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20-102775-Preliminary TIR-07-15-2020-V1
Wall Short Plat Preliminary Technical Information Report June 17, 2020 Prepared for Jeff and Karen Wall 30201 18th Ave SW Federal Way, WA 98023 Submitted by ESM Consulting Engineers, LLC 33400 8th Avenue S, Suite 205 Federal Way, WA 98003 253.838.6113 tel 253.838.7104 fax www.esmcivil.com 06/17/2020 i TABLE OF CONTENTS 1. Project Overview 1 2. Conditions and Requirements Summary 7 3. Offsite Analysis 8 FIGURES 1.1 Vicinity Map 1.2 Existing Site Conditions 1.3 Proposed Site Conditions 1.4 Soils Map 3.1 Downstream Analysis Flow Path APPENDICES A. Hydrology Model Output \\esm8\engr\esm-jobs\1879\001\015\stormreport\documents\rprt-001.docx Wall Short Plat Preliminary TIR Page 2 June 17, 2020 1. PROJECT OVERVIEW The proposed Wall Short Plat project is located in Section 1, Township 23 South, Range 3 East, City of Federal Way, WA 98023, 30201 118th Ave SW, King County Parcel Number 0121039146 and is zoned RS15.0. The project proposes a 6-Lot single family lot + 3-Tract subdivision resulting in the construction of 5 new single-family residences with driveways, private road improvements, utility services, and landscaping. Refer to Figure 1.1 for a vicinity map. The existing site is 6.25 acres and currently developed with an existing single-family residence with access from 18th Ave SW at the southeast corner of the property. The property also contains existing gravel driveway through an easement from 20th Pl SW providing access to an existing residence on a separate property to the north. This existing easement will be improved to provide access to 3 of the proposed lots, lots 2, 3, and 4. Additional access is proposed from the end of the 19th Pl SW cul-de-sac from the south through a new access easement on parcel 0850500040. This will provide access to lots 1 and 5. The existing residence will be future Lot 6 and maintain its existing access, 18th Ave SW, a private roadway. Vegetation on the site is dense comprised primarily of mature trees and an established brush understory. Grade elevations descend from southeast to northwest; approximately 80 to 90 feet of elevation change occurs across the property (approximate gradients of 30 to 35 percent). A stream, as identified by Raedeke Associates, Inc. in the Critical Areas Assessment & Buffer mitigations plan, dated October 16, 2019, is present in the western site area, meandering in and out of the 20th Pl SW Right-of-Way (ROW). A 100-foot buffer has been established which lays the western portion of the project parcel. This report is part of the required drainage review as identified by FWRC 16.25.020 in accordance with the 2016 King County Surface Water Design Manual (SWDM) and the 2016 Federal Way addendum to the King County Surface Water Design Manual (Federal Way Addendum). The Federal Way Flow Control and Water Quality Applications Maps show that this project is in a conservation flow control area (Level 2 Flow Control) and Enhanced Basic Water Quality Area, respectively. In support of the preliminary drainage plan for the short plat, a Geotechnical study has been completed by Earth Solutions NW, LLC dated August 3, 2017. This report can be found with the short plat application submittal package. Wall Short Plat Preliminary TIR Page 3 June 17, 2020 FIGURE 1.1 – TIR WORKSHEET TIR Worksheet can be provided with the Final TIR if requested by the City. Wall Short Plat Preliminary TIR Page 4 June 17, 2020 SHEET OF DRAWN : DATE : DRAWING NAME : JOB NO.\\\\Esm8\\engr\\ESM-JOBS\\1879\\001\\015\\StormReport\\Drawings\\EN-02.dwgFile:Plotted By: Tony Ableman(425) 297-9900 (253) 838-6113 www.esmcivil.com Land Planning Landscape Architecture Land Surveying Project ManagementPublic Works Civil Engineering Κ Κ Β Εδχδθκ ςξ+ ς≅ 87//2 223// 7σγ ≅υδ Ρ+ Ρτησδ 1/4 Β Ν Μ Ρ Τ Κ Σ Η Μ Φ ∆ Μ Φ Η Μ ∆ ∆ Θ Ρ FEDERAL WAY EVERETT ΕΗΦΤΘ∆ 0−2 , ΥΗΒΗΜΗΣΞ Λ≅Ο 1879-001-015 EN-02 TMA 03-30-2018 1 1 NOT TO SCALE20TH PL SW21ST PL SW18TH AVE SW17TH AVE SW12TH LN SWSW 304TH ST 16TH AVE SWSW MARINE VIEW DR SW 300TH ST11TH AVE SWSW 300TH PL SW 301ST ST 12TH PL SWSW 307TH ST SW 306TH LN23RD AVE SWSW 300TH ST SW 301ST PL25TH AVE SW24TH AVE SWSW 305TH ST SW 306TH PL 26TH AVE S28TH AVE SWSW DASH POINT RD509 ΡΗΣ∆ PUGET SOUND Wall Short Plat Preliminary TIR Page 5 June 17, 2020 Wall Short Plat Preliminary TIR Page 6 June 17, 2020 Wall Short Plat Preliminary TIR Page 7 June 17, 2020 Hydrologic Soil Group—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/22/2018 Page 1 of 45242100524220052423005242400524250052426005242700524280052421005242200524230052424005242500524260052427005242800548200548300548400548500548600548700 548200 548300 548400 548500 548600 548700 47° 20' 15'' N 122° 21' 47'' W47° 20' 15'' N122° 21' 16'' W47° 19' 47'' N 122° 21' 47'' W47° 19' 47'' N 122° 21' 16'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 200 400 800 1200 Feet 0 50 100 200 300 Meters Map Scale: 1:4,180 if printed on A portrait (8.5" x 11") sheet. Soil Map may not be valid at this scale. Figure 1.4 - Soils Map Project Site Wall Short Plat Preliminary TIR Page 8 June 17, 2020 MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Rating Polygons A A/D B B/D C C/D D Not rated or not available Soil Rating Lines A A/D B B/D C C/D D Not rated or not available Soil Rating Points A A/D B B/D C C/D D Not rated or not available 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 13, Sep 7, 2017 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Aug 1, 2011—Jul 15, 2014 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. Hydrologic Soil Group—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/22/2018 Page 2 of 4 Wall Short Plat Preliminary TIR Page 9 June 17, 2020 Hydrologic Soil Group Map unit symbol Map unit name Rating Acres in AOI Percent of AOI AgB Alderwood gravelly sandy loam, 0 to 8 percent slopes B 37.1 77.6% AgD Alderwood gravelly sandy loam, 15 to 30 percent slopes B 7.4 15.4% Cb Coastal beaches 2.7 5.7% Totals for Area of Interest 47.9 100.0% Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Hydrologic Soil Group—King County Area, Washington Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/22/2018 Page 3 of 4 Wall Short Plat Preliminary TIR Page 10 June 17, 2020 2. CONDITIONS AND REQUIREMENTS SUMMARY The proposed project is subject to Stormwater Review as a result of being subject to a City of Federal way development permit and results in more than 2,000 sf of new impervious surface according to FWRC 16.15.010. This review includes KCSWDM’s Small Project Drainage Requirements, Core Requirements 1-9, and Special Requirements 1-5. Core Requirement #1 - Discharge at Natural Location This site is made up of one parcel within two Threshold Discharge Areas (TDA), each with its own natural discharge location. TDA #1: Most of the site naturally drains to the northwest corner of the property within the existing stream running along the west side of the site. Similarly, all runoff from the developed threshold discharge basin will be directed to this channel for discharge at the natural discharge location. TDA #2: A smaller area in the northeast corner of the site naturally sheet flow off the site in this corner. Most of this area tributary to the second natural discharge area will be outside of the short plat improvements (i.e. new impervious areas). These existing site conditions will largely be maintained in this area. Core Requirement #2 - Offsite Analysis A Level 1 downstream analysis has been provided in Section 3 of this report. Core Requirement #3 - Flow Control The site is located within a Conservation Flow Control Area requiring at minimum Level 2 flow control standards per Section 1.2.3.1.B of the KCSWDM and the Federal Way Addendum Flow Control Application map. The flow control standard will be necessary for TDA #1 will be met through the use of an above ground detention system within Tract B to release stormwater at approved rates. Threshold Discharge Area #2 is small enough to not increase the historic flows by 0.15 cfs using 15-minute time steps for the 100-year storm event. See Section 4 for more information. Core Requirement #4 - Conveyance System The conveyance system calculations will be provided with the final TIR. Core Requirement #5 - Erosion and Sediment Control A SWPPP will be provided with the final TIR. Core Requirement #6 - Maintenance & Operations The Operations and Maintenance manual will be provided with the final TIR. Core Requirement #7 - Financial Guarantees & Liability A completed bond quantity worksheet will be provided with the final TIR. Wall Short Plat Preliminary TIR Page 11 June 17, 2020 Core Requirement #8 - Water Quality Facilities The King County Water Quality Application Map shows that this project is in an Enhanced Basic Water Quality Area. This requirement is proposed to be met with a water quality treatment facility, specifically a Modular Wetland. Refer to Figure 1.3, Proposed Conditions, for an exact location of the device. Water Quality calculations will be provided with the final TIR. Core Requirement #9 - Flow Control BMPs The proposed project a subdivision project within the UGA. Implementation of flow control BMPs for associated plat infrastructure improvements/target surfaces (e.g. access tract) are to be evaluated per Section 1.2.9.3.1&2 and occur concurrently as part of the plat. This evaluation can be found below for the plat infrastructure. BMPs associated with the individual lot improvements will be deferred until a permit is obtained for construction on the lots, with the exception of Lot 1. Full dispersion is proposed to manage the rooftop and driveway for Lot 1. Requirement #1 • Full Dispersion Full Dispersion (Section C.2.1) has been evaluated for the project site. There is insufficient onsite native vegetated flow path to which target impervious surfaces may be dispersed over grades less than 15 percent. Therefore, minimum design requirement #1 (specified in Section C.2.1.1) cannot be met; hence, this BMP is infeasible for the project site. However, full dispersion is feasible for the future development of Lot 1. This BMP will be stipulated by recorded document to be installed. The credit per Table 1.2.9.A of the SWDM to reduce the size of the required flow control and water quality facilities has been applied as a result. (refer to Figure 1.3B Proposed Conditions for the dispersion device’s location). Requirement #2 • Full Infiltration Full infiltration (Section C.2.2 or Section 5.2, whichever is applicable) has been evaluated. A geotechnical study has been prepared by Earth Solutions NW, LLC, dated August 2, 2017, states, “It is our [ESNW] opinion infiltration is not feasible from a geotechnical standpoint... [on-site] soils should be considered impervious for practicable stormwater design purposes”. Therefore, full infiltration is infeasible. • Limited Infiltration The minimum requirements for limited infiltration are the same as those for full infiltration, except infiltration depressions are excluded and existing soils in the location of the infiltration device may be fine sands, loamy sands, sandy loams, or loams as opposed to only medium sands or better. However, as stated above, the soils onsite are considered impervious for practicable stormwater design purposes. Therefore, limited infiltration has been deemed infeasible. Wall Short Plat Preliminary TIR Page 12 June 17, 2020 • Bioretention The Geotechnical report does not recommend infiltration onsite as stated above, under Full Infiltration. According to the first infeasibility criteria, this citation disqualifies the project for bioretention and is therefore infeasible. • Permeable Pavement Again, the Geotechnical report does not recommend infiltration onsite as stated above. According to the first infeasibility criteria, this citation disqualifies the project for permeable pavement and is therefore infeasible. Requirement #3 • Basic Dispersion The third criteria of the second minimum design requirements for basic dispersion cannot be met: the slope of the flowpath segment must be no steeper than 15% for any 20-foot reach of the flowpath segment. The only lot that meets this requirement is Lot 1, which is using full dispersion, therefore basic dispersion is infeasible. Requirement #4 • New Pervious Surfaces The soil moisture holding capacity of new pervious surfaces will be protected in accordance with KCC 16.82.100 (F) and (G) where the former requires that the duff layer or native topsoil be retained to the maximum extent practicable and the latter requires a soil amendment to mitigate for lost moisture holding capacity where compaction or removal of some or all of the duff layer or underlying topsoil has occurred. Special Requirement #1 - Other Adopted Area-Specific Requirements This project is not in a designated Critical Drainage Area or in an area included in an adopted master drainage plan, basin plan, salmon conservation plan, stormwater compliance plan, flood hazard management plan, lake management plan, or shared facility drainage plan. Special Requirement #1 does not apply. Special Requirement #2 - Flood Hazard Area Delineation This project is not adjacent to a flood hazard area for a river, stream, lake, wetland, closed depression, marine shoreline, or a King County-mapped channel migration zone. Special Requirement #2 does apply. Special Requirement #3 - Flood Protection Facilities This project does not rely on/modify or construct an existing or proposed flood protection facility (such as a levee or revetment) for protection against hazards posed by erosion or inundation, respectively. Special Requirement #3 does not apply. Wall Short Plat Preliminary TIR Page 13 June 17, 2020 Special Requirement #3 - Flood Protection Facilities This project does not require a commercial building or commercial site development permit. Special Requirement #4 does not apply. Special Requirement #5 - Oil Control This project is not a redevelopment project and does not classify as a high-use site. The SWDM defines a high-use site as an area within a commercial or industrial site that typically generates or is subject to runoff containing high concentrations of oil due to high traffic turnover, on-site vehicle or heavy or station equipment use, or the frequent transfer of liquid petroleum or coal derivative products. Special Requirement #5 does not apply. Wall Short Plat Preliminary TIR Page 14 June 17, 2020 3. OFFSITE ANALYSIS Task 1 - Study Area Definition and Maps The offsite analysis includes a site map showing property lines, topography, study area and site boundaries, downstream flowpath for a distance of one mile, and potential/existing problems. Task 2 - Resource Review The following resources have been reviewed for existing/potential problems in the study area (upstream and one mile downstream of the project site) with a summary of findings have been provided below: • Adopted Basin Plans - None noted • Floodplain/Floodway FEMA Maps – None noted • Other Offsite Analysis Reports - None noted • Sensitive Areas Folio - None noted • DNRP Drainage Complaints and Studies – The site does not have any drainage complaints. King County had four closed cases downstream of the project. Refer to Figure 3.1 Downstream Analysis for the location of the City of Federal Way’s drainage complaints and the following page for a list of cases and reasons why these problems are not applicable to the project. • Road Drainage Problems – None noted • King County Soils Survey - Refer to Figure 1.4 for a soils map • Wetlands Inventory - None noted • Migrating River Studies - None noted • Section 303d List of Polluted Waters – The site’s stormwater runoff is tributary to the Puget Sound at the terminus of 20th PL SW. This outfall is only listed as a Category 1 waterbody for bacteria. • King County Designated Water Quality Problems - None noted • Stormwater Compliance Plans - None noted Figure 4: At the terminus of 20th PL SW, the outfall is only listed as a Category 1 waterbody for bacteria. Wall Short Plat Preliminary TIR Page 15 June 17, 2020 Task 3 - Field Inspection (Level 1 Inspection) A Level 1 Downstream Analysis was completed by ESM Consulting Engineers, LLC in the afternoon on March 19, 2018, when it was sunny and 55°F. TDA #1 Threshold discharge area #1 is the largest area of the 2 and also the location of the proposed detention pond. Stormwater runoff from a private driveway south (upstream) of the project drains onto the site (Point #1). A stream enters through the southwestern boundary along 20th PL SW (Point 2) and exits out of the parcel’s northwest corner, conveying the site’s runoff a quarter mile downstream to the Puget Sound. Heavy vegetation along the slopes of 20th PL SW, mitigates potential erosion problems (Figure 3). Point #1: Looking North, a private driveway’s stormwater runoff draining onto the site Point 2: The stream enters from the southwestern boundary along 20th PL SW Figure 3: There is heavy vegetation along the slopes of 20th PL SW Wall Short Plat Preliminary TIR Page 16 June 17, 2020 Further downstream, the stream continues onto another parcel (Point 4) and into a control structure (Point 5) before dispersing onto the beach and into the Puget Sound (Point 6). Point 4: Towards the outfall, the stream becomes wider Point 5: Before discharging to the Puget Sound, the stream enters a control structure Point 6: After the control structure, the stream disperses onto the beach and then flows into the Puget Sound Wall Short Plat Preliminary TIR Page 17 June 17, 2020 TDA #2 Threshold Discharge Area two is made up of a relatively smaller area in the north east corner of the project. See Figures 1.3A and 1.3B. This area has been classified as a second threshold discharge area as discharge enters an entirely separate downstream corridor all the way down to the Puget Sound. Water does not meet up within a quarter mile downstream of TDA #1. The land use characteristics of TDA #2 will remain unchanged and the downstream characteristics should not be impacted by the proposed short plat. The downstream corridor of TDA #2 is entirely within parcels 0121039046 and 0121039021, both between the project side and the Puget Sound, approximately 1,650 feet downstream. Both parcels are located on private property and therefore a field investigation has not been completed at this time. Based on available aerial photographs, the downstream flow path is through heavily vegetated with large trees and underbrush until water reach the lower area near the Puget Sound where the area has been cleared for landscaping and single-family residences. Along the flow path, the water may slightly channelize into shallow flow as well as open up to having more sheet flow characteristics before reaching the Puget sound. A map pf the downstream corridors for both TDA #1 and TDA #2 have been shown in Figure 3.1 at the end of this section. Task 4 - Drainage Description and Problem Descriptions There did not appear to be any existing or potential drainage and water quality problems identified during the Level 1 downstream analysis. The following is a brief narrative on the four potential downstream drainage problems that require special attention in the SWDM: severe erosion, severe flooding, conveyance system nuisance, and potential impacts to Wetland Hydrology problems. • Severe Erosion A section of the stream along 20th Pl SW is in an erosion hazard area; however, the heavy vegetation in and along the stream appears to mitigate erosion/incision undercutting adjacent slopes as none is evident. • Severe Flooding This potential problem is not expected to be an issue since the stream is in a valley that is 10 to 40 feet deeper than the road adjacent to it. Wall Short Plat Preliminary TIR Page 18 June 17, 2020 • Conveyance system Nuisance Conveyance system nuisance means a flooding or erosion problem that does not constitute a “severe flooding problem” or “severe erosion problem” and that results from the overflow of a constructed conveyance system for runoff events less than or equal to a 10-year event. There are several conveyance pipes connecting sections of the stream together, two of which are critical pipes, an 18” and 36”, and have been analyzed for sufficient capacity during the 10-year storm event on the following page. • Potential Impacts to Wetland Hydrology There are no wetlands identified on or off the site and therefore potential impacts to wetland hydrology does not apply to this project. Wall Short Plat Preliminary TIR Page 19 June 17, 2020 King County Figure 3.1 - Downstream Analysis ±The information included on this map has been compiled by King County staff from a variety of sources and is subject to change without notice. King Countymakes no representations or warranties, express or implied, as to accuracy, completeness, timeliness, or rights to the use of such information. This document isnot intended for use as a survey product. King County shall not be liable for any general, special, indirect, incidental, or consequential damages including,but not limited to, lost revenues or lost profits resulting from the use or misuse of the information contained on this map. Any sale of this map or information onthis map is prohibited except by written permission of King County. Legend Parcels index contours - 100 foot contours - 5 foot (below 1000feet) and 10 foot Potential landslide hazardareas (2016, see explanation--->) Potential landslide hazardareas 50-foot buffer (2016) Landslide hazards,incorporated KC (1990) Potential steep slope hazardareas (2016, see explanation--->) Erosion hazard (1990 SAO) Seismic hazard (1990 SAO) Coal mine hazard (1990 SAO) Stream (1990 SAO) class 1 class 2 perennial class 2 salmonid class 3 unclassified Wetland (1990 SAO) Sensitive area notice on title FEMA preliminary 100-yearfloodplain Streams Downstream Flowpath Drainage Complaint S-1013-0014 S-0217-0110 S-0117-0092 S-0118-0051 Wall Short Plat Preliminary TIR Page 20 June 17, 2020 TRESHOLD DISCHARGE AREA #1 TRESHOLD DISCHARGE AREA #2 NATURAL DISCHARGE LOCATION #1 NATURAL DISCHARGE LOCATION #2 DOWNSTREAM FLOW PATH #2 (APPROX 1,650 FEET)DOWNSTREAM FLOW PATH #2 (APPROX 1,650 FEET) PUGET SOUND PUGET SOUND 4. FLOW CONTROL & WATER QUALITY FACILITY ANALYSIS AND DESIGN 4.1 Existing Site Hydrology (Part A): The existing site is 6.25 acres, vegetation is dense comprised primarily of mature trees and an established brush understory. Grade elevations descend from southeast to northwest; approximately 80 to 90 feet of elevation change occurs across the property (approximate gradients of 30 to 35 percent). A stream, as identified by Raedeke Associates, Inc. on January 16, 2018 (the final report will be provided with a later submittal), is present in the western site area, meandering in and out of the 20th Pl SW Right-of-Way (ROW). A 100-foot buffer has been established which impacts the western portion of the project parcel. Upon examination of the existing topography, it is apparent the site is made up of two separate threshold discharge areas (TDAs). A majority of the undeveloped area and the existing single-family residence are tributary to the existing stream along 20th Pl SW making up TDA #1. The second TDA consists of the existing landscaping, future Tract C and a small portion of undeveloped area. The heavy forest and ground cover allows for most stormwater to be absorbed into the forest floor, evapotranspiration, or intercepted by the tree canopy. The existing single-family residence and associated landscaping will remain undisturbed and defined as non-target surfaces per page 1-17 of the SWDM as a result of being construction prior to January 8, 2001. 4.2 Developed Site Hydrology (Part B): Threshold Discharge Area #1 Most all development will take place with TDA #1. The proposed onsite stormwater conveyance system will collect and convey runoff from all of the developed areas of the project site to a above ground flow control facility within Tract B of the short plat. Rooftop and driveway runoff (with the exception of Lot 1 and 6) will be collected and tightlined to the pond. Surface flow from landscaping and forested areas will be intercepted in a roadside ditch along the new access tract and conveyed to the pond as well. Within the open areas of landscaping and forested areas, there is decreased opportunity for stormwater to turn into surface runoff. Having indirectly connected surfaces of landscaping and heavily wooded areas changes the characteristics of surface flow compared to a pervious area that is 100% landscaping. This is due to continued stormwater absorption into the forest floor, transpiration from the tree canopy, and decreased times of concentration. As a result, to provide a better representation of surface water characteristics within the open spaces, pervious areas Wall Short Plat Preliminary TIR Page 21 June 17, 2020 were modeled as a Pasture land cover. This is to represent the average between lawn and forested land cover. A full dispersion trench flow control BMP for lot 1 has been proposed to be conditioned with the recorded of the short plat. Dispersed from lot 1 will also be intercepted within the roadside ditch if necessary. The existing developed residence, a non-target surface will flow through the new detention facility and has been accounted for in the detention analysis. The flow control facility will discharge to water down to the base of the existing stream set-aside in a new tract (Tract A) of the short plat. Table 4.1 and Table 4.2 outlines the predeveloped and developed basin areas used in the hydrology model for TDA #1. TABLE 4.1 – Predeveloped TDA #1 Threshold Discharge Area #1 Total (sf) Impervious, Flat (sf) C, Lawn, Mod (sf) C, Pasture, Mod (sf) C, Forest Steep (sf C, Forest Mod (sf) Tract A (Non-target) 40,062 0 0 0 40,062 0 Tract B 32,298 0 0 0 0 34,982 Lot 1 37,744 0 0 0 0 37,744 Lots 2-5 90,783 0 0 0 0 90,783 Lot 6 (Non-target) 27,629 6,522 0 21,107 0 0 Access Easement 2,140 0 0 0 0 2,140 TOTAL (sf) 230,656 6,522 0 21,107 40,062 165,649 TOTAL (ac) 5.30 0.15 0 0.48 0.92 3.80 TABLE 4.2 – Developed TDA #1 Threshold Discharge Area #1 Total (sf) Impervious, Flat (sf) C, Lawn, Mod (sf) C, Pasture, Mod (sf) C, Forest Steep (sf) C, Forest Mod (sf) Tract A (Non-target+) 40,062 0 0 0 40,062 0 Tract B 32,298 12,829 19,469 0 0 0 Lot 1* 37,744 0 0 16,684 0 21,060 Lots 2-5** 90,783 16,000 0 74,783 0 0 Lot 6 (Non-target++) 27,629 6,522 0 21,107 0 0 Access Easement 2,140 2,140 0 0 0 0 TOTAL (sf) 230,656 37,491 19,469 112,574 40,062 21,060 TOTAL (ac) 5.30 0.86 0.45 2.58 0.92 0.48 *Includes 4,000 Full dispersion forest Credit for the 4,000 assumed impervious area per SWDM Table 1.2.9.A **Assumes 4,000 sf of impervious surface/lot per SWDM page 1-17, remaining pasture +vault bypass ++non-target vault flow-through Wall Short Plat Preliminary TIR Page 22 June 17, 2020 Threshold Discharge Area #2 The area within TDA #2 will remain undisturbed and continue to direct any generated surface water to the northeast. Being a separate threshold discharge area project threshold apply to this area as if it were a second project. No flow control or water quality facilities are proposed for this TDA. The TDA, approximately 1.00 acres is made up of non-target surfaces that will not be disturbed as a part of the short plat. A non-target surface is defined on page 1-17 of the SWDM as being a surface constructed prior to January 8, 2001. 4.3 Performance Standards (Part C): The following performance standards apply to TDA#1. Threshold Discharge Area #2 is made up of non-target impervious surfaces and not subject to any of the performance standards. Hydrology Model The approved hydrology model used for this project is the 2012 Western Washington Hydrology Model (WWHM) software, which incorporates all the methods required for determining compliance with the flow control and water quality standards specified below. Flow Control (Sec 1.2.3.2 SWDM) Per the King County Flow Control Applications Map, the project site is required to release stormwater at rates compliant with Conservation Flow Control (Level 2) standards. According to Section 3.2.2.2: DURATIONS AND PEAKS FOR FLOW CONTROL STANDARDS of the SWDM, those standards specify that the mitigated peak flow rates match the predeveloped 2-year and 10-year peak flow rates. Additionally, the standard specifies that the developed discharge durations must be strictly below the predeveloped discharge durations from 50% of the 2-year peak flow to the 2-year peak flow and continue approximating discharge durations to the full 50- year peak flow without a duration increase greater than 10%. The Log Pearson Type III extreme value analysis (specified in Bulletin 17B) was used to determine the peak flows due to the skew of the rainfall data and the 100-year peak flow being beyond the length of the available data record within the SeaTac rain gauge. Bypass (Sec 1.2.3.2.E) On some sites, topography can make it difficult or costly to collect all target surface runoff for conveyance to the onsite flow control facility. Section 1.2.3.2.E of the SWDM allows for bypass of the flow control facility. Compensatory mitigation by the flow control facility must be provided so that the net effect at the point of convergence downstream is the same with or without the bypass. Tract A will bypass the facility as it is downstream of the detention pond. This area will also remain forested and undisturbed as a part of the short plat. Wall Short Plat Preliminary TIR Page 23 June 17, 2020 Water Quality (Sec 1.2.8.1) Stormwater treatment is required for this project as a result of exceeding the 5,000 sf PGIS site area threshold. to provide a water quality benefit to stormwater coming in contact with pollution generating impervious surfaces (PGIS) prior to discharging from the project site. The current Federal Way Water Quality Application map provides the area-specific water quality standard for the project site. The project falls within the Enhanced Water Quality Area at a minimum. Enhanced treatment areas are subject to the Basic level of protection (TSS removal) and the additional measures needed to achieve a higher level of metals removed. This include 80% removal of total suspended solids (TSS), >30% dissolved copper, and >60% dissolved zinc removal for flows or volumes up to and including the Water Quality design flow or volume for a typical rainfall year. Following detention the design water quality flow rate is the 2-year release rate from the detention facility per Section 6.2.1 of the SWDM. Preceding detention the flow rate from the drainage basin at or below which 91% of the total runoff volume will be treated. These flows are determined using WWHM. A treatment system is proposed downstream of detention prior to release of stormwater to the stream in Tract A. Further design information during application of the construction drawings. Conveyance Per Section 1.2.4.1 of the SWDM, new conveyance systems shall be designed with sufficient capacity to convey and contain (at a minimum) the developed site conditions 25-year peak flow without creating or aggravating a severe flooding or erosion problem. See Section 5 of this report for more information. Discharge Per Section 1.2.1 of the SWDM, the project will discharge stormwater to the natural discharge location of the project site and in a manner that will not create a significant adverse impact to downhill properties or drainage facilities. Wall Short Plat Preliminary TIR Page 24 June 17, 2020 4.4 Flow Control System (Part D): The onsite flow control facility consists of a detention pond, 3-orifice riser control structure, and dispersion trench for detained outflow. The riser and detention volume have been sized to release detained stormwater at rates compliant with Level 2 control standards. No BMP credits are used in the sizing of the flow control facility with the exception of the dispersion trench proposed for Lot 1 allowing for the impervious surfaces on lot 1 to be modeled as forest. Table 4.3 shows required and provided storage volumes. The design of the detention facility is shown on sheet UT-04 of the submitted plan set. The hydrology model files used for this project site will be included on digital storage media with the submittal of the final report. The automatic report output from WWHM is included in Appendix A of this report. 4.5 Water Quality System (Part E): As discussed in Section 4.3, Enhanced Basic water quality treatment standards apply to this project. The Department of Ecology provides a list of treatment devices that have received a General Use Level Designation (GULD). Prior to discharge from the project site, stormwater from the project parcels will be treated by a GULD facility. The treatment facility/structure will most likely be placed following detention. Further detail will be provided during preparation of the construction permit application. Table 4.3 Stormwater Detention Volume Total (Cu Ft) Modeled 20,212 Provided 23,000 Volumetric increase above modeled +12% Wall Short Plat Preliminary TIR Page 25 June 17, 2020 5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN Onsite Conveyance System: Runoff from the developed project site will be collected from the developed lot lawns, roofs, drive aisles, and parking area by the proposed the conveyance system located throughout the project. The proposed stormwater drainage system is composed of catch basin structures with 6-inch and 12-inch diameter pipes. A conveyance and backwater analysis will be completed to verify the capacity of the critical pipes in the system. The proposed stormwater drainage system will be designed to convey the 25- year peak flow rate generated by the developed tributary basin as required in the SWDM. Conveyance calculations will be provided with the final TIR. Wall Short Plat Preliminary TIR Page 26 June 17, 2020 6. SPECIAL REPORTS AND STUDIES Geotechnical Study Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report, October 19, 2017; prepared by Associated Earth Sciences, Inc., This study has been included as a part of the project submittal package. Critical Area Study Critical Areas Assessment & Buffer Mitigation Plan; October 16, 2019; prepared by Raedeke Associates, Inc. This study has been included as a part of the project submittal package. Wall Short Plat Preliminary TIR Page 27 June 17, 2020 7. OTHER PERMITS Building permits will be required for this project, together with permits for utility connections. Wall Short Plat Preliminary TIR Page 28 June 17, 2020 8. ESC ANALYSIS AND DESIGN The Erosion and Sedimentation Control will be provided with the final TIR. Wall Short Plat Preliminary TIR Page 29 June 17, 2020 9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT The Bond Quantities worksheet will be provided with the final TIR. Wall Short Plat Preliminary TIR Page 30 June 17, 2020 10. OPERATIONS AND MAINTENANCE The Operations and Maintenance manual will be provided with the final TIR. Wall Short Plat Preliminary TIR Page 31 June 17, 2020 APPENDIX A – HYDROLOGY MODEL OUTPUT WWHM2012 PROJECT REPORT Detention 10/24/2018 4:15:39 PM Page 2 General Model Information Project Name:Detention Site Name: Site Address: City: Report Date:10/24/2018 Gage:Seatac Data Start:1948/10/01 Data End:2009/09/30 Timestep:15 Minute Precip Scale:1.000 Version Date:2018/07/12 Version:4.2.15 POC Thresholds Low Flow Threshold for POC1:50 Percent of the 2 Year High Flow Threshold for POC1:50 Year Detention 10/24/2018 4:15:39 PM Page 3 Landuse Basin Data Predeveloped Land Use Basin 1 Bypass:No GroundWater:No Pervious Land Use acre C, Forest, Mod 3.74 C, Forest, Steep 0.92 C, Pasture, Mod 0.48 Pervious Total 5.14 Impervious Land Use acre ROADS FLAT 0.15 Impervious Total 0.15 Basin Total 5.29 Element Flows To: Surface Interflow Groundwater Detention 10/24/2018 4:15:39 PM Page 4 Mitigated Land Use Basin 1 Bypass:No GroundWater:No Pervious Land Use acre C, Forest, Mod 0.48 C, Pasture, Mod 2.58 C, Lawn, Mod 0.45 C, Forest, Steep 0.92 Pervious Total 4.43 Impervious Land Use acre ROADS FLAT 0.86 Impervious Total 0.86 Basin Total 5.29 Element Flows To: Surface Interflow Groundwater Trapezoidal Pond 1 Trapezoidal Pond 1 Detention 10/24/2018 4:15:39 PM Page 5 Routing Elements Predeveloped Routing Detention 10/24/2018 4:15:39 PM Page 6 Mitigated Routing Trapezoidal Pond 1 Bottom Length:63.02 ft. Bottom Width:31.51 ft. Depth:7 ft. Volume at riser head:0.4636 acre-feet. Side slope 1:2 To 1 Side slope 2:2 To 1 Side slope 3:2 To 1 Side slope 4:2 To 1 Discharge Structure Riser Height:6 ft. Riser Diameter:18 in. Orifice 1 Diameter:1.41 in.Elevation:0 ft. Orifice 2 Diameter:2.34 in.Elevation:4.302 ft. Orifice 3 Diameter:1.49 in.Elevation:5.25875000000003 ft. Element Flows To: Outlet 1 Outlet 2 Pond Hydraulic Table Stage(feet)Area(ac.)Volume(ac-ft.)Discharge(cfs)Infilt(cfs) 0.0000 0.045 0.000 0.000 0.000 0.0778 0.046 0.003 0.015 0.000 0.1556 0.046 0.007 0.021 0.000 0.2333 0.047 0.010 0.026 0.000 0.3111 0.048 0.014 0.030 0.000 0.3889 0.049 0.018 0.033 0.000 0.4667 0.049 0.022 0.036 0.000 0.5444 0.050 0.026 0.039 0.000 0.6222 0.051 0.030 0.042 0.000 0.7000 0.051 0.034 0.045 0.000 0.7778 0.052 0.038 0.047 0.000 0.8556 0.053 0.042 0.049 0.000 0.9333 0.054 0.046 0.052 0.000 1.0111 0.054 0.050 0.054 0.000 1.0889 0.055 0.054 0.056 0.000 1.1667 0.056 0.059 0.058 0.000 1.2444 0.057 0.063 0.060 0.000 1.3222 0.057 0.068 0.062 0.000 1.4000 0.058 0.072 0.063 0.000 1.4778 0.059 0.077 0.065 0.000 1.5556 0.060 0.081 0.067 0.000 1.6333 0.060 0.086 0.069 0.000 1.7111 0.061 0.091 0.070 0.000 1.7889 0.062 0.096 0.072 0.000 1.8667 0.063 0.101 0.073 0.000 1.9444 0.063 0.105 0.075 0.000 2.0222 0.064 0.110 0.076 0.000 2.1000 0.065 0.116 0.078 0.000 2.1778 0.066 0.121 0.079 0.000 2.2556 0.067 0.126 0.081 0.000 2.3333 0.067 0.131 0.082 0.000 2.4111 0.068 0.136 0.083 0.000 2.4889 0.069 0.142 0.085 0.000 Detention 10/24/2018 4:15:39 PM Page 7 2.5667 0.070 0.147 0.086 0.000 2.6444 0.071 0.153 0.087 0.000 2.7222 0.071 0.158 0.089 0.000 2.8000 0.072 0.164 0.090 0.000 2.8778 0.073 0.170 0.091 0.000 2.9556 0.074 0.175 0.092 0.000 3.0333 0.075 0.181 0.094 0.000 3.1111 0.076 0.187 0.095 0.000 3.1889 0.077 0.193 0.096 0.000 3.2667 0.077 0.199 0.097 0.000 3.3444 0.078 0.205 0.098 0.000 3.4222 0.079 0.211 0.099 0.000 3.5000 0.080 0.218 0.100 0.000 3.5778 0.081 0.224 0.102 0.000 3.6556 0.082 0.230 0.103 0.000 3.7333 0.083 0.237 0.104 0.000 3.8111 0.084 0.243 0.105 0.000 3.8889 0.084 0.250 0.106 0.000 3.9667 0.085 0.256 0.107 0.000 4.0444 0.086 0.263 0.108 0.000 4.1222 0.087 0.270 0.109 0.000 4.2000 0.088 0.277 0.110 0.000 4.2778 0.089 0.284 0.111 0.000 4.3556 0.090 0.291 0.147 0.000 4.4333 0.091 0.298 0.167 0.000 4.5111 0.092 0.305 0.182 0.000 4.5889 0.093 0.312 0.195 0.000 4.6667 0.094 0.319 0.206 0.000 4.7444 0.095 0.327 0.216 0.000 4.8222 0.096 0.334 0.225 0.000 4.9000 0.096 0.342 0.234 0.000 4.9778 0.097 0.349 0.242 0.000 5.0556 0.098 0.357 0.250 0.000 5.1333 0.099 0.364 0.257 0.000 5.2111 0.100 0.372 0.264 0.000 5.2889 0.101 0.380 0.282 0.000 5.3667 0.102 0.388 0.298 0.000 5.4444 0.103 0.396 0.310 0.000 5.5222 0.104 0.404 0.321 0.000 5.6000 0.105 0.412 0.332 0.000 5.6778 0.106 0.421 0.341 0.000 5.7556 0.107 0.429 0.351 0.000 5.8333 0.108 0.437 0.359 0.000 5.9111 0.109 0.446 0.368 0.000 5.9889 0.110 0.455 0.376 0.000 6.0667 0.111 0.463 0.658 0.000 6.1444 0.112 0.472 1.261 0.000 6.2222 0.113 0.481 2.036 0.000 6.3000 0.114 0.490 2.908 0.000 6.3778 0.115 0.499 3.800 0.000 6.4556 0.116 0.508 4.637 0.000 6.5333 0.118 0.517 5.352 0.000 6.6111 0.119 0.526 5.902 0.000 6.6889 0.120 0.535 6.289 0.000 6.7667 0.121 0.545 6.652 0.000 6.8444 0.122 0.554 6.966 0.000 6.9222 0.123 0.564 7.265 0.000 7.0000 0.124 0.573 7.553 0.000 Detention 10/24/2018 4:15:39 PM Page 8 7.0778 0.125 0.583 7.829 0.000 Detention 10/24/2018 4:15:39 PM Page 9 Analysis Results POC 1 + Predeveloped x Mitigated Predeveloped Landuse Totals for POC #1 Total Pervious Area:5.14 Total Impervious Area:0.15 Mitigated Landuse Totals for POC #1 Total Pervious Area:4.43 Total Impervious Area:0.86 Flow Frequency Method:Log Pearson Type III 17B Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.203047 5 year 0.321526 10 year 0.408847 25 year 0.528242 50 year 0.623324 100 year 0.723386 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.120296 5 year 0.190675 10 year 0.247547 25 year 0.332181 50 year 0.405216 100 year 0.487426 Annual Peaks Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.278 0.095 1950 0.290 0.177 1951 0.387 0.357 1952 0.148 0.081 1953 0.115 0.079 1954 0.154 0.106 1955 0.258 0.156 1956 0.223 0.190 1957 0.217 0.096 1958 0.184 0.105 Detention 10/24/2018 4:16:15 PM Page 10 1959 0.153 0.096 1960 0.303 0.263 1961 0.159 0.091 1962 0.108 0.076 1963 0.150 0.101 1964 0.201 0.100 1965 0.159 0.102 1966 0.135 0.087 1967 0.326 0.140 1968 0.194 0.091 1969 0.183 0.093 1970 0.163 0.094 1971 0.186 0.104 1972 0.302 0.250 1973 0.147 0.093 1974 0.185 0.105 1975 0.257 0.108 1976 0.185 0.107 1977 0.063 0.054 1978 0.154 0.091 1979 0.084 0.071 1980 0.418 0.208 1981 0.137 0.087 1982 0.314 0.204 1983 0.209 0.109 1984 0.140 0.079 1985 0.080 0.072 1986 0.320 0.231 1987 0.303 0.237 1988 0.123 0.087 1989 0.086 0.063 1990 0.737 0.286 1991 0.412 0.321 1992 0.175 0.099 1993 0.148 0.094 1994 0.067 0.054 1995 0.199 0.155 1996 0.472 0.345 1997 0.360 0.306 1998 0.138 0.077 1999 0.441 0.210 2000 0.160 0.084 2001 0.065 0.054 2002 0.197 0.140 2003 0.287 0.082 2004 0.312 0.263 2005 0.225 0.119 2006 0.232 0.108 2007 0.578 0.260 2008 0.637 0.368 2009 0.320 0.209 Ranked Annual Peaks Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.7366 0.3684 2 0.6365 0.3569 3 0.5783 0.3451 Detention 10/24/2018 4:16:15 PM Page 11 4 0.4721 0.3212 5 0.4411 0.3056 6 0.4178 0.2861 7 0.4120 0.2631 8 0.3867 0.2630 9 0.3604 0.2598 10 0.3256 0.2504 11 0.3198 0.2375 12 0.3196 0.2306 13 0.3140 0.2096 14 0.3122 0.2087 15 0.3031 0.2078 16 0.3025 0.2043 17 0.3019 0.1900 18 0.2903 0.1771 19 0.2871 0.1563 20 0.2777 0.1553 21 0.2584 0.1405 22 0.2565 0.1404 23 0.2317 0.1191 24 0.2252 0.1094 25 0.2230 0.1081 26 0.2174 0.1077 27 0.2095 0.1072 28 0.2009 0.1060 29 0.1993 0.1053 30 0.1971 0.1045 31 0.1941 0.1044 32 0.1860 0.1017 33 0.1852 0.1014 34 0.1847 0.1002 35 0.1842 0.0987 36 0.1827 0.0965 37 0.1746 0.0964 38 0.1629 0.0947 39 0.1600 0.0939 40 0.1594 0.0935 41 0.1592 0.0934 42 0.1543 0.0926 43 0.1542 0.0914 44 0.1530 0.0914 45 0.1505 0.0910 46 0.1479 0.0875 47 0.1478 0.0868 48 0.1465 0.0866 49 0.1402 0.0842 50 0.1381 0.0822 51 0.1374 0.0813 52 0.1354 0.0791 53 0.1234 0.0786 54 0.1149 0.0773 55 0.1075 0.0760 56 0.0863 0.0720 57 0.0837 0.0707 58 0.0804 0.0628 59 0.0669 0.0543 60 0.0653 0.0536 61 0.0630 0.0536 Detention 10/24/2018 4:16:15 PM Page 12 Detention 10/24/2018 4:16:15 PM Page 13 Duration Flows The Facility PASSED Flow(cfs)Predev Mit Percentage Pass/Fail 0.1015 11890 10397 87 Pass 0.1068 10617 7542 71 Pass 0.1121 9330 4090 43 Pass 0.1173 8342 3848 46 Pass 0.1226 7501 3660 48 Pass 0.1279 6767 3525 52 Pass 0.1331 6006 3360 55 Pass 0.1384 5482 3247 59 Pass 0.1437 4960 3142 63 Pass 0.1490 4500 3039 67 Pass 0.1542 4025 2866 71 Pass 0.1595 3696 2733 73 Pass 0.1648 3401 2633 77 Pass 0.1700 3056 2490 81 Pass 0.1753 2813 2346 83 Pass 0.1806 2590 2233 86 Pass 0.1859 2387 2108 88 Pass 0.1911 2131 1922 90 Pass 0.1964 1933 1811 93 Pass 0.2017 1756 1674 95 Pass 0.2069 1599 1562 97 Pass 0.2122 1420 1415 99 Pass 0.2175 1275 1329 104 Pass 0.2227 1170 1246 106 Pass 0.2280 1050 1151 109 Pass 0.2333 975 1043 106 Pass 0.2386 908 917 100 Pass 0.2438 853 824 96 Pass 0.2491 791 695 87 Pass 0.2544 743 592 79 Pass 0.2596 678 502 74 Pass 0.2649 628 415 66 Pass 0.2702 568 390 68 Pass 0.2755 512 370 72 Pass 0.2807 462 354 76 Pass 0.2860 408 330 80 Pass 0.2913 359 290 80 Pass 0.2965 324 266 82 Pass 0.3018 288 238 82 Pass 0.3071 248 207 83 Pass 0.3124 219 185 84 Pass 0.3176 191 168 87 Pass 0.3229 168 143 85 Pass 0.3282 143 130 90 Pass 0.3334 128 116 90 Pass 0.3387 118 101 85 Pass 0.3440 109 76 69 Pass 0.3492 92 55 59 Pass 0.3545 78 40 51 Pass 0.3598 66 24 36 Pass 0.3651 52 13 25 Pass 0.3703 48 0 0 Pass 0.3756 43 0 0 Pass Detention 10/24/2018 4:16:15 PM Page 14 0.3809 41 0 0 Pass 0.3861 37 0 0 Pass 0.3914 33 0 0 Pass 0.3967 29 0 0 Pass 0.4020 24 0 0 Pass 0.4072 22 0 0 Pass 0.4125 21 0 0 Pass 0.4178 18 0 0 Pass 0.4230 15 0 0 Pass 0.4283 12 0 0 Pass 0.4336 10 0 0 Pass 0.4388 9 0 0 Pass 0.4441 8 0 0 Pass 0.4494 8 0 0 Pass 0.4547 7 0 0 Pass 0.4599 7 0 0 Pass 0.4652 7 0 0 Pass 0.4705 7 0 0 Pass 0.4757 6 0 0 Pass 0.4810 6 0 0 Pass 0.4863 6 0 0 Pass 0.4916 6 0 0 Pass 0.4968 6 0 0 Pass 0.5021 6 0 0 Pass 0.5074 5 0 0 Pass 0.5126 5 0 0 Pass 0.5179 5 0 0 Pass 0.5232 5 0 0 Pass 0.5285 5 0 0 Pass 0.5337 5 0 0 Pass 0.5390 5 0 0 Pass 0.5443 4 0 0 Pass 0.5495 4 0 0 Pass 0.5548 4 0 0 Pass 0.5601 4 0 0 Pass 0.5653 4 0 0 Pass 0.5706 4 0 0 Pass 0.5759 4 0 0 Pass 0.5812 3 0 0 Pass 0.5864 3 0 0 Pass 0.5917 3 0 0 Pass 0.5970 3 0 0 Pass 0.6022 3 0 0 Pass 0.6075 3 0 0 Pass 0.6128 3 0 0 Pass 0.6181 3 0 0 Pass 0.6233 2 0 0 Pass Detention 10/24/2018 4:16:15 PM Page 15 Water Quality Water Quality BMP Flow and Volume for POC #1 On-line facility volume:0 acre-feet On-line facility target flow:0 cfs. Adjusted for 15 min:0 cfs. Off-line facility target flow:0 cfs. Adjusted for 15 min:0 cfs. Detention 10/24/2018 4:16:15 PM Page 16 LID Report Detention 10/24/2018 4:16:41 PM Page 17 Model Default Modifications Total of 0 changes have been made. PERLND Changes No PERLND changes have been made. IMPLND Changes No IMPLND changes have been made. Detention 10/24/2018 4:16:41 PM Page 18 Appendix Predeveloped Schematic Detention 10/24/2018 4:16:43 PM Page 19 Mitigated Schematic Detention 10/24/2018 4:16:44 PM Page 20 Predeveloped UCI File RUN GLOBAL WWHM4 model simulation START 1948 10 01 END 2009 09 30 RUN INTERP OUTPUT LEVEL 3 0 RESUME 0 RUN 1 UNIT SYSTEM 1 END GLOBAL FILES <File> <Un#> <-----------File Name------------------------------>*** <-ID-> *** WDM 26 Detention.wdm MESSU 25 PreDetention.MES 27 PreDetention.L61 28 PreDetention.L62 30 POCDetention1.dat END FILES OPN SEQUENCE INGRP INDELT 00:15 PERLND 11 PERLND 12 PERLND 14 IMPLND 1 COPY 501 DISPLY 1 END INGRP END OPN SEQUENCE DISPLY DISPLY-INFO1 # - #<----------Title----------->***TRAN PIVL DIG1 FIL1 PYR DIG2 FIL2 YRND 1 Basin 1 MAX 1 2 30 9 END DISPLY-INFO1 END DISPLY COPY TIMESERIES # - # NPT NMN *** 1 1 1 501 1 1 END TIMESERIES END COPY GENER OPCODE # # OPCD *** END OPCODE PARM # # K *** END PARM END GENER PERLND GEN-INFO <PLS ><-------Name------->NBLKS Unit-systems Printer *** # - # User t-series Engl Metr *** in out *** 11 C, Forest, Mod 1 1 1 1 27 0 12 C, Forest, Steep 1 1 1 1 27 0 14 C, Pasture, Mod 1 1 1 1 27 0 END GEN-INFO *** Section PWATER*** ACTIVITY <PLS > ************* Active Sections ***************************** # - # ATMP SNOW PWAT SED PST PWG PQAL MSTL PEST NITR PHOS TRAC *** 11 0 0 1 0 0 0 0 0 0 0 0 0 12 0 0 1 0 0 0 0 0 0 0 0 0 14 0 0 1 0 0 0 0 0 0 0 0 0 END ACTIVITY Detention 10/24/2018 4:16:44 PM Page 30 Predeveloped HSPF Message File Detention 10/24/2018 4:16:44 PM Page 31 Mitigated HSPF Message File Detention 10/24/2018 4:16:44 PM Page 32 Disclaimer Legal Notice This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc. be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc. or their authorized representatives have been advised of the possibility of such damages. Software Copyright © by : Clear Creek Solutions, Inc. 2005-2018; All Rights Reserved. Clear Creek Solutions, Inc. 6200 Capitol Blvd. Ste F Olympia, WA. 98501 Toll Free 1(866)943-0304 Local (360)943-0304 www.clearcreeksolutions.com