storage ModeListed below Elevation Cum.Store (feet) (cubic -feet) 10.00 0 10.50 1,290 11.00 5,590 Device Routing Invert Outlet Devices #1 Primary 10.00' Swale Infiltration Model Head (feet) 0.00 0.50 1.00 Disch. (cfs) 0.000 0.360 0.770 Primary OutFlow Max=0.73 cfs @ 12.63 hrs HW=10.95' (Free Discharge) L1=Swale Infiltration Model (Custom Controls 0.73 cfs) CAUsers\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Type 111 24-hr Barn Cty 25 Yr Rainfall=5.92" Prepared by Down Cape Engineering Inc HydroCADO 10 20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 10 Pond 1 P: Grass Lined Swale Storage and Infiltration Model Hydrograph w 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1b 1b I 10 1a zu n 44 co c Time (hours) Pond 1 P: Grass Lined Swale Storage and Infiltration Model Stage -Discharge M 1 0 0.05 0.1 0.15 0.2 0.25 0.3 U.3b u.4 u.40 u.D D.DD D.D Discharge (cfs) ■ Inflow p Primary Primary CAUsers\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Type /// 24-hr Barn Cty 25 Yr Rainfall=5.92" Prepared by Down Cape Engineering Inc HydroCADO 10.20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC Page 11 w Pond 1 P: Grass Lined Swale Storage and Infiltration Model Stage -Area -Storage Storage (cubic -feet) ■ Storage C:\Users\Public\Documents\HydroCAD\Projects\ HydroCAD Farm Pasture Grass Swale 25-094 Brahmst Farm Swale DA1 Table of Contents Prepared by Down Cape Engineering Inc HydroCADO 10.20-7a s/n 13540 © 2025 HydroCAD Software Solutions LLC TABLE OF CONTENTS Proiect Reports 1 Routing Diagram 2 Rainfall Events Listing (selected events) 3 Area Listing (all nodes) 4 Soil Listing (all nodes) 5 Ground Covers (all nodes) 6 Notes Listing (all nodes) Barn Ctv 25 Yr Event 7 Node Listing 8 Subcat 1S: Pasture Area leading to Swale- DA1 9 Pond 1 P: Grass Lined Swale Storage and Infiltration Model