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Home My WebLink AboutStorm Drainage Calcs HYDROLOGY REPORT for LAVENDER PLACE SUBDIVISION SOUTH BLOOMERANG AVENUE & EAST LAKE HAZEL ROAD Meridian, Idaho PREPARED BY: Breckon Land Design, Inc. 6661 N Glenwood Street Garden City, Idaho 83714 p: (208) 376-5153 f: (208) 376-6528 SS�ptjAL E*6, T ER £ b54 $120 Nr MP ti Project No. 24029 June 28, 2024 1 PURPOSE The purpose of this report is to confirm that the on-site storm water management system design for the Lavender Place Subdivision is adequate for the specified design storms per the City of Meridian and Idaho Department of Environmental Quality standards. SITE DESCRIPTION The subject property is located on the Northeast Corner of East Lake Hazel Road and South Bloomerang Avenue in the City of Meridian, Idaho. The limits of construction are approximately 2.1 acres, as shown on the attached drainage basin map and the construction documents. See Appendix A for the post development drainage basin map. Currently, the site is relatively flat; therefore, only a small amount of runoff leaves the property to the existing bordering roadways. The proposed improvements consist of residential lots, residential buildings, associated structures, and utility and roadway improvements servicing the subdivision. Stormwater within the limits of construction will be contained on site to the extent possible with the addition of the proposed improvements. Soil properties and infiltration rates were obtained from a Geotechnical Engineering letter for the Proposed Subdivision by Innovate Geotechnical, dated May 29, 2024. See Appendix D for a copy of the letter. DESIGN CRITERIA The system is designed to accommodate the 100-year design storm using Infiltration Swales and Seepage Beds in accordance with recommendations given in section 3.2.E Infiltration Facilities of the City of Boise in the Stormwater Management Design Manual, December 2019. The storm water runoff flows were calculated using the Rational Method with a weighted post development runoff coefficient calculated for each drainage area. The rainfall intensity used was taken from the Intensity-Duration- Frequency Curve for the City of Boise in the Stormwater Management Design Manual, December 2019. See Appendix B for a copy of this curve. PEAK FLOW ANALYSIS OUTLINE The proposed storm drainage conveyance facilities are sized to provide the necessary capacity to convey the design storm, as required. The following steps were taken to design the storm conveyance facilities. 1. Calculate the individual drainage basin areas (A), the Runoff Coefficient (C) and Time of Concentration (Tc) values for the developed basins. Combine basins, as appropriate, to determine flow at specific Design Points. 2. Determine the peak flow for each basin and Design Point using the Rational Equation (Q=CIA), utilizing the Rainfall Intensity (1)from the appropriate Intensity-Duration-Frequency curves based on the Tc value. 3. Verify capacity of the inlets, storm drain lines, sand and grease traps and other storm water conveyance facilities to accommodate the peak flows. PEAK FLOW ANALYSIS RESULTS The proposed storm drainage conveyance system has been sized to accommodate the 100-year peak flow rates. All facilities have been verified to adequately pass the peak flow and convey the stormwater to a facility for infiltration so that off-site runoff is well below pre-development conditions. See Appendix C for detailed calculations. 2 DESIGN STORM RETENTION VOLUME ANALYSIS OUTLINE The proposed storm drainage retention facilities are sized to provide the necessary capacity to store the 100- year storm event, to provide for initial settlement of sediments in the runoff and dispose by infiltration within the required time frame. The following steps were taken to design the storm drain facility. 1. Calculate the individual drainage basin areas (A) and estimate the Runoff Coefficient (C) for the developed basins. Combine basins, as appropriate, to determine volume at specific Design Points. 2. Determine the Rainfall Intensity (1)from the appropriate Intensity-Duration-Frequency curves based on the design storm event and storm duration (T) of one hour. 3. Size the drainage facility per the Modified Rational Method to accommodate the required volume using the following equation V= C*I*T*A, providing for freeboard and sedimentation. 4. Verify capacity of facilities for design volume and maximum drain time. 100 YEAR RETENTION RESULTS The proposed drainage facilities have been designed to adequately detain and dispose of the 100-year design storm event. See Appendix C for detailed calculations. PRE- DEVELOPMENT DRAINAGE BASIN AND DESIGN POINT ANALYSIS A Pre-development analysis was not performed to demonstrate pre-development site conditions, as drainage is proposed to be retained on-site to the extent possible. As the design intent is to retain the stormwater discharge from the project on-site, the off-site discharge will be reduced significantly with the proposed improvements. POST DEVELOPMENT DRAINAGE BASIN AND DESIGN POINT ANALYSIS For the post-development analysis, the drainage patterns for the proposed site have been divided into five (5) basin areas as shown on the Drainage Area Map in Appendix A. The selected runoff coefficients are a composite of impervious surfaces (C=0.95) and pervious areas (C=0.15). Each basin denotes an area of the project that drains to a specific point defined by a design point. See Appendix C for detailed calculations. Seepage Bed DB-1 is the point of discharge for Drainage Basin 1 and serves approximately 0.73 acres consisting of impervious surfaces and pervious surface drainage. Stormwater runoff from Drainage Basin 1 includes sheet flow and gutter flow draining to a catch basin. The flow is intercepted by the inlet and directed to a Sand & Grease Trap and subsequently the Seepage Bed. Design flows are stored until they infiltrate. See Appendix C for sizing calculations. Seepage Bed DB-2 is the point of discharge for Drainage Basin 2 and serves approximately 0.36 acres consisting of impervious surfaces and pervious surface drainage. Stormwater runoff from Drainage Basin 2 includes sheet flow and gutter flow draining to a catch basin. The flow is intercepted by the inlet and directed to a Sand & Grease Trap and subsequently the Seepage Bed. Design flows are stored until they infiltrate. See Appendix C for sizing calculations. Seepage Bed DB-3 is the point of discharge for Drainage Basin 3 and serves approximately 0.34 acres consisting of impervious surfaces and pervious surface drainage. Stormwater runoff from Drainage Basin 3 includes sheet flow and gutter flow draining to a catch basin. The flow is intercepted by the inlet and directed to a Sand & Grease Trap and subsequently the Seepage Bed. Design flows are stored until they infiltrate. See Appendix C for sizing calculations. Seepage Bed DB-4 is the point of discharge for Drainage Basin 4 and serves approximately 0.92 acres consisting of impervious surfaces and pervious surface drainage. Stormwater runoff from Drainage Basin 4 includes sheet flow and gutter flow draining to a catch basin. The flow is intercepted by the inlet and directed a Sand & Grease Trap and subsequently the Seepage Bed. Design flows are stored until they infiltrate. See Appendix C for sizing calculations. 3 Multiple Drainage Swales DB-5 is the point of discharge for Drainage Basin 5. Drainage Basin 5 covers approximately 0.32 acres consisting of impervious surfaces and pervious surface drainage. Stormwater runoff from Drainage Basin 5 includes sheet flow to multiple swales. The swales are located above a continuous sand window running the entire length of the swale locations. The flow to the stormwater swales is stored in the swale until it infiltrates into the ground. See Appendix C for sizing calculations. 4 Appendix A Drainage Basin Map ORIGINAL SURVEY ON g� o o �o Pleb PI / \ _ ss\ssIff M .I I � Mtn �\ --- � ` ,. ; = D B 1 - � 6 D B 5 U I � \\� DRAINAGE BASIN#5 _ � —`__„_"�___ _____, TOTAL AC 031 - °#0 / IMPERMOUS'.016 AC `,�',y _ PERVIOUS.0.15AC ME Ni DRAINAGE BASIN#1 t' _ ?'�1"�w^r R ERA( TOT&AS IMPERVIOUS: - = D B 2 _ _- - // N a - -- _ - W U) 1 b, CO Q w c w we•• - L DRAINAGE BASIN#4 - \� �'iTt. .T J P V 00 w 1 '� �� g• _ TOTAL AC:0.9 1 / --'--'�. IMPERVIOUS:0.3 AC / `\ �� `Y Y-Z ^*" �] T Q Q DRANAGE BASIN#3 - \ 98'• __ / / _ - ` LJ•J _ raraL Ac 9.a3 v� - ! a' IMPERMOU5016 AC. We•. pOAINAGE BASIN#2 Lu W B 0 Y Z Q PERVIOUS 016 AC �'— _ TOTALAC:0.36 Q IF - _ •� _ IMPERVIDUS:0.23 AS PERVIOUS:0.13 AC — _ - //) LJ.J J e8 W Fr Q ❑-as ❑ _ -El c) a ��a E —— ——— T m FP 112" 7��iL, w, 2 2—GPTz-- �P El LAVENDER PLACE DRAINAGE AREASji �j e a PAz�o.: a- OB/26/2624 A x DA0.1 1 2 1 3 141 5 1 6 1 7 1 8 9 10 11 12 13 14 15 16 Appendix 6 Intensity-Duration-Frequency Curve Hydrologic and Hydraulic Graphs APPENDIX D 10.0 8.0 6.0 4.0 2.0 1.7 RUNOFF EVENT FREQUENCY IN YEARS 1.0 E L 0 0.8 0.6 Uj a 0.4 V !� Z } 0,2 Z INDICATES MAXIMUM PRECIPITATION AT BOISE WEATHER STATICIN BETWEEN 1902 AND 1961 Z 0.1 .0 8 .v6 .04 .02 10 12 15 20 30 40 50 60 2 3 4 5 6 8 10 12 18 24 MINUTES 0 H 0 U R 5 - - DURATION FIGURE D.1 RAINFALL INTENSITY, DURATION AND FREQUENCY RELATIONSHIP 2019 Boise Stormwater Design Manual p-� Appendix C Storm Water Calculations Drainage Facility Calculations Version 2.0 24029 Wvender Place Brandt Elwell H.Kent Magleby,P.E. r 1 - �. • • City of Meridian Standards Drainage Area S�rface Area Runoff Coefficient Equivalent Area Impervious Area 22,488 0.95 21,364 _. ......... ................................ Pervious Area 9,262 - 0.15 1,389 Drainage Area Summary Significant Flow Events "Drainage A 31,750 sf 0.52 0.97 1.20 1.39 1.63 Drainage Area A 0.73 acres Weighted Runoff Coefficient C 0.72 Time of Concentration Tc 10 min •- Q100 J .- Q100 . . Seepage Bed Drainage Weighted Runoff Coefficient(C) Captured Flow(Qa,) Equivalent Captured Area(A.pt)=CxA Infiltration rate Storage Volume Required(V_j 0.72 1.63 cfs 22,753 ft2 8.00 in/hr 4,034 ft 3 Required Storage for Captured Volume (Design Storm) Qdev=CIA Vin=tQdev Vout=tQperc Vstorage=Vin-Vout Time(min) Time(sec) ntens�ty(ir/hr)�l Q dev.(ft3/S) V in(ft') V OUt(ft) V Storage(ft) 10 600 2.58 1.36 815.31 168.89 646 ...................................................... 15 900 2.18 1.15 1033.36 253.33 780 ........................................................................................E....................................................................................................................................................................................i...................................................................� ....................... 20 1200 1.81 0.95 1143.97 337.78 806 .............................................................. ____. ____. _____. ..................... ___. ____.____. __.................. 30 1800 1.58 : 0.83 1497.90 506.67 991 ............................................................i........................................................1................................... ....................i................................................................................................................. ....................... 40 2400 1.51 0.80 1908.72 675.56 1,233 ......................................... __......._.. _.........___ ___.................. 50 3000 1.25 0.66 1975.08 844.44 1,131 ............................................................:........................................................_............................. ....................................................................................................................................... ............................. 60 3600 1.15 0.61 2180.49 1013.33 1,167 ......... ......... ................... . _........._.. _. ..... ____... _____ ........ __ .................. 120 7200 1.07 0.56 4057.60 2026.67 2,031 180 10800 1.00 0.53 5688.23 3040.00 2,648 ...................................... ____ _. ......... ...... ...... ................................... ................... 240 14400......... ........ 0.96 0.51 7280.93 4053.33 ; 3,228 .. ........ ...... ........ ........ ....... ........ .................. 300 18000 0.96 0.51 9101.16 5066.67 4,034 360 21600 0.54 0.28 6143.28 6080.00 63 .......................................................................................................................... ........................... 480 28800 0.40 0.21 6067.44 8106.67 -2,039 ........ _. ................. 600 36000 0.25 0.13 4740.19 10133.33 -5,393 ............................................:..... ......... ......... ......... .................. 720 43200 0.16 0.08 3640.46 12160.00 -8,520 1080 64800 0.10 0.05 3412.94 18240.00 -14,827 1440 ....................86400................... 0.00 0.00 0.00 24320.00 '. -24 320 Seepage Bed DB- 1 Seepage Bed Area 30 1,520 Seepage Bed Volume Minimum Storage Volume 4,034 ft.3 Percent of Voids within Gravel i40% Recovery Time 08 Infiltration Rate 8.00 in/hr .................................................................................................................s.............................................................. ............................. Storage Required 4 034.5 ft. 3 Seepage Bed Dimensions Percolation Rate 1,013.3 ft.'/hr Surface Area 1,520 k.' Recovery Time 4 hr ...........................................................................................................................:...................................................... Cover over Gravel 1 k. ................................................................................................................................................................................. Depth of Gravel 6.6 k. Depth of Sand 1.5 ft. Depth • & Grease Trap Calculations 1 750 Gallon Catch Basin Sediment Box Type C 3 ft.2 0 1500 Gallon Sand&Grease Trap by Amcor Precast or Approved Equal 0 ft.2 .................. 0 Drainage Basin 0 ft.2 Area Through Baffle 3 ft.' 100 Year Peak Flow at 10 minute Duration 1.63 cfs Velocity through Baffle 0.54 ft/s Drainage Facility Calculations Version 2.0 24029 Wvender Place Brandt Elwell H.Kent Magleby,P.E. r 2 - �. • • City of Meridian Standards Drainage Area S�rface Area Runoff Coefficient Equivalent Area Impervious Area 9,856 0.95 9,363 _.......... ............ ................................ ___ ......... , Pervious Area 5,907 - 0.25 1,477 Drainage Area Summary Significant Flow Events Drainage Area A 15,763 sf 0.25 0.46 0.57 0.66 0.78 Drainage Area A 0.36 acres Weighted Runoff Coefficient C 0.69 Time of Concentration Tc 10 min .- Q100 Q100 . . Seepage Bed l Drainage Basin Weighted Runoff Coefficient(C) Captured Flow(Qa,) Equivalent Captured Area(A.pt)=CxA Infiltration rate Storage Volume Required N-a.) 0.69 0.78 cfs 10,840 ft2 8.00 m/hr 1,859 ft' Required Storage for Captured Volume (Design Storm) Qdev=CIA Vin=tQdev Vout=tQperc Vstorage=Vin-Vout Time(min) Time(sec) ntens�ty(in/hr)=1 •. 10 600 2.58 0.65 388.43 82.56 306 .... ..................__.. 15 900 2.18 0.55 492.31 123.83 368 ........................................................................................E........................................................................................................................i...............................................................................................................................i.. ........................... 20 1200 1.81 0.45 545.01 165.11 380 ........................................................ ____ ____. ___. ____.____. __.................. 30 1800 1.58 0.40 713.63 247.67 466 ..................................................................................................... ..........i............................................................i...................................................... ......................... 40 2400 1.51 0.38 909.35 330.22 579 .......................................... __......._.. __' .............-- .................... 50 3000 1.25 0.31 940.97 412.78 528 ...................................................................................................................._............................. ....i................................................................................................................................................... ........................... 60 3600 1.15 0.29 1038.83 495.33 543 ......... ......... - _........._.. ___- __._. __ - ___.................. 120 7200 1.07 0.27 1933.12 990.67 942 180 10800 1.00 0.25 2709.99 1486.00 1,224 ...................................... ____- -... __. ................. 240 14400 0.96 0.24 3468.78 1981.33 ; 1,487 ............................................ ......_ ........ ......... ................. 300 15000 0.96 0.24 4335.98 2476.67 1,859 ----.: ................... 360 21600 0.54 0.14 2926.79 2972.00 -45 r .................... ......... .................. 480 28800 0.40 0.10 2890.65 3962.67 -1,072 600 36000 0.25 0.06 2258.32 4953.33 -2,695 ....... ......... ......... ........ ......... .................. 720 43200 0.16 0.04 1734.39 5944.00 -4,210 1080 64800 0.10 0.03 1625.99 8916.00 7,290 .................... ................... 1440 86400 0.00 0.00 0.00 11888.00 '. -11 888 Seepage Bed DB-2 Seepage Bed Area 20 743 Seepage Bed Volume Minimum Storage Volume 1,859 ft.3 Percent of Voids within Gravel i40% Recovery Time Infiltration Rate 8.00 in/hr .................................................................................................................s............................................................................................ Storage Required 1 859.3 ft. 3 Seepage Bed Dimensions Percolation Rate 495.3 ft'/hr Surface Area 743 k.' ...........................................................................................................................:...................................................... Cover over Gravel 1 k. ................................................................................................................................................................................. Depth of Gravel 6.3 k. Depth of Sand 1.5 ft. Depth • & Grease Trap Calculations 1 750 Gallon Catch Basin Sediment Box Type C 3 ft.2 0 1500 Gallon Sand&Grease Trap by Amcor Precast or Approved Equal 0 ft.z .............. 0 Drainage Basin 0 ft.z Area Through Baffle 3 ff.' 100 Year Peak Flow at 10 minute Duration 0.78 cfs Velocity through Baffle 0.26 ft/s Drainage Facility Calculations Version 2.0 24029 Wvender Place Brandt Elwell H.Kent Magleby,P.E. r 3 - �. • • City of Meridian Standards Drainage Area S�rface Area Runoff Coefficient Equivalent Area Impervious Area 6,711 0.95 6,385 _.......... ............ _.................................................... Pervious Area _ 7,914 - 0.25 1,979 Drainage Area Summary Significant Flow Events Drainage Area A 14,635 sf 0.19 0.36 0.44 0.51 0.50 Drainage Area A 0.34 acre 11 s Weighted Runoff Coefficient C 0.57 Time of Concentration Tc 10 min •- Q100 - Q100 . . Seepage Bed l DrainageBasin Weighted Runoff Coefficient(C) Captured Flow(Qa,) Equivalent Captured Area(A.pt)=CxA Infiltration rate Storage Volume Required N-a.) 0.57 0.50 cfs 8,364 ft, 8.00 m/hr 1,545 ft' Required Storage for Captured Volume (Design Storm) Qdev=CIA Vin=tQdev Vout=tQperc Vstorage=Vin-Vout Time(min) Time(sec) ntens�ty(ir/hr)-1 •. 10 600 2.58 0.50 299.69 60.00 240 .............................................................................i...................................... ___ 15 900 2.18 0.42 379.84 90.00 290 .................................................................................................................................................................................................................i.................................................................................................................................... ........................... 20 1200 1.81 0.35 420.50 120.00 300 .............................................................. ____. ____. ____. ____ ____.____. ....____ ___.................. 30 1800 1.58 0.31 550.60 180.00 371 ................................................................................................................... ......... ................................i................................................. .... ........................... 40 2400 1.51 0.29 701.61 240.00 462 . ........................................... ----. __........._ _ __ ____.___. ___.................. 50 3000 1.25 0.24 726.00 300.00 426 ......... ......... ......... ..._ ......... ....................................................................................... ............................... 60 3600 1.15 0.22 801.50 360.00 442 ......... ......... - _...........__ __. ___- __._. _. - ____................... 120 7200 1.07 0.21 1491.50 720.00 771 180 10800 1.00 0.19 2090.88 1080.00 1,011 ...................................... ___.- 240 14400 0.96 0.19 2676.33 1440.00 1,236 ............................................ ........ ........ ....... ......... ................. 300 18000 0.96 0.19 3345.41 1800.00 1,545 360 21600 0.54 0.10 2258.15 2160.00 98 ..... ......... ......... ......... ......... ......... .................. 480 28800 0.40 0.08 2230.27 2880.00 -650 600 36000 0.25 0.05 1742.40 3600.00 -1,858 ....:................................................. ........ ......... .................. 720 43200 0.16 0.03 1338.16 4320.00 -2,982 1080 64800 0.10 0.02 1254.53 6480.00 5,225 1440 86400 0.00 0.00 0.00 8640.00 '. -8 640 Seepage Bed DB-3 Seepage Bed Area 20 540 Seepage Bed Volume Minimum Storage Volume 1,545 ft Percent of Voids within Gravel i40% Recovery Time Infiltration Rate 8.00 in/hr .................................................................................................................s............................................................................................ Storage Required 1,545.4 ft. 3 Seepage Bed Dimensions Percolation Rate 360.0 ft'/hr Surface Area 540 k.' Recovery Time 4 hr ...........................................................................................................................:...................................................... Cover over Gravel 1 k. ................................................................................................................................................................................. Depth of Gravel 7.2 k. Depth of Sand 1.5 ft. Depth • & Grease Trap Calculations 1 750 Gallon Catch Basin Sediment Box Type C 3 ft.z 0 1500 Gallon Sand&Grease Trap by Amcor Precast or Approved Equal 0 ft.z .............. 0 Drainage Basin 0 ft.z Area Through Baffle 3 ff.' 100 Year Peak Flow at 10 minute Duration 0.50 cfs Velocity through Baffle 0.17 ft/s Drainage Facility Calculations Version 2.0 24029 Wvender Place Brandt Elwell H.Kent Magleby,P.E. r 4 - �. • • City of Meridian Standards Drainage Area S�rface Area Runoff Coefficient Equivalent Area Impervious Area 33,776 0.95 32,087 Pervious Area 6,113 0.15.........__... 917 Drainage Area Summary Significant Flow Events Drainage Area A 39,889 sf 0.75 1.40 1.74 2.02 2.37 Drainage Area A 0.92 acres Weighted Runoff Coefficient C 0.83 Time of Concentration Tc 10 min .- Q100 Q100 . . Seepage Bed l Drainage Basin Weighted Runoff Coefficient(C) Captured Flow(Qa,) Equivalent Captured Area(A.pt)=CxA Infiltration rate Storage Volume Required N-a.) 0.83 2.37 cfs 33,004 ft2 8.00 m/hr 5,712 ft, Required Storage for Captured Volume (Design Storm) Qdev=CIA Vin=tQdev Vout=tQperc Vstorage=Vin-Vout Time(min) Time(sec) ntens�ty(ir/hr)-1 • 10 600 2.58 1.97 1182.65 249.67 933 ....................................................................................................................._.. 15 900 2.18 1.67 1498.94 374.51 1,124 ................................i..........................................................................................................................i............................................ .....................i...................................................................... ......................................................... 20 1200 1.81 1.38 1659.38 499.34 1,160 ......... ............ ......... .......... ........ ___.. ___. ____. __. ____. ____. .................. 30 1800 1.58 1.21 2172.77 749.01 1,424 ............................................................i................................................................................................................................................................................... .................... .............................................................................................................. 40 2400 1.51 1.15 2768.68 998.68 1,770 . ........................................... --- ____..........____. ......... ____. ___. __....__ __ ____. _____.......... 50 3000 1.25 0.95 2864.94 1248.36 1,617 ...................................................................................................................._........................................................................................................................:................... ........................................................................................................ 60 3600 1.15 0.88 3162.90 1498.03 1,665 ......... ......... ................... . ___............ _. ........ ...... ___. __ .........-- ___................. 120 7200 1.07 0.82 5885.74 2996.05 2,890 180 10800 1.00 0.76 8251.04 4494.08 3,757 ...................................... ____- .... .........-- 240 14400 0.96 0.73 ! 10561.33 5992.11 ; 4,569 ............................................ ........ ........ ......... ................. 300 18000 0.96 0.73 13201.66 7490.13 ! 5,712 360 21600 0.54 0.41 8911.12 8988.16 -77 ......... ......... ......... ......... ......... .................. 480 28800 0.40 0.31 8801.11 11984.21 -3,183 600 36000 0.25 0.19 6875.86 14980.27 -8,104 ...... ......... ......... ........ ......... .................. 720 43200 0.16 0.12 5280.66 17976.32 -12,696 1080 64800 0.10 0.08 4950.62 26964.48 22,014 ........................................................ .... ....................................................................;, 1440 86400 0.00 0.00 0.00 35952.64 -35,953 Seepage Bed DB-4 Seepage Bed Area 32 2,247 Seepage Bed Volume Minimum Storage Volume 1,712 ...................................................................................................................... Percent of Voids within Gravel 40% Recovery Time Requlred Seepage Bed Volume 14,279 ft. Infiltration Rate 8.00 in/hr .................................................................................................................s............................................................................................ Storage Required 5 711.5 ft. 3 Seepage Bed Dimensions Percolation Rate 1,498.0 ft'/hr Surface Area 2,247 k.' ...........................................................................................................................:...................................................... Cover over Gravel 1 k. Depth of Gravel 6.4 k. Depth of Sand 1.5 ft. Depth • & Grease Trap Calculations 1 750 Gallon Catch Basin Sediment Box Type C 3 ft.z 0 1500 Gallon Sand&Grease Trap by Amcor Precast or Approved Equal 0 ft.z .............. 0 Drainage Basin 0 ft.z Area Through Baffle 3 ff.' 100 Year Peak Flow at 10 minute Duration 2.37 cfs Velocity through Baffle 0.79 ft/s Drainage Facility Calculations Version 2.0 24029 Wvender Place Brandt Elwell H.Kent Magleby,P.E. r 5 - • • • City of Meridian Standards Drainage Area S�rface Area Runoff Coefficient Equivalent Area Impervious Area 7,138 0.95 6,781 ....._. ................................ _... ___ ......... ......... ......... ......... Pervious Area 6,732 0.15 1,010 Drainage Area Summary Significant Flow Events Drainage Area A 13,870 sf 0.18 0.33 0.41 0.48 0.56 Drainage Area A 0.32 acres Weighted Runoff Coefficient C 0.56 Time of Concentration Tc 10 min •- Q100 - Q100 • Swale DrainageBasin Weighted Runoff Coefficient(C) Captured Flow(Qa,) Equivalent Captured Area(A.pt)=CxA Infiltration rate Storage Volume Required M-) 0.56 0.56 cfs 7,791 ft, 8 m/hr 716 ft' Required Storage for Captured Volume (Design Storm) Qdev=CIA Vin=tQdev Vout=tQperc Vstorage=Vin-Vout Time(min) Time(sec) ntens�ty(ir/hr)-1 • 10 600 2.58 0.47 279.17 80.00 199 ....................................................................................................................._.. 15 900 2.18 0.39 353.84 120.00 234 ...............................................................................................................................................................................................................i.................................................................................................................................. ........................... 20 1200 1.81 0.33 391.71 160.00 232 ........................................................ ____ ____. ___. ____. ____. ___.................. 30 1800 1.58 0.28 512.90 240.00 273 ..................................................................................................... ..................................................................i...................................................... ........................... 40 2400 1.51 0.27 653.57 320.00 334 . .......................................... __......._.. ___.................. 50 3000 1.25 0.23 676.29 400.00 276 ...................................................................................................................._............................. ............................................................................................................................................................. ............................... 60 3600 1.15 0.21 746.63 480.00 267 ........................................................... - _........._.. ___- __._. __.. _ _ .................. 120 7200 1.07 0.19 1389.38 960.00 429 180 10800 1.00 0.18 1947.73 1440.00 508 ...................................... ____- __ .................. 240 14400 0.96 0.17 2493.09 1920.00 573 ............................................ ........ ........ .............................................. .................. 300 18000 0.96 0.17 3116.36 2400.00 716 360 21600 0.54 0.10 2103.54 2880.00 -776 ..... ......... ......... ......... ......... ......... .................. 480 28800 0.40 0.07 2077.57 3840.00 -1,762 ........ ................. 600 36000 0.25 0.05 1623.10 4800.00.:..... -3,177 ......... ......... ........ ......... .................. 720 43200 0.16 0.03 1246.54 5760.00 -4,513 ................. 1080 64800 0.10 0.02 1168.64 8640.00 ': -7,471 1440 86400 0.00 0.00 0.00 11520.00 -11520 Drainage Swale - 5 Drainage Swale Area Width(ft.) Length(ft.) Area(ft.') Surface Area 6 190 1,140 __.......... ......... ......... ......... ......... ......... Base Area 2 175 420 Effective Infiltration Area 2 300 720 Minimum Depth of Swale - VS _ 716 f.3 = 1.0 ft. Effective Infiltration Area 720 ft. Depth of Swale Recovery Time Infiltration Rate 8.0 in/hr .... .... .... ... ... Design Storage Depth of Swale 1.0 .ft. Storage Required 716.4 ft a .......... ........ ......... .... ..... ... ... Depth of Freeboard Required 0.0 ft. Percolation Rate 480.0 ft a/hr Total DepthI A Appendix D Geotechnical Report INNUVATE GEEITECHNICAL Page 1 of 1 May 29, 2024 Westpark Companies Attention: Taylor Merrill Subject: Geotechnical Engineering Letter Lavender Place Subdivision W Bloomerang Ave and E Lake Hazel Road Ada County, Idaho IGEO Project No. 324043 INTRODUCTION This letter presents recommendations for infiltration for the Lavender Place Subdivision. The proposed improvements are outlined in the provided Grading and Drainage Plan prepared by Breckon Land Design dated May 16, 2024. RECOMMENDATIONS Infiltration We understand infiltration facilities are proposed at various locations indicated in the referenced drawing. IGEO performed 3 test pits on site on May 20, 2024 to assess subsurface conditions related to infiltration design. The results of our subsurface evaluation indicated silty/clayey fill soils and clayey/cemented soils to a depth of approximately 4.5 feet to 8.5 feet below the existing ground surface. These soils are unsuitable for infiltration design. Below this layer, our subsurface evaluation indicated sand and gravel soils to a depth of 11 feet below the existing ground surface. We recommend a design infiltration rate of 8 inches/hour be used for the sand and gravel soils on site. Our professional services have been performed, our findings obtained, and our letter prepared in accordance with generally accepted geotechnical engineering principles and practices. This warranty is in lieu of all other warranties, either expressed or implied. Sincerely, AL Innovate Geotechnical �� y(,T-NS60 cc Seth P. Olsen, P.E. s, 0 Senior Geotechnical Engineer OF FXN P. GEOTECHNICAL ENGINEERING 2006 E Franklin Rd.,#110 MERIDIAN,ID (208)484-1090