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HomeMy WebLinkAboutStormwater Memorandum - PLN General - 4/17/2009 TO BE KEPT IN THE PARCEL FILE MEMORANDUM TO: FROM: Mike Whalen, AIA Adam Lee, EIT; Tye Simpson, PE COMPANY: DATE: Mike Whalen AIA April 17, 2009 SUBJECT: JOB NUMBER: Stormwater Memorandum - Harmony Hill 09.37.080 Retreat Center Introduction The site address is 7362 E State Route 106, in Union, Mason County, Washington. The project involves the construction of two two-story cabins and the reconstruction of a two- story annex building, including some gravel paths and site landscaping. The site is located within Mason County, which has adopted the Department of Ecology's drainage standards, and is therefore subject to the drainage requirements detailed in the 1992 DOE Surface Water Management Manual for Western Washington (SWMMWW). Per the Public Works pre-application review, "The site already has a developed stormwater plan. Applicant needs to have an engineering firm review that plan to determine if the stormwater features have sufficient capacity for the proposed changes or whether new Seattle capacity needs to be added." Review of the existing stormwater facilities indicated that Bangkok the improvements described in this memo would adequately mitigate the additional runoff Guam from the proposed development. Hong Kong Honolulu Using the SWMMW, stormwater dispersion and temporary erosion control facilities were Philippines designed for the site per the small site requirements. As the new impervious surface is Shenzhen below 5,000 square feet of new and replaced impervious surface, and the land disturbing Singapore activities will be less than 1 acre, no stormwater detention or quality facilities were required for the project. The proposed stormwater best management practices (BMPs) improvements for the site were designed to mitigate any downstream effects per the County's requirements. This memorandum outlines the code requirements and sizing calculations used for the design. Stormwater Modeling To determine runoff rates and to size stormwater facilities the rational method was used. See Appendix A for the hydrologic and design calculations. Belt Collins Northwest LLC 1925 Post Alley,Suite 3B■Seattle,Washington 98101 ■USA T/206 285 4395■F/206 285 4516■seattie@beltcollins.com■www.beltcollins.com I } MEMORANDUM Hillside Cabin Area The hillside cabins are new buildings adding approximately 1,000 SF of new impervious surface each. Additionally, gravel paths accessing the cabins will be constructed. To mitigate the additional runoff from the cabins, drainage swales and a 25 foot long flow dispersion trench will be used to dissipate the runoff. The dispersal trench was located to provide a 100 foot native vegetated flow path as outline in the SWMMW. The natural drainage pattern in the area of disturbance will be modified slightly but the overall drainage pattern will not be changed. Annex Building Area Since there was an existing building in the location of the reconstructed annex building, and the proposed building will occupy the same footprint there is no additional impervious surface from the construction of this building. The existing building does not have gutters and downspouts. The proposed building will have gutters, downspouts and a tightline conveyance system. The system will outfall to the existing downstream ditch where runoff from the existing building is collected. However, to provide erosion protection of the existing ditch, rock protection will be added to the new outfall. Temporary Erosion and Sediment Control Prior to construction, temporary erosion and sediment control BMPs will be implemented to help prevent erosion of the site. Silt fencing, silt dikes, and plastic covering of stockpiles will be used to reduce sediment laden runoff from discharging from the site. iC- S IAipS .0 �F WASgIN oy 32233 ISTER� �4'w �S�ONAL EN�'l Appendix A: Hydrologic and Design Calculations "MON Project: T Project#: Client: Date: l Location: , © Page of Be1tC011 s AV-S By: i C -A V =Q., T j Seattle Bangkok Guam Hong Kong Honolulu Philippines Shenzhen Singapore Belt Collins Northwest LLC 1925 Post Alley,Suite 3B■Seattle,Washington 98101-1028■USA T/206 285 4395■F/206 285 4516■northwest@beltcollins.com•www.beitcollins.com L } Project: I 1 ENV ( k C Project #: Client: C� Date: i loq Location: K/1 Page -` f BeItColfinS Subject: IRkebmifJAR� wm if By: 4z I GULV t f 1Z►N6- D:3PER, QAI ) P--CAJ C i) F I+ I g / q T�- - bD 1l _60(k � Ola)( — - - - Zc C2, , 2 4 - 1417 Seattle { Bangkok 0.12 c� Guam Hong Kong r ! Honokiki ! G�S �.-�? �Tk'.?lU � Philippines Shenzhen V _ I Singapore 1-emu. 6A P i / I I i a Belt Collins Northwest LLC 1926 Post Alley,Suite 3B a Seattle,Washington 98101-1028■USA T/206 285 4395■F/206 285 4516■northwest@beltcollins.com■www.beltcollins.com � MEM TO BE KEPT IN THE Wff' :: .D TO: PARCEL FILE FROM: MAY 14 «;rj Mike Whalen, AIA Adam Lee, EIT C�X Tye Simpson, PE M Cr C 01AKIN ING COMPANY: DATE: Mike Whalen AIA May 11, 2009 SUBJECT: JOB NUMBER: Stormwater Memorandum 09.37.080 Harmony Hill Retreat Center Creekside Addition & Hillside Cabins Introduction The site address is 7362 E State Route 106, in Union, Mason County, Washington. The project involves the construction of two two-story cabins and the reconstruction of a two- story annex building, including some gravel paths and site landscaping. The site is located within Mason County, which has adopted the Department of Ecology's drainage standards, and is therefore subject to the drainage requirements detailed in the 1992 DOE Surface Water Management Manual for Western Washington (SWMMWW). In the Public Works pre-application review responses, "The site already has a developed stormwater plan. Applicant needs to have an engineering firm review that plan to determine if the stormwater features have sufficient capacity for the proposed changes or whether new capacity needs to be added." Review of the existing stormwater facilities Seattle indicated that the improvements described in this memo would adequately mitigate the Bangkok additional runoff from the proposed development. Guam Hong Kong Using the SWMMW, stormwater dispersion and temporary erosion control facilities were Honolulu designed for the site per the small site requirements. As the new impervious surface is Philippines below 5,000 square feet of new and replaced impervious surface, and the land disturbing Shenzhen activities will be less than one acre, no stormwater detention or quality facilities were Singapore required for the project. The proposed stormwater best management practices (BMPs) improvements for the site were designed to mitigate any downstream effects per the County's requirements. This memorandum outlines the stormwater requirements for the project per the Mason County Stormwater Checklist. The checklist along with our project specific responses is attached in Appendix A. Those checklist items identified as applicable to this project are addressed in more detail below. Stormwater Conveyance and Control To calculate peak runoff rates the rational method was used. Included in Appendix B are these hydrologic calculations along with the hydraulic calculations for the project. MEMORANDUM Streambank Erosion Control Hillside Cabin Area The hillside cabins are new buildings adding approximately 1,000 square feet (SF) of new impervious surface each. Additionally, gravel paths accessing the cabins will be constructed. To mitigate the additional runoff from the cabins, drainage swales and a 25 foot long flow dispersion trench will be used to dissipate the runoff. The dispersal trench was located to provide a 100 foot native vegetated flow path as outline in the SWMMW. The natural drainage pattern in the area of disturbance will be modified slightly but the overall drainage pattern will not be changed. In addition modular gravel paving will be provided to facilitate infiltration of runoff of the added gravel paths and paving. Annex Building Area Since there was an existing building in the location of the reconstructed annex building, and the proposed building will occupy the same footprint there is no additional impervious surface from the construction of this building. The existing building does not have gutters and downspouts. The proposed building will have gutters, downspouts and a tightline conveyance system. The system will outfall to the existing downstream ditch where runoff from the existing building is collected. However, to provide erosion protection of the existing ditch, rock protection will be added to the new outfall. Temporary Erosion and Sediment Control Prior to construction, temporary erosion and sediment control BMPs will be implemented to help prevent erosion of the site. Compost socks, silt dikes, and plastic covering of stockpiles will be used to reduce sediment laden runoff from discharging from the site. Construction vehicle access will travel to the work area through the existing gravel roads, which will act as a stabilized rock construction entrance and will reduce the tracking of sediment onto public roads. Exposed soils left unworked for more than 3 days will be covered. Preservation of Natural Drainage Systems Natural drainage patterns will be maintained by using pervious paving surfaces and infiltrating and dispersing the stormwater from the building roofs into the surrounding forested area. Stormwater discharges from the site flow to their existing drainage locations. K. S I �� pF WAs O� �1 O� O15j), 16 32233 �0 R I STER �SSIONAL Belt Collins Northwest,LLC.11925 Post Alley,Suite 3B I Seattle,WA 98101 USA Tel:206 285 4395 I Fax:206 285 4516 1 www.beltcollins.com I beltcollinsnw@beltcollins.com Belt Collins is an Equal Opportunity Employer 1 Appendix A: Mason County Stormwater Checklist i Mason County Stormwater Checklist County Project#_PAR2008-00051 Project Name—Harmony Hill Retreat Center: Creekside Addition& Hillside Cabins_ Twnsp,Sec,RangeT22N, R3W, S33 Minimum Requirements of 1992 Stormwater Manual The stormwater checklist identifies the minimum requirements of the 1992 Stormwater Manual. The checklist is intended to identify the locations within the plan that addresses the minimum requirements. Mason County will not perform a technical evaluation of the submittal. Rather, the checklist provides a guide to allow Mason County to review the submittal and determine if the applicant has addressed the minimum features that make up a stormwater plan. It is incumbent upon the applicant and his/her engineer to fulfill all the applicable requirements of the 1992 Stormwater Plan as it relates to the proposed project. Review by Mason County is intended to determine if the plan has addressed the minimum requirements and should not be considered a review for technical accuracy of the plan. During construction of the project,the stormwater plan engineer of record or his/her authorized representative shall inspect the site to ensure the stormwater plan is being implemented as designed. Upon completion of the project,the engineer or his authorized representative shall be required to certify that the stormwater plan has been implemented as designed. Failure to meet the minimum requirements could result in delay or rejection of the application until the deficiencies are corrected. �X #1: Erosion and Sediment Control.All new development and redevelopment that includes land disturbing activities of one acre or greater shall comply with erosion and sediment control requirements 1 through 14, below. Compliance with the Erosion and Sediment Control Requirements shall be demonstrated through implementation of a large parcel erosion and sediment control plan. Al l new development and redevelopment that includes land disturbing activities of less than one acre shall comply with the small parcel minimum requirements found in Section 14.48.130, Compliance with the small parcel requirements shall be demonstrated through implementation of a small parcel erosion and sediment control plan. Erosion and Sediment Control Requirement#1: Stabilization and Sediment Trapping. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#2: Delineate Clearing and Easement Limits. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#3: Protection of Adjacent Properties. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#4: Timing and Stabilization of Sediment Trapping Measures. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. 1 Erosion and Sediment Control Requirement#5: Cut and Fill Slopes. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#6: Controlling Off-Site Erosion. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#7: Stabilization of Temporary Conveyance Channels and Outlets. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#8: Storm Drain Inlet Protection. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#9: Underground Utility Construction. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#10: Construction Access Routes. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#11: Removal of Temporary BMPs. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#12: Dewatering Construction Sites. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#13: Control of Pollutants Other than Sediment on Construction Sites. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#14: Maintenance. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. Erosion and Sediment Control Requirement#15: Financial Liability. See page/paragraph NA—See Stormwater Memorandum page 2,paragraph 3. X❑#2: Preservation of Natural Drainage Systems. Natural drainage patterns shall be maintained, and discharges from the site shall occur at the natural location to the maximum extent practicable. See page/paragraph_See Stormwater Memorandum page 2, paragraph 4. ❑#3: Source Control of Pollution. Source control BMPs shall be applied to all projects to the maximum extent practicable. Source control BMPs shall be selected, designed, and maintained according to an approved manual. See page/paragraph NA—See Stormwater Memorandum ❑#4: Runoff Treatment BMPs. All projects shall provide treatment of stormwater. Treatment BMPs shall be sized to capture and treat the water quality storm, defined as the six-month, twenty-four hour storm. The first priority for treatment of stormwater shall be to infiltrate as much as possible of the water quality design storm into the ground. Pretreatment of stormwater prior to infiltration into the ground may be required. See page/paragraph NA— See Stormwater Memorandum 2 ❑X #5: Streambank Erosion Control. The requirement below applies only to situations where stormwater runoff is discharged directly or indirectly to a stream, and must be met in addition to meeting the requirements in minimum requirement#4, Runoff Treatment BMP's. See page/paragraph See Stormwater Memorandum page 2, paragraph 1-2. ❑#6: Wetlands. Stormwater discharges to wetlands shall maintain the wetland's natural hydroperiod and flows to the extent needed to preserve or enhance its existing functions and values. Prior to proposing discharge of higher volumes of stormwater to a wetland, alternative discharge, detention, and infiltration practices located in areas outside the wetland shall be evaluated and employed by the project engineer where feasible and practicable. See page/paragraph NA— See Stormwater Memorandum ❑#7: Water Quality Sensitive Areas. Where the Mason County commissioners or their designee determine that the minimum requirements do not provide adequate protection of water quality sensitive areas, either on-site or within the basin, more stringent controls shall be required to protect water quality. An adopted and implemented basin plan (minimum requirement#9) may be used to develop requirements for water quality sensitive areas that are tailored to a specific basin. See page/paragraph NA— See Stormwater Memorandum ❑#8: Off-Site Analysis and Mitigation. Downstream Analysis May Trigger Additional Requirements. The project engineer shall provide a detailed qualitative analysis of the flow path of the discharge from the project site to the receiving water. This requirement shall apply to all projects where a drainage and erosion control plan is prepared, including those proposing retention facilities. This analysis shall include flow routing, and provide existing pipe and channel sizes and estimated capacities. In addition, the project engineer shall discuss any known or expected downstream erosion, flooding, or water quality problems, including those that may be caused by interflow from the proposed retention facility. The director or designee shall have the discretion to specify the distance and level of detail to be provided by the project engineer. In making this determination, the director or designee shall consider such factors as the relative size of the new development, availability of other hydrologic work for the drainage area, and the extent to which stormwater generated on the project site is to be infiltrated. See page/paragraph NA— See Stormwater Memorandum ❑#9: Basin Planning. Basin plan supersedes Manual See page/paragraph NA— See Stormwater Memorandum ❑#10: Operation and Maintenance. An operation and maintenance schedule shall be provided for all proposed stormwater facilities and BMPs, and the party(or parties) responsible for maintenance and operation shall be identified. An operation and maintenance (O&M)covenant will be required to cover all privately owned and maintained stormwater facilities approved by the director. O&M Covenant forms are available at the Mason County public works office. A copy of the completed instrument shall be recorded with the county auditors' office by the proponent and a copy of the recorded instrument is required to be submitted to the public works department prior to final approval of the completed permanent stormwater facilities. See page/paragraph NA— See Stormwater Memorandum 0#11: Financial Liability. Performance bonding, or other appropriate instruments shall be required for all projects to ensure compliance with these standards. See page/paragraph NA— See Stormwater Memorandum 3 I certify that the stormwater plan submitted for this project fulfills the applicable provisions of the 1992 Stormwater Manual. Engin er Date Applicant Date K. Slap of WAsk�� 32233 4 �0 I STERN �ssralvaL C. 4 Appendix B: Hydrologic and Design Calculations i _....._... Project: § Project#: i ` Client: Date: 'f /' j'I Location: .; Page d of ,--- Subject: - By: BeItCofts _ I i �f Jjy b jT S� �`J' ...,.> + / -mil �.. �1.l( „.;-' ✓,'. l 111 �' ) .• 79,Ot / seaale �a Bangkok Guam Hong Kong r JJ Honolulu ( Philippines 1 Fly Shenzhen 6:T Singapore ' Ate"' 't.,.�•'1"'It T� �:"` r d rF .�' `,•°'' ` _ ,>, 'T'..�" Y'°'� ';: Belt Collins Northwest LLC 1925 Post Alley,Suite 3B■Seattle,Washington 98101-1028■USA T/206 285 4395•F/206 285 4516■northwest@beitcollins.com•www.beltcollins.com � r Project: Project#: Client: --�— — — Date: I; Location Page �of rY7t k g 1. Beattie Bangkok Guam t_ t Hong Kong i d lviv Honolulu S Philippines Shenzhen Singapore (P P o � �: Belt Collins Northwest LLC 1925 Post Alley,Suite 3B•Seattle,Washington 98101-1028•USA T/206 285 4395■F/206 285 4516■northwest@beltcollins.com•www.beltcollins.com