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Home My WebLink AboutRS_OLD TIR_Walmart OGP 2_210218_v1.pdf6162 S. Willow Drive, Suite 320 Greenwood Village, CO 80111 303.770.8884 •GallowayUS.com January 11th, 2021 Mr. Justin Johnson Community & Economic Development Civil Construction 1055 South Grady Way Renton, WA 98057 RE: Drainage Conformance Letter for Technical Information Report – Retail Expansion – Walmart Store #2516-240 Dear Mr. Johnson, The purpose of this letter is to demonstrate that the drainage characteristics of the Pickup expansion improvements for the Walmart Supercenter, Store #2516 in Renton, WA is in conformance with the Technical Information Report – Retail Expansion dated February 25th, 2010 (per stamp date) by Pac Land. SUMMARY The subject property is located at 743 Rainier Ave S, Renton, WA 98057, a previously developed lot (part of the previous Walmart expansion) that consists of an existing Walmart with associated paved parking, totaling an area of 13.65 Acres. It is bound to the north by SW 7th Street, east by Rainier Ave S, and south & west by adjacent businesses. The project area consists of approximately 0.30 Acres. The existing area is comprised of paved parking lot, sidewalk, and landscape islands. Improvements to the site include the addition of a Pickup building expansion (4,547 SF), conversion of landscape islands to pavement (824 SF), installation of a Pickup canopy, re-striping of existing parking lot, relocation of existing waterline and hydrant, relocation of storm sewer, the addition of 2 manholes, and relocation of an existing light pole. DRAINAGE BASIN PATTERNS Improvements to the site will not change the existing drainage basin areas and drainage shall follow the existing drainage patterns for basins 2a and 2b outlined in the Technical Information Report – Retail Expansion report (Note: these areas are indicated as proposed conditions within the existing report). Portions of the landscape areas in proposed drainage basins 2a and 2b are being modified as part of this project and will become impervious pavement. Total additional areas for basins 2a and 2b are 824 SF between the basis. In addition to the changes illustrated above, 4,547 SF of existing impervious area (sidewalk and pavement) will be removed and replaced by the proposed building expansion. Roof drains from the proposed building expansion will tie into the existing catch basin to the north. The existing roof drain in this area will be removed and a larger 10” PVC drain will be installed. Existing basin and proposed basin areas are shown in the tables on the following page. Wal-Mart Stores Inc. Walmart #2516 Pickup Expansion 01-11-2021 Galloway & Company, Inc.Page 2 of 4 Existing basin areas per Technical Information Report – Retail Expansion report. (Note: These are listed as Proposed Sub-Basin areas in Table 2 of the above-mentioned report.) Proposed basin areas based on new construction. Surface Proposed Sub-Basins 1 2a 2b 3 Roof 150,244 4,209 338 0 Garden Center 0 4,701 0 0 Landscaping 0 50,921 8,833 5,112 Pavement (PGIS) 2,548 272,843 27,048 37,433 Sidewalk 0 24,776 3,620 1,927 Gravel 0 0 0 0 Total 152,792 357,450 39,839 44,472 SITE HYDROLOGY Existing drainage patterns outlined in the Technical Information Report – Retail Expansion shall remain unchanged with the current improvements.Existing basins 2a and 2b were broken down from the existing report and runoff rates can be found below. Proposed drainage patterns will follow the existing drainage patterns. Drainage basin 2a will maintain the same area as noted previously, however, pervious areas (landscape island areas) will be 1 2a 2b 3 Roof 150,244 0 0 0 Garden Center 0 4,701 0 0 Landscaping 0 51,795 8,783 5,112 Pavement (PGIS)2,548 276,524 27,496 37,433 Sidewalk 0 24,430 3,560 1,927 Gravel 0 0 0 0 Total 152,792 357,450 39,839 44,472 Surface Existing Sub-Basins Event Peak Q Rate (CFS) 2-year - 24 Hr 3.3151 10-year 5.1105 100-year 7.0812 Ex. Basin 2a Runoff Rates SBUH Method Event Peak Q Rate (CFS) 2-year - 24 Hr 0.3495 10-year 0.551 100-year 0.7726 Ex. Basin 2b Runoff Rates SBUH Method Wal-Mart Stores Inc. Walmart #2516 Pickup Expansion 01-11-2021 Galloway & Company, Inc.Page 3 of 4 exchanged for impervious pavement areas and building roof area in exchange for pavement areas. Drainage basin 2b will also maintain the same area, however, impervious areas will be exchanged for pervious areas. The changes in the proposed drainage basins produced the following flow information. Detailed calculations can be found in Appendix B of this Report. RETENTION/DETENTION ANALYSIS As stated in the Retention/Detention System section of the Technical Information Report – Retail Expansion, unless the 24-hour storm increases by more than 0.50 cfs from the pre-developed peak runoff rate, the project is exempt from runoff control. Given that Basin 2a only increases by 0.0057 cfs and Basin 2b only decreases by 0.0004 cfs, this project is exempt under the provision. BIOSWALE TREATMENT The existing biofiltration swale in the northwestern portion of the site will still maintain the treatment of the Basin 2b area. As noted in the Technical Information Report – Retail Expansion report, additional water quality was provided for the additional pavement area. This bioswale outlined in the New PGIS section of the Technical Information Report – Retail Expansion report, shall facilitate in providing the additional water quality. Per the existing report, the bioswale for Basin 2b was overdesigned for the required conditions and given the minimal increase in flow rates, the existing bioswale will suffice in providing the required water quality for the proposed development. The existing bioswale design was verified using the following parameters: The resultant swale design requires a minimum bottom width of 7.58 feet and a minimum treatment length of 90.9 feet. Given that the swale was designed with an 8-foot bottom width and a 115 foot treatment length, with a 1.0% longitudinal slope, the swale design fits into the parameters. Calculations showing the existing bioswale characteristics accommodating the additional flows can be found in Appendix C. Event Peak Q Rate (CFS) 2-year - 24 Hr 3.3208 10-year 5.1157 100-year 7.0858 Prop. Basin 2a Runoff Rates SBUH Method Event Peak Q Rate (CFS) 2-year - 24 Hr 0.3491 10-year 0.5505 100-year 0.7721 Prop. Basin 2b Runoff Rates SBUH Method Criteria Value 2-yr, 24-hr flow 0.3491 cfs Design Flow Depth 0.25 feet Mannings Coeff 0.35 Residence Time 9 minutes Side Slope 3.0 H:V Bioswale Design Criteria - Single Event Wal-Mart Stores Inc. Walmart #2516 Pickup Expansion 01-11-2021 Galloway & Company, Inc.Page 4 of 4 CONCLUSIONS Given the minor increase in flows (0.0057 cfs) for Basin 2a and the decrease in flows (0.0004 cfs) for Basin 2b, as well as the fact that the bioswale can accommodate the additional flow rates as designed; the improvements outlined in this conformance letter comply with the existing drainage report and therefore no additional modifications are necessary. Sincerely, GALLOWAY Brandon D. Alley, PE Principal - Galloway US BrandonAlley@gallowayUS.com NOTE: See attached documents for further drainage information: Addendum A: Drainage Map Addendum B: Drainage Detention Calculations Addendum C: Bioswale Calculations 01/12/2021 APPENDIX A PROPOSED DRAINAGE MAP WWW W W WW SD SD 1 inch = 100 ft. ( IN FEET ) GRAPHIC SCALE 0100 100 200 40050 RENTON, WAWAL-MART STORES, INC.BENTONVILLE, AR WAL-MART SUPERCENTER #2516-240SHEET JOB No. SCALE DATE CHECKED DRAWN REVISIONS BY PB2002516.20 01/08/21 AS NOTED JES JSB 1 A 2 3 B NOT FOR CONSTRUCTIONGallowayUS.com6162 S. Willow Drive, Suite 320Greenwood Village, CO 80111303.770.8884DMAP H:\RHA\WA, Renton - RHA002516.20 - 743 Rainier Ave. S\3 Permit CDs\3.04 Grading-Drainage Studies\3.04.2 Proposed Drainage Reports-Info\RHA2516_Drainage Map_REV1 - 2019-02-11 - Sid Biddle DRAINAGE MAP EXISTING SUB-BASIN DATA APPENDIX B DRAINAGE DETENTION CALCULATIONS Appended on: Friday, January 8, 2021 11:31:19 AM Existing 2a Event Summary Record Id: Existing 2a Licensed to: Unlicensed Copy Event Peak Q (cfs)Peak T (hrs)Hyd Vol (acft)Area (ac)Method 2 yr 24 hr 3.3151 8.00 1.0936 8.2058 SBUH 10 year 5.1105 8.00 1.6964 8.2058 SBUH 100 year 7.0812 8.00 2.3726 8.2058 SBUH All results based on storm duration of 24.0 hours. This is ok if all precipitations are appropriate for the storm duration. If some design event precipitations are for different duration storms, those results are incorrect Design Method SBUH Rainfall type TYPE1A.RAC Hyd Intv 10.00 min Peaking Factor 484.00 Storm Duration 24.00 hrs Abstraction Coeff 0.20 Pervious Area 8.2058 ac DCIA 0.00 ac Pervious CN 96.26 DC CN 0.00 Pervious TC 5.00 min DC TC 0.00 min Pervious CN Calc Description SubArea Sub cn Impervious surfaces (pavements, roofs, etc)7.0168 ac 98.00 Open spaces, lawns,parks (>75% grass)1.189 ac 86.00 Pervious Composited CN (AMC 2)96.2612 Pervious TC Calc Type Description Length Slope Coeff Misc TT Sheet Smooth Surfaces 30.00 ft 2.0%0.011 2.50 in 0.5232 min Int Channel CMP Pipe (n=0.024 200.00 ft 0.5%0.024 2.2202 min Pervious TC 2.7434 min Page 1 of 1 1/8/2021file:///H:/RHA/WA,%20Renton%20-%20RHA002516.20%20-%20743%20Rainier%20Ave... Appended on: Friday, January 8, 2021 11:49:17 AM Existing 2b Event Summary Record Id: Existing 2b Licensed to: Unlicensed Copy Event Peak Q (cfs)Peak T (hrs)Hyd Vol (acft)Area (ac)Method 2 yr 24 hr 0.3495 8.00 0.1155 0.915 SBUH 10 year 0.551 8.00 0.182 0.915 SBUH 100 year 0.7726 8.00 0.257 0.915 SBUH All results based on storm duration of 24.0 hours. This is ok if all precipitations are appropriate for the storm duration. If some design event precipitations are for different duration storms, those results are incorrect Design Method SBUH Rainfall type TYPE1A.RAC Hyd Intv 10.00 min Peaking Factor 484.00 Storm Duration 24.00 hrs Abstraction Coeff 0.20 Pervious Area 0.91 ac DCIA 0.00 ac Pervious CN 95.35 DC CN 0.00 Pervious TC 5.00 min DC TC 0.00 min Pervious CN Calc Description SubArea Sub cn Impervious surfaces (pavements, roofs, etc)0.713 ac 98.00 Open spaces, lawns,parks (>75% grass)0.202 ac 86.00 Pervious Composited CN (AMC 2)95.3508 Pervious TC Calc Type Description Length Slope Coeff Misc TT Sheet Smooth Surfaces 30.00 ft 2.0%0.011 0.00 in 0.5232 min Int Channel CMP Pipe (n=0.024)200.00 ft 0.5%0.024 2.2202 min Pervious TC 2.7434 min Page 1 of 1 1/8/2021file:///H:/RHA/WA,%20Renton%20-%20RHA002516.20%20-%20743%20Rainier%20Ave... Appended on: Friday, January 8, 2021 11:50:29 AM Proposed 2a Event Summary Record Id: Proposed 2a Licensed to: Unlicensed Copy Event Peak Q (cfs)Peak T (hrs)Hyd Vol (acft)Area (ac)Method 2 yr 24 hr 3.3208 8.00 1.0955 8.2058 SBUH 10 year 5.1157 8.00 1.6985 8.2058 SBUH 100 year 7.0858 8.00 2.3749 8.2058 SBUH All results based on storm duration of 24.0 hours. This is ok if all precipitations are appropriate for the storm duration. If some design event precipitations are for different duration storms, those results are incorrect Design Method SBUH Rainfall type TYPE1A.RAC Hyd Intv 10.00 min Peaking Factor 484.00 Storm Duration 24.00 hrs Abstraction Coeff 0.20 Pervious Area 8.2058 ac DCIA 0.00 ac Pervious CN 96.29 DC CN 0.00 Pervious TC 5.00 min DC TC 0.00 min Pervious CN Calc Description SubArea Sub cn Impervious surfaces (pavements, roofs, etc)7.0369 ac 98.00 Open spaces, lawns,parks (>75% grass)1.1689 ac 86.00 Pervious Composited CN (AMC 2)96.2906 Pervious TC Calc Type Description Length Slope Coeff Misc TT Sheet Smooth Surfaces 30.00 ft 2.0%0.011 2.50 in 0.5232 min Int Channel CMP Pipe (n=0.024)200.00 ft 0.5%0.024 2.2202 min Pervious TC 2.7434 min Page 1 of 1 1/8/2021file:///H:/RHA/WA,%20Renton%20-%20RHA002516.20%20-%20743%20Rainier%20Ave... Appended on: Friday, January 8, 2021 11:51:09 AM Proposed 2b Event Summary Record Id: Proposed 2b Licensed to: Unlicensed Copy Event Peak Q (cfs)Peak T (hrs)Hyd Vol (acft)Area (ac)Method 2 yr 24 hr 0.3491 8.00 0.1153 0.9146 SBUH 10 year 0.5505 8.00 0.1818 0.9146 SBUH 100 year 0.7721 8.00 0.2568 0.9146 SBUH All results based on storm duration of 24.0 hours. This is ok if all precipitations are appropriate for the storm duration. If some design event precipitations are for different duration storms, those results are incorrect Design Method SBUH Rainfall type TYPE1A.RAC Hyd Intv 10.00 min Peaking Factor 484.00 Storm Duration 24.00 hrs Abstraction Coeff 0.20 Pervious Area 0.9146 ac DCIA 0.00 ac Pervious CN 95.34 DC CN 0.00 Pervious TC 5.00 min DC TC 0.00 min Pervious CN Calc Description SubArea Sub cn Impervious surfaces (pavements, roofs, etc)0.7118 ac 98.00 Open spaces, lawns,parks (>75% grass)0.2028 ac 86.00 Pervious Composited CN (AMC 2)95.3392 Pervious TC Calc Type Description Length Slope Coeff Misc TT Sheet Smooth Surfaces 30.00 ft 2.0%0.011 2.50 in 0.5232 min Int Channel CMP Pipe (n=0.024)200.00 ft 0.5%0.024 2.2202 min Pervious TC 2.7434 min Page 1 of 1 1/8/2021file:///H:/RHA/WA,%20Renton%20-%20RHA002516.20%20-%20743%20Rainier%20Ave... APPENDIX C BIOSWALE CALCULATIONS 1/8/2021 JSB RCJ Step# P-1 0.3491 cfs Water Quality Design Flow P-2 0.01 ft/ft Swale Slope < 2%, underdrains required P-3 D-1 0.25 ft D-2 0.35 D-3 3 H:V D-4a 7.58 ft D-4b 9.08 ft D-5 2.08 sf 90.6 = Length (Feet) Required D-6 0.17 fps 90.9 = Length (Feet) w/ 9 min rest time (this is still long enough) D-7 Yes Okay - V<1.5 fps proceed to SC-1 D-8 D Table III-6.2 (D or E) D-9 1.70 Table III - 6.6, n =nwq from step D-2 D-10 -ft D-11 - D-12 -% D-13 -fps D-14 -sf D-15a -ft D-15b -ft D-16 If biofiltration swale is still to wide, see DOE page III-6-17 for more info and options BIOSWALE DESIGN (SBUH) Date Designer Project Manager WMT Pickup Expansion #2516 PB2002516.20 On-Site (Redev) Project Name Project Number Swale Title Channel Width T=b+2*y*z b=(A/y)-Z*y First Approx for VR Hydraulic Radius R=(V*R)/Vmax Actual VR VR=(1.486*(R1.6668)*S0.5)/n % Difference Actual V V=(VR/R) Channel X-Sec Area A=Q/A New Swale Bottom Width (b) Degree of retardence Design Steps (D) Design Flow Depth (y) Mannings Roughness Coeff (ᶯwq) Side Slope (Z) Swale bottom width (b) b=(Q*n)/(1.486*y1.667*s0.5)-(Z*y) Top width (T) T=b+2*y*z Area (A) A=(b*y)+(Z*y2) Flow Velocity v=Q/A Is Design Adequate Qwq (2-yr, 24-Hr) Slope of Swale (s) Select Vegetation to be used Grass Legume Mix Preliminary Steps (P) Step# SC-1 0.77 cfs SC-2a Good (Good or Fair) SC-2b 0.25 ft SC-3 D Table III-6.2 (D or E) SC-4 3.00 fps Table III-6.3 SC-5 0.043 (Good initial choice 0.04) SC-6 2.70 Table III - 6.6 SC-7 0.9 ft SC-8 2.90 SC-9 7.38%% SC-10 3.22 fps SC-11 0.24 sf SC-12 Yes Swale is Stable 1.2 ft 8.00 ft 15.2 ft SC-13 1.2 ft 115 SC-14 2.2 ft SC-15 1.16 ft SC-16a 32.12 sf SC-16b 15.06 cfs Okay, Capacity > 100-yr, 24-Hr Storm R=((b*y*t)+(Z*y*t2))/(b+2yt*(Z2+1)0.5) Channel X-Sec Area Channel Capacity Q=(1.486*A*R0.677*s0.5)/n Hydraulic Radius A=Q/V Compare A with Orig New y = New b = b=(Q*n)/(1.486*y1.667*s0.5)-(Z*y) New T = T=b+2*y*z Depth of Flow y=(t-b)/2z Total Swale Depth yt = y+1 X-Sec Area Degree of Retardance Max allowed Velocity Trial Mannings n First Approx for VR Hydraulic Radius R=(V*R)/Vmax Actual VR VR=(1.486*R1.667*s0.5)/n % difference Actual V V=VR/R Qwq (100-yr - 24-Hr) Condition of vegetation Height of grass @ first storm Stability Check Steps (SC)