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Home My WebLink About2019.03.04; French Hospital Hydrology Report French Hospital Medical Center 1911 Johnson Ave San Luis Obispo, CA March 04, 2019 AV Job # 181067 Hydrology Report and Post Construction Stormwater Control Plan Prepared for: Dignity Health San Luis Obispo, CA Prepared By: Ashley & Vance Engineering, Inc. 1413 Monterey Street San Luis Obispo, CA 93401 805.545.0010 French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 1 March 04, 2019 Attn: Lee Brennan Cuningham Group Architects 8665 Hayden Place Culver City, CA 90232 Subject: French Hospital Medical Center Re: Hydrology Report and Stormwater Control Plan Lee: Enclosed is the Hydrology Report for the above-referenced project. The study calculations were prepared using the Rational Method per City of San Luis Obispo standards, and the Santa Barbara Urban Hydrograph Method per the Central Coast Regional Water Quality Control Board Standards. As detailed in the project conclusion, this project is well suited to meet hydrologic and Low Impact Design standards and goals. Please contact me for any clarifications or supporting information you need with reference to this report. Regards, Zach Brown Ken Brown, PE Ashley & Vance Engineering French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 2 Contents Introduction and Background ....................................................................................................................... 3 Existing Conditions .................................................................................................................................... 3 Proposed Project ....................................................................................................................................... 3 Site Location Description .............................................................................................................................. 4 Site Location Map ..................................................................................................................................... 5 Water Quality Analysis .................................................................................................................................. 5 Drainage Management Area Description ................................................................................................. 5 Performance requirement 1-Site Design and Runoff Reduction .............................................................. 6 Performance requirement 2-Water Quality Treatment ........................................................................... 6 Performance requirement 3-Runoff Retention ........................................................................................ 6 Performance requirement 4-Peak Management ..................................................................................... 6 Hydrologic Analysis ....................................................................................................................................... 6 Hydraulic Calculations ................................................................................................................................... 6 Geotechnical and Structural Analysis ........................................................................................................... 6 Risk Assessment / Impacts ............................................................................................................................ 7 Enhancement / Mitigation ............................................................................................................................ 7 Unusual or Special Conditions ...................................................................................................................... 7 Report Conclusion ......................................................................................................................................... 7 French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 3 Introduction and Background The intent of this report is to address the requirements of the City of San Luis Obispo’s Drainage Design Manual, as well as the requirements of the Regional Water Quality Control Board’s (RWQCB) Post Construction Stormwater Management Requirements (PCMR) for Development Projects in the Central Coast Region. Calculations to determine the amount of water requiring treatment and retention were performed using the Santa Barbara Urban Hydrograph Method per the Water Board requirements. Calculations to verify sizing of drainage conveyance devices were based on the Rational Method per the Drainage Design Manual. This project will result in the replacement of over 22,000 square feet of impervious surface area and is in Watershed Management Zone 3, and as a result, is required to meet Performance Requirements 1, 2, and 4. Additionally, Resolution ARC-1024-16 is applicable to a portion of the project covered in this report, all work associated with the Parking Structure and Pharmacy Building, also described as the “Parking Structure.” That resolution contains Condition of Approval 25, below, that applies to this Hydrology Report and Stormwater Control Plan: 25. The revisions to the medical arts building [Parking Structure and Pharmacy Building] is considered part of the larger campus entitlements. As such, this portion of the project is not subject to the current Post Construction Stormwater Regulations. The architect of record or engineer of record shall provide an entitlement summary to clarify that compliance is limited to the previous regulations. This project is subject to the previous Low Impact Development Standards. The civil plans and landscape plans shall show and note how the roof drainage and drainage from the parking structure roof deck will be disconnected or otherwise conveyed to landscape planters/basins, vegetated swales, or areas of porous paving prior to discharge and collection in the campus drainage piping system. Runoff from trash enclosure areas shall be treated as required per City Engineering Standard 10108. Existing Conditions The existing site consists of a working medical campus and surrounding parking lots. Some parking areas will be removed and replaced with the “Parking Structure” building and the “Patient Tower” building. The parking removed will be replaced within the Parking Structure building. The site is predominantly on the southeast (Parking Structure), and southwest (Patient Tower) of the existing medical campus. The site slopes to the south and southwest, towards an open space. There is an existing onsite storm drainage system that currently outfalls to this open space. The onsite soils are Type D per the Web Soil Survey, and existing cover is predominantly buildings or parking with interspersed landscaping. Proposed Project The proposed project includes replacing existing parking areas with two new medical buildings and related hardscape and landscape. French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 4 Patient Tower Runoff from the Patient Tower portion of the site will be treated and infiltrated directly in planters or the storage chambers that will be added beneath the parking lot to the southwest of the building. Once the underground chambers are full, water will flow through a new storm drain to a new manhole that connects to the nearby existing storm drain pipe that conveys water to the open space to the west of the building. The Patient Tower portion of the site results in an increase of less than 10,000 square feet of impervious area, but the runoff will be reduced below pre-project levels due to the addition of stormwater storage chambers. Parking Structure Per Resolution ARC-1024-16, this portion of the project “is considered part of the larger campus entitlements. As such, this portion of the project is not subject to the current Post Construction Stormwater Regulations.” However, this portion of the project “is subject to the previous Low Impact Development Standards,” and so roof drainage and other hardscape drainage will be conveyed to landscaped planters and vegetated swales prior to discharge into the existing storm drainage system. The Parking Structure portion of the site does not result in an increase in impervious area, and any runoff will first be conveyed to landscaped planters and vegetated swales for treatment and infiltration, so runoff will match or be below pre-project levels. Runoff for this portion of the site will be directed into the existing storm drainage system after flowing through the landscaped planters and vegetated swales, per Condition 25 of Resolution ARC-1024-16. Site Location Description The site is located on the southwest of Johnson Avenue between Breck Street and Ella Street. French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 5 Site Location Map Water Quality Analysis This project is located within Watershed Management Zone 3 and thus is required to retain the 85 th percentile storm for this region. The site will be analyzed using HydroCAD and the Santa Barbara Urban Hydrograph Method. Drainage Management Area Description The site is divided up into DMAs based on the surface type and location. Runoff from each DMA will receive treatment prior to leaving the site. The Final version of this report will show all DMAs on drainage exhibits. Patient Tower For the Patient Tower portion of the site, roof and parking lot runoff will be directed to landscaped planters, where water will be infiltrated through a bioinfiltration media, and into an underdrain that will then be conveyed to the underground chamber system. These chambers will infiltrate some water into the native ground and provide a controlled outfall into the existing storm drainage system that will be less than the pre-project flows. Parking Structure For the Parking Structure portion of the site, roof and parking lot runoff will be directed to landscaped planters, where water will be infiltrated through a bioinfiltration media, and into an underdrain that will French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 6 then be conveyed to the existing storm drainage system. Since this portion of the project does not increase impervious areas and adds landscaped infiltration areas, post-project flows will be less than the pre-project flows. Performance Requirements Per Resolution ARC-1024-16, the Parking Structure portion of this project is not subject to the current Performance Requirements, but is subject to Low Impact Design requirements. Therefore, the Stormwater Control Plan for Post Construction Requirements Worksheet, attached, includes areas for only the Patient Tower Portion of the project. Performance requirement 1-Site Design and Runoff Reduction The Patient Tower portion of this project will direct driveway, walkways, patios, and/or parking areas into vegetated areas to the maximum extent feasible. Portions of the site slope to landscaping prior to entering catch basins allowing runoff to slow down and percolate into native soils. Performance requirement 2-Water Quality Treatment Runoff from the Patient Tower Building, flatwork and parking areas onsite will receive treatment in inground planters or directly in the stormwater chambers. Sizing for the individual treatment devices will be provided in the final report, and will be based on the 24-hour 85th percentile event. Performance requirement 3-Runoff Retention Since this project is located in Watershed Management Zone 3, Performance Requirement 3 is not applicable. Performance requirement 4-Peak Management The patient tower portion of this project results in an increase in impervious area of less than 10,000 square feet, however, it will also include a chamber storage system. This will result in a decrease in runoff discharged to the existing storm drainage system. Hydrologic Analysis Due to the inclusion of a chamber storage system has been reduced for all design storms. Hydraulic Calculations In the final report, individual drainage conveyance and storage devices will be sized and the calculations included here. Geotechnical and Structural Analysis No Geotechnical concerns at this time due to this project. French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 7 Risk Assessment / Impacts The risk to downstream improvements is negligible. Impacts are negligible as project flows are decreased from the pre-project condition. Enhancement / Mitigation No downstream enhancements/mitigations are required as a result of this project. Unusual or Special Conditions There are no notable special conditions or unusual site conditions associated with this project. Report Conclusion The proposed project meets the City criteria for Hydrology and Storm Water Management. This report was prepared in compliance with:  SLO Water Management Plan Volume III Drainage Design Manual  SLO Uniform Design Criteria Std 1010.B French Hospital Medical Center Hydrology Report and Post Construction Stormwater Control Plan Page | 8 Stormwater Control Plan for Post Construction Requirements Worksheet Stormwater Control Plan for Post Construction Requirements Exhibit 4 Application Submittal Where directions state “Done” that means no additional information or forms below that point needs to be filled out or furnished. See Exhibits for Watershed Management Zones, Basins, & Urban Sustainability Areas Use “n/a” where information requested is not applicable. If you are unsure regarding how to fill out any of the information, please come in and request assistance from a staff person. Project Information Step 1 Applicant Name: Application No: Project Name: Location Address: Location APN: Site Zoning: Project Type:  Commercial Detached Single Family Residential Industrial Multi-unit Residential Mixed Use Public Project Phase: Project Description: Total Project Site Area = (a) Total New Impervious Surface Area = (b) Total Replaced Impervious Surface Area = (c) Total Existing Impervious Area = (d) Total Impervious Area of Completed Project = (e) Net Impervious Area: (a+b) – (c-d) = OR where (c-d) is a negative number: (a+b) = Your project is NOT subject to Post Construction Requirements if… Step 2  Area (a+b) of project is < 2,500 square feet – Done OR  Area (a+b) of project is > 2,500 square feet, and is a project type listed below ( type) – Done  Road & parking surface repair – slurry & fog & crack seal, pothole & spot patching, overlay & resurfacing & other damage repair with no expansion  Road & parking shoulder grading  Road & parking cleaning, repairing, maintaining, reshaping, regarding drainage systems  Sidewalk & bike path / lane project – no other impervious surface created and runoff is directed to vegetated area  Curb & gutter improvement or replacement – no other impervious created  Underground utility project – surface replaced in kind  Utility vaults – Ex: lift stations, backflows  Fuel storage – above ground with spill containment  Photovoltaic systems – on existing impervious surface, over pervious surface with vegetated cover, buffer strip at the most down gradient row of panels  Second story – no increase in building footprint  Decks & stairs & walkways – raised with space below for drainage  Temporary structures – in place less than 6 months  Trails and pathways, where no other impervious surfaces are replaced or created, and built to direct stormwater runoff to adjacent vegetated areas. Stormwater Control Plan for Post Construction Requirements Otherwise, your project is subject to the Post Construction Requirements Project Site Details Step 3 Watershed Management Zone: Urban Sustainability Area Name:  Meet USA Conditions  See Area calculations in Step 1 to compare to thresholds in each Step below  Where directions state “Go To”, fill out and attach the referenced Form and any supporting documents Step 4 Project is > 2,500 square feet  Yes ‐ Go To Requirement 1 – Site Design & Runoff Reduction ‐ Form 1 AND THEN Go To Step 5 Step 5 Detached single family residential project where Area (e) is > 15,000 square feet OR Project where Area (e) > 5,000 square feet  Yes ‐ Go To Requirement 2 – Water Quality Treatment ‐ Form 2 AND THEN Go To Step 6  No ‐ Done Step 6 Detached single family residential project where Area (e) > 15,000 square feet OR Project where Area (a+b) > 15,000 square feet AND is in Watershed Management Zone 4,7,10 over a ground water basin OR in Zone 1,2,5,6,8,9  Yes ‐ Go To Requirement 3 – Runoff Retention ‐ Form 3 AND THEN Go To Step 7  No ‐ Done Step 7 Project where Area (a+b) > 22,000 square feet AND is in Watershed Management Zone 1,2,3,6,9  Yes ‐ Go To Requirement 4 – Peak Management ‐ Form 4  No ‐ Done Exhibits 1. Watershed Management Zones 2. Groundwater Basin Location 3. Watershed Management Zone Revision Request 4. Urban Sustainability Conditions and Maps of Approved Areas Stormwater Control Plan for Post Construction Requirements Form 1 Requirement 1 – Site Design and Runoff Reduction: Identify the strategies used to reduce runoff through site design. Strategies 1-5 required. Describe or attach simple plan details for 1. – 5. 1.Limit disturbance of creeks and natural drainage features and setback development from these features. 2.Minimize compaction of highly permeable soils 3.Minimize clearing of native vegetation and grading, conserving natural areas and maximizing undisturbed areas, and developing along natural landforms. 4.Minimize impervious surfaces including roadways and parking lots 5.Do one of the following:  Direct roof run off into cistern, rain barrel, or vegetated area Direct driveway, walkways, patios, and/or parking area into vegetated area Construct surfaces (bike lanes, walks, driveways, parking areas,patios) with permeable surfaces 6.Other (Optional): Identify strategy(s) and describe or show how it will be done in the project. Stormwater Control Plan for Post Construction Requirements Form 2 Requirement 2 – Water Quality Treatment: (Reference Post Construction Stormwater Management Requirements for Development Projects in the Central Coast Region – Adopted July 12, 2013 California Regional Water Quality Control Board Central Coast Region – for details regarding requirements – Section B.3 and Section C. Alternative Compliance.) Treatment Location   On Site  Off Site - Alternative Compliance Measure Used   1. Harvesting, infiltration, evapotranspiration  2. Bio-filtration Treatment (Document inability to use 1.)  3. Non-Retention Based Treatment (Document inability to use 1. or 2.) Description of structural controls: Alternative compliance measures: Attachments  Attach treatment/sizing calculations, including any volume treated with off-site compliance.  Attach construction and planting details and specifications for bio-filtration options  Attach documentation regarding Treatment Measure selection  Attach infeasibility analysis where alternative compliance is proposed. Certification I certify that the systems selected and sized, as demonstrated in the attached calculations, meet the Water Quality Treatment required for this project per the Post Construction Requirements adopted by the Central Coast Regional Water Quality Control Board. Where identified in the attached documentation, Water Quality Treatment will be met through alternative compliance. Signature Date Stormwater Control Plan for Post Construction Requirements Form 3 Requirement 3 – Runoff Retention: (Reference Post Construction Stormwater Management Requirements for Development Projects in the Central Coast Region – Adopted July 12, 2013 California Regional Water Quality Control Board Central Coast Region – for details regarding requirements – Section B.4 and Section C. Alternative Compliance.)  If a revision to the site’s Watershed Management Zone is being requested, attach Watershed Management Revision Request Form (Exhibit) and supporting documentation.  Rainfall maps are available from the Regional Water Quality Control Board Site Assessment Measures Summary  Attach documentation of the following information:  Site topography  Development envelope  Hydrologic features including natural areas, wetlands, watercourses, seeps, springs, and required setbacks  Vegetative cover including trees  Open space requirements  Location of groundwater wells used for drinking water  Depth to seasonal high groundwater  Soil types and hydrologic soil groups  Depth to impervious layer such as bedrock  Presence of unique geology (e.g. karst)  Geotechnical hazards  Existing structures, utilities, and drainage infrastructure including municipal storm drain system components  Existing easements and covenants  Documented soil or groundwater contamination  Source and estimated stormwater run-on from offsite, coming to project area  Drainage Management Areas (B.4.d.iii)  Drainage management strategies by Drainage Management Area  Runoff reduction measures and any structural control measures by Drainage Management Area (or full site as appropriate)  Technical infeasibility limits on-site compliance  10% of equivalent impervious surface area is dedicated to retention based stormwater control measures – No alternative compliance for retention Runoff volume - compliance not achieved on-site:  Alternative compliance for retention proposed Runoff volume – compliance not achieved onsite: Runoff volume – alternative compliance used: Analysis and Sizing  Attach calculated Tributary Areas and Design Volumes per the Post Construction Stormwater Management Requirements – Attachment D  Adjustment made for redevelopment  Adjustment made for being in, and meeting requirements of, an Urban Sustainability Area Stormwater Control Plan for Post Construction Requirements Form 3 Control Mechanism Site in Zone 1, 4, 7, and/or 10 and over groundwater basin  95th percentile event retained via infiltration  Finding of technical infeasibility – Structural Stormwater Measure proposed Site in Zone 2  95th percentile event retained via storage, harvesting, infiltration, and/or evapotranspiration  Finding of technical infeasibility – Structural Stormwater Measure proposed Site in Zone 5 and/or 8  85th percentile event retained via infiltration  Finding of technical infeasibility – Structural Stormwater Measure proposed Site in Zone 6 and/or 9  85th percentile event retained via storage, harvesting, infiltration, and/or evapotranspiration  Finding of technical infeasibility – Structural Stormwater Measure proposed Attachments  Attach Attachment D calculations for hydrologic analysis and stormwater control measure sizing  Attach discussion of technical infeasibility for structural stormwater measure, where proposed in lieu of preferred storage, harvesting, infiltration, and/or evapotranspiration, include justification for any non- retention based controls  Attach documentation of technical infeasibility for on-site compliance, including a site specific hydrologic and/or design analysis supporting findings  Attach description of alternative compliance project including a summary description of pollutant and flow reduction comparing the expected aggregate results of the alternate project to the results that would otherwise have been achieved by meeting the numeric performance requirements onsite.  Attach Attachment E calculations for retention requirement adjustment for technical infeasibility  Attach Attachment F calculations for off-site retention requirements  Attach agreement for alternative compliance site use, for purposes of achieving compliance  Attach Operations and Maintenance Plan for all stormwater control measures (include any Peak Management facilities) Post-Construction Owner Identification At the time of completion of the construction work, and the shift to post-construction stormwater controls, the below listed owner is responsible for Operations and Maintenance of stormwater control measures: (If responsibilities are divided, list all responsible owners and associated measures.) Certification I certify that the systems selected, sized, and designed as demonstrated in the attached calculations, meet the Runoff Retention Performance Requirement for this project per the Post Construction Requirements adopted by the Central Coast Regional Water Quality Control Board. Where identified in the attached documentation, Runoff Retention will be met through alternative compliance. Signature Date Stormwater Control Plan for Post Construction Requirements Form 4 Requirement 4 – Peak Management (Reference Post Construction Stormwater Management Requirements for Development Projects in the Central Coast Region – Adopted July 12, 2013 California Regional Water Quality Control Board Central Coast Region – for details regarding requirements – Section B.5) Show any stormwater control measures used to meet the requirements of this section, in the documentation and attachments required for Retention (Form 3), including in all mapping and Operations and Maintenance materials. Peak Management Compliance  Post-development peak flows, discharged from the site, do not exceed pre-project peak flows for the 2 through 10 years storm events.  Technical infeasibility limits on-site compliance  Alternative compliance for retention proposed Runoff volume – compliance not achieved onsite: Runoff volume – alternative compliance used: Attachments  Attach calculations showing pre-project discharge and post-project discharge for the 2 through 10 year storm events  Attach documentation of technical infeasibility for on-site compliance, including a site specific hydrologic and/or design analysis supporting findings  Attach description of alternative compliance project including a summary description of pollutant and flow reduction comparing the expected aggregate results of the alternate project to the results that would otherwise have been achieved by meeting the numeric performance requirements onsite.  Attach agreement for alternative compliance site use, for purposes of achieving compliance Post-Construction Owner Identification At the time of completion of the construction work, and the shift to post-construction stormwater controls, the below listed owner is responsible for Operations and Maintenance of the peak management control measures: (If responsibilities are divided, list all responsible owners and associated measures.) Certification I certify that the systems selected, sized, and designed as demonstrated in the attached calculations, meet the Peak Management requirements for this project per the Post Construction Requirements adopted by the Central Coast Regional Water Quality Control Board. Where identified in the attached documentation, Peak Management will be met through alternative compliance. Signature Date