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HomeMy WebLinkAbout21-100017_Stormwater Prelim TIR_12-1-2020_V1PRELIMINARY TECHNICAL INFORMATION REPORT SURFACE WATER MANAGEMENT 12-1-2020 HENRY-MOORE REASONABLE USE Prepared for: City of Federal Way Community Development Project Folder#: 20-104505 PRELIMINARY FOR AGENCY REVIEW ONLY Surface Water Preliminary TIR Page 1 CONTENTS 1. PROJECT OVERVIEW .................................................................................................... 2 2. CONDITIONS AND REQUIREMENTS SUMMARY ........................................................ 14 3. OFFSITE ANALYSYST – LEVEL 1 ................................................................................. 21 4. FLOW CONTROL, LID, WQ FACILITY ANALYSIS, AND DESIGN .................................. 21 5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ....................................................... 25 6. SPECIAL REPORTS AND STUDY .................................................................................. 27 7. OTHER PERMITS ......................................................................................................... 28 8. CSWPP ANALYSIS AND DESIGN ................................................................................. 29 9. BOND QUANITITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANTS 30 10. OPERATION AND MAINTENANCE MANUAL .......................................................... 31 APPENDIX A - MAPS 1 - USGS TOPO MAP 2 - FEDERAL WAY CRITICAL AREAS MAP PDA_646 3 - USDA SOILS MAP AND INFORMATION 4 - KING COUNTY I-MAP OF DRAINAGE COMPLAINTS 5 - FEMA FIRMETTE - FLOOD HAZARD MAP 6 - FEMA FLOODPLAIN MAP 7 - FEDERAL WAY DRAINAGE BASIN MAP 8 - FEDERAL WAY FLOW CONTROL APPLICATION MAP 9- FEDERAL WAY GEOLOGICAL HAZARDS MAP 10- FEDERAL WAY SURFACE WATER RESOURCES MAP 11 - FEDERAL WAY WATER QUALITY APPLICATIONS MAP APPENDIX B - CRITICAL AREAS DELINEATION AND MITIGATION PLAN APPENDIX C - SEPA CHECK LIST APPENDIX D - PLANS Surface Water Preliminary TIR Page 2 1. PROJECT OVERVIEW G E N E R A L P RO J E C T I N F O R M AT I ON The project site encompasses 1.76 acres and is divided into three existing lots within RS 15.0 zoning, surrounded by an older residential neighborhood. All three lots are encumbered by a stream, stream buffers, wetlands, and wetland buffers. There is no portion of the project site outside of the stream buffers or wetland buffers. The application seeks reasonable use for construction in the buffers of a single-family residence on each lot. Parcel A – 142103-9043, 0.83 AC (36,744 SF) Parcel B – 142103-9087, 0.5 AC (22,082 SF) Parcel C – 142103-9069, 0.41 AC (18,000 SF) Bordering the site to the west, south, and east are existing single-family houses. A large Category II wetland complex lies on portions of the site to the north and in a private native growth protection tract adjacent to the north boundary. The site slopes gently from south to north with Joes Creek flowing through the middle and capturing stormwater from surrounding higher elevations. Joes Creek funnels runoff from the surrounding neighborhood through the site to the wetland where the creek disperses. The site improvements proposed are to construct three single family houses, each with less than a 1600 SF footprint, on each lot. The existing gravel driveway will be replaced with a paved driveway to each lot for the minimum required width within private road easements. Public water and sewer utilities exist in Hoyt Road to be extended to the property. Figure 1.1: TIR WORKSHEET (following 5 pages) Surface Water Preliminary TIR Page 3 Surface Water Preliminary TIR Page 4 Surface Water Preliminary TIR Page 5 Surface Water Preliminary TIR Page 6 Surface Water Preliminary TIR Page 7 Surface Water Preliminary TIR Page 8 P RO J E C T S I T E The site pictured below encompasses over one and three quarters acres in the Lower Puget Sound Basin and is located at 33305 SW 43rd Avenue along southwest 333rd Place, west of southwest 43rd Avenue and Hoyt Road, in the City of Federal Way. The site is comprised of three residential lots and there are wetlands on the northern portion of the site and a creek through the middle dividing the drainage area. All three lots are located within the 165’ wetland buffers and stream buffers. Figure 1.2A: Site Map The site topography is generally flat with slopes less than 5% and gently sloping towards Joes Creek from both the east and west sides. Joes Creek is listed as “unclassified” and is a narrow drainage that flows from south to north through the middle of the site all year around. The creek collects all the stormwater from the site and surrounding higher elevation properties to the west, south, and east, and flows to a single discharge point along the north boundary. Downstream, the creek disperses into a large wetland complex. The creek path follows the wetlands and eventually discharges to Lorene Lake. Surface Water Preliminary TIR Page 9 Figure 1.2B: Joes Creek flows from the south through the middle of the site and discharges north to the wetland. The total drainage area of the site is 76,826 SF with stormwater naturally collected by the creek which splits the site nearly in half. The drainage area of the site on the northwest side of the creek consists of 40,858 SF and is an unmanaged forest with natural vegetation such as Bigleaf Maples, Douglas-firs and Himalayan Blackberry. The drainage area of the site on the southeast side of the creek consists of 35,968 SF previously cleared with prior residential uses. The three lots had once contained two mobile homes and a house along with the existing gravel driveway. The structures have been removed but the gravel driveway remains. The vegetation on the southeast portion of the site mainly consists of a wild groundcover, a sparse number of trees, and shrubs. No construction is proposed for the 40,858 SF portion of the site on the west/northwest side of the creek. The area proposed for the construction of three single family residences and a paved access will be limited to the 35,968 SF portion of the site, on the east/southeast side of the creek. Access to the site is by Surface Water Preliminary TIR Page 10 333rd Place, a gravel road in a 15’ wide road easement from SW 43rd Avenue to the site. The gravel road extends through the site and ends near the east bank of Joes Creek. The following surfaces are proposed with the redevelopment of the site: DESCRIPTION TYPE AREA (SF) METHOD/BMP Pavement (onsite, max 20-ft wide) PGIS 4939 FULL DISPERSION/BMP T5.30 Pavement (offsite, 10-ft wide strip) PGIS 1900 STRIP (SHEET FLOW) DISPERSION /BMP T5.12 LOT -9043 - SFR Non-PGIS 1486 FULL DISPERSION/BMP T5.30 LOT -9087 - SFR Non-PGIS 1300 FULL DISPERSION/BMP T5.30 LOT -9069 - SFR Non-PGIS 1563 FULL DISPERSION/BMP T5.30 LOT -9043 -Lawn PGPS 1500 SIMPLE DETENTION POND/KCSWDM Chapter 5.1.7 LOT -9087- Lawn PGPS 900 SIMPLE DETENTION POND/KCSWDM Chapter 5.1.7 LOT -9069 - Lawn PGPS 1500 SIMPLE DETENTION POND/KCSWDM Chapter 5.1.7 PROPOSED DISPOSITION OF STORMWATER RUN-OFF AFTER DEVELOPMENT BY:  Full Dispersion: 9,288 SF of PGIS and Non-PGIS  Strip Dispersion: 1,900 SF of PGIS (offsite only)  Simple Detention Pond: 3,900 SF of PGPS Surface Water Preliminary TIR Page 11 Figure 1.3: Drainage Basins, Subbasins, and Site Characteristics- Actual location of Joes Creek identified with thick blue line and the area highlighted is where soil excavation occurred for soil investigation shown in the site photo below. Surface Water Preliminary TIR Page 12 S O I L S The site is generally flat with slopes between 0-8% with an exception in the southwest corner and small slope between 8%-15%. A site investigation found soil consistent with the USDA map in Figure 4 on the next page. The soils are classified as Alderwood Gravelly Sandy Loam; AgB and AgC. The site is predominately recognized as AgB due to the flat topography. Soil samples from the southeast drainage area (southeast side of the creek) where construction is proposed found the soil to be consistent across the three lots with exception of the wetland. Soils found in the wetland are covered in the Wetland Report which is included in Section 6. The soil outside of the wetland does not contain clay and consisted predominately of small rock fragments creating a sandy loam mixture. Surface Water Preliminary TIR Page 13 The hand dug excavation pictured below was performed near the south property line on the far west lot (lot- 9043), just east of the creek confluence point by about 12 feet. The flat area shown is characteristic of the southeast drainage area where construction is proposed. No cobble or stones were found within the excavation area. Underlying the gravelly sandy loam mixture are consolidated material creating a hard pan where groundwater perches during the wet season. The consolidated hard pan materials were found at a depth approximately 3.5’ deep. Above the hardpan, the soils are well drained, friable and non-sticky. Site photo showing area of soil excavation. Perspective is looking south while standing near the east property line of Lot A. Figure 1.3: USDA Soils Map (complete map with soil identifications attached in Appendix A) South property line (lot-9043) -orange property stake below is on the opposite creek bank of Joes Creek. Hand-dug Excavation Pit JOES CREEK Surface Water Preliminary TIR Page 14 2. CONDITIONS AND REQUIREMENTS SUMMARY G E N E R A L R E Q U I R E M E N T S The site is encumbered with wetlands, wetland buffers, a stream, and stream buffers. A Critical Areas Report has been completed and provided in Section 6. The wetland lies on the northern portion of the site while Joes Creek, and unclassified stream, divides the site through the middle from south to north. All three lots are within the 165-ft wetland buffer and stream buffer. The project proposes intrusion to the buffers for ‘Reasonable Use’ for each single-family lot. The existing lots are large lots zoned RS15.0 as shown below. The scope of this project does not include a land division process. The project is subject to Process III Project Approval for Reasonable Use. The project design shall protect the natural environment and surrounding critical areas. Figure 2. 1: City of Federal Way Zoning Map Surface Water Preliminary TIR Page 15 A stream crossing is proposed to extend a sewer force main to public sewer. The sewer will be laterally drilled a minimum of 4’ below the creek bed pursuant to FWRC 19.145.320 2(g). The project is subject to environmental review and an environmental checklist. A SEPA checklist has been submitted as part of the ‘Reasonable Use’ process and for the utility crossing and is included in Appendix B. A paved driveway in a private road easement will provide for access to each lot. The stormwater runoff from the new pollution generating surfaces shall be treated by allowing the run-off to disperse and flow naturally through naturally vegetated paths using Full Dispersion methods before discharging to the creek or wetland. Individual single-family site plans with erosion and sediment control plans will be submitted with each building permit application for review and approval prior to constructing any improvements. Roof downspouts will collect the non-pollution generating run-off and disperse the flow for Full Dispersion through a naturally vegetated flow path to control runoff from the site. Grass lawns will be kept to a minimum of less than 1500 SF of pollution generating pervious surface per lot and will be controlled and filtered through ‘Simple Detention Ponds for Cleared Areas’ meeting the requirements of Chapter 5.1.7 in the 2016 King County Surface Water Design Manual. Surface water runoff control and water quality treatment per the 2016 King County Surface Water Design Manual are required. The project must comply with all nine core and five special requirements where relevant. Project requirements protect the natural areas which and are important to the environment. The project design will be for natural vegetated flow paths to provide water quality and flow control in treating the stormwater runoff before leaving the site. The project is not allowed to impact habitat, water quality, stormwater retention, groundwater recharge, or generate erosion . The Core Requirements 1-9 and Special Requirements 1-5 will be addressed as described in the following table: Surface Water Preliminary TIR Page 16 SUMMARY TABLE: REQUIREMENTS: How being met if applicable: Core Reqmt #1: Discharge at the Natural Location No change to the natural discharge location. Core Reqmt #2: Offsite Analysis Level 1 offsite analysis completed (Section 3) Core Reqmt #3: Flow Control Full Dispersion (Section 4) Core Reqmt #4: Conveyance System Gravel channels and oversized drainpipes for roof downspouts for capacity exceeding the peak flow during a 100-year rainfall event. (Sizing analysis in Section 5) Core Reqmt #5: Erosion and Sediment Control ESC measures during construction and in final design. Core Reqmt #6: Maintenance and Operations Property owners will be responsible to maintain natural vegetated areas. Core Reqmt #7: Financial Guarantees and Liability Private Facilities/NVFP Area Covenant Core Reqmt #8: Water Quality Full Dispersion (Section 4) Core Reqmt #9: Flow Control BMPs T5.30 Full Dispersion (onsite), T5.12 Sheet Flow Dispersion (offsite), Simple Detention Special Reqmt #1: Other Adopted Area-Specific Requirements Critical Areas: wetlands and stream to be protected. The project area lies within the South Lower Puget Sound Subbasin. Special Reqmt #2: Flood Hazard Area Delineation N/A – Not within the 100-yr flood plain. Special Reqmt #3: Flood Protection Facilities N/A – Not within an area of flooding. Special Reqmt #4: Source Control N/A - Single Family Special Reqmt #5: Oil Control N/A – Sindle Family C O R E R E QU I R E ME N T # 1 – D I S C H A RG E AT T H E N AT U R A L L O C AT I ON The runoff from the site is collected and concentrated in a natural drainage named Joes Creek, an unclassified stream flowing from south to north through the middle of the site. The creek naturally discharges at a singular point on the north end of the property into a Category III wetland. Upstream, the channel collects the drainage from higher elevation properties to the south and funnels the flow through to the north where it disperses within a wetland. After leaving the site, the downstream flow travels 0.4 miles through the wetland Surface Water Preliminary TIR Page 17 and then another 0.2 miles in a secondary channel from where it discharges into a series of two lakes known as the Twin Lakes: first discharging into Lorene Lake and then into Jeane Lake. The project proposes no changes to the natural discharge location. Figure 4: USGS - FWS WETLAND – NATURAL DISCHARGE POINT (Note, thicker blue line is actual location of Joes Creek also represented by the thin blue line in the topo map. ) Surface Water Preliminary TIR Page 18 C O R E R E QU I R E ME N T # 2 – OF F S I T E A N A LYS I S Due to critical sensitive areas a level 1 offsite analysis is required. The analysis has been completed and is included in Chapter 3. C O R E R E QU I R E ME N T # 3 – F L OW C O N T RO L Flow control is required for all new impervious, new pervious, and replaced pervious surfaces within the redevelopment of the three lots. The onsite impervious surfaces fall within the wetland buffer and stream buffer. Runoff from these surfaces will be controlled using Full Dispersion methods per BMP 5.30. Pervious surfaces such as lawns will be controlled using simple detention ponds as described in the KCSWDM, Section 5.1.7. C O R E R E QU I R E ME N T # 4 – C ON V E YA N C E S Y S T E M The conveyance systems for this project are simple and will consist of collecting the roof downspouts and short drainage channels alongside a portion of the 20-ft wide driveway. Curbs and rolled edges will be used to directing sheet flow to dispersion areas for discharge in a naturally vegetated path. C O R E R E QU I R E ME N T # 5 – E ROS I O N A N D S E D I M E N T C ON T RO L Single family erosion control measures recognized by the City of Federal Way will be used during construction to contain and stabilized denuded soils. In addition, pollutant control measures recommended in the critical areas study by the project’s environmental expert will be followed to protect the surrounding wetlands and creek. After construction, vegetation shall be established and maintained to enhance the stream bank and natural areas. C O R E R E QU I R E ME N T # 6 – M A I N T E N A N C E A N D O PE R AT I O N S Natural vegetation areas require little maintenance once established and are to flourish naturally when they remain untouched. In order to maintain the requirements that natural areas be maintained with no adverse impact by future home owners, an easement including a covenant running with the land shall be executed to identify the reserve areas and to notify the future owners of the duty for protection of the surrounding environment. C O R E R E QU I R E ME N T # 7 – F I N A N C I A L G UA R A N T E E S A N D L I A B I L I T Y All proposed drainage facilities are to be privately owned and maintained by the applicant or future property owner. Covenants will be recorded on the properties disclosing liability of subsequent owners to retain natural areas. C O R E R E QU I R E ME N T # 8 – WAT E R Q UA L I T Y Water quality is required for all new and replaced pollutant generating impervious and pollutant generating pervious surfaces. Full Dispersion methods per BMP 5.30 will be used for treating the runoff from all onsite impervious surfaces within the wetland buffer and stream buffer. Simple detention ponds will be used to capture lawn runoff and filter out any potential pollutants prior to discharge to natural areas. Surface Water Preliminary TIR Page 19 C O R E R E QU I R E ME N T # 9 – F L OW C O N T RO L B M P S BMPs to meet the performance standards for a Low Impact Development: Full Dispersion Applications, Restricted Footprint, Minimum Disturbance Foundations, and Native Growth Retention BMP T5.30 Full Dipsersion NVFP – PGIS and NPGIS (onsite) BMP T5.12 Sheet Flow Dispersion – PGIS (offsite 10-ft pavement strip) Simple Detention – PG Pervious Surfaces (grass lawns) S P E C I A L R E Q U I RE M E N T # 1 – OT H E R A D O P T E D R E Q U I R E M E N T S Sensitive areas are defined and delineated in the critical areas report in Section 6. These areas will be protected and enhanced per the recommendations of the environmental expert to mitigate for reasonable use of the buffer areas and provide stream corridor protection. Along with the recognized critical areas on the site, the project is located in the South Lower Puget Sound Sub- basin of the Lower Puget Sound Basin and identified in the Hylebos Creek and Lower Puget Sound Basin Plan as area11A, subcatchment J2. No significant problems or adverse impacts were identified within the subcatchment. The plan recognizes flooding has occurred downstream of the project in the Twin Lakes development area. No flooding is indicated for the project area or areas adjacent to it. Flow controls are proposed for the project which will not adversely impact the existing flood conditions downstream. No other adopted plans or requirements were found applicable to the site. Surface Water Preliminary TIR Page 20 S P E C I A L R E Q U I RE M E N T # 2 – F L OO D H A ZA R D A R E A D E L I N E AT I O N This project is not within a 100-year floodplain or area mapped for a Flood Hazard Reduction Plan. The FEMA map is attached in the Appendix. Therefore, additional requirements from a Flood Hazard Reduction Plan do not apply. S P E C I A L R E Q U I RE M E N T # 3 – F L OO D P ROT E C T I ON FAC I L I T I E S Not Applicable. The project is not within the 100-year flood plain. S P E C I A L R E Q U I RE M E N T # 4 – S O U RC E C O N T ROL Not Applicable. Does not apply to single family. S P E C I A L R E Q U I RE M E N T # 5 – O I L C O N T RO L Not Applicable. Does not apply to single family. Surface Water Preliminary TIR Page 21 3. OFFSITE ANALYSIS O F F S I T E A N A LY S I S L E V E L 1 A minimum Level 1 downstream analysis is required for a distance of at least 0.25 mile from the project. The discharge leaving the site, concentrated by Joes Creek, flows downstream through a wetland that extends Surface Water Preliminary TIR Page 22 over 0.4 miles long before it is conveyed in a secondary channel about 0.2 miles long and then discharges to a series of lakes. Downstream Water Quality: The State’s Water Quality Atlas does not report any water quality issues for the portion of Joes Creek downstream of the site before it discharges into a series of lakes, over half a mile away. FEMA Maps – A FEMA map was reviewed (attached in Appendix) and the site is not within a flood plain as it is covered by “Zone X - outside the 500-year flood plain”. FEMA indicates it is an area of minimal flood hazard. Basin Plan - The South Lower Puget Sound Subbasin plan indicates the nearest area of known flooding along Joes Creek occurred 0.6 miles downstream in the development area surrounding the Twin Lakes also known as Jeane Lake and Lorene Lake. Geologic Hazards – Slope hazards do not exist on the site or within 200-ft. Site survey and topographic data shows the majority of the site is relatively flat and slightly rolling. There is a manmade 20% slope in the southwest corner of the site that shows no evidence of movement. Record drawings indicate fill was added in that area to create the slope during the construction of the neighboring houses to the west. The 1965 aerial below shows the site and original homestead prior to being surrounded by larger developments. Note that fill was imposed on the property in the 90’s with house construction to the west and is shown on record drawings. It appears that prior to development that occurred later that likely water flowed naturally through the site but was not channeled into the drainage by what is now called Joe’s Creek. The project does not propose to impact any areas west of the creek. Downstream Drainage Problems (within 0.25 miles): 1. Conveyance System Nuisance Problem: No record was found of any downstream drainage problems requiring special attention. 2. Server Erosion Problem: No record was found of any downstream drainage problems requiring special attention. 3. Server Flooding Problem: No record was found of any other downstream drainage problems requiring special attention. 4. Potential Impacts to Wetland Hydrology: Subject to critical area review. There will be no intrusion into the wetland with the proposed project. Intrusion into the wetland buffers are unavoidable. Mitigation efforts and BMP’s for flow control and water quality will be used to prevent impacts to the wetland hydrology. Refer to Section 6 for the complete Critical Areas Report and analysis. The downstream wetland starts on the north end of the project site and encompasses an area of over 16 acres extending 0.4 miles past the site. The project proposes will intrude into the buffers of the wetland. This is unavoidable because no portion of the site lies area outside of the wetland buffer. The Critical Areas Report provided in Section 6 gives a detailed analysis of the wetland. Mitigation efforts are proposed by the project Surface Water Preliminary TIR Page 23 to protect the wetland biology from stormwater runoff and will prevent any significant alteration in the wetland hydrology. The project proposes to use Full Dispersion methods per BMP T5.30 to fully dissipate the stormwater runoff from the 20’ driveway and residential downspouts through a naturally vegetated forested path before leaving the site. This will control the flow and provide for water quality from pollution generating impervious surface and the non-pollution generating impervious surfaces proposed onsite with the project. Small lawns are proposed for each lot which will be flow controlled using a ‘Simple Detention Pond for Cleared Areas’ as identified in Section 5.1.7 of the KCSWDM. These ponds will be constructed at the base of each grassy lawn to control the flow and filter out pollutants from the pollutant generating pervious surfaces. Along with the BMPs, trees and vegetation will be retained within the natural forested areas beyond the project areas on both sides of the creek. The impervious surfaces will make up less than 14% of the total natural vegetative areas within the Total Drainage Area. The proposed project will not have any adverse impact on the downstream hydrology. All maps are attached in Appendix A. 1965 AERIAL PHOTO OF SITE: Surface Water Preliminary TIR Page 24 4. FLOW CONTROL, LID, WQ FACILITY ANALYSIS, AND DESIGN Full Dispersion Sheet Flow Dispersion Simple Detention Ponds STORMWATER PLANS (included in Appendix D) C O R E R E QU I R E ME N T # 3 – F L OW C O N T RO L The City of Federal Way Flow Control Applications Map indicates the area is in the Lower Puget Sound Basin and the application of Conservation Flow Controls is required. The minimum requirement is for historic site conditions to be met for Level 2 flow control standards to match 50% of 2 year through 50 year peaks and to match the 2 year and 10 year peaks and additional flow controls may be required since the downstream discharges to a wetland. The project proposes native vegetation in areas beyond the clearing limits as shown on the plans to retain approximately 64,062 SF of natural vegetation within the combined three lots 76,826 SF, making up 83% of the site. The 17% of the site not containing natural vegetation is the sum of 9,194 SF of impervious (roof and driveway) at 12% and 3,900 SF (grass lawns) of replaced pervious surface at 5% of the Total Discharge Area. Full Dispersion methods are proposed for the total 9,194 SF of new impervious surface to be created with the build out of all three lots. Grass lawns are pollutant generating pervious surfaces. Areas proposed for grass lawns are constrained by the Joes Creek and the site topography. Full dispersion of these areas cannot be met given the length of flow path required and the closeness of the creek. Simple Detention Ponds are proposed at the base of each lawn to decrease the velocity of stormwater run-off and protect the creek from erosion, sediment, and pollutants. The proposed project with proposed flow control methods will not concentrate flows to the creek or increase the flow downstream at the natural discharge location from the site. Surface Water Preliminary TIR Page 25 5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN C O R E R E QU I R E ME N T # 4 – C ON V E YA N C E S Y S T E M Roof Down Spouts – The maximum size roof area to be captured is less than 1600 SF. Pipe diameters have been sized conservatively to accommodate the 100-year storm event of 4.2 inches over a 24-hour period. For practical purposes, a conservative peak rain intensity of 4.0 in/hr was used to calculate maximum flow and ensure the proposed drainpipes have sufficient capacity to handle the peak flow. According to data published for Seattle Public Utilities1 which includes surrounding regions such as Federal Way, the maximum intensity of a 100-year rainfall event in the area is 3.23 in/hr when comparing 5-minute intervals. Therefore, the peak rain fall intensity of 4.0 in/hr being used here is taking a conservative approach. REF: http://climatechange.seattle.gov/wp-content/uploads/2018/01/Seattle-IDF-Curve-Update-TM_12-29- 2017.pdf Roof downspouts will be collected in 4-inch diameter drains along the house footers where they will flow to a yard catch basin. From the yard catch basin, the collected flows will be conveyed in a 6-inch drainpipe to either a 50-ft or a 10-ft dispersion trench with naturally vegetated flow path where shown on the plans. The conveyance pipe from the yard catch basin to the dispersion trench will be a 6-inch diameter drainpipe at a 0.0052 slope. The capacity for this pipe is 243 gpm. If the peak rainfall of 4.0 inches were to fall over the course of an hour, the flow in the 6-inch pipe would be 70 gpm. The capacity in the 6-inch pipe is 243 gpm which is 3.5 times the amount needed to handle the peak rainfall intensity. In other words, based on the pipe size and slope for a capacity for 243 gpm and a roof area of 1600 SF, the rainfall intensity could be as much as 14.6 in/hr before it exceeded the capacity of the drain line. Ditches - The roadside channels will be constructed of a drain rock bed with a 6-inch perforated underdrain pipe. A minimum slope of 0.005 will be used to construct the base of the channel and allow flow to the 50-ft notched board dispersion trench and naturally vegetated flow path as shown on the plans. The flat gravel channel will allow flows to dissipate to control erosion as the flow makes it way towards the dispersion trench. The underdrain shall be 6-inch perforated pipe with a capacity of 233 gpm which well exceeds the capacity needed to capture the peak flow of the 100-year rain event for a 1,000 SF area of sheet flow to the 1 Intensity Duration Frequency Curves and Trends for the City of Seattle, Technical Memorandum, December 29, 2017 completed by Tetra Tech for Seattle Public Utilities CSO reduction Program. Surface Water Preliminary TIR Page 26 channel. Based on a conservative peak intensity rainfall of 4.0 inches/hour, the channel has over 5 times the capacity needed to handle the peak intensity rainfall for a 100-year event. The oversized conveyance systems will help to control flows and allow for storage and dissipation. They will also help in long term maintenance to avoid the potential for debris collection to cause flooding. SUMMARY TABLE: RUNOFF AREA CONVEYANCE SYST PIPE DIA/SLOPE CAPACITY 100 YR PK FLOW (MAX GPM) ROOF <1600SF Yard CB w/ Drainpipe to Dispersion and NVFP 6” /0.0052 243 GPM 70 GPM @ 4.0 in/hr PAVEMENT <1000 SF Gravel Channel with Underdrain to Dispersion and NVFP 6”/0.005 233 GPM 43 GPM @ 4.0 in/hr Surface Water Preliminary TIR Page 27 6. SPECIAL REPORTS AND STUDY CRITICAL AREAS DELINEATION AND MITIGATION PLAN See Wetland Report submitted. Prepared by: Mark Heckert, Beaver Creek Environmental, LLC POB 731695 Puyallup, WA 98373 mheckert@Q.com (253) 732-6515 Dated: December 1, 2020 SEPA See SEPA Checklist submitted. Prepared by: Michelle Henry, PE P.M.Development@outlook.com (360) 281-8195 Dated: November 23, 2020 Surface Water Preliminary TIR Page 28 7. OTHER PERMITS Building Applications and Permits Utility Connection Permits Surface Water Preliminary TIR Page 29 8. CSWPP ANALYSIS AND DESIGN The site was previously occupied by single family dwelling units and cleared by previous occupants. There will be minor clearing needed to widen the existing roadway. Excavation for new dwelling units will be kept to a minimum with stepped foundations. A final erosion control plan will be submitted with each single-family lot through the building application and permit process. Surface Water Preliminary TIR Page 30 9. BOND QUANITITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANTS Covenants are to be recorded for each lot to retain naturally vegetated areas for flow control and water quality. Surface Water Preliminary TIR Page 31 10. OPERATION AND MAINTENANCE MANUAL Natural vegetation areas require little maintenance once established and are to flourish naturally when they remain untouched. In order to maintain the requirements that natural areas be maintained with no adverse impact by future home owners, an easement including a covenant running with the land shall be executed to identify the reserve areas and to notify the future owners of the duty for protection of the surrounding environment.