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Home My WebLink AboutRS_Drainage_Report_20210521_v1.pdfCivil Engineers ● Structural Engineers ● Landscape Architects ● Community Planners ● Land Surveyors Technical Information Report PREPARED FOR: Studio19 Architects 207 ½ 1st Avenue South, Suite 300 Seattle, WA 98104-3462 PROJECT: Watershed Affordable Housing Apartments 615 and 617 Williams Avenue South Renton WA 2200335.10 PREPARED BY: Jesse Newman, EIT Project Engineer REVIEWED BY: Charlie Palmer, PE Project Manager DATE: November 2020 Revised May 2021 Technical Information Report PREPARED FOR: Studio19 Architects 207 ½ 1st Avenue South, Suite 300 Seattle, WA 98104-3462 PROJECT: Watershed Affordable Housing Apartments 615 and 617 Williams Avenue South Renton WA 2200335.10 PREPARED BY: Jesse Newman, EIT Project Engineer REVIEWED BY: Charlie Palmer, PE Project Manager DATE: November 2020 Revised May 2021 I hereby state that this Technical Information Report for the Watershed Affordable Housing Apartments project has been prepared by me or under my supervision, and meets the standard of care and expertise that is usual and customary in this community for professional engineers. I understand that City of Renton does not and will not assume liability for the sufficiency, suitability, or performance of drainage facilities prepared by me. 05/03/2021 Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Table of Contents 1.0 Project Overview.........................................................................................................................1-1 1.1 Purpose and Scope .........................................................................................................1-1 1.2 Existing Conditions ..........................................................................................................1-1 1.3 Post-Development Conditions .........................................................................................1-1 Section 1.0 Figures Figure 1-1 ...................................TIR Worksheet Figure 1-2 ...................................Vicinity Map Figure 1-3A.................................Predeveloped Site Area Map Figure 1-3B.................................Proposed Area Map Figure 1-4 ...................................Soils Map 2.0 Conditions and Requirements Summary.................................................................................2-1 2.1 Core Requirements..........................................................................................................2-1 2.1.1 CR 1 – Discharge at the Natural Location ..........................................................2-1 2.1.2 CR 2 – Offsite Analysis.......................................................................................2-1 2.1.3 CR 3 – Flow Control ...........................................................................................2-1 2.1.4 CR 4 – Conveyance System...............................................................................2-1 2.1.5 CR 5 – Erosion and Sediment Control................................................................2-1 2.1.6 CR 6 – Maintenance and Operations .................................................................2-2 2.1.7 CR 7 – Financial Guarantees and Liability .........................................................2-2 2.1.8 CR 8 – Water Quality..........................................................................................2-2 2.2 Special Requirements......................................................................................................2-2 2.2.1 SR 1 – Other Adopted Area-Specific Requirements...........................................2-2 2.2.2 SR 2 – Flood Hazard Area Delineation...............................................................2-2 2.2.3 SR 3 – Flood Protection Facilities.......................................................................2-2 2.2.4 SR 4 – Source Controls......................................................................................2-2 2.2.5 SR 5 – Oil Control...............................................................................................2-2 2.2.6 SR 6 – Aquifer Protection ...................................................................................2-2 Section 2.0 Figures Figure 2-1 ...................................FEMA Floodplains Map Figure 2-2 ...................................Groundwater Protection Map 3.0 Offsite Analysis...........................................................................................................................3-1 3.1 Purpose and Scope .........................................................................................................3-1 3.2 Existing Conditions ..........................................................................................................3-1 3.3 Offsite Analysis................................................................................................................3-1 Technical Information Report Watershed Affordable Housing Apartments 2200335.10 3.3.1 Downstream Analysis .........................................................................................3-1 3.3.2 Upstream Analysis..............................................................................................3-7 3.4 Conclusion.......................................................................................................................3-7 Section 3.0 Figures Figure 3-1 ..................................Vicinity Map Figure 3-2 ...................................Existing Conditions Map Figures 3-3A through 3-3F .........Offsite Analysis Maps Figure 3-4 ...................................Offsite Analysis Drainage Table Figure 3-5 ...................................Offsite Drainage Photos Figure 3-6 ...................................FEMA Flood Insurance Rate Map Figure 3-7 ...................................Critical Area Map Figure 3-8 ...................................Soil Survey Figure 3-9 ...................................WA DOE Clean Water Act Section 303(d) Map Figure 3-10 .................................Wetland Inventory Map 4.0 Flow Control and Water Quality Facility Analysis and Design...............................................4-1 4.1 Flow Control.....................................................................................................................4-1 4.1.1 Existing Site Hydrology (Part A) .........................................................................4-1 4.1.2 Developed Site Hydrology (Part B).....................................................................4-1 4.1.3 Performance Standards (Part C) ........................................................................4-2 4.1.4 Flow Control System (Part D).............................................................................4-4 4.2 Water Quality System (Part E).........................................................................................4-5 Section 4.0 Figures Figure 4-1 ...................................MGSFlood Report Figure 4-2 ...................................Proposed Area Map Figure 4-3 ...................................Water Quality Area Map Figure 4-4 ...................................BioPod Sizing Chart 5.0 Conveyance System Analysis and Design...............................................................................5-1 Section 5.0 Figures Figures will be provided later in design. 6.0 Special Reports and Studies.....................................................................................................6-1 Section 6.0 Figures Figure 6-1 ...................................Geotechnical Report 7.0 Other Permits..............................................................................................................................7-1 8.0 CSWPPP Analysis and Design..................................................................................................8-1 8.1 ESC Plan Analysis and Design (Part A) ..........................................................................8-1 8.1.1 ESC Maintenance...............................................................................................8-1 8.1.2 ESC Supervisor ..................................................................................................8-1 Technical Information Report Watershed Affordable Housing Apartments 2200335.10 8.1.3 Documentation....................................................................................................8-2 8.1.4 Review Timing ....................................................................................................8-2 8.2 Stormwater Pollution Prevention and Spill (SWPPS) Plan Design (Part B) ....................8-2 8.2.1 Pollution and Spill Prevention Source Controls and BMPs.................................8-2 8.2.2 Responsible Personnel and Contact Information ...............................................8-4 8.2.3 Pollution and Spill Prevention Worksheets.........................................................8-4 8.2.4 Disposal Methods ...............................................................................................8-4 Section 8.0 Figures Erosion Control will be designed at a later submittal. 9.0 Bond Quantities, Facility Summaries, and Declaration of Covenant ....................................9-1 Section 9.0 Figures Figures will be provided later in design. 10.0 Operations and Maintenance Plan..........................................................................................10-1 10.1 Facility Descriptions.......................................................................................................10-1 10.2 Maintenance Tasks........................................................................................................10-1 10.3 Maintenance Requirements...........................................................................................10-2 Section 10.0 Figures Figures will be provided later in design. 11.0 Conclusion................................................................................................................................11-1 Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 1 Project Overview Technical Information Report Watershed Affordable Housing Apartments 1-1 2200335.10 1.0 Project Overview 1.1 Purpose and Scope This report accompanies the civil engineering plans and documents for the Watershed Affordable Housing Apartments located at 617 Williams Avenue South, Renton, Washington. The site is divided into four parcels, Title Parcel A – 172305-9069 (southeast), Title Parcel B – 172305-9136 (northeast), Title Parcel C – 172305-9136 (west), and Title Parcel D – 182305-9282 (southwest). The project site is approximately 1.34 acres in size; with the right-of-way dedication, the project site measures 1.286 acres. Refer to Figure 1-1 for the TIR Worksheet and Figure 1-2 for a Vicinity Map. The project proposes to construct two buildings (one with 50 residential units and one with 28 residential units) over at-grade parking. In addition, the project proposes to construct exterior parking. The buildings will be served with new water, sewer, fire, and storm utilities to support the buildings. Frontage improvements include new sidewalk along South Grady Way and Williams Avenue South. Street trees will be planted in an amenity strip along South Grady Way and in tree pits along Williams Avenue South. In addition, a walking path/emergency access drive will connect South Grady Way to Burnett Linear Park, running west of the building. The site is located within the city of Renton, which has adopted the 2017 King County Surface Water Design Manual, herein referenced as the City of Renton Surface Water Design Manual (CORSWDM). Per the CORSWDM, Flow Control Best Management Practices (BMPs), Peak Rate Flow Control, and Enhanced Basic Water Quality Menus will apply to the proposed project. 1.2 Existing Conditions The existing site is developed with one existing condominium, a gravel driveway, and an existing billboard. The site is vegetated with lawn and sparse trees. Access to the site is from the gravel driveway at the southwest portion of the project site along South Grady Way and from the parking area at the northeast corner along Williams Avenue South. The topography of the site is relatively flat and slightly undulating, with less than 1-foot fall from one end to the other. Based on a geotechnical report dated June 10, 2020, in general, site soils beneath the topsoil consist of very loose to medium dense alluvial, and floodplain deposits consisting of non-plastic silt (ML), silty fine sand (SM), and sand (SP, SP-SM) were encountered extending to the maximum exploration depth of 46.5 feet. Per the King County Soil Survey Map (Figure 1-4), site soils consist primarily of Urban Land (Ur) soils. Per the Geotechnical Report, the groundwater table was observed at an average depth of 6 to 9 feet below existing grades on one visit, and between 9 and 10 feet on another visit. Refer to the attached Geotechnical Report titled, “Updated Geotechnical Engineering Study,” by Earth Solutions NW, Figure 6-1. The site drains to a catch basin found at the southwest corner of the site near South Grady Way. The existing drainage patterns are analyzed and discussed in the Level One Downstream Analysis in Section 3.0. Refer to the attached Predeveloped Site Area Map, Figure 1-3A. 1.3 Post-Development Conditions The project proposes to construct an apartment building over at-grade parking, new exterior parking, and an emergency access drive/pedestrian walk on the west portion of the property. In addition, the project proposes to construct new curb and sidewalk along South Grady Way and Williams Avenue South. Drainage along Williams Avenue South and South Grady Way will be improved by sloping the gutters more steeply. Technical Information Report Watershed Affordable Housing Apartments 1-2 2200335.10 Stormwater from the new parking area and a portion of the emergency access road will be conveyed to a BioPod system. After the BioPod treats the runoff for water quality, flows will be conveyed to a detention pipe system. The detention pipe will also accept flows from the new building roof and areas that could not be treated for water quality. A portion of the parking lot is situated in such a way that runoff cannot be collected and conveyed to the water quality system; however, runoff can be conveyed to the detention pipe. Following detention, stormwater will discharge via pipe to the existing public downstream conveyance system along South Grady Way. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 1.0 Figures Figure 1-1.........TIR Worksheet Figure 1-2.........Vicinity Map Figure 1-3A.......Predeveloped Site Area Map Figure 1-3B.......Proposed Area Map Figure 1-4.........Soils Map Jesse Newman AHBL 206-267-2425 Watershed Affordable Housing Apartments 617 Williams Ave S. Renton, Washington. 23N 5E Section 18 and 17 Black River Fill Flat Minimal Vertical Limitations, only 4.9-feetfrom rim to invert of discharge location. TDA 1 1 Peak Rate Flow Control June 8, 2020 NA BIOPOD PIPE TANK 1200 6th AvenueSuite 1620Seattle, WA 98101206.267.2425 TEL206.267.2429 FAXWATERSHED AFFORDABLE HOUSING APARTMENTSVICINITY MAPEX1-2GRAPHIC SCALE050010001" = 500 FEET250NPROJECT SITE 1200 6th Avenue Suite 1620 Seattle, WA 98101 206.267.2425 TEL 206.267.2429 FAX DRAWN BY:DATE:JOB NO.: WATERSHED AFFORDABLE HOUSING APARTMENTS PREDEVELOPED SITE AREA MAP JN 11/2020 20201106 EX 1-3A 12"CON. 14DL8"CON. 14DL 18"CON. 25DL 14"CON. 18DL 48"DEC. 40DL 48"DEC. 40DL 60"DEC. 50DL 36"DEC. 45DL 32"DEC. 40DL 60"DEC. 50DL 72"DEC. 55DL 8"DEC. 20DL GRAPHIC SCALE 0 40 80 1" = 40 FEET 20 N LEGEND Description Quantity Unit EXISTING IMPERVIOUS AREA 0.299 ac TOTAL PROJECT SITE AREA WITH DEDICATION 1.286 ac TOTAL PROJECT SITE AREA 1.336 ac PROJECT SITE DISCHARGE LOCATION EXISTING ROW LINE 1200 6th Avenue Suite 1620 Seattle, WA 98101 206.267.2425 TEL 206.267.2429 FAX DRAWN BY:DATE:JOB NO.: 12"CON. 14DL8"CON. 14DL 18"CON. 25DL 14"CON. 18DL 48"DEC. 40DL 48"DEC. 40DL EG: 30.14 UP UP 32 Watershed Affordable Housing Apartments Proposed Area Map JN 11/2020 2200335 FIG 1-3B GRAPHIC SCALE 0 40 80 1" = 40 FEET 20 N LEGEND Description Quantity Unit BYPASS GRASS (GRASS PAVE)0.001 ac BYPASS IMPERVIOUS 0.025 ac BYPASS TOTAL 0.041 ac GRASS (GRASS PAVE)0.023 ac NEW + REPLACED IMPERVIOUS 1.118 ac TOTAL PROJECT SITE AREA WITH DEDICATION 1.286 ac DISCHARGE LOCATION LIMITS OF DEVELOPMENT PROPOSED BUILDING 3-FT DIAMETER FLOW CONTROL TANK WQ TREATMENT WITH BIOPOD WQ TREATMENT WITH BIOPOD NEW IMP PGIS BYPASS AND PASTURE TOTAL NEW PLUS REPLACED IMP= 1.118 sc TOTAL SITE AREA= 1.286 ac TOTAL PASTURE= 1.286 ac (total proj sitre area) - 1.118 ac (New + Repl IMP) - 0.041 ac (bypass total area) - 0.023 ac (grass pave) =0.104 ac BYPASS BYPASS IMP =0.025 ac BYPASS PASTURE 0.041 ac- 0.025-0.001 ac (BYPASS GRASS) = 0.015 ac Soil Map—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/15/2020 Page 1 of 3525810052581105258120525813052581405258150525816052581705258180525819052582005258210525822052581005258110525812052581305258140525815052581605258170525818052581905258200525821052582205258230559690559700559710559720559730559740559750559760559770559780 559690 559700 559710 559720 559730 559740 559750 559760 559770 559780 47° 28' 29'' N 122° 12' 28'' W47° 28' 29'' N122° 12' 23'' W47° 28' 24'' N 122° 12' 28'' W47° 28' 24'' N 122° 12' 23'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 30 60 120 180 Feet 0 5 10 20 30 Meters Map Scale: 1:666 if printed on A portrait (8.5" x 11") sheet. Soil Map may not be valid at this scale. MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: King County Area, Washington Survey Area Data: Version 15, Sep 16, 2019 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Jul 1, 2019—Jul 25, 2019 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/15/2020 Page 2 of 3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI Ur Urban land 1.4 100.0% Totals for Area of Interest 1.4 100.0% Soil Map—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/15/2020 Page 3 of 3 Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 2 Conditions and Requirements Summary Technical Information Report Watershed Affordable Housing Apartments 2-1 2200335.10 2.0 Conditions and Requirements Summary 2.1 Core Requirements 2.1.1 CR 1 – Discharge at the Natural Location The project site naturally drains to a catch basin at the southwest corner of the site via conveyance and sheet flow. The proposed project will pipe developed flows to the same drainage facility. This meets the requirement for discharging at the natural location. 2.1.2 CR 2 – Offsite Analysis AHBL staff performed a Level One Downstream Analysis for the project. The analysis included: Defining and mapping the study area. Reviewing available information on the study area. Field inspecting the study area. Describing the existing drainage system, including its existing and predicted drainage and water quality problems. Refer to Section 3.0 for the full offsite analysis. 2.1.3 CR 3 – Flow Control This project is in a Peak Rate Flow Control Standard area, which requires matching existing site conditions for the 2-, 10-, and 100-year peak rate runoff. Because there were no observed drainage problems within 1 mile downstream, there are no further flow control requirements. Flow control and flow control BMPs are further discussed in Section 4.0, Flow Control and Water Quality Facility Analysis and Design. 2.1.4 CR 4 – Conveyance System The project has designed all new conveyance systems to convey the 25-year storm event and to not overtop in the 100-year storm event. If overtopping occurs, the drainage will be routed to the natural discharge location without creating a severe flooding or severe erosion problem. The design and calculations for the new conveyance system are included in Section 5.0. The project will not change the flow characteristics of the existing conveyance systems because flow control will be provided prior to discharging to the existing public system. For flow control analysis, refer to Section 4.0, Flow Control and Water Quality Facility Analysis and Design. Spill control is required because the project constructs a new onsite conveyance system that collects runoff from non-rooftop pollution generating impervious surface (PGIS). Spill control will be provided by use of a tee section in a manhole. 2.1.5 CR 5 – Erosion and Sediment Control An erosion and sediment control plan will be developed for this site in accordance with the CORSWDM and to conform to the requirements of the Construction Stormwater National Pollutant Discharge Elimination System (NPDES) permit issued by the Department of Ecology (Ecology). The preliminary erosion and sediment control plan is described further in Section 8.0 and in the project plans. Technical Information Report Watershed Affordable Housing Apartments 2-2 2200335.10 2.1.6 CR 6 – Maintenance and Operations The onsite drainage facilities will be privately maintained by the owner. An Operations and Maintenance Manual will be provided in Section 10.0 at a later submittal. 2.1.7 CR 7 – Financial Guarantees and Liability This project will provide a Drainage Facilities Restoration and Site Stabilization Financial Guarantee. Bond Quantity Worksheets will be provided in Section 9.0 at a later submittal. 2.1.8 CR 8 – Water Quality The project site is subject to the Enhanced Basic Water Quality Treatment Menu per the CORSWDM. Design of these water quality facilities is discussed further in Section 4.0. 2.2 Special Requirements 2.2.1 SR 1 – Other Adopted Area-Specific Requirements To our knowledge, no other adopted area-specific requirements apply to the project site. 2.2.2 SR 2 – Flood Hazard Area Delineation Floodplain/Floodway (FEMA) Maps: The project site is located in Zone X per Flood Insurance Rate Map No. 53033C0977G, Panel 977, which is determined to be outside the 500-year (0.02 percent reoccurrence) floodplain. Refer to Figure 2-1 of this section for the FEMA Floodplains Map. 2.2.3 SR 3 – Flood Protection Facilities The project does not contain, will not construct, and is not adjacent to any existing flood protection facilities. 2.2.4 SR 4 – Source Controls The proposed project consists of a parking lot and apartments. The King County Stormwater Pollution Prevention Manual (KCSPPM) will be referenced for source control measures, in addition to erosion and sediment control measures, during construction. For construction source controls, see Section 8.0 CSWPPP Analysis and Design. Post-construction source controls will be supplied at a later submittal in Section 10.0, Operations and Maintenance Plan. 2.2.5 SR 5 – Oil Control The project does not fit the definition of a high-use site; therefore, it is not subject to oil control requirements. 2.2.6 SR 6 – Aquifer Protection The project does not fall under Zone 1 or Zone 2 of the Aquifer Protection Area; thus, this requirement is not applicable. Refer to Figure 2-2, Groundwater Protection Map. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 2.0 Figures Figure 2-1.........FEMA Floodplains Map Figure 2-2.........Groundwater Protection Map 1200 6th Avenue Suite 1620 Seattle, WA 98101 206.267.2425 TEL 206.267.2429 FAX COMMUNITY NUMBER PANEL SUFFIX CONTAINS: (SEE MAP INDEX FOR FIRM PANEL LAYOUT) Federal Emergency Management Agency FLOOD INSURANCE RATE MAPFIRM Notice to User: The Map Number shown belowshould be used when placing map orders; theCommunity Number shown above should beused on insurance applications for the subject community. MAP REVISED MAP NUMBER53033C0977G KING COUNTY,WASHINGTONAND INCORPORATED AREAS KING COUNTY 530071 0977 GRENTON, CITY OF 530088 0977 G NFIP NATIONAL FLOOD INSURANCE PROGRAMPANEL 0977G PANEL 977 OF 1700 PRELIMINARY 9/15/2017 D :,,SSB E A C O N WAYSWHITWORTHMOSESAVENUE¥k405 ST SSMITHERSAVENUE S WYGRANTMORRIS4TH LANE SSS T REET AVENUE530088 CEDAR4T H AVENUES ZONE AH MILLS HIGHRENTONS AVENUEAVENUEWHITWORTHSMORRISAVEAVENUEMILLS PedestrianBridge S GRADYSG R ADYAVENUE S9THAVENUEWAY S5TH S S 10TH STR R O A DRAILRO ADLOGAWILLIAMST A L B O T ZONEX AVENUECITY OF R ENTON BURNETTSSTREET AVENUE7T H SSMITHERSAVENUESTREETSS3RDSTREETSS RA ILROAD AVENUEAVENUEWHITWORTHSS 41 ROAD3RD STREET SS 19 AVENUEMAINW A Y SY0624 X6TH SLANEBENSONWELLSSÑ WATERSHED AFFORDABLE HOUSING APARTMENTS FEMA FLOODMAPS EXHIBIT 2-1 OTHER AREAS ZONE X ZONE D mile; and areas protected by levees from 1% annual chance flood. Areas determined to be outside the 0.2% annual chance floodplain. Areas in which flood hazards are undetermined, but possible. 250 10000FEET 150 3000METERS MAP SCALE 1" = 500'500 150 COMMUNITY NUMBER PANEL SUFFIX CONTAINS: (SEE MAP INDEX FOR FIRM PANEL LAYOUT) Federal Emergency Management Agency FLOOD INSURANCE RATE MAPFIRM Notice to User: The Map Number shown belowshould be used when placing map orders; theCommunity Number shown above should beused on insurance applications for the subject community. MAP REVISED MAP NUMBER53033C0977G KING COUNTY,WASHINGTONAND INCORPORATED AREAS KING COUNTY 530071 0977 G RENTON, CITY OF 530088 0977 G NFIP NATIONAL FLOOD INSURANCE PROGRAMPANEL 0977G PANEL 977 OF 1700 PRELIMINARY 9/15/2017 PROJECT SITE PROJECT SITE Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 3 Offsite Analysis Technical Information Report Watershed Affordable Housing Apartments 3-1 2200335.10 3.0 Offsite Analysis 3.1 Purpose and Scope The project encompasses four tax parcels at and surrounding 617 Williams Avenue South in Renton, Washington. site is divided into four parcels, Title Parcel A – 172305-9069 (southeast), Title Parcel B – 172305-9136 (northeast), Title Parcel C – 172305-9136 (west), and Title Parcel D – 182305-9282 (southwest). The project is bordered by Williams Avenue South to the east, South Grady Way to the south, and parcel properties to the north and west. The combined parcel size is 1.34 acres and the project site is a zero-lot development. The project proposes to construct two buildings, one with 50 residential units and one with 28 residential units, over at-grade parking. In addition, the project proposes to construct exterior parking onsite. The buildings will be served with new water, sewer, fire, and storm utilities to support the buildings. This report has been prepared to review the upstream and downstream conditions, research any downstream challenges, review any potential downstream impacts, provide a field inspection report, and provide recommendations for mitigation. 3.2 Existing Conditions The existing site is developed with one existing condominium, a gravel driveway, and lawn. In addition, there are six trees located on the site. Access to the site is from the gravel driveway at the southwest portion of the project site along South Grady Way and from the parking area at the northeast corner along Williams Avenue South. The site is relatively flat and slightly undulating, with less than 1-foot fall from one end to the other. The site drains to a catch basin found at the southwest corner of the site near South Grady Way. Refer to Figure 3-2 for the Existing Conditions Map for topographical and aerial depiction of project site. Soils for the downstream analysis area, including the project site, are typically of the Urban Land or Woodinville Silt Loam variety, per the King County Soil Survey Map (Figure 3-8). Refer to the attached Geotechnical Report (Figure 6-1) for further soil information. 3.3 Offsite Analysis 3.3.1 Downstream Analysis Task 1 – Study Area Definition Maps The project is in the Black River Subbasin and has one Threshold Discharge Area. Stormwater drains to the existing City of Renton storm line that borders the east and south edge of the project site. Stormwater in this existing system travels south on Williams Avenue South in a series of pipe and catch basins, and then drains west along South Grady Way to a catch basin located at the south edge of the project site. The study area includes the downstream drainage path from the project site for 1 mile downstream. Field inspection was extended 0.25 mile downstream from the project site. The upstream was analyzed for identifying potential impacts to the site and drainage systems. Refer to Figures 3-3A through 3-3F, 3-4, and 3-5 for maps, the offsite analysis table, and photos. Technical Information Report Watershed Affordable Housing Apartments 3-2 2200335.10 Task 2 – Resource Review The following resources were reviewed to discover any existing or potential problems in the study area: Adopted Basin Plans: The project site is located within the Black River Subbasin, which is part of the Green River North Basin, as identified by the CSI Project: Green River North Sub Regional Planning Area, dated May 2003. Floodplain/Floodway (FEMA) Maps: The project site is in Zone X per Flood Insurance Rate Map No. 53033C0977G, Panel 977, which is determined to be outside the 500-year (0.02 percent reoccurrence) floodplain. Refer to Figure 3-6. Other Offsite Analysis Reports: No other known offsite analysis reports have been identified for the areas immediately upstream or downstream of this project site. Sensitive Areas Folio: Refer to Figure 3-7 for the Critical Area Map, per City of Renton GIS. The project in within a seismic hazard area; otherwise, the City of Renton GIS does not show any other sensitive areas near the project site or within 1-mile downstream of the project. In addition, there are riparian wetlands surrounding the Black River (see Wetland Inventory Map, Figure 3-10. Drainage Complaints: There was a drainage complaint listed on the City of Renton GIS at 645 Shattuck Avenue near the associated drainage system. The complaint was that the sidewalk was flooding, per discussion with a City of Renton Public Works Maintenance employee. The drainage complaint has been closed since April 18, 2018. Based on the aerial imagery, it appears that the sidewalk in this area has been replaced. USDA Soils Survey: Soils for the downstream analysis area, including the project site, are typically of the Urban Land (58.4%) or Woodinville Silt Loam variety (40.8%), per the King County Soil Survey Map (Figure 3-8). Wetlands Inventory: There is a non-delineated wetland surrounding the Black River. Refer to the Wetlands Inventory Map, Exhibit 3-10. Section 303d List of Polluted Waters: No polluted waters are located within 1 mile downstream of the project site per Ecology’s Clean Water Act Section 303d list of polluted waters and published maps. There was a 303d listed waterbody approximately 1.4 miles downstream from the project site. Refer to Figure 3-9 for a copy of the area map. Task 3 – Field Inspect the Study Area (Level 1) A Level 1 (qualitative) downstream field inspection was completed on June 8, 2020. Weather was sunny, with temperatures around 70° F. The site was dry. The Level 1 inspection included the following tasks: 1.Investigate any problems reported or observed during the resource review – No problems were found downstream of the project area. 2.Locate all existing/potential constrictions or lack of capacity in the existing drainage system – At one location, based on City of Renton GIS, the drainage system went from 42-inch diameter pipe to 36-inch diameter pipe. No drainage complaints have been reported in this area. Technical Information Report Watershed Affordable Housing Apartments 3-3 2200335.10 3.Identify all existing/potential downstream drainage problems as defined in Section 1.2.2.1 – All existing and potential drainage problems are listed in the drainage table, Figure 3-4. 4.Identify existing/potential overtopping, scouring, bank sloughing, or sedimentation – None of these problems were identified during field inspection. 5.Identify significant destruction of aquatic habitat or organisms – No areas of destruction were identified, including siltation, bank erosion, or incision in a stream. 6.Collect qualitative data on features such as land use, impervious surfaces, topography, and soil types – Data reviewed during resource review were confirmed during the field inspection. 7.Collect information on pipe sizes, channel characteristics, drainage structures, and relevant critical areas – This information is identified in Task 4; however, some of the catch basins were located in a busy roadway and we were not able to collect information. 8.Verify tributary basins delineated in Task 1 – Tributary basins were confirmed from Task 1. 9.Contact neighboring property owners or residents in the area about past or existing drainage problems, and describe these in the report – We spoke with a City employee, Gary Fink, who stated that there were no known drainage issues in the area. 10.Note the date and weather conditions at the time of inspection – Noted above. Task 4 – Describe the Drainage System, and Its Existing and Predicted Drainage and Water Quality Problems The field inspection including walking the downstream flow path from the south site discharge location to just beyond the 1 mile downstream. The Offsite Analysis Maps, Offsite Analysis Drainage Table, and Offsite Drainage Photos from the field inspection are provided in Figures 3-3A through 3-3F (Offsite Analysis Maps), Figure 3-4 (Offsite Analysis Drainage Table), and Figure 3-5 (Offsite Drainage Photos), respectively. A written description of the downstream drainage system is given below. The downstream inspection began at the catch basin located near the southwest corner of the site adjacent to South Grady Way. Runoff from the project site is directed to this catch basin via surface flow. The inlet appeared to be surrounded with a slightly bare area, with little or no vegetation; this does have potential to introduce sediment into the drainage system. In addition, sediment was observed in the catch basin. The sediment did not appear to hinder the flow characteristics of the drainage system; there was a small amount of sediment at the invert of the pipe. Refer to Element 1 in the Downstream Analysis Map (Figure 3-3B), Offsite Analysis Drainage Table (Figure 3-4), and Offsite Drainage Photos (Figure 3-5). Drainage Problem Review at Element 1: 1. Description of the problem – Little or no vegetation found directly surrounding inlet and sediment in standing water. 2. Magnitude of damage caused by the drainage problem – No damage was observed. There were some signs of sediment in the catch basin. There were no signs of erosion around the catch basin, and no signs of flooding or significant destruction of aquatic habitat to organisms. 3. General frequency and duration of drainage problem – There are no known issues with this area. Technical Information Report Watershed Affordable Housing Apartments 3-4 2200335.10 4. Return frequency of storm or flow of the water when the problem occurs (option for Level 1) – Frequency and duration of the drainage problems have not been reported. 5. Water surface elevation when problem occurs – No water has been observed to be backed up. 6. Names and concerns of involved parties (optional) – No involved parties have identified a concern. 7. Current mitigation of drainage problem – None identified. 8. Possible cause of the drainage problem – Lack of vegetation surrounding the catch basin. 9. Will project aggravate problem – No. The improved condition will use peak flow control standards, assuming existing conditions. It is expected that the detained flows may experience a net decrease in peak flows and durations. In addition, the landscaping in this area will all be new. Based on reviews of the nine areas above, the identified potential existing drainage problem may not meet the definition of any of the four defined problem types in Section 1.2.2.1.1 of the CORSWDM. The inlet discharges to the southwest in a 24-inch concrete pipe to another catch basin located at the southwest corner of the property line. This structure has a single vine of a blackberry bush growing inside of it, and there appeared to be a small amount of soil at the bottom of the catch basin. Refer to Element 2 in the Downstream Analysis Map (Figure 3-3B), Offsite Analysis Drainage Table (Figure 3-4), and Offsite Drainage Photos (Figure 3-5). Drainage Problem Review at Element 2 1. Description of the problem – The catch basin has a single blackberry vine growing within it. If left unchecked, more blackberry vines could sprout up, creating a potential drainage problem. 2. Magnitude of damage caused by the drainage problem – No damage was observed. There were some signs of sediment in the catch basin. There were no signs of flooding or significant destruction of aquatic habitat to organisms. 3. General frequency and duration of drainage problem – There are no known issues with this area. 4. Return frequency of storm or flow of the water when the problem occurs (option for Level 1) – Frequency and duration of the drainage problems have not been reported. 5. Water surface elevation when problem occurs – No water has been observed to be backed up. 6. Names and concerns of involved parties (optional) – No involved parties have identified a concern. 7. Current mitigation of drainage problem – None identified. 8. Possible cause of the drainage problem – Vegetation inside the catch basin. Technical Information Report Watershed Affordable Housing Apartments 3-5 2200335.10 9. Will project aggravate problem – No. The improved condition will use peak flow control standards, assuming existing conditions. It is expected that the detained flows may experience a net decrease in peak flows and durations. This catch basin will be cleared during construction. Based on reviews of the nine areas above, the identified potential existing drainage problem may not meet the definition of any of the four defined problem types in Section 1.2.2.1.1 of the CORSWDM. Drainage flows west in a 24-inch concrete pipe to a catch basin located on South 7th Street, found east of Burnett Avenue South. The structure appeared to be in good condition. An orange liquid was observed entering the catch basin from the inlet pipe; this orange material is most likely iron oxidized bacteria. As seen in the following Elements, this liquid was observed throughout the drainage system. Refer to Element 3 in the Downstream Analysis Map (Figure 3-3B), Offsite Analysis Drainage Table (Figure 3-4), and Offsite Drainage Photos (Figure 3-5). Drainage continues west in a 24-inch concrete pipe, to the next catch basin, approximately 25 feet downstream. This catch basin appeared to be in good condition. Refer to Element 4 in the Downstream Analysis Map (Figure 3-3B), Offsite Analysis Drainage Table (Figure 3-4), and Offsite Drainage Photos (Figure 3-5). Drainage then travels beneath South 7th Street through a 24-inch ductile iron pipe to a solid- rimmed catch basin in Burnett Linear Park. This catch basin appeared to be in good condition. Refer to Element 5 in the Downstream Analysis Map (Figure 3-3B), Offsite Analysis Drainage Table (Figure 3-4), and Offsite Drainage Photos (Figure 3-5). Afterward, drainage travels west along South 7th Street through a series of catch basins and 36-inch polyethylene pipes. Elements 6 through 8 were not observed due to safety concerns with accessing the catch basins in the roadway. The City of Renton GIS was consulted to determine the characteristics of the existing system. Refer to Elements 6 through 8 in the Downstream Analysis Map (Figure 3-3B) and the Offsite Analysis Drainage Table (Figure 3-4). Element 8 drains through a 36-inch pipe to a catch basin located at an alley found between Smithers Avenue South and Morris Avenue South. This structure appeared to be in good condition. Refer Element 9 in the Downstream Analysis Map (Figure 3-3B), Offsite Analysis Drainage Table (Figure 3-4), and Offsite Drainage Photos (Figure 3-5). Similar to Elements 6 through 8, Elements 10 through 14 were not observed due to safety concerns with accessing the catch basins in the roadway. According to City of Renton GIS, drainage continues west through a series of 36-inch polypropylene pipes until Element 11. At Element 11, the outlet size changes to a 42-inch polypropylene pipe. At Element 12, the outlet pipe size changes back to a 36-inch polypropylene pipe. This introduces a potential constriction of flows at this location. Drainage Problem Review at Element 12 1. Description of the problem – The pipe size changes from 42-inch to 36-inch. 2. Magnitude of damage caused by the drainage problem – No damage was observed around the catch basins. No drainage complaints have been filed in the area. 3. General frequency and duration of drainage problem – There are no known issues with this area. Technical Information Report Watershed Affordable Housing Apartments 3-6 2200335.10 4. Return frequency of storm or flow of the water when the problem occurs (option for Level 1) – Frequency and duration of the drainage problems have not been reported. 5. Water surface elevation when problem occurs – No water has been observed to be backed up. 6. Names and concerns of involved parties (optional) – No involved parties have identified a concern. 7. Current mitigation of drainage problem – None identified. 8. Possible cause of the drainage problem – Pipe size changing from larger to smaller diameter. 9. Will project aggravate problem – No. The improved condition will use peak flow control standards, assuming existing conditions. It is expected that the detained flows may experience a net decrease in peak flows and durations. 10. Based on reviews of the nine areas above, the identified potential existing drainage problem may not meet the definition of any of the four defined problem types in Section 1.2.2.1.1 of the CORSWDM. Following Element 12, drainage continues west along South 7th Street in a 36-inch polypropylene pipe to a catch basin located east of Whitworth Lane South. The catch basin structure appeared to be in good condition. Drainage continues west on South 7th Street to the 0.25-mile point downstream from the project site to a catch basin at Whitworth Lane South. This structure appeared to be in working order. According to City of Renton GIS, drainage continues west along South 7th Street through a series of catch basins and increasing pipe diameters ranging from 36-inch to 60-inch at the outfall to the Black River drainage basin. Refer to Element 17 in the Downstream Analysis Map (Figure 3-3B), Offsite Analysis Drainage Table (Figure 3-4), and Offsite Drainage Photos (Figure 3-5). The field investigation ended at this point. Task 5 – Mitigation of Existing or Potential Problems The downstream path is entirely comprised of a manmade system composed of pipes and catch basins. The piped conveyance system appears to have adequate capacity. Pipe sizes appear to generally increase as flow continues downstream. Where present, flows appeared to be shallow and slow. No signs of flooding or overtopping of structures were observed. Based on the review of the areas discussed in Task 4, the identified potential existing drainage problems may not meet the definition of any of the four defined problem types in Section 1.2.2.1.1 of the CORSWDM. As such, mitigation of potential or existing problems will be provided through Flow Control and Water Quality Management. Flow control BMPs will not be proposed. The project will use a detention tank to match peak flows of the 2-, 10-, and 100-year events. In addition, the project will use a bipod system to treat water quality. The project is not expected to aggravate any downstream drainage issues. Technical Information Report Watershed Affordable Housing Apartments 3-7 2200335.10 3.3.2 Upstream Analysis The project site is in a large flat area with little to no drainage entering the project site from adjacent properties. It was determined that upstream drainage will not adversely impact the site drainage. 3.4 Conclusion This analysis is based on data and records either supplied to or obtained by AHBL, Inc. These documents are referenced within the text of the analysis. The analysis has been prepared using procedures and practices within the standard accepted practices of the industry. We conclude that this analysis represents the field conditions as observed and researched by AHBL. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 3.0 Figures Figure 3-1 ..................................Vicinity Map Figure 3-2...................................Existing Conditions Map Figures 3-3A through 3-3F.........Offsite Analysis Maps Figure 3-4...................................Offsite Analysis Drainage Table Figure 3-5...................................Offsite Drainage Photos Figure 3-6...................................FEMA Flood Insurance Rate Map Figure 3-7...................................Critical Area Map Figure 3-8...................................Soil Survey Figure 3-9...................................WA DOE Clean Water Act Section 303(d) Map Figure 3-10.................................Wetland Inventory Map 1200 6th AvenueSuite 1620Seattle, WA 98101206.267.2425 TEL206.267.2429 FAXWATERSHED AFFORDABLE HOUSING APARTMENTSVICINITY MAPEX3-1GRAPHIC SCALE050010001" = 500 FEET250NPROJECT SITE 1,128 94 Existing Conditions Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. None 6/9/2020 Legend 64032 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 64 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Labels City and County Boundary Addresses Parcels Streets Points of Interest Parks Waterbodies 2019.sid Red: Band_1 Green: Band_2 Blue: Band_3 Map PROJECT SITE STORMWATER DISCHARGE LOCATION EX 3-2 9,0281505 Downstream Analysis Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. THIS MAP IS NOT TO BE USED FOR NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 06/05/2020 Legend 1023 0 512 1023 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov Pump Stations Discharge Points Stormwater Mains Culverts Open Drain Facility Outlines Private Pump Stations Private Discharge Points Private Pipes Private Culverts Private Open Drain Private Facility Outlines Stormwater Ponds Streets Parks Waterbodies 2019.sid Red: Band_1 Green: Band_2 Blue: Band_3 Map FLOW PATH PROJECT SITE TDA 1 DISCHARGE TO BLACK RIVER, APPRX. 1.4 MILES DOWNSTREAM. 1- MILE DOWNSTREAM 1/4 MILE DOWNSTREAM EX 3-3A 2,257376 Downstream Analysis Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. THIS MAP IS NOT TO BE USED FOR NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 06/05/2020 Legend 256 0 128 256 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov Parcels Network Structures Access Riser Inlet Manhole Utility Vault Clean Out Unknown Control Structures Pump Stations Discharge Points Water Quality Detention Facilities Pond Tank Vault Bioswale Wetland Other Stormwater Mains Culverts Open Drain Virtual Drainlines Facility Outlines Private Network Structures Access Riser Inlet Manhole Clean Out Utility Vault Unknown Private Control Structures Private Pump Stations Private Discharge Points Private Water Quality Private Detention Facilities Tank Wetland Filter Strip Infiltration Trench Vault Pond Bioswale Stormtech Chamber Other Private Pipes Private Culverts Private Open Drain Private Facility Outlines Flow Control BMPs Fences Stormwater Ponds Streets Points of Interest Parks Waterbodies 2019.sid 1/4 MILE DOWNSTREAM 1 2 FLOW PATH 3 4 5 6 7 8 9 10 11-13 1415 16 FLOW PATH CONTINUED 3-3C LEGEND Description 1/4 MILE FLOW PATH a DRAINAGE ELEMENT SEE PHOTOS PROJECT SITE DISCHARGE LOCATION EX 3-3B 2,257376 Downstream Analysis Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. THIS MAP IS NOT TO BE USED FOR NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 06/05/2020 Legend 256 0 128 256 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov Parcels Network Structures Access Riser Inlet Manhole Utility Vault Clean Out Unknown Control Structures Pump Stations Discharge Points Water Quality Detention Facilities Pond Tank Vault Bioswale Wetland Other Stormwater Mains Culverts Open Drain Virtual Drainlines Facility Outlines Private Network Structures Access Riser Inlet Manhole Clean Out Utility Vault Unknown Private Control Structures Private Pump Stations Private Discharge Points Private Water Quality Private Detention Facilities Tank Wetland Filter Strip Infiltration Trench Vault Pond Bioswale Stormtech Chamber Other Private Pipes Private Culverts Private Open Drain Private Facility Outlines Flow Control BMPs Fences Stormwater Ponds Streets Points of Interest Parks Waterbodies 2019.sid 0.61 MILES FLOW PATH CONTINUED 3-3D FLOW PATH EX 3-3C 2,257376 Downstream Analysis Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. THIS MAP IS NOT TO BE USED FOR NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 06/05/2020 Legend 256 0 128 256 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov Parcels Network Structures Access Riser Inlet Manhole Utility Vault Clean Out Unknown Control Structures Pump Stations Discharge Points Water Quality Detention Facilities Pond Tank Vault Bioswale Wetland Other Stormwater Mains Culverts Open Drain Virtual Drainlines Facility Outlines Private Network Structures Access Riser Inlet Manhole Clean Out Utility Vault Unknown Private Control Structures Private Pump Stations Private Discharge Points Private Water Quality Private Detention Facilities Tank Wetland Filter Strip Infiltration Trench Vault Pond Bioswale Stormtech Chamber Other Private Pipes Private Culverts Private Open Drain Private Facility Outlines Flow Control BMPs Fences Stormwater Ponds Streets Points of Interest Parks Waterbodies 2019.sid FLOW PATH FLOW PATH CONTINUED 3-3E 0.95 MILES DOWNSTREAM EX 3-3D 2,257376 Downstream Analysis Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. THIS MAP IS NOT TO BE USED FOR NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 06/05/2020 Legend 256 0 128 256 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov Parcels Network Structures Access Riser Inlet Manhole Utility Vault Clean Out Unknown Control Structures Pump Stations Discharge Points Water Quality Detention Facilities Pond Tank Vault Bioswale Wetland Other Stormwater Mains Culverts Open Drain Virtual Drainlines Facility Outlines Private Network Structures Access Riser Inlet Manhole Clean Out Utility Vault Unknown Private Control Structures Private Pump Stations Private Discharge Points Private Water Quality Private Detention Facilities Tank Wetland Filter Strip Infiltration Trench Vault Pond Bioswale Stormtech Chamber Other Private Pipes Private Culverts Private Open Drain Private Facility Outlines Flow Control BMPs Fences Stormwater Ponds Streets Points of Interest Parks Waterbodies 2019.sid 1 MILE DOWNSTREAM FLOW PATH CONTINUED 3-3F FLOW PATH EX 3-3E 2,257376 Downstream Analysis Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. THIS MAP IS NOT TO BE USED FOR NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 06/05/2020 Legend 256 0 128 256 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov Parcels Network Structures Access Riser Inlet Manhole Utility Vault Clean Out Unknown Control Structures Pump Stations Discharge Points Water Quality Detention Facilities Pond Tank Vault Bioswale Wetland Other Stormwater Mains Culverts Open Drain Virtual Drainlines Facility Outlines Private Network Structures Access Riser Inlet Manhole Clean Out Utility Vault Unknown Private Control Structures Private Pump Stations Private Discharge Points Private Water Quality Private Detention Facilities Tank Wetland Filter Strip Infiltration Trench Vault Pond Bioswale Stormtech Chamber Other Private Pipes Private Culverts Private Open Drain Private Facility Outlines Flow Control BMPs Fences Stormwater Ponds Streets Points of Interest Parks Waterbodies 2019.sid DISCHARGE TO BLACK RIVER 17 LEGEND Description a DRAINAGE ELEMENT DRAINAGE ELEMENT OBSERVED, SEE PHOTOS FLOW PATH EX 3-3F DATE:6/8/2020BY:Jesse Newman#DESCRIPTIONINOUTEXISTINGPOTENTIALPHOTOSOBSERVATIONS OF FIELD INSPECTOR1 Catch Basin Inlet 24" CONC. 24" CONC. 0-2% 0'Inlet appeared to be surrounded by a bare area, where little grass was growing, water appeared to have sediment in it.Element 1 PhotosStanding water in catch basin has some sediment visible, inlet surrounded by vegetation2 Catch Basin Inlet 24" CONC. 24" CONC. 0-2% 51'Catch basin has vegetation growing inside Element 2 Photos Black berry bush growing from inside the CB.3 Catch Basin Inlet 24" CONC. 24" CONC. 0-2% 344'Element 3 PhotosSome orage liquid observed. Potentially from iron oxidizing bacteria.4 Catch Basin Solid LID 24" CONC. 24" DIP 0-2% 389'Element 4 PhotosSome orage liquid observed. Potentially from iron oxidizing bacteria.5 Catch Basin Solid LID 24" DIP 36" PP 0-2% 435'Element 5 PhotosSome orage liquid observed. Potentially from iron oxidizing bacteria.6 Catch Basin Solid LID 36" PP 36" PP 0-2% 523'Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.7 Catch Basin Inlet 36" PP 36" PP 0-2% 598'Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.8 Catch Basin Solid LID 36" PP 36" PP 0-2% 654'Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.9 Catch Basin Inlet 36" PP 36" PP 0-2% 797'Element 9 Photos10 Catch Basin Inlet 36" PP 36" PP 0-2% 923'Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.11 Catch Basin Solid LID 36" PP 42" DIP 0-2% 940'Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.12 Catch Basin Solid LID 42" DIP 36" PP 0-2% 951'Constriction of storm Pipe from 42" to 36"Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.13 Catch Basin Solid LID 36" PP 36" CONC. 0-2% 965'Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.14 Catch Basin Solid LID 36" CONC. 36" PP 0-2% 1073'Not observed due to safety concerns accessing CB in roadway. (sizes from City of Renton GIS.15 Catch Basin Solid Inlet 36" PP 36" PP 0-2% 1254'Element 15 Photos16 Catch Basin Solid Inlet 36" PP 36" PP 0-2% 1363'Element 16 Photos Some orage liquid observed. Potentially from iron oxidizing bacteria.17 Outfall to Black River 60-72" Corrugated Metal, oval 2-4% 1.4 milesElement 17 PhotosDRAINAGE ELEMENT (SEE MAP)PROBLEMSDISTANCE FROM SITE DISCHARGEAPPROXIMATE SLOPE OUTEX 3-4Watershed Affordable Housing ApartmentsOffsite Drainage Table EXHIBIT 3-5 OFFSITE DRAINAGE PHOTOSELEMENT 1 PHOTOS ELEMENT 2 PHOTOS ELEMENT 3 PHOTOS ELEMENT 4 PHOTO ELEMENT 5 PHOTOS ELEMENT 9 PHOTOS ELEMENT 16 PHOTOS ELEMENT 17 PHOTOS 1200 6th Avenue Suite 1620 Seattle, WA 98101 206.267.2425 TEL 206.267.2429 FAX COMMUNITY NUMBER PANEL SUFFIX CONTAINS: (SEE MAP INDEX FOR FIRM PANEL LAYOUT) Federal Emergency Management Agency FLOOD INSURANCE RATE MAPFIRM Notice to User: The Map Number shown belowshould be used when placing map orders; theCommunity Number shown above should beused on insurance applications for the subject community. MAP REVISED MAP NUMBER53033C0977G KING COUNTY,WASHINGTONAND INCORPORATED AREAS KING COUNTY 530071 0977 GRENTON, CITY OF 530088 0977 G NFIP NATIONAL FLOOD INSURANCE PROGRAMPANEL 0977G PANEL 977 OF 1700 PRELIMINARY 9/15/2017 D :,,SSB E A C O N WAYSWHITWORTHMOSESAVENUE¥k405 ST SSMITHERSAVENUE S WYGRANTMORRIS4TH LANE SSS T REET AVENUE530088 CEDAR4T H AVENUES ZONE AH MILLS HIGHRENTONS AVENUEAVENUEWHITWORTHSMORRISAVEAVENUEMILLS PedestrianBridge S GRADYSG R ADYAVENUE S9THAVENUEWAY S5TH S S 10TH STR R O A DRAILRO ADLOGAWILLIAMST A L B O T ZONEX AVENUECITY OF R ENTON BURNETTSSTREET AVENUE7T H SSMITHERSAVENUESTREETSS3RDSTREETSS RA ILROAD AVENUEAVENUEWHITWORTHSS 41 ROAD3RD STREET SS 19 AVENUEMAINW A Y SY0624 X6TH SLANEBENSONWELLSSÑ WATERSHED AFFORDABLE HOUSING APARTMENTS DRAWING_TITLE EXHIBIT 3-6 OTHER AREAS ZONE X ZONE D mile; and areas protected by levees from 1% annual chance flood. Areas determined to be outside the 0.2% annual chance floodplain. Areas in which flood hazards are undetermined, but possible. 250 10000FEET 150 3000METERS MAP SCALE 1" = 500'500 150 COMMUNITY NUMBER PANEL SUFFIX CONTAINS: (SEE MAP INDEX FOR FIRM PANEL LAYOUT) Federal Emergency Management Agency FLOOD INSURANCE RATE MAPFIRM Notice to User: The Map Number shown belowshould be used when placing map orders; theCommunity Number shown above should beused on insurance applications for the subject community. MAP REVISED MAP NUMBER53033C0977G KING COUNTY,WASHINGTONAND INCORPORATED AREAS KING COUNTY 530071 0977 G RENTON, CITY OF 530088 0977 G NFIP NATIONAL FLOOD INSURANCE PROGRAMPANEL 0977G PANEL 977 OF 1700 PRELIMINARY 9/15/2017 PROJECT SITE FEMA FLOOD INSURANCE RATE MAP 9,0281505 CRITICAL AREA MAP This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. THIS MAP IS NOT TO BE USED FOR NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 06/05/2020 Legend 1023 0 512 1023 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov Parcels Erosion Hazard - High Floodway Special Flood Hazard Areas (100 year flood) Other Flood Areas (Zone X - 500 year flood) Channel Migration Zones (CMZ) as mapped by King County Moderate Hazard Area Potential Hazard Area Severe Hazard Area Landslide Very High High Moderate Unclassified Slope City of Renton >15% & <=25% >25% & <=40% (Sensitive) >40% & <=90% (Protected) >90% (Protected) Environment Designations Natural Shoreline High Intensity Shoreline Isolated High Intensity Shoreline Residential Urban Conservancy Jurisdictions Streams (Classified) <all other values> Type S Shoreline Type F Fish Type Np Non-Fish Type Ns Non-Fish Seasonal Unclassified Not Visited Wetlands Seismic Hazard Areas Faults Streets Parks Waterbodies Map PROJECT SITE DISCHARGE TO BLACK RIVER APPROXIMATELY 1.4 MILES DOWNSTREAM EX 3-7 Soil Map—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/5/2020 Page 1 of 35257500525770052579005258100525830052585005258700525890052575005257700525790052581005258300525850052587005258900557300557500557700557900558100558300558500558700558900559100559300559500559700559900 557300 557500 557700 557900 558100 558300 558500 558700 558900 559100 559300 559500 559700 559900 47° 28' 54'' N 122° 14' 27'' W47° 28' 54'' N122° 12' 15'' W47° 28' 5'' N 122° 14' 27'' W47° 28' 5'' N 122° 12' 15'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 350 700 1400 2100 Feet 0 100 200 400 600 Meters Map Scale: 1:7,400 if printed on B landscape (17" x 11") sheet. Soil Map may not be valid at this scale. EXHIBIT 3-8 SOIL SURVEY MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: King County Area, Washington Survey Area Data: Version 15, Sep 16, 2019 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Jun 29, 2019—Jul 25, 2019 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/5/2020 Page 2 of 3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI Py Puyallup fine sandy loam 0.4 0.8% Ur Urban land 30.6 58.4% Wo Woodinville silt loam 21.4 40.8% Totals for Area of Interest 52.5 100.0% Soil Map—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/5/2020 Page 3 of 3 Water Quality Atlas Map Sources: Esri, HERE, Garmin, Intermap, increment P Corp., GEBCO, USGS,FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, EsriJapan, METI, Esri China (Hong Kong), (c) OpenStreetMap contributors, and Ju ne 5, 202 0 0 0.25 0.50.125Miles K AssessedWaters/Sediment Water Category 5 - 303d Category 4C Category 4B Category 4A Category 2 Category 1 Sedim ent Category 5 - 303d Category 4C Category 4B Category 4A Category 2 Category 1 DRAINAGE PATH PROJECT SITE APPROXIMATELY 1.4 MILES DOWN STREAM EXHIBIT 3-9 WA DOE Clean Water Act Section 303(d) Map 4,800 400 City of Renton Wetland Inventory This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. None 6/9/2020 Legend 2720136 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 272 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Labels City and County Boundary Parcels Environment Designations Natural Shoreline High Intensity Shoreline Isolated High Intensity Shoreline Residential Urban Conservancy Jurisdictions Streams (Classified) <all other values> Type S Shoreline Type F Fish Type Np Non-Fish Type Ns Non-Fish Seasonal Unclassified Not Visited Wetlands Streets Points of Interest Parks Waterbodies 2019.sid Red: Band_1 Green: Band_2 Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 4 Flow Control and Water Quality Facility Analysis and Design Technical Information Report Watershed Affordable Housing Apartments 4-1 2200335.10 4.0 Flow Control and Water Quality Facility Analysis and Design 4.1 Flow Control 4.1.1 Existing Site Hydrology (Part A) Based on a geotechnical report dated June 10, 2020, in general, site soils beneath the topsoil, consist of very loose to medium dense alluvial and floodplain deposits consisting of non-plastic silt (ML). Silty fine sand (SM) and sand (SP, SP-SM) were encountered extending to the maximum exploration depth of 46.5 feet. Per the King County Soil Survey Map (Figure 1-4) site soils consist primarily of Urban Land (Ur) soils. Per the geotechnical report, seasonal high groundwater was observed at 6 to 9 feet below grade. The existing site was modelled as till grass and impervious area. MGSFlood was used to design the storm drainage facilities. The predeveloped site was modeled in the existing condition because the site is in the peak rate flow control area shown on the City of Renton GIS map. Refer to Figure 1-3A for the Predeveloped Site Area Map. The annual peak flows were determined using the information above and the surface conditions below. Table 1 - Existing Site Hydrology Description Target Area (Acres)Total Area (Acres) Till Grass 0.987 0.987 Impervious 0.299 0.299 Total Area 1.286 1.286 4.1.2 Developed Site Hydrology (Part B) The project proposes to construct storm detention facilities to the Peak Rate Flow Control standard. The developed inflow to the detention facility includes all developed target areas tributary to the facility. Additionally, bypass areas were developed for target areas that are not physically capable of being conveyed to the detention facility. Site soils are modelled as till soils. The project proposes to design a 36-inch pipe tank to address the Peak Rate Flow Control requirements. In addition, the site proposes grass pave in strips totaling 0.023 acre. The grass pave area was modeled as 100 percent grass per the CORSWDM, Table 1.2.9.A. This facility will release surface water runoff to the existing public stormwater conveyance system. The annual peak flows for the proposed site were determined using the basin information above and the surface conditions below. Refer to Figure 4-2 for the Proposed Area Map. Technical Information Report Watershed Affordable Housing Apartments 4-2 2200335.10 Table 2 - Developed Site Hydrology Description Target Detained Area (Acres) Target Bypass Area (Acres) Total Area (Acres) Till Forest ------ Till Grass (Grass Pave)0.023 0.001 0.024 Till Pasture 0.104 0.015 .119 Impervious 1.118 0.025 1.143 Total Area 1.245 0.041 1.286 4.1.3 Performance Standards (Part C) Area-Specific Flow Control Facility Standard This project is in a Peak Rate Flow Control Standard area, which requires matching developed discharge rates to the existing peak discharge rates for the 2-, 10-, and 100-year return periods. Refer to Section 4.1.4 for flow calculation results. Existing site conditions are assumed to be till grass and impervious area. Flow Control BMP Requirements Flow Control BMPs are required per CR 3. The project site totals 1.286 acres, including all parcels. The developed site includes 1.143 acres of impervious area, including new and replaced impervious areas and bypass areas. This results in 88 percent impervious in the proposed conditions. The project falls under the Large Lot BMP Requirements. Refer to Figure 4-2 for the Proposed Area Map, which includes the total imperious surface for the site. Below is a summary of the Flow Control BMPs (per CORSWDM Section 1.2.9.2.2) that the project will assess: 1.Full dispersion: This is infeasible because there is not enough native vegetated surface. 2.Target Impervious Surface Flow Control BMPs: Per CORSWDM, Table C.1.3.A - Onsite BMP Application Rates, because the project site is above 85 percent impervious surface coverage, the onsite BMP application is not required. o Full Infiltration: This BMP is infeasible because we cannot meet the minimum design requirements. Per Section C2.2.2 1a. of the CORSWDM, these soil conditions must be met: existing soil must be coarse sands or cobbles or medium sands. In general, the soils layers that could be used for infiltration consist of silty sand and inorganic silts. o Limited Infiltration: This BMP is infeasible because we cannot meet the minimum design requirements. Per Section C2.2.2 1a. of the CORSWDM, these soil conditions must be met: existing soil must be coarse sands or cobbles or medium sands. In general, the soils layers that could be used for infiltration consist of silty sand and inorganic silts. Technical Information Report Watershed Affordable Housing Apartments 4-3 2200335.10 o Basic Dispersion: This BMP is infeasible. The minimum distance of vegetated flow path (50 feet) cannot be met. In addition, this project is on very flat terrain and surrounded by impervious area. o Rain Garden: This BMP is infeasible. Per CORSWDM Section C.2 - Onsite BMPs Infeasibility Criteria, for a bioretention planter that accepts greater than 5,000 square feet of PGIS, a minimum of 3 feet of vertical separation is required below bioretention. Per the Geotechnical Report, seasonal high groundwater depth ranges from 6 to 9 feet below grade and may be higher during larger rain events. This does not yield enough vertical separation between the rain garden and the seasonal high ground water. o Permeable Pavement: The paved portion of the site cannot be permeable pavement because the surface is a parking lot and driving isle that will generate PGIS. In addition, the site soils have not been analyzed to meet the soil suitability qualities for providing treatment listed in the infeasibility criteria in Section C.2.7, Criterion 20. Further, the owner does not have the budget for the maintenance required to clean the pores from clogging. o Rainwater Harvesting: This BMP is infeasible; no irrigation systems are proposed. o Vegetated Roof: The project will have a green roof. o Reduced Impervious Surface Credit: This BMP is not achievable because none of the proposed techniques are feasible for the site’s intended use. o Native Growth Retention Credit: This BMP is not achievable because there is no presence of native growth in the existing conditions to retain. o Perforated Pipe Connection for Roof Drains: Perforated pipe connections were not provided for the roof drain connections due to the non-infiltrative soil type that characterizes the site. The project is not required to mitigate any area according to Table C1.3.A of the CORSWDM. Conveyance System Capacity Standards The onsite stormwater networks will be sized to adequately convey the fully developed, 25-year storm event, as stipulated by the CORSWDM, and to not overtop during the 100-year storm event. Area-Specific Water Quality Treatment Menu In accordance with the CORSWDM, onsite flows will be treated to the specifications provided by the Enhanced Basic Water Quality standards. The goal of this treatment menu is to reduce total suspended solids (TSS) by 80 percent and to reduce zinc concentration by 50 percent for a typical rainfall year. This goal will be accomplished by providing a proprietary system approved by Ecology, a BioPod system. Source Controls The proposed project consists of a new parking lot, portable classrooms, and a covered play area. The CORSWDM will be referenced for source control measures, in addition to erosion and sediment control measures, during construction. For construction source controls, refer to Technical Information Report Watershed Affordable Housing Apartments 4-4 2200335.10 Section 8.0 CSWPPP Analysis and Design. For post-construction source controls, refer to Section 10.0, Operations and Maintenance Plan. Oil Controls Not applicable. 4.1.4 Flow Control System (Part D) Flow Control BMPs A vegetated green roof will be used. Detention Sizing Detention and control structure requirements were designed using the MGSFlood program and hydrology summarized in Sections 4.1.1 and 4.1.2. For the results, refer to Figure 4-1, MGSFlood Report. The system requirements are summarized as follows: Table 3 – Detention Pipe Requirements Required Provided Detention Volume 3,141 cf 3,298 cf Live Storage Depth 2 ft 2 ft Outlet Control Structure Orifice #1 Diameter 2-11/16 in 2-11/16 in Orifice #2 Diameter 2-3/8 in 2-3/8 in Orifice #3 Diameter 2 in 2 in Orifice #1 Height 100.50 ft 100.50 ft Orifice #2 Height 101.70 ft 101.70 ft Orifice #3 Height 102.00 ft 102.00 ft Point of Compliance Review The developed peak flows for the 2-, 10-, and 100-year storms match that of the existing peak flows. Refer to Figure 4-1, MGSFlood Report for results. Discharge Requirements The project meets the natural discharge location requirement and is not subject to discharge requirements 1 through 3, per the following: 1. The project discharges to a manmade conveyance system. 2. The project is not located within a Landslide Hazard Drainage Area. 3. The project is not adjacent to or does not contain a landslide, steep slope, or erosion hazard area. Technical Information Report Watershed Affordable Housing Apartments 4-5 2200335.10 4.2 Water Quality System (Part E) The new PGIS for the proposed site includes 16,422 square feet (0.377 acre) of area that will be treated for water quality prior to detention. Roughly 4,914 square feet (0.113 acre) of target pollution generating surface cannot feasibly be conveyed to the water quality facility. In addition, about three-quarters of the 5,000-square foot area are located beneath a building. The area that cannot be treated is less than the 5,000 square feet of new plus replaced PGIS. Onsite runoff from PGIS will be treated to the Enhanced Basic Water Quality standards of the CORSWDM using three different BioPods, a proprietary system with Ecology approval to treat enhanced basic water quality. MGSFlood was used to determine the online flows, and a chart from the manufacturer was used to size the BioPod system. Refer to Figure 4-3 for the Water Quality Area Map and Figure 4-4 for the BioPod Sizing Chart. A summary of the results is tabulated below: Area from WQ Area Map Required Online Discharge Rate Provided Size from BioPod Sizing Chart Treatment Flow Capacity from Figure 4-5, BioPod Sizing Chart WQ #1 0.01 cfs 4’ x 6’0.074 cfs WQ #2 0.02 cfs 4’ x 6’0.074 cfs WQ #3 0.03 cfs 4’ x 6’0.074 cfs Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 4.0 Figures Figure 4-1.........MGSFlood Report Figure 4-2.........Proposed Area Map Figure 4-3.........Water Quality Area Map Figure 4-4.........BioPod Sizing Chart ————————————————————————————————— MGS FLOOD PROJECT REPORT Program Version: MGSFlood 4.54 Program License Number: 201710010 Project Simulation Performed on: 05/03/2021 2:18 PM Report Generation Date: 05/03/2021 2:18 PM ————————————————————————————————— Input File Name: Grady Way 3' Tank.fld Project Name: Grady Way Apartments Analysis Title: Preliminary Model Comments: ———————————————— PRECIPITATION INPUT ———————————————— Computational Time Step (Minutes): 15 Extended Precipitation Time Series Selected Climatic Region Number: 3 Full Period of Record Available used for Routing Precipitation Station : 95004005 Puget West 40 in_5min 10/01/1939-10/01/2097 Evaporation Station : 951040 Puget West 40 in MAP Evaporation Scale Factor : 0.750 HSPF Parameter Region Number: 1 HSPF Parameter Region Name : USGS Default ********** Default HSPF Parameters Used (Not Modified by User) *************** ********************** WATERSHED DEFINITION *********************** Predevelopment/Post Development Tributary Area Summary Predeveloped Post Developed Total Subbasin Area (acres) 1.286 1.286 Area of Links that Include Precip/Evap (acres) 0.000 0.000 Total (acres) 1.286 1.286 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Existing Predeveloped ---------- -------Area (Acres) -------- Till Grass 0.987 Impervious 0.299 ---------------------------------------------- Subbasin Total 1.286 Watershed Affordable Housing Apartments FIG 4-1 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 5 ---------- Subbasin : NEW IMP PGIS BYPASS AND PASTURE AND GRASS PAVE ---------- -------Area (Acres) -------- Till Pasture 0.104 Till Grass 0.023 Impervious 0.741 ---------------------------------------------- Subbasin Total 0.868 ---------- Subbasin : Bypass ---------- -------Area (Acres) -------- Till Pasture 0.015 Till Grass 0.001 Impervious 0.025 ---------------------------------------------- Subbasin Total 0.041 ---------- Subbasin : WQ 1 ---------- -------Area (Acres) -------- Impervious 0.053 ---------------------------------------------- Subbasin Total 0.053 ---------- Subbasin : WQ 2 ---------- -------Area (Acres) -------- Impervious 0.132 ---------------------------------------------- Subbasin Total 0.132 ---------- Subbasin : WQ 3 ---------- -------Area (Acres) -------- Impervious 0.192 ---------------------------------------------- Subbasin Total 0.192 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 1 ------------------------------------------ Link Name: POC 1 Link Type: Copy Downstream Link: None ************************* LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 5 ------------------------------------------ Link Name: TDA 1 Link Type: Copy Downstream Link: None ------------------------------------------ Link Name: Pipe Tank Link Type: Structure Downstream Link Name: TDA 1 User Specified Elevation Volume Table Used Elevation (ft) Pond Volume (cu-ft) 100.00 0. 100.10 31. 100.20 109. 100.30 185. 100.40 271. 100.50 399. 100.60 504. 100.70 616. 100.80 771. 100.90 894. 101.00 1019. 101.10 1191. 101.20 1322. 101.30 1455. 101.40 1635. 101.50 1770. 101.60 1905. 101.70 2084. 101.80 2217. 101.90 2349. 102.00 2521. 102.10 2646. 102.20 2768. 102.30 2924. 102.40 3035. 102.50 3141. 102.60 3269. 102.70 3355. 102.80 3430. 102.90 3508. 103.00 3540. Hydraulic Conductivity (in/hr) : 0.00 Massmann Regression Used to Estimate Hydralic Gradient Depth to Water Table (ft) : 100.00 Bio-Fouling Potential : Low Maintenance : Average or Better Riser Geometry Riser Structure Type : Circular Riser Diameter (in) : 18.00 Common Length (ft) : 0.000 Riser Crest Elevation : 102.50 ft Hydraulic Structure Geometry Number of Devices: 3 ---Device Number 1 --- Device Type : Circular Orifice Control Elevation (ft) : 100.50 Diameter (in) : 2.69 Orientation : Horizontal Elbow : Yes ---Device Number 2 --- Device Type : Circular Orifice Control Elevation (ft) : 101.70 Diameter (in) : 2.38 Orientation : Horizontal Elbow : Yes ---Device Number 3 --- Device Type : Circular Orifice Control Elevation (ft) : 102.00 Diameter (in) : 2.00 Orientation : Horizontal Elbow : Yes ------------------------------------------ Link Name: WQ 1 Link Type: Copy Downstream Link Name: Pipe Tank ------------------------------------------ Link Name: WQ 2 Link Type: Copy Downstream Link Name: Pipe Tank ------------------------------------------ Link Name: WQ 3 Link Type: Copy Downstream Link Name: Pipe Tank **********************FLOOD FREQUENCY AND DURATION STATISTICS******************* ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 Number of Links: 1 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 5 Number of Links: 5 ***********Groundwater Recharge Summary ************* Recharge is computed as input to Perlnd Groundwater Plus Infiltration in Structures Total Predeveloped Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Existing Predevelope 116.858 Link: POC 1 0.000 _____________________________________ Total: 116.858 Total Post Developed Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: NEW IMP PGIS BYPASS 20.588 Subbasin: Bypass 2.695 Subbasin: WQ 1 0.000 Subbasin: WQ 2 0.000 Subbasin: WQ 3 0.000 Link: TDA 1 0.000 Link: Pipe Tank Not Computed Link: WQ 1 Not Applicable Link: WQ 2 Not Applicable Link: WQ 3 Not Applicable _____________________________________ Total: 23.284 Total Predevelopment Recharge is Greater than Post Developed Average Recharge Per Year, (Number of Years= 158) Predeveloped: 0.740 ac-ft/year, Post Developed: 0.147 ac-ft/year ***********Water Quality Facility Data ************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 1 ********** Link: POC 1 ********** Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 347.16 Inflow Volume Including PPT-Evap (ac-ft): 347.16 Total Runoff Infiltrated (ac-ft): 0.00, 0.00% Total Runoff Filtered (ac-ft): 0.00, 0.00% Primary Outflow To Downstream System (ac-ft): 347.16 Secondary Outflow To Downstream System (ac-ft): 0.00 Volume Lost to ET (ac-ft): 0.00 Percent Treated (Infiltrated+Filtered+ET)/Total Volume: 0.00% ----------------------SCENARIO: POSTDEVELOPED Number of Links: 5 ********** Link: TDA 1 ********** Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 543.17 Inflow Volume Including PPT-Evap (ac-ft): 543.17 Total Runoff Infiltrated (ac-ft): 0.00, 0.00% Total Runoff Filtered (ac-ft): 0.00, 0.00% Primary Outflow To Downstream System (ac-ft): 543.17 Secondary Outflow To Downstream System (ac-ft): 0.00 Volume Lost to ET (ac-ft): 0.00 Percent Treated (Infiltrated+Filtered+ET)/Total Volume: 0.00% ********** Link: WQ 1 ********** 15-Minute Timestep, Water Quality Treatment Design Discharge On-line Design Discharge Rate (91% Exceedance): 0.01 cfs Off-line Design Discharge Rate (91% Exceedance): 0.00 cfs ********** Link: WQ 2 ********** 15-Minute Timestep, Water Quality Treatment Design Discharge On-line Design Discharge Rate (91% Exceedance): 0.02 cfs Off-line Design Discharge Rate (91% Exceedance): 0.01 cfs ********** Link: WQ 3 ********** 15-Minute Timestep, Water Quality Treatment Design Discharge On-line Design Discharge Rate (91% Exceedance): 0.03 cfs Off-line Design Discharge Rate (91% Exceedance): 0.02 cfs ***********Compliance Point Results ************* Scenario Predeveloped Compliance Link: POC 1 Scenario Postdeveloped Compliance Link: TDA 1 *** Point of Compliance Flow Frequency Data *** Recurrence Interval Computed Using Gringorten Plotting Position Predevelopment Runoff Postdevelopment Runoff Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs) ---------------------------------------------------------------------------------------------------------------------- 2-Year 0.208 2-Year 0.198 5-Year 0.305 5-Year 0.294 10-Year 0.381 10-Year 0.358 25-Year 0.495 25-Year 0.411 MATCHING PREDEVELOPED RUNOFF DISCHARGE FOR 2, 10, AND 100 YR EVENTS 50-Year 0.602 50-Year 0.443 100-Year 0.627 100-Year 0.553 200-Year 0.695 200-Year 0.602 500-Year 0.785 500-Year 0.666 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals MATCHING PREDEVELOPED RUNOFF DISCHARGE 100 YR EVENTS 1200 6th Avenue Suite 1620 Seattle, WA 98101 206.267.2425 TEL 206.267.2429 FAX DRAWN BY:DATE:JOB NO.: 12"CON. 14DL8"CON. 14DL 18"CON. 25DL 14"CON. 18DL 48"DEC. 40DL 48"DEC. 40DL EG: 30.14 UP UP 32 Watershed Affordable Housing Apartments Proposed Area Map JN 11/2020 2200335 FIG 4-2 GRAPHIC SCALE 0 40 80 1" = 40 FEET 20 N LEGEND Description Quantity Unit BYPASS GRASS (GRASS PAVE)0.001 ac BYPASS IMPERVIOUS 0.025 ac BYPASS TOTAL 0.041 ac GRASS (GRASS PAVE)0.023 ac NEW + REPLACED IMPERVIOUS 1.118 ac TOTAL PROJECT SITE AREA WITH DEDICATION 1.286 ac DISCHARGE LOCATION LIMITS OF DEVELOPMENT PROPOSED BUILDING 3-FT DIAMETER FLOW CONTROL TANK WQ TREATMENT WITH BIOPOD WQ TREATMENT WITH BIOPOD NEW IMP PGIS BYPASS AND PASTURE TOTAL NEW PLUS REPLACED IMP= 1.118 sc TOTAL SITE AREA= 1.286 ac TOTAL PASTURE= 1.286 ac (total proj sitre area) - 1.118 ac (New + Repl IMP) - 0.041 ac (bypass total area) - 0.023 ac (grass pave) =0.104 ac BYPASS BYPASS IMP =0.025 ac BYPASS PASTURE 0.041 ac- 0.025-0.001 ac (BYPASS GRASS) = 0.015 ac 1200 6th Avenue Suite 1620 Seattle, WA 98101 206.267.2425 TEL 206.267.2429 FAX DRAWN BY:DATE:JOB NO.: 12"CON. 14DL8"CON. 14DL 18"CON. 25DL 14"CON. 18DL 48"DEC. 40DL 48"DEC. 40DL EG: 30.14 UP UP 32 Watershed Affordable Housing Apartments Water Quality Area Map Name Date Job No. FIG 4-3 GRAPHIC SCALE 0 40 80 1" = 40 FEET 20 N LEGEND Description Quantity Unit PGIS- BYPASS 0.117 ac PGIS WQ 1 0.053 ac PGIS WQ 2 0.132 ac PGIS WQ 3 0.192 ac DISCHARGE LOCATION LIMITS OF DEVELOPMENT PROPOSED BUILDING 3-FT DIAMETER FLOW CONTROL TANK WQ 1 TREATMENT WITH BIOPOD WQ 2 TREATMENT WITH BIOPOD WQ 3 TREATMENT WITH BIOPOD DRAWN BY:DATE:JOB NO.: 1200 6th Avenue, Suite 1620 Seattle, WA 98101 206.267.2425 TEL 206.267.2429 FAX Watershed Affordable Housing Apartments Biopod Sizing Chart Jesse N 07/01/2020 2200335.10 FIG 4-4 Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 5 Conveyance System Analysis and Design Technical Information Report Watershed Affordable Housing Apartments 5-1 2200335.10 5.0 Conveyance System Analysis and Design Conveyance will be sized to convey the 25-year storm event and checked for adequate capacity for the 100-year event during final design. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 5.0 Figures Figures will be provided later in design. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 6 Special Reports and Studies Technical Information Report Watershed Affordable Housing Apartments 6-1 2200335.10 6.0 Special Reports and Studies “Updated Geotechnical Engineering Study” dated June 10, 2020, by Earth Solutions NW. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 6.0 Figures Figure 6-1.........Geotechnical Report Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 7 Other Permits Technical Information Report Watershed Affordable Housing Apartments 7-1 2200335.10 7.0 Other Permits A Construction Stormwater NPDES Permit is necessary for the proposed construction. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 8 CSWPPP Analysis and Design Technical Information Report Watershed Affordable Housing Apartments 8-1 2200335.10 8.0 CSWPPP Analysis and Design The proposed development will comply with guidelines set forth in the CORSWDM and in conformance with the required Construction Stormwater NPDES Permit. The plan will include erosion/sedimentation control features designed to prevent sediment-laden runoff from leaving the site or from adversely affecting critical water resources during construction. 8.1 ESC Plan Analysis and Design (Part A) The erosion potential of the site is influenced by four major factors: soil characteristics, vegetative cover, topography, and climate. Erosion/sedimentation control is achieved by a combination of structural measures, cover measures, and construction practices that are tailored to fit the specific site. The following measures will be used to control sedimentation/erosion processes: Clearing Limits: Areas of clearing will be delineated on a TESC plan. Areas will be delineated in the field with high visibility construction flags or temporary construction fencing. Cover Measures: Disturbed areas shall be permanently or temporarily covered, as required in Section D.4.2 of the CORSWDM. Temporary measures include plastic sheeting and mulch. Permanent measures include hydroseed. Perimeter Protection: Silt fencing and straw wattles will be used for perimeter protection. Traffic Area Stabilization: Existing paved surfaces will be used for the construction entrance to the maximum extent feasible. A construction entrance will be supplied in areas that are unpaved. Wheel washing and/or street sweeping will be provided as necessary to prevent tracking of sediment onto adjacent surfaces. Dust control measures will be implemented when exposed soils are dry to the point that wind transport is possible. Sediment Retention: Inlet protection will be provided on all new and existing catch basins downstream of construction activities. Surface Water Control: This will be designed at a later submittal. Wet Season Requirements: Construction activities will occur during the months of May through August and will thus not trigger wet season requirements. The Contractor shall ensure that the site is stabilized prior to a predicted storm event with plastic sheeting or mulch. Critical Areas Restrictions: No critical areas are located onsite. 8.1.1 ESC Maintenance All ESC measures shall be maintained and reviewed on a regular basis, as prescribed in the maintenance requirements of each BMP. 8.1.2 ESC Supervisor The Contractor shall designate an ESC supervisor who shall be responsible for maintenance and review of ESC and for compliance with all permit conditions relating to ESC. The ESC supervisor must be available for rapid response to ESC problems. The ESC supervisor is required to be a Technical Information Report Watershed Affordable Housing Apartments 8-2 2200335.10 Certified Erosion and Sedimentation Control Lead (CESCL) with demonstrated expertise in ESC to perform these reviews and to be responsible for ESC implementation. 8.1.3 Documentation The ESC supervisor shall review the site at least once a month during the dry season, weekly during the wet season, and within 24 hours of significant storms. Written records of these reviews shall be kept onsite, with copies submitted to the King County Department of Development and Environmental Services (DDES) within 48 hours. If DDES requires that a written record be maintained, a standard ESC Maintenance Report will be included closer to final design. A copy of all required maintenance reports shall be kept onsite throughout the duration of construction. 8.1.4 Review Timing During the wet season, weekly reviews shall be carried out every 6 to 8 calendar days. During the dry season, monthly reviews shall be carried out within 3 days of the calendar day for the last inspection (e.g., if an inspection occurred on June 6, then the next inspection must occur between July 3 and July 9). Reviews shall also take place within 24 hours of significant storms. In general, a significant storm is one with more than 0.5 inch of rain in 24 hours or less. Another indication that a storm is “significant” is if gullies form as a result of the runoff. 8.2 Stormwater Pollution Prevention and Spill (SWPPS) Plan Design (Part B) The below SWPPS Plan shall be completed by the Contractor for use during construction of the project. Blanks are intentional because they are part of the Contractor’s means and methods. The SWPPS Plan includes three elements: a site plan, a pollution prevention report, and a spill prevention and cleanup report. This report includes identifying the expected sources of potential pollution and spills that may occur during construction, and works to develop a plan to prevent pollution and spills. It also develops a plan to mitigate spills that may occur. The SWPPS Plan will be kept onsite at all times during construction. The general contractor will be responsible to ensure that subcontractors are aware of the SWPPS Plan and a form or record will be provided stating that all subcontractors have read and agree to the SWPPS Plan. An employee training worksheet will be provided for the Contractors use. A SWPPS Site Plan will be developed by the Contractor. The SWPPS Site Plan, Pollution Prevention Report, and Spill Prevention and Cleanup Report have been preliminarily developed and BMPs have been selected based on Section 2.3.1.4 of the CORSWDM and the KCSPPM. 8.2.1 Pollution and Spill Prevention Source Controls and BMPs The sources of pollution and spills have been identified below, and the BMPs to be used for each source for prevention of both pollution and spills have been listed below: Liquids that will be handled or stored onsite are the following: _________. Containers will be stored as shown on the SWPPS Site Plan and include the following types and sizes: ___________. Tight-fitting lids shall be placed on all containers containing liquids. Containers shall be covered with plastic sheeting during rain events. Drip pans or absorbent materials shall be placed beneath all mounted container taps and at all potential drip and spill locations during filling and unloading of containers. Containers shall be stored such that if a container leaks or spills, the contents will not be discharged, flow, or be washed into the storm drainage system, surface water, or groundwater. Appropriate spill cleanup materials shall be stored and maintained near the container storage area. Storage area shall be swept and cleaned as Technical Information Report Watershed Affordable Housing Apartments 8-3 2200335.10 needed. Area shall not be hosed down such that water drains to the storm drainage system or neighboring areas. Containers shall be checked daily for leaks and spills and replaced, as necessary. All spilled liquids will be collected and disposed of properly. Spill control devices shall be routinely inspected on a weekly basis. Dry pesticides and fertilizers shall be covered with plastic sheeting or stored in a sealed container. Materials shall be stored on pallets or another raised method to prevent contact with stormwater runoff. Alternatively, the materials shall be contained in a manner such that if the container leaks or spills, the contents will not discharge, flow, or be washed into the storm drainage system, surface waters, or groundwater. Maintenance requirements are the same as liquid materials described above. Chemicals that will be handled or stored onsite are the following: _________. Containers will be stored as shown on the SWPPS Site Plan and include the following types and sizes: ___________. BMPs and maintenance requirements are the same as liquids with the addition of the following ______. Soil, sand, and other erodible materials shall be stored onsite in a contractor-designated location. Materials shall be covered with plastic sheeting per standard detail. Perimeter controls shall be constructed as shown on the TESC plan to prevent eroded materials from leaving the project site. Fueling shall not occur onsite. If fueling does occur onsite, the Contractor shall develop a containment plan for spills and provide lighting and signage if fueling occurs at night in conformance with the KCSPPM. Maintenance and repair of vehicles shall not occur onsite. If maintenance or repair of vehicles does occur onsite, the Contractor shall develop a spill prevention plan in conformance with the KCSPPM. Truck wheel washing shall occur in a controlled manner, such that runoff is collected and disposed of in a legal manner. Rinsing of hand tools shall occur in a designated location and water for washing shall be collected and disposed of in a legal manner. Contaminated soils shall be covered with plastic sheeting or contained to prevent stormwater from carrying pollutants away to surface or ground waters. Appropriate spill cleanup materials, such as brooms, dustpans, vacuum sweepers, etc., shall be stored and maintained near the storage area. Storage area shall be swept and cleaned as needed. Area shall not be hosed down such that water drains to the storm drainage system, groundwater, surface water, or neighboring areas. During concrete and asphalt construction, the Contractor shall provide the following BMPs or equivalent measures, methods or practices as required: 1. Drip pans, ground cloths, heavy cardboard, or plywood wherever concrete, asphalt, and asphalt emulsion chunks and drips are likely to fall unintentionally, such as beneath extraction points from mixing equipment. 2. Storm drains shall be covered to prevent concrete and asphalt from entering the storm system. Technical Information Report Watershed Affordable Housing Apartments 8-4 2200335.10 3. Concrete, concrete slurry and rinse water shall be contained and collected and shall not be washed or allowed to discharge into storm drain, ditch, or neighboring parcels. All collected runoff shall be properly disposed of. 4. Contractor shall designate an area where application and mixing equipment cleaning will be conducted. Rinse water and slurry shall be collected, contained, and disposed of in a legal manner. 5. Routine maintenance: the pouring area shall be swept at the end of each day or more frequently if needed. Loose aggregate chunks and dust shall be collected. Areas shall not be hosed down. The Contractor may provide the following optional BMPs if the above do not provide adequate source controls: 1. Cover portable mixing equipment with an awning or plastic sheeting to prevent contact with rainfall. 2. Provide catch basin inserts configured for pollutant removal. pH elevated water shall not be discharged from the site. Contractor shall monitor stormwater for pH prior to discharging from the site. Contractor shall implement a pH treatment plan if pH is not within the natural range. 8.2.2 Responsible Personnel and Contact Information The Contractor shall designate a CESCL and post their contact name, company, and phone number and/or email in a readily visible location onsite. [name] with [company] shall be responsible for pollution and spill prevention and cleanup and can be contacted at [phone] or [email] . Contractor shall fill out a Pollution Prevention Team Worksheet that will be supplied later. 8.2.3 Pollution and Spill Prevention Worksheets Pollution prevention, BMP implementation reports, material inventory worksheets, pollutant source identification worksheet, and spill/leak report will be supplied closer to final design. 8.2.4 Disposal Methods Contractor shall dispose of contaminated soils and water in a legal manner. Options include the following: _______________. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 8.0 Figures Erosion Control will be designed at a later submittal. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 9 Bond Quantities, Facility Summaries, and Declaration of Covenant Technical Information Report Watershed Affordable Housing Apartments 9-1 2200335.10 9.0 Bond Quantities, Facility Summaries, and Declaration of Covenant This section will be completed later in design. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 9.0 Figures Figures will be provided later in design. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 10 Operations and Maintenance Plan Technical Information Report Watershed Affordable Housing Apartments 10-1 2200335.10 10.0 Operations and Maintenance Plan The drainage facilities detailed in this report will be privately owned and maintained. 10.1 Facility Descriptions Control Structure The control structure is a manhole on the downstream end (south end) of the pond. The control structure includes a flow restrictor attached to the outlet pipe that looks like a tee. The bottom of the tee has an orifice plate, and one downturned elbow also has orifice plates attached to the bottom. The top of the tee is open. The purpose of the flow restrictor is to control the release of flows to the downstream system, which prevents flooding and excessive erosion. The manhole also includes a jailhouse overflow, which is an opening with vertical bars set on the side and near the top of the structure. In order to function properly, the control structure should be kept free of excessive accumulated sediment, trash, and debris. Damage to the structure or the flow restrictor should be repaired or replaced. The orifice plates and flow restrictor tee should be free of obstructions. If the orifice plates are damaged or missing, it should be repaired or replaced. The inlet and outlet pipes should be free of sediment accumulation, trash, and debris. The jailhouse should be clear of blockages and the grates should be intact. Conveyance Systems Pipes transport stormwater runoff from developed portions of the property to the detention pond, and then to the downstream points of connection. To work properly, pipes must be kept free of silt and other debris. If pipes become blocked, surface flooding will occur. Debris Barrier Debris barriers are a metal trash rack located over the entrance to a pipe or culvert. A debris barrier may also be a conical structure constructed of metal bars and/or rods placed over a catch basin. Trash and debris should be removed so that the barrier is clear to receive stormwater flows. Damaged or missing bars should be repaired or replaced. Catch Basins and Area Drains Catch basins collect surface drainage and direct it into storm conveyance pipes. They help prevent downstream drainage problems by trapping sediment and other debris that would otherwise flow downstream with the runoff. It is important to keep catch basins clean so that accumulated silt is not flushed out during a significant storm. In addition, if the outflow pipe becomes blocked with debris, surface flooding will occur. All catch basins should be inspected at least once each year and after major storms. Area drains convey runoff directly into conveyance pipes. To prevent surface flooding, their surface grates must be kept free of litter and debris. If dirt or other sediment gets into the pipes and they become blocked, the pipes will need to be cleaned, either manually or using a Vactor truck. 10.2 Maintenance Tasks A stormwater Facility Maintenance Checklist will be supplied later. Technical Information Report Watershed Affordable Housing Apartments 10-2 2200335.10 10.3 Maintenance Requirements Maintenance requirements will be supplied later. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 10.0 Figures Figures will be provided later in design. Technical Information Report Watershed Affordable Housing Apartments 2200335.10 Section 11 Conclusion Technical Information Report Watershed Affordable Housing Apartments 11-1 2200335.10 11.0 Conclusion This site has been designed to meet the 2017 CORSWDM. The site incorporates detention and water quality facilities to treat stormwater draining from the site. Flow calculations/modeling used City of Renton standards for sizing stormwater conveyance networks and treatment facilities. It was determined using these criteria that: Detention facilities have been designed to meet the required Peak Rate Flow Control Standard. Water quality facilities have been designed to meet the required Enhanced Basic Water Quality Treatment Level for the site. Pipe networks will be designed to be of adequate size to effectively convey the 25-year storm event and to contain the 100-year storm event. Calculations will be supplied closer to final design. This analysis is based on data and records either supplied to or obtained by AHBL. These documents are referenced within the text of the analysis. The analysis has been prepared using procedures and practices within the standard accepted practices of the industry. We conclude that this project, as schematically represented, will not create any new problems within the downstream drainage system. This project will not noticeably aggravate any existing downstream problems due to either water quality or quantity. AHBL, Inc. Jesse Newman, EIT Project Engineer JN/lsk November 2020 Revised May 2021 Q:\2020\2200335\WORDPROC\Reports\20210503 Rpt (TIR) 2200335.10.docx