Loading...
Approved Stormwater Report PLN2021-00388929 220th Street SW 2-Lot Short Plat Edmonds, WA 98026 Stormwater Site Plan Report Prepared for: Select Homes Date: May 19, 2021 Revised: January 6, 2022 Revised: May 23, 2022 Revised: September 19, 2022 9/19/2022 Prepared by: Rob Long, PE RAM Engineering, Inc. 16531 13th Ave W, Suite A108, Lynnwood WA 98037 (425) 678-6960 RAMengineeringinc.com Job No. 20-016 RESUB CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT SEP 23 2022 COMPLIES WITH APPLICABLE CITY STORMWATER CODE 10/03/2022 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report 8929 220th Street SW 2-Lot Short Plat Introduction: This summary report provides site design information for a residential 2-lot short plat development by Select Homes. This report includes storm drainage analysis to support permit review and approval. The property is located at 8929 220th Street SW, in the northwest 1/4 of Section 30, T 27 N, R 4 E, W.M. Site Address: 8929 220th Street SW Edmonds, WA 98026 Tax Parcel Number: 00380300101000 Applicant: Select Homes 16513 13th Ave W, Suite A108, Lynnwood WA 98037 Contact: Kayla Clark Nichols (425) 742-6044 TABLE OF CONTENTS SECTION PAGES A. Project Overview .............................................................................................................................. 7 B. Existing Conditions Summary ........................................................................................................... 2 C. Developed Site Hydrology .............................................................................................................. 16 D. Soils Report ..................................................................................................................................... 18 E. Construction SWPPPP Requirements ................................................................................................ 2 F. Operation and Maintenance Guidelines ............................................................................................ 9 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page A-1 A. PROJECT OVERVIEW: This report provides engineering information for the proposed construction of a 2-lot residential short plat on a 26,746 sf (0.614 ac) lot; the project is located at 8929 220th Street SW in the City of Edmonds. The applicant, Select Homes proposes to remove the existing residence and all existing hard surface to construct three single family residences on the project site. This report provides the evaluation for the proposed single family residence development. Summary of Minimum Requirements for Category 2 Project: Minimum Requirement #1 – Preparation of Stormwater Site Plan. The proposed site development consists of disturbing about 0.439 acres of land and creating/replacing about 9,234 sf of hard surface area for a 2- lot residential short plat. Note, to retain existing trees onsite, about 0.196 acres of the northerly portion of the site will not be developed as part of a “conservation subdivision design” (ECDC 20.75.048). The conservation subdivision design clusters and concentrates the development area of the project to retain existing trees onsite. The project is classified as a Category 2 project per the City’s classification system. Per ECDC 18.30, Category 2 projects must comply with Minimum Requirements No. 1 through No. 9. The civil site development plans and this report have been prepared to address the projects impacts. Minimum Requirement #2 – Construction Stormwater Pollution Prevention (SWPPP). A construction Stormwater Pollution Prevention Plan (SWPPP) has been incorporated into the site development plans. A summary of the site’s erosion control measures that evaluates the typical 13 elements of a SWPPP are included in section E. The total site disturbance area of the project is less than one acre, thus a formal Notice of Intent application for NPDES coverage will not be made to the Department of Ecology. Minimum Requirement #3 – Source Control of Pollution. Specific source controls are not required for single family residential sites. General requirements for these sites include preventing the discharge of pollutants to the City’s storm drainage system per Edmonds City Code Chapter 7.200 (Illicit Discharges). This includes common household items such as pesticides, herbicides, fertilizers, detergents and fluids from vehicle maintenance. Single family residences shall incorporate DOE’s S411 BMPs for landscape and lawn vegetation management. Lawn and vegetation management can include control of objectionable weeds, insects, mold, bacteria, and other pests with pesticides. Examples include weed control on golf course lawns, access roads, and utility corridors and during landscaping; sap stain and insect control on lumber and logs; rooftop moss removal; killing nuisance rodents; fungicide application to patio decks, and residential lawn/plant care. It is possible to release toxic pesticides such as pentachlorophenol, carbamates, and organometallics to the environment by leaching and dripping from treated parts, container leaks, product misuse, and outside storage of pesticide contaminated materials and equipment. Poor management of the vegetation and poor application of pesticides or fertilizers can cause appreciable stormwater contamination. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page A-2 Minimum Requirement #4 – Preservation of Natural Drainage Systems and Outfalls. The site generally slopes in a west/northwesterly direction towards private properties. Stormwater flows leave the site and flow across neighboring private properties flowing in a west/northwesterly direction towards Westgate Pond about 200 feet west of the project site. Westgate Pond discharges west to 92nd Ave SE and then south to 220th St SW; at the corner of 92nd Ave SE and 220th St SW the flows then flow west along 220th St SW. The portion of the site, south of the existing house does flow southwesterly and discharges to the stormwater pipe conveyance system along the north side of 220th St SW. There are some minor upstream areas east of the site that has potential to run-on from the neighboring property to the east; this area is relatively small and will have a de minimis impact to the site development. See additional offsite/upstream discussion in Section C below. The developed site will discharge the project’s collected stormwater directly to 220th St SW due to the fact the west/northwesterly drainage path of the site flows to and across private property with no drainage easements. The stormwater flows that discharge to 220th St SW flow westerly and merge with the Westgate Pond flows about 540 ft downstream (west) of the site. The merge of the stormwater systems is about 540 ft from the site and thus are considered to be within the same threshold discharge basin (within ¼ mile, thus no diversion). The project will maintain the existing downstream basin with only a minor sub-basin re-rout of stormwater flows. See additional offsite downstream discussion provided in Section C below. The site is located within the upper reach of the Westgate Pond watershed basin per the City of Edmonds maps (see section C). Minimum Requirement #5 – Onsite Stormwater Management. The proposed project is a category 2 project, thus is subject to the City’s on-site stormwater management BMPs found in “List No. 2”. The project proposes to cluster the site development and retain a large, treed area as part of a “conservation subdivision design”. The clustering of the houses, tree retention, and proposed grading limits the viability of infiltration and dispersion BMP methods for the project. A site-specific geotechnical evaluation of the site’s soils demonstrates that infiltration is a not a viable option for stormwater management onsite due to the relatively impermeable confining glacial till soils and adequate flow length paths are not available for the individual building lots for any dispersion type BMP. Therefore, individual lot stormwater BMP detention pipes (Edmonds Stormwater Addendum Section 6.3) will be utilized to manage the project’s new/replaced hard surfaces. BMP T5.13: Post-Construction Soil Quality and Depth will be applied to the disturbed pervious areas of the site development. A summary table of the project Stormwater Management BMP are provided below. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page A-3 Stormwater Management BMP List No. 2 Evaluation Lawn and landscaped areas: BMP Viable Limitations / Infeasibility Criteria BMP T5.13: Post-Construction Soil Quality and Depth (Volume V, Chapter 5) Yes Limitation: Considered infeasible on slopes greater than 33 percent are present., BMP T5.13 shall be applied to the site post construction. Roofs: BMP T5.30: Full Dispersion (Volume V, Chapter 5) BMP T5.10A: Downspout Full Infiltration Systems (Volume III, Chapter 3) No No Infeasibility: Infeasible due to the lack of adequate vegetative flow paths, the shallow depth to low permeability soil and limited setback space due to clustered development. See additional discussion in the Geotechnical letter (Jan. 4, 2022) by Earth Solutions Northwest. Bioretention (Volume V, Chapter 7) No Infeasibility: Infeasible due to the shallow depth to low permeability soil and limited setback space due to clustered development. BMP T5.10B: Downspout Dispersion Systems (Volume III, Chapter 3) No See BMP T5.30 dispersion infeasibility discussion above. BMP T5.10C: Perforated Stub-out (Volume III, Chapter 3) No Infeasibility: Not recommended due to shallow depth to dense unweathered till. Detention Vaults or Pipes (Edmonds Stormwater Addendum) Yes Individual lot detention pipes will be used to meet Minimum Requirement No. 5. Other Hard Surfaces: BMP T5.30: Full Dispersion (Volume V, Chapter 5) No See BMP T5.30 dispersion infeasibility discussion above. BMP T5.15: Permeable Pavement (Volume V, Chapter 5) No See BMP T5.10A infiltration-based infeasibility discussion above. Bioretention (Volume V, Chapter 7) No See BMP T5.10A infiltration-based infeasibility discussion above. BMP T5.12: Sheet Flow Dispersion BMP T5.11: Concentrated Flow Dispersion (Volume V, Chapter 5) No See BMP T5.30 dispersion infeasibility discussion above. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page A-4 There is also a portion of right-of-way (ROW) frontage area that will be redevelopment (920 sf of hard surface). These ROW improvements will replacement/expand of the frontage sidewalk to 7 ft and install a new access driveway cut; there are no feasible or viable stormwater BMP for these areas. To account for the 920 sf of ROW improvements, 460 sf of hard surface area shall be accounted for in each individual lot stormwater BMP detention pipe system. Note, per the City’s Stormwater Addendum section 6.3.1, the city does allow for an area up to 1,000 sf not to be managed if BMPs are infeasible. As discussed above, a large area will be reserved for tree retention in the northerly portion of lot 2 as part of a “conservation subdivision design” for the project. A conservation subdivision design is a form of stormwater management BMP due to the fact it reduces hard surface areas and retains existing tree canopies on site. See the civil site development plans and Section C for additional stormwater BMP evaluation and construction details. A summary table of the project Stormwater Management BMP areas are provided below. Onsite Stormwater Management BMP Summary:* Project Site Category: Category 2 Discharge Location: City's Municipal Separate Storm Sewer System (MS4) Type of Area Square Footage Stormwater Facility Lawn and Landscape Forest/Tree Retention Lot 1- 5,184 sf Lot 2- 4,729 sf Lot 2- 8,519 sf BMP T5.13: Post-Construction Soil Quality BMP T5.13: Post-Construction Soil Quality Tree Retention Area Roofs Lot 1- 2,575 sf Lot 2- 2,617 sf Stormwater BMP Detention Pipe Stormwater BMP Detention Pipe Hard Surfaces ROW Improvements = Lot 1- 1,897 sf Lot 2- 1,225 sf 920 sf Stormwater BMP Detention Pipe Stormwater BMP Detention Pipe Less than 1,000 sf and no viable BMP Add 460 sf of hard surface to each lot system 25% Retrofit 0 sf N.A. *See development conditions map provided in Section C below. Minimum Requirement #6 – Runoff Treatment. The project does not propose to create/replace more than 5,000 sf of pollution-generation impervious surface onsite. The project will create about 3,322 sf pollution generation hard surfaces (road and driveway areas); a majority of the site’s impervious surface is rooftop and walkway/patio areas (non-pollution generation surfaces). Thus, no basic or enhanced water quality treatment system is required. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page A-5 Minimum Requirement #7 – Flow Control. As listed above in MR#5 all proposed new/replaced hard surfaces and disturbed pervious surfaces (lawn/landscape areas) will be mitigate with the application of stormwater BMPs. The project is exempt from flow control (MR 7) due to the fact the site development will not create and/or replace more than 10,000 sf of unmitigated hard surface; nor does the proposed developed conditions exceed a 0.15 cfs (using a 15-minute time step) increase in the 100-year recurrence interval flow between the pre-existing conditions (prior to 1977) and the proposed developed conditions (ECDC 18.30.060.D.7.b.iii 100-year flow frequency threshold). A full overview of the proposed storm drainage control facilities calculations is provided in section C. Minimum Requirement #8 – Wetland Protection. No wetlands are known to exist on or adjacent to the site. Minimum Requirement #9 – Operation and Maintenance. An operation and maintenance summary is provided in section F. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page A-6 Parcel (Vicinity) Map: RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page A-7 Site Plan: RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page B-1 B. EXISTING CONDITIONS SUMMARY Existing Conditions. The total site consists of 26,746 sf (0.614 acres) parcel and is currently occupied by an existing single-family residence with a loop access driveway off 220th St SW. The existing house is surrounded by grass lawn and landscape areas with several large mature trees throughout the site. The site is surrounded by single family residences to the west, north, and east; and 220th St SW to the south boundary of the site (see aerial photo below). The site generally slopes in the west/northwesterly direction in a range between 2% and 30%. No define drainage courses are found on-site and stormwater runoff would sheet flow west/northwesterly direction. No stream or wetlands were discovered on or immediately adjacent to the site; Westgate pond is about 200 feet west of the project site. Additional discussion of the local drainage basin and downstream path is discussed in Section C of this report. Soils: In accordance with the project’s site-specific geotechnical engineering study by Earth Solutions NW, LLC the on-site soils consist of silty sand (USCS: SM) glacial till deposits. The subject site is underlain by glacial till deposits (Qvt, Vashon till soils). Full soils description is included in the project’s site specific geotechnical engineering study by Earth Solutions NW, LLC. Aerial Photo (City of Edmonds GIS, 2017 photo): RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page B-2 Existing Conditions Map: RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-1 C. DEVELOPED SITE HYDROLOGY: Developed Conditions. The proposed project includes the construction of three single residence and associated driveways and utilities. All existing structures and impervious (hard) surface onsite will be removed/replaced with the site development; landscaping and grass lawn around the new residence will stabilize the site upon building construction. The project proposes to create and/or replace 12,915 sf of hard surface with the complete site development, all existing hard surface will be removed with the project site development. The following is a summary of the proposed new/replaced hard surface areas: Total Hard Surface to Mitigated = 9,234 sf (0.211 ac) Lot 1 Hard Surface to Mitigate = 4,932 sf Roof Area = 2,575 sf Walkway/Patio Area = 50 sf Driveway Area = 540 sf Shared Private Driveway = 1,307 sf 50% of Project Frontage Improvements = 460 sf Lot 2 Hard Surface to Mitigate = 4,302 sf Roof Area = 2,617 sf Walkway/Patio Area = 50 sf Driveway Area = 1,175 sf 50% of Project Frontage Improvements = 460 sf Total Pervious Surface (Lawn/Forest) = 18,432 sf (0.423 ac) Lot Lawn/Landscape Area = 9,913 sf Forest/Tree Retention Area = 8,519 sf RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-2 Developed Conditions Map: RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-3 In accordance with the Edmonds City Code (18.30) Category 2 projects shall meet MR 5 and MR 7 (see discussion in Section A above) and provide on-site stormwater management and flow control. To meet these criteria, the proposed hard surfaces of the site shall implement BMPs from “List No. 2” or meet the LID performance standard. The project proposes to cluster the site development and retain a large, treed area as part of a “conservation subdivision design”. The cluster, tree retention, and proposed grading limits the viability of infiltration and dispersion BMP methods for the project. Therefore, individual lot stormwater BMP detention pipes (Edmonds Stormwater Addendum Section 6.3) will be utilized to manage the project’s new/replaced hard surfaces. BMP T5.13: Post-Construction Soil Quality and Depth will be applied to the disturbed pervious areas of the site development. The total proposed new replaced hard surface of 9,234 sf will be mitigated with individual lot detention pipe systems. Lot 1 detention pipe system will mitigate 4,932 sf of hard surface and the Lot 2 detention system will mitigate 4,302 sf of hard surface (see hard surface breakdown above). The onsite detention pipe systems have been sized in accordance with the Edmonds Stormwater Addendum simplified sizing approach, below is a summary of each system: Detention Pipe Sizing Calculations: Lot 1 Lot 2  Hard Surface = 4,932 sf 4,302 sf  Required 36” Detention Pipe = 39.46 ft 34.42 ft (0.008 x 4,932 sf)1 (0.008 x 4,302 sf)1  Provided 36” Detention Pipe = 40 ft 35 ft 1. Per Edmonds Stormwater Addendum simplified sizing approach Section 6.3.1. Additionally, all disturbed pervious (lawn and landscape) areas will preserve or restore the health and water- holding capacity of the soils by compost-amending (Per DOE BMP T5.13: Post-Construction Soil Quality and Depth). See attached City of Edmonds Standard Detail SD-642 criteria for BMP T5.13: Post-Construction Soil Quality and Depth. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-4 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-5 Stormwater Modeling Calculations. As discussed in section A above, the project is exempt from flow control (MR 7) due to the fact the site development will not create and/or replace more than 10,000 sf of non-mitigated (stormwater BMPS) hard surface; nor does the proposed developed conditions exceed a 0.15 cfs (using a 15-minute time step) increase in the 100-year recurrence interval flow between the pre-existing conditions (prior to 1977, existing house onsite was built in 1952) and the proposed developed conditions (ECDC 18.30.060.D.7.b.iii 100-year flow frequency threshold). Existing Conditions Summary (See existing conditions map in Section B above): Existing House = 0.046 ac. (2,013 sf) Existing Driveway = 0.041 ac. (1,802 sf) Existing Walk/Patio = 0.003 ac. (145 sf) Total Hard Surface = 0.090 ac. (Conservatively the existing sheds will not be included in the existing conditions) Existing Lawn = 0.152 ac. (6,606 sf) Existing Forest = 0.393 ac. (17,100 sf) Total Pervious Surface = 0.545 ac. Developed Conditions Summary (See existing conditions map in Section C above): Lot 1 and Lot 2 Roof = 0.119 ac. (5,192 sf) Lot 1 and Lot 2 Driveways = 0.069 ac. (3,022 sf) Lot 1 and Lot 2 Walk/Patio = 0.002 ac. (100 sf) Frontage Sidewalk = 0.021 ac. (920 sf) Total Hard Surface = 0.211 ac. Lot 1 and 2 Lawn = 0.228 ac. (9,913 sf) Tree Retention Forest = 0.196 ac. (8,519 sf) Total Pervious Surface = 0.424 ac. 100-year Recurrence Flow Summary:  Pre-Existing Condition1 100-year flow = 0.121 cfs (15-minume time step)  Developed Condition 100-year flow = 0.178 cfs (15-minume time step)  100-year Recurrence Flow increase = 0.057 cfs 1. Existing house was built in 1952 (prior to 1977), thus included in pre-existing condition. Pre-existing and proposed developed runoff rate calculations were performed using the DOE 2012 Western Washington Hydrology Model (WWHM2012) continuous runoff model. WWHM2012 Input/Output Data: RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-6 ___________________________________________________________________ Project Name: 20-016 Storm modeling- 100 yr Site Name: 20-016 Site Address: 8929 220th St SW City : Edmonds Report Date: 1/5/2022 MGS Regoin : Puget East Data Start : 1901/10/1 Data End : 2058/09/30 DOT Data Number: 03 Version Date: 2021/08/18 Version : 4.2.18 ___________________________________________________________________ PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Mod 0.393 C, Lawn, Flat 0.152 Pervious Total 0.545 Impervious Land Use acre ROOF TOPS FLAT 0.046 DRIVEWAYS FLAT 0.041 SIDEWALKS FLAT 0.003 Impervious Total 0.090 Basin Total 0.635 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Pervious Land Use acre C, Pasture, Flat 0.228 C, Forest, Mod 0.196 Pervious Total 0.424 Impervious Land Use acre ROOF TOPS FLAT 0.119 DRIVEWAYS FLAT 0.069 SIDEWALKS FLAT 0.023 Impervious Total 0.211 Basin Total 0.635 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-7 Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.545 Total Impervious Area:0.09 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:0.424 Total Impervious Area:0.211 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.034286 5 year 0.050503 10 year 0.063701 25 year 0.083525 50 year 0.100819 100 year 0.120493 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.06695 5 year 0.091053 10 year 0.109113 25 year 0.134451 50 year 0.155241 100 year 0.177749 Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-8 1902 0.032 0.075 1903 0.036 0.082 1904 0.059 0.097 1905 0.022 0.044 1906 0.021 0.047 1907 0.040 0.068 1908 0.029 0.057 1909 0.030 0.068 1910 0.045 0.071 1911 0.033 0.070 1912 0.110 0.144 1913 0.026 0.046 1914 0.116 0.186 1915 0.022 0.043 1916 0.034 0.080 1917 0.020 0.041 1918 0.028 0.063 1919 0.020 0.038 1920 0.030 0.057 1921 0.027 0.047 1922 0.037 0.065 1923 0.031 0.057 1924 0.039 0.088 1925 0.020 0.041 1926 0.035 0.081 1927 0.027 0.060 1928 0.028 0.054 1929 0.044 0.086 1930 0.047 0.102 1931 0.026 0.047 1932 0.028 0.055 1933 0.030 0.053 1934 0.064 0.090 1935 0.022 0.044 1936 0.031 0.063 1937 0.049 0.081 1938 0.025 0.046 1939 0.027 0.060 1940 0.048 0.102 1941 0.043 0.099 1942 0.045 0.081 1943 0.036 0.072 1944 0.064 0.105 1945 0.037 0.077 1946 0.034 0.062 1947 0.021 0.047 1948 0.042 0.067 1949 0.049 0.106 1950 0.023 0.046 1951 0.039 0.090 1952 0.093 0.121 1953 0.081 0.110 1954 0.031 0.060 1955 0.023 0.045 1956 0.019 0.043 1957 0.027 0.056 1958 0.057 0.080 1959 0.046 0.073 1960 0.024 0.050 1961 0.092 0.151 1962 0.030 0.061 1963 0.018 0.042 1964 0.095 0.145 1965 0.047 0.082 1966 0.025 0.053 1967 0.042 0.074 1968 0.026 0.053 1969 0.028 0.053 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-9 1970 0.043 0.073 1971 0.044 0.070 1972 0.170 0.243 1973 0.054 0.122 1974 0.043 0.086 1975 0.074 0.101 1976 0.055 0.093 1977 0.019 0.039 1978 0.049 0.075 1979 0.033 0.066 1980 0.041 0.075 1981 0.037 0.078 1982 0.026 0.054 1983 0.039 0.076 1984 0.034 0.069 1985 0.046 0.080 1986 0.025 0.044 1987 0.047 0.077 1988 0.025 0.047 1989 0.024 0.056 1990 0.031 0.056 1991 0.036 0.071 1992 0.039 0.073 1993 0.039 0.089 1994 0.036 0.060 1995 0.019 0.040 1996 0.044 0.066 1997 0.027 0.055 1998 0.036 0.067 1999 0.033 0.075 2000 0.035 0.068 2001 0.033 0.077 2002 0.074 0.107 2003 0.030 0.053 2004 0.038 0.081 2005 0.083 0.160 2006 0.028 0.061 2007 0.040 0.077 2008 0.030 0.064 2009 0.024 0.052 2010 0.031 0.065 2011 0.026 0.060 2012 0.034 0.064 2013 0.032 0.063 2014 0.023 0.052 2015 0.106 0.147 2016 0.024 0.052 2017 0.046 0.095 2018 0.058 0.080 2019 0.083 0.115 2020 0.049 0.081 2021 0.042 0.073 2022 0.046 0.088 2023 0.048 0.109 2024 0.112 0.155 2025 0.032 0.075 2026 0.036 0.073 2027 0.038 0.082 2028 0.015 0.034 2029 0.029 0.053 2030 0.051 0.101 2031 0.016 0.037 2032 0.025 0.053 2033 0.023 0.055 2034 0.023 0.048 2035 0.048 0.075 2036 0.035 0.058 2037 0.031 0.072 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-10 2038 0.044 0.071 2039 0.052 0.118 2040 0.027 0.056 2041 0.029 0.061 2042 0.058 0.086 2043 0.038 0.085 2044 0.033 0.062 2045 0.030 0.055 2046 0.028 0.050 2047 0.027 0.064 2048 0.024 0.054 2049 0.039 0.085 2050 0.027 0.052 2051 0.050 0.086 2052 0.027 0.061 2053 0.025 0.055 2054 0.081 0.117 2055 0.026 0.060 2056 0.037 0.081 2057 0.021 0.044 2058 0.035 0.081 ___________________________________________________________________ Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.1699 0.2426 2 0.1156 0.1865 3 0.1117 0.1602 4 0.1099 0.1550 5 0.1063 0.1508 6 0.0948 0.1471 7 0.0929 0.1447 8 0.0915 0.1444 9 0.0835 0.1223 10 0.0825 0.1205 11 0.0815 0.1184 12 0.0809 0.1167 13 0.0738 0.1150 14 0.0738 0.1099 15 0.0640 0.1085 16 0.0638 0.1067 17 0.0594 0.1060 18 0.0584 0.1053 19 0.0577 0.1022 20 0.0569 0.1021 21 0.0550 0.1012 22 0.0538 0.1008 23 0.0519 0.0987 24 0.0507 0.0970 25 0.0496 0.0953 26 0.0490 0.0929 27 0.0486 0.0903 28 0.0486 0.0900 29 0.0486 0.0893 30 0.0482 0.0882 31 0.0477 0.0877 32 0.0475 0.0863 33 0.0471 0.0863 34 0.0470 0.0859 35 0.0469 0.0855 36 0.0460 0.0852 37 0.0459 0.0851 38 0.0458 0.0820 39 0.0455 0.0816 40 0.0452 0.0816 41 0.0451 0.0815 42 0.0445 0.0814 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-11 43 0.0439 0.0814 44 0.0439 0.0813 45 0.0438 0.0809 46 0.0434 0.0807 47 0.0428 0.0805 48 0.0428 0.0804 49 0.0425 0.0803 50 0.0422 0.0803 51 0.0420 0.0796 52 0.0410 0.0775 53 0.0401 0.0775 54 0.0396 0.0771 55 0.0393 0.0768 56 0.0391 0.0768 57 0.0388 0.0759 58 0.0388 0.0754 59 0.0387 0.0753 60 0.0386 0.0751 61 0.0380 0.0750 62 0.0379 0.0749 63 0.0379 0.0748 64 0.0373 0.0741 65 0.0370 0.0734 66 0.0367 0.0733 67 0.0366 0.0730 68 0.0364 0.0729 69 0.0362 0.0729 70 0.0359 0.0724 71 0.0359 0.0723 72 0.0359 0.0709 73 0.0357 0.0708 74 0.0350 0.0706 75 0.0349 0.0705 76 0.0346 0.0704 77 0.0346 0.0687 78 0.0345 0.0684 79 0.0344 0.0684 80 0.0343 0.0681 81 0.0341 0.0675 82 0.0334 0.0667 83 0.0333 0.0660 84 0.0331 0.0658 85 0.0327 0.0654 86 0.0326 0.0649 87 0.0322 0.0644 88 0.0322 0.0641 89 0.0321 0.0638 90 0.0312 0.0633 91 0.0312 0.0632 92 0.0311 0.0627 93 0.0310 0.0623 94 0.0309 0.0621 95 0.0307 0.0614 96 0.0304 0.0608 97 0.0304 0.0607 98 0.0302 0.0605 99 0.0301 0.0604 100 0.0300 0.0604 101 0.0300 0.0599 102 0.0296 0.0598 103 0.0294 0.0597 104 0.0289 0.0597 105 0.0287 0.0580 106 0.0285 0.0573 107 0.0283 0.0572 108 0.0281 0.0567 109 0.0281 0.0565 110 0.0279 0.0562 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-12 111 0.0276 0.0561 112 0.0275 0.0559 113 0.0273 0.0552 114 0.0273 0.0551 115 0.0271 0.0548 116 0.0270 0.0547 117 0.0269 0.0545 118 0.0269 0.0540 119 0.0268 0.0539 120 0.0266 0.0537 121 0.0263 0.0533 122 0.0263 0.0532 123 0.0261 0.0532 124 0.0261 0.0531 125 0.0259 0.0531 126 0.0255 0.0529 127 0.0255 0.0525 128 0.0252 0.0523 129 0.0250 0.0517 130 0.0247 0.0516 131 0.0245 0.0515 132 0.0245 0.0504 133 0.0241 0.0498 134 0.0239 0.0482 135 0.0238 0.0475 136 0.0238 0.0474 137 0.0236 0.0468 138 0.0235 0.0468 139 0.0235 0.0467 140 0.0234 0.0463 141 0.0234 0.0458 142 0.0234 0.0456 143 0.0218 0.0452 144 0.0217 0.0445 145 0.0215 0.0445 146 0.0213 0.0444 147 0.0210 0.0438 148 0.0205 0.0434 149 0.0201 0.0425 150 0.0197 0.0422 151 0.0195 0.0415 152 0.0194 0.0408 153 0.0188 0.0401 154 0.0186 0.0388 155 0.0182 0.0381 156 0.0164 0.0365 157 0.0148 0.0344 ___________________________________________________________________ Perlnd and Implnd Changes No changes have been made. ___________________________________________________________________ This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc. be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc. or their authorized representatives have been advised of the possibility of such damages. Software Copyright © by : Clear Creek Solutions, Inc. 2005-2022; All Rights Reserved. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-13 Offsite Analysis. The site is located in the Westgate Pond watershed basin. The Westgate Pond basin is a relatively small urban basin that collects stormwater runoff from primarily residential areas in the City Edmonds and ultimately discharges runoffs west/northwesterly to Puget Sound via the Shell Creek see attached watershed map). Upstream Evaluation: There are some minor upstream areas east of the site that has potential to contribute stormwater run-on from the neighboring property to the east. The upstream area (about 5,000 sf) primarily consists of open lawn area of the adjacent property to the east (8917 220th St SW). Due to the relatively small and vegetated lawn area, no significant run-on flows are anticipated. See the existing conditions map provided above in Section B. Downstream Evaluation: The natural flow path of the site generally slopes west/northwesterly direction across private property towards Westgate Pond about 200 feet west of the site. The Westgate Pond discharges to the west to 92nd Ave W and then flows south to 220th St SW. Due to the fact the current flow path of the site to the west/northwest flows across private properties and the subject site has no easements across the neighboring private properties; the project’s detention tank systems will discharge to the existing storm drainage system along the north side of 220th St SW. The 220th St SW pipe system flows west and merges with the Westgate Pond discharge +/-540 ft for the site at the corner of 220th St SW and 92nd Ave W. From the merge point of the developed site drainage and the Westgate Pond discharge (+/-540 ft from the site), the system continues west along 220th St SW to 95th Ave W and turns north to 217th St SW. The system flows west along 217th St SW to 96th St SW. At 96th St SW the system is directed north in a collection of 18” to 24” pipes to a discharge to Shell Creek. The Shell Creek discharge point is about 1.1 miles downstream of the site. Shell Creek continues in a northwesterly direction and eventually discharges into the Puget Sound about 2.3 miles from the site. See C-4 Downstream Drainage Facility Map, delineating the downstream path to the discharge point to Shell Creek. A majority of the downstream conveyance system within a ¼ mile of the site consists of a man-made pipe and catch basin system; some areas of open channel are found near the conveyance system into and out of Westgate Pond. No apparent flooding or system inadequacies within a ¼ mile downstream of the site were discovered during site visits or research of the downstream system. See attached watershed and downstream drainage facility map for the local downstream path. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-14 Watershed Basin Map RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-15 Downstream Drainage Facility Map (City of Edmonds GIS) RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page C-16 Offsite Stormwater Pipe Conveyance Capacity Evaluation. The proposed project stormwater systems will discharge to the existing City stormwater conveyance system (MS4) within 220th St SW. A majority of site naturally slopes in a northwesterly direction and does not directly discharge to 220th St SW; the area south of the existing house does flows southwesterly and does currently contribute runoff flows to 220th St SW. The northwesterly discharge does eventually route west and back south to the 220th St SW west of 92nd Ave W (west of the site, see Offsite Basin Analysis above). Thus, the site will contribute stormwater flows to a portion of the 220th MS4 system that currently does not convey site stormwater flows. The MS4 system from the site to the natural downstream “merge point” consists of 12” smooth-walled pipes. A conservative assumption has been made that the MS4 has a minimum slope of 0.5% and a flow capacity of 2.94 cfs (see pipe flow calculation below). The discharge to the MS4 along 220th St SW will be from the two individual stormwater BMP detention pipes located on lots 1 and 2 of the short plat. The peak discharge from the individual lot 36” detention pipes will be from a ½” orifice of the control tee for each system. The peak rate for a ½” orifice with 36” of head is about 0.011 cfs. Therefore, two systems would have a peak discharge of about 0.022 cfs (two BMP detention pipe systems). The increase flows of the peak discharge (0.022 cfs) is about 0.75% (0.022/2.94) of the 12” pipe system capacity within 220th St SW. Thus, the system will have minimal impact on the overall capacity of the 220th St SW MS4. Orifice Flow Equation: A = (3.14)(.5/12)2 / 4 = 0.00136 ft2 Q = (0.6)(0.00136)[(2)(32.2)(3)]^.5 = 0.011 cfs RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page D-1 D. SOILS REPORT: Geotechnical Evaluation Letter Prepared by: Earth Solutions NW, LLC Updated: January 4, 2022 Pages: 17 RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page E-1 E. CONSTRUCTION SWPPP REQUIREMENTS: The project will require grading to construct the proposed buildings and utilities (including the stormwater detention pipe system). Standard erosion control measures are proposed to be used during construction. The primary erosion and sediment control BMP during construction will be proper soil stabilization methods. Exposed soils shall be stabilized by application of effective BMPs that protect the soil from the erosive forces of raindrops, flowing water, and wind. Applicable practices include, but not limited to, temporary and permanent seeding, sodding, mulching, plastic covering, erosion control fabrics, matting, the early application of gravel base on areas to be paved, and dust control. The contractor shall select a soil stabilization method best suited for the situation. Stockpiles must be stabilized and protected with sediment trapping measures. In addition, site containment of exposed soils shall be sustained by using silt fence barriers along the down-slope boundaries of the site’s disturbance areas. See the site development plan for details. Construction Stormwater Pollution Prevention Plan (SWPPP): The following is a summary of the site’s erosion control measures that evaluates the typical DOE 13 elements of a SWPPP: Element 1: Mark Clearing Limits: Clearing limits have been delineated on the engineering site development plans. Element 2: Establish Construction Access: A construction access has been delineated on the engineering site development plans. Element 3: Control Flow Rates: During construction silt fencing will provide attenuation of site runoff and upon project completion and stabilization (established lawns and landscape of exposed soils), the infiltration storm system will provide flow control. Element 4: Install Sediment Controls: Filter fabric fence has been delineated and detailed on the engineering site development plans. At a minimum, silt fence will be installed along the down gradient perimeter of the disturbed area that will receive sediment-laden runoff. Element 5: Stabilize Soils: Soils will be stabilized per the TESC notes listed on the engineering site development plans. Element 6: Protect Slopes: Exposed slopes shall be stabilized per the TESC notes listed on the engineering site development plans. Element 7: Protect Drain Inlets: Drain inlet protection will be installed on all catch basins that will receive sediment-laden runoff. See the engineering site development plans for locations and detail. Element 8: Stabilize Channels and Outlets: This element is not applicable since there are no temporary channels or outlets proposed. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page E-2 Element 9: Control Pollutants: Pollutants shall be managed as described in the TESC notes listed on the engineering site development plans. Element 10: Control De-Watering: Highly turbid or contaminated de-watering water shall be handled separately from stormwater. The water from all de-watering systems for trenches and foundations shall be treated or disposed prior to discharging from the site. Element 11: Maintain BMP’s: BMPs shall be maintained and removed at the end of the project as follows: i. All temporary and permanent erosion and sediment control BMPs shall be inspected, maintained and repaired in accordance with the Drainage Manual or as approved or required by the City to assure continued performance of their intended function in accordance with BMP specifications. ii. The applicant may remove temporary BMPs when they are no longer needed. iii. All temporary erosion and sediment control BMPs shall be removed within 30 days after construction is completed and the City has determined that the site is stabilized. Element 12: Manage the Project: The owner of the site is responsible for managing the installation and maintenance of the site BMPs. Element 13. Protect Low Impact Development BMPs: Compaction (i.e., vehicle and equipment traffic or storage; and/or placement of stockpiles) to the soils in the area of the stormwater BMPs (Post- Construction Soil Quality and Depth BMP T5.13 shall be avoided during construction. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-1 F. OPERATION AND MAINTENANCE GUIDELINES: These guidelines are intended to provide operation and maintenance instructions for the project’s storm drainage control facilities. The owner is responsible for maintenance of storm drainage facilities within the property (private property owner system); the owner is not responsible for maintenance within the public right-of-way (City system). Private Property Owners: Private property owners are responsible for properly maintaining the stormwater infrastructure on their property to ensure it operates as designed. The City has developed an inspection program to ensure private property owners are properly maintaining their stormwater systems. City System: City crews perform maintenance activities on the entire storm drainage system, including inspection and cleaning of catch basins, street sweeping, emergency flooding response, creek maintenance, inspection and monitoring of private stormwater detention systems. This manual is not comprehensive; although it explains the intended operation of the various components of the drainage system, and suggests a routine of inspection and maintenance, it cannot anticipate every problem. Once a historical record of maintenance is established, it may be prudent to alter the routine. It is recommended that maintenance records be kept, and that the records be reviewed periodically. Concept of Operation: The drainage design is shown and described in the site development engineering plans and report. The approved site development plans and report should be retained by the owner and used as a reference to identify stormwater BMP facilities outlined in this manual. Conveyance Systems. The design objective in pipe sizing was to convey the large storm events without the water surface exceeding the ground elevation. Frequent overtopping of the pipe system in an area might indicate a downstream blockage. Overtopping of the drain systems at the catch-basins is an indication that maintenance is required. Stormwater Storage Detention Pipe (Tank) System. The onsite detention system consists of an underground detention pipe. Onsite storm water runoff from the impervious surfaces of the lots is routed to the detention pipe and discharged to the City’s storm pipe system in 220th St SW. The detention pipe is a typical component of a storm water control system for new development. Standard maintenance procedures should be followed. The following pages outline standard general maintenance criteria for the project’s drainage facilities. For additional and updated maintenance information visit the Washington State Department of Ecology’s web-site at: http://www.ecy.wa.gov/programs/wq/wqhome.html RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-2 Appendix V-A: BMP Maintenance Tables Ecology intends the facility-specific maintenance standards contained in this section to be conditions for determining if maintenance actions are required as identified through inspection. Recognizing that Permittees have limited maintenance funds and time, Ecology does not require that a Permittee perform all these maintenance activities on all their stormwater BMPs. We leave the determination of importance of each maintenance activity and its priority within the stormwater program to the Permittee. We do expect, however, that sufficient maintenance will occur to ensure that the BMPs continue to operate as designed to protect ground and surface waters. Ecology doesn’t intend that these measures identify the facility's required condition at all times between inspections. In other words, exceedance of these conditions at any time between inspections and/or maintenance does not automatically constitute a violation of these standards. However, based upon inspection observations, the Permittee shall adjust inspection and maintenance schedules to minimize the length of time that a facility is in a condition that requires a maintenance action. Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) Maintenance Component Defect Conditions When Maintenance is Needed Results Expected When Maintenance is Performed Storage Area Plugged Air Vents One-half of the cross section of a vent is blocked at any point or the vent is damaged. Vents open and functioning. Debris and Sediment Accumulated sediment depth exceeds 10% of the diameter of the storage area for 1/2 length of storage vault or any point depth exceeds 15% of diameter. (Example: 72-inch storage tank would require cleaning when sediment reaches depth of 7 inches for more than 1/2 length of tank.) All sediment and debris removed from storage area. Joints Between Tank/Pipe Section Any openings or voids allowing material to be transported into facility. All joint between tank/pipe sections are sealed. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-3 Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) Maintenance Component Defect Conditions When Maintenance is Needed Results Expected When Maintenance is Performed (Will require engineering analysis to determine structural stability). Tank Pipe Bent Out of Shape Any part of tank/pipe is bent out of shape more than 10% of its design shape. (Review required by engineer to determine structural stability). Tank/pipe repaired or replaced to design. Vault Structure Includes Cracks in Wall, Bottom, Damage to Frame and/or Top Slab Cracks wider than 1/2-inch and any evidence of soil particles entering the structure through the cracks, or maintenance/inspection personnel determines that the vault is not structurally sound. Cracks wider than 1/2-inch at the joint of any inlet/outlet pipe or any evidence of soil particles entering the vault through the walls. Vault replaced or repaired to design specifications and is structurally sound. No cracks more than 1/4-inch wide at the joint of the inlet/outlet pipe. Manhole Cover Not in Place Cover is missing or only partially in place. Any open manhole requires maintenance. Manhole is closed. Locking Mechanism Not Working Mechanism cannot be opened by one maintenance person with proper tools. Bolts into frame have less than 1/2 inch of thread (may not apply to self-locking lids). Mechanism opens with proper tools. Cover Difficult to Remove One maintenance person cannot remove lid after applying normal lifting pressure. Intent is to keep cover from sealing off access to maintenance. Cover can be removed and reinstalled by one maintenance person. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-4 Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) Maintenance Component Defect Conditions When Maintenance is Needed Results Expected When Maintenance is Performed Ladder Rungs Unsafe Ladder is unsafe due to missing rungs, misalignment, not securely attached to structure wall, rust, or cracks. Ladder meets design standards. Allows maintenance person safe access. Catch Basins See Table V-A.5: Maintenance Standards - Catch Basins See Table V-A.5: Maintenance Standards - Catch Basins See Table V-A.5: Maintenance Standards - Catch Basins Table V-A.4: Maintenance Standards - Control Structure/Flow Restrictor Maintenance Component Defect Condition When Maintenance is Needed Results Expected When Maintenance is Performed General Trash and Debris (Includes Sediment) Material exceeds 25% of sump depth or 1 foot below orifice plate. Control structure orifice is not blocked. All trash and debris removed. Structural Damage Structure is not securely attached to manhole wall. Structure is not in upright position (allow up to 10% from plumb). Connections to outlet pipe are not watertight and show signs of rust. Structure securely attached to wall and outlet pipe. Structure in correct position. Connections to outlet pipe are watertight; structure repaired or replaced and works as designed. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-5 Table V-A.4: Maintenance Standards - Control Structure/Flow Restrictor Maintenance Component Defect Condition When Maintenance is Needed Results Expected When Maintenance is Performed Any holes - other than designed holes - in the structure. Structure has no holes other than designed holes. Cleanout Gate Damaged or Missing Cleanout gate is not watertight or is missing. Gate cannot be moved up and down by one maintenance person. Chain/rod leading to gate is missing or damaged. Gate is rusted over 50% of its surface area. Gate is watertight and works as designed. Gate moves up and down easily and is watertight. Chain is in place and works as designed. Gate is repaired or replaced to meet design standards. Orifice Plate Damaged or Missing Control device is not working properly due to missing, out of place, or bent orifice plate. Plate is in place and works as designed. Obstructions Any trash, debris, sediment, or vegetation blocking the plate. Plate is free of all obstructions and works as designed. Overflow Pipe Obstructions Any trash or debris blocking (or having the potential of blocking) the overflow pipe. Pipe is free of all obstructions and works as designed. Manhole See Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) See Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) See Table V-A.3: Maintenance Standards - Closed Detention Systems (Tanks/Vaults) RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-6 Table V-A.4: Maintenance Standards - Control Structure/Flow Restrictor Maintenance Component Defect Condition When Maintenance is Needed Results Expected When Maintenance is Performed Catch Basin See Table V-A.5: Maintenance Standards - Catch Basins See Table V-A.5: Maintenance Standards - Catch Basins See Table V-A.5: Maintenance Standards - Catch Basins Table V-A.5: Maintenance Standards - Catch Basins Maintenance Component Defect Conditions When Maintenance is Needed Results Expected When Maintenance is performed General Trash & Debris Trash or debris which is located immediately in front of the catch basin opening or is blocking inletting capacity of the basin by more than 10%. Trash or debris (in the basin) that exceeds 60 percent of the sump depth as measured from the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of six inches clearance from the debris surface to the invert of the lowest pipe. Trash or debris in any inlet or outlet pipe blocking more than 1/3 of its height. Dead animals or vegetation that could generate odors that could No Trash or debris located immediately in front of catch basin or on grate opening. No trash or debris in the catch basin. Inlet and outlet pipes free of trash or debris. No dead animals or vegetation present within the catch basin. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-7 Table V-A.5: Maintenance Standards - Catch Basins Maintenance Component Defect Conditions When Maintenance is Needed Results Expected When Maintenance is performed cause complaints or dangerous gases (e.g., methane). Sediment Sediment (in the basin) that exceeds 60 percent of the sump depth as measured from the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of 6 inches clearance from the sediment surface to the invert of the lowest pipe. No sediment in the catch basin Structure Damage to Frame and/or Top Slab Top slab has holes larger than 2 square inches or cracks wider than 1/4 inch. (Intent is to make sure no material is running into basin). Frame not sitting flush on top slab, i.e., separation of more than 3/4 inch of the frame from the top slab. Frame not securely attached Top slab is free of holes and cracks. Frame is sitting flush on the riser rings or top slab and firmly attached. Fractures or Cracks in Basin Walls/ Bottom Maintenance person judges that structure is unsound. Grout fillet has separated or cracked wider than 1/2 inch and longer than 1 foot at the joint of any inlet/outlet pipe or any evidence of soil particles entering catch basin through cracks. Basin replaced or repaired to design standards. Pipe is regrouted and secure at basin wall. RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-8 Table V-A.5: Maintenance Standards - Catch Basins Maintenance Component Defect Conditions When Maintenance is Needed Results Expected When Maintenance is performed Settlement/ Misalignment If failure of basin has created a safety, function, or design problem. Basin replaced or repaired to design standards. Vegetation Vegetation growing across and blocking more than 10% of the basin opening. Vegetation growing in inlet/outlet pipe joints that is more than six inches tall and less than six inches apart. No vegetation blocking opening to basin. No vegetation or root growth present. Contamination and Pollution See Table V-A.1: Maintenance Standards - Detention Ponds No pollution present. Catch Basin Cover Cover Not in Place Cover is missing or only partially in place. Any open catch basin requires maintenance. Cover/grate is in place, meets design standards, and is secured Locking Mechanism Not Working Mechanism cannot be opened by one maintenance person with proper tools. Bolts into frame have less than 1/2 inch of thread. Mechanism opens with proper tools. Cover Difficult to Remove One maintenance person cannot remove lid after applying normal lifting pressure. (Intent is keep cover from sealing off access to maintenance.) Cover can be removed by one maintenance person. Ladder Ladder Rungs Unsafe Ladder is unsafe due to missing rungs, not securely attached to Ladder meets design standards and allows RAM Engineering, Inc. Select Homes: 2-Lot SP, 8929 220th Street SW RAM No. 20-016 Stormwater Site Plan Report Page F-9 Table V-A.5: Maintenance Standards - Catch Basins Maintenance Component Defect Conditions When Maintenance is Needed Results Expected When Maintenance is performed basin wall, misalignment, rust, cracks, or sharp edges. maintenance person safe access. Metal Grates (If Applicable) Grate opening Unsafe Grate with opening wider than 7/8 inch. Grate opening meets design standards. Trash and Debris Trash and debris that is blocking more than 20% of grate surface inletting capacity. Grate free of trash and debris. Damaged or Missing. Grate missing or broken member(s) of the grate. Grate is in place, meets the design standards, and is installed and aligned with the flow path.