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HomeMy WebLinkAboutMOD-0806-2021 Project Drainage ReportABOVE GRADE ENGINEERING address: 245 Higuera Street San Luis Obispo, CA 93401 phone: (805) 540-51 15 web: www.abovegradeengineering.com PROJECT DRAINAGE REPORT FOR Westmont Subdivision 468 Westmont Ave. San Luis Obispo, California 93405 Prepared By: Above Grade Engineering, Inc. Scott Stokes, PE 58256 November 11, 2021 Job Number 21146 www.abovegradeengineering.com A California Corporation . Scoff Stokes PE # 58256 Westmont Subdivision Project 21146 Table of Contents Page 2 INTRODUCTION & BACKGROUND: ........................................................................................... 3 PRE -CONSTRUCTION: ................................................................................................................... 3 POST -CONSTRUCTION: ................................................................................................................ 4 DESIGNMETHODOLOGY: ............................................................................................................ 4 OVERFLOWROUTES.....................................................................................................................7 CONCLUSION.................................................................................................................................. 7 Tables Table 1: Western Property Line Runoff Rates...................................................................................5 Table 2: Peak Management Results...................................................................................................7 Exhibits / Technical Appendix Exhibit 1: Pre -Development Conditions Exhibit 2: Post -Development Conditions Attachment A: Perme4ble Paver Detail Attachment B: SWCP Application (Stormwater Control Plan for Post Construction Requirements) www.abovegradeengineering.com A California Corporation . Scott Stokes PE # 58256 Westmont Subdivision Project 21146 Page 3 d O A CAL FIRE San Luis® Obispo County Fire- wesunonl Ave 1 Westmont Avenue 468 Westmont Ave, San Stanford or Luis Obispo, CA 93405 Mental Marketing Marlme°r O 555 Westmonty Apartments Nativity of Our Lady g4 °ayDr ©Catholic Church n o = q i O Fel-Mar or I Dartmouth or Bella Montana Homes L � S Highland or v ,� Q H gh�and Dr v � O Highland or Q Highland Dr 0 C Dragonfly C Power Signs & Graphics 1 emporanly closed Sign shop Q o c Warren Way Tolosa Way INTRODUCTION & BACKGROUND: This project is located at 468 Westmont Avenue in- San Luis Obispo, California. The existing site of approximately 4.98 acres is located just on the northern edge of the City limits. Access to the site is currently from Stanford Drive but both Cuesta Drive and Westmont Avenue terminate at the edge of the property. The proposed development is to subdivide the existing parcel into 41 new smaller parcels by implementing the California Density Bonus Law. The proposed drainage concept for this project is to convey the runoff generated by each individual lot to its own respective permeable paver section where the runoff can be treated and retained to the level required. Once reaching capacity, overflow will go to the central storm drain system which will eventually terminate on the edge of the wetland area covering the existing drainage swale. This same drainage concept will also address the requirements set forth by the Waterway Management Plan. PRE -CONSTRUCTION: The existing lot consists of slopes between 5% and 50%. The site is currently developed and holds an existing house, used by college students, an asphalt driveway and two areas covered in compacted decomposed granite. The remainder of the lot contains a handful of concrete walkways but mostly existing vegetation. Existing vegetation includes local grasses and a large www.abovegradeengineering.com A California Corporation • Scott Stokes PE # 58256 Westmont Subdivision Project 21146 Page 4 sum of trees located in and around the existing drainage swale as well as a handful of trees located along the southern property line. Runoff generated by the project site generally flows from the northeast to the southwest if located to the east of the drainage swale whereas on the west side of the drainage swale, drainage patterns flow from the northwest to the southeast. Once the runoff reaches the drainage swale, it will cross the southern properly line where it will continue to flow south until it reaches the concrete channel located within the backyards of the homes encompassed by Princeton Place, Stanford Drive and north of Highland Drive. The eastern section of Westmont Avenue terminates at the edge of the eastern property line and some of its runoff historically drained onto the project site. POST -CONSTRUCTION: The proposed development is to subdivide the existing parcel into 41 new parcels. Out of the 41 new lots, 2 of the lots will contain the existing residence and the remaining 38 will be developed to provide new residential homes and their associated parking areas. The proposed drainage concept for this project is to convey the runoff generated by each new residential building and a portion of the private access road to a permeable paver section to be treated and retained (see Exhibit-2). With a new and replaced impervious area greater than 22,000 square feet but less than, the project will be required to meet: • PR-1 Site Design & Runoff Reduction • PR-2 Water Quality Treatment • PR-3 Runoff Retention • PR-4 Peak Management The project is also subject to the requirements set forth by the Waterway Management Plan which will require that any additional runoff that is created by the new development to be addressed and retained on -site so as not to increase the impacts upon the downstream drainage system. DESIGN METHODOLOGY: PR-1 Site Design & Runoff Reduction Performance Requirement #1 requires that improvements made to an existing site do no increase runoff values that historically crossed over property lines. Historically, runoff created by the project site drained to the drainage swale located near the western edge which eventually drainage across the southern property line (see Exhibit-1). www.abovegradeengineering.com A California Corporation . Scott Stokes PE # 58256 Westmont Subdivision Project 21146 Page 5 The proposed improvements subdivide the original parcel into 41 new parcels. Runoff from each of the new lots shall receive treatment, retention and detention via the permeable paver section located in the front parking lot or adjacent shared access road (see Exhibit-2 & Exhibit-3). In order to confirm that the new development does not increase the runoff from historical rates, analysis of both the pre and post development of the site has been completed. Conditions for the pre and post development reflected in Exhibit-1 thru Exhibit-3 were used to evaluate and compare the two. In Table 1 below, the contributing areas, associated surfaces types and runoff volumes have been tabulated. Table 1: Western Property Line Runoff Rates Condition DMA Impervious (sqft) Pervious (sqft) Runoff Volumes (cf) 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr Pre 1 thru 35 21,897 149,488 3,626 4,626 5,388 6,248 6,985 7,640 36 thru 38 0 45,485 889 1,133 1,320 1,531 1,712 1,872 Post 1 thru 35 73,346 98,039 4,634 5,911 6,884 7,983 8,925 9,763 36 thru 38 14,574 30,911 1,120 1,428 1,663 1,929 2,157 2,359 Difference 1 thru 35 1,008 1,285 1,496 1,735 1,940 2,123 36 thru 38 231 295 343 398 445 487 Using the analysis and results tabulated in Table 1 above, the combined volumes of all permeable paver sections shall equate to a minimum 2,123 cubic feet and 487 cubic feet for the properties east and west of the drainage swale respectively. In addition to not increasing runoff across property lines, Performance Requirement #1 also requires that at least one measure used to minimize stormwater runoff be implemented into the site design. As mentioned before, the project will be incorporating permeable pavers into the parking areas and shared driveway areas (see Exhibit-2). Runoff created by each new residence and a portion of the private access road will be directed to their respective permeable paver section. PR-2 Water Quality Treatment The impervious areas created shall receive treatment for the 85t" percentile, 24-hour storm and based on the location of the project site, this equates to a rainfall depth of 1.2-inches. Treatment will be achieved by capturing and routing the runoff generated by the private access road, sidewalks and roof drains to the immediately adjacent permeable paver section. The drain rock layer of the permeable paver section will be sized to provide the necessary treatment volume within its pore spaces. The minimum depth of each individual Drainage Management Area (DMA) was determined using the calculation below. Example: DMA - 1 Ai,,,,p = 2,105 : area of impervious, sq f t d8s = 1.2 : rainfall depth, in C = 0.90 : impervious runoff coefficient www.abovegradeengineering.com A California Corporation . Scott Stokes PE ff 58256 Westmont Subdivision Project 21146 Page 6 Vreq: required storage volume Vreq=Aimp(d85/12)C = 2,105(1.2/12)(0.9) = 189 ft3 AP = 997 : permeable paver area, sq f t d = unknown : required stone depth, f t Vreq = Apd(0.4) : required storage volume d = Vreq/(Ap X 0.4) = 189/(997 X 0.4) = 0.47 f t Using this same analysis for each DMA resulted in the majority requiring a minimum stone depth that was smaller than 1.0-ft. After analyzing each DMA for the minimum stone depth with respect to treatment volume, a similar analysis will be required to determine the minimum stone depth to contain the required retention volume. PR-3 Runoff Retention The impervious areas created shall retain the 95th percentile, 24-hour storm and based on the location of the project site, this equates to a rainfall depth of 2.0-inches. Retention will be achieved by capturing and routing the runoff generated by the private access road, sidewalks and roof drains to the immediately adjacent permeable paver section. The drain rock layer of the permeable paver section will be sized to provide the necessary retention volume within its pore spaces. The minimum depth of each individual Drainage Management Area (DMA) was determined using the calculation below. Example: DMA — 1 Aimp-new = 2,105 : area of impervious, sq f t Airnp_rep = 0 : replaced impervious area, sq f t d95 = 2.0 : rainfall depth, in C = 0.90 : impervious runoff coefficient Vreq: required storage volume Vreq = [Aimp-new + (0.5 X Aimp-rep*d95/12)C Vreq = [2,105 + (0.5 X 0)](2/12)0.9 = 316 ft3 AP = 997 : permeable paver area, sq f t d = unknown : required stone depth, f t Vreq = Apd(0.4) : required storage volume d = Vreq/(Ap X 0.4) = 316/(997 X 0.4) = 0.79 f t Using this same analysis for each DMA resulted in the majority requiring a minimum stone depth that was smaller than 1.0-ft with the exceptions being DMA-9 and DMA-37 and DMA-3 8. With this determination each permeable paver section, with the exception of the 3 mentioned above, will contain a minimum of 12-inches below the perforated pipe located at the top of the stone layer. Refer to provided Exhibit-2 and Exhibit-3 for all of the minimum permeable paver stone depths. www.abovegradeengineering.com A California Corporation . Scott Stokes PE # 58256 Westmont Subdivision Project 21146 PR-4 Peak Mana eg ment Page 7 As previously mentioned in the PR-1 section, the project will be required to detain the necessary runoff volumes in order to mimic historical runoff conditions. In order to achieve this requirement, the combined retention volumes of each permeable paver section calculated in PR-3 Runoff Retention above will need to equal or exceed the volume difference calculated in Table 1. The combined permeable paver section volume and results from Table 1 have been tabulate in Table 2 below. Table 2: Peak Management Results Table 1 Results Total Volume for 100- r (cf) 2,610 Permeable Paver Total Volume c 14,661 As we can see from Table 2, the combined volumes provided by each permeable paver section will hold almost 5.6 times the volume required per the Table 1 results thus meeting the Peak Management requirement. This determination also shows that with the proposed permeable paver sections, the Waterway Management Plan requirements will also be met. OVERFLOW ROUTES Each respective permeable paver section will contain a centrally located perforated pipe placed at the top of the stone layer. The perforations of the pipe will allow excess runoff to be routed to the proposed central storm drain system where the excess runoff will be routed towards the eastern edge of the existing drainage swale. Once reaching this edge, the storm drain system will discharge into the drainage swale to maintain historical drainage patterns. In the off chance that the perforated pipe is unable to function properly, excess runoff will build up until the permeable paver section is no longer able to accept runoff. At this point, the runoff will either drain to the adjacent landscape where it will continue to the rear of the property or the runoff will drain onto the private access road. CONCLUSION With the proposed permeable paver sections, the project will meet the requirements set forth by the Waterway Management Plan, Performance Requirement #1 thru Performance Requirement #4. www.abovegradeengineering.com A California Corporation . Scott Stokes PE # 58256 Westmont Subdivision Project 21146 EXHIBITS Page 8 www.abovegradeengineering.com A California Corporation . Scott Stokes PE # 58256 r A ►❖.❖.❖.❖.❖, .......... .......... .......... DMA-2 1) aim%= PRE -CONDITIONS: TOTAL RUNOFF COEFFICIENT DMA No. AC/CONC BLDG (sqft) LANDSCAPE DG (sqft) TOTAL (sqft) Runoff (sqft) (sqft) Coefficient 1 9,691 9,601 149,488 2,605 171.385 0.54 2 0 0 45,485 0 45,485 0.50 .0 0 30 .0 120 dVZO VAM %/////////,(IN FEET) �_T LEGEND ® BUILDING OUTLINE FLATWORK (AC OR CONC) COMPACTED DG DMA-# OUTLINE ►,M DMA-36 1\\ 1 omn-3s �. \ POST -CONDITIONS DRAINAGE MANAGEMENT AREAS (DMAs) J / POST- CON DITIONS:TREATMENT& RETENTION DMA No. AC/CONC (sqft) BLDG(sgft) LANDSCAPE (sqft) PAVERS (sqft) TOTAL (sqft) Impervious Area (sqft) 36 233 0 21,376 0 21,609 233 37 4,304 7,920 6,352 2,193 20,769 12,224 38 533 1,584 665 325 3,107 2,117 ReplacedRetention Impervious Treatment Minimum DMA No. Area (sqft) Volume (cf) Impervious Volume (cf) Volume (cf) (sqft) 36 233 21 0 35 35 37 12,224 1,100 0 1,834 1,834 38 2,117 191 0 318 318 POST -CONDITIONS: PFRMFARI F PAVFR DFSIGN Minimum Specified Paver Area Required Actual DMA No. Stone Depth Stone Depth (sqft) Volume (cf) Volume (cf) ft ft 36 _ 209 37 2,193 1,834 2.17 1,904 38 325 318 244 2.50 325 DMA-36THRU DMA-38: RUNOFF ANALYSIS Condition Runoff Coefficient Runoff Volume (cf) 2-yr 5-vr 10-vr 25-yr 50-yr 100-yr POST 0.63 1.120 1.428 1,663 1,929 2.157 2,359 ' PRE 0.50 889 1.133 295 1,320 343 1,531 1,712 1,872 DIFFERENCE 231 398 445 487 FN 60 0 30 60 120 ( IN FEET) 1 INCH = 60 FT ® BUILDING OUTLINE FLATWORK (AC OR CONIC) ® PERMEABLE PAVER DMA-k OUTLINE DMA-36 D _ DMA-37 -27 MA-2 1" 11 26 j ```�: DMA-35 DMA-35�~�� 5 DMA-9 / 24 r J / ` 23 A DMA-36 1 �) 22 DMA-20 DMA- A ' 21 DMA-8 _ --�= STORM DRAIN SIZING: CONTRIBUTING DMAs LEGEND ® ® DMA-# OUTLINE lOI ABOVE sl1RniyisinN GRADESTORM DRAIN ANALYSIS SCALE: 1 "=60' (ON 11 x17)_ ENGINEERING JOB NUMBER: 21146 245 Hi uera Sheet BY: DLL 60 0 30 60 120 9 San Luis Obispo, CA 93AOI DATE: 11 /11 /2021 (805) 5AO-5115 SHEET: ( IN FEET) abovegn7deengineering.com 1 INCH = 60 FT ACahbmlaCwpEX-4 ordlon %w %km PE i 58256 AC/GONG LANDSCAPE PAVERS Imperv ious Runoff DMA No. BLDG (sgft) TOTAL (sgft) (s ft) Is ft s ft) Area Is ftl Coefficient 1THRU 7 4.320 7,700 1,139 6,841 20,000 12,020 078 9THRU 18 & 16,062 17,600 5,917 15,047 54,626 33,662 0.77 29THRU 34 25 THRU 28 3,390 4,400 905 3,861 12,556 7,790 0.78 9THRU34 24,137 27,368 8087 23,738 83.330 51.505 0.78 Runoff Impervious Min. Pipe DMA No 2- r cfs Y ( ) 25- r cfs y ( ) 50- r cfs v ( ) 10Q r cfs v ( ) Coefficient Area (acres) Dia. (in) Westmont Subdivision Project 21146 Attachment A Page 13 www.abovegradeengineering.com A California Corporation . Scott Stokes PE # 58256 (318' MIN GAP SPACING BETWEEN Y ' PER PLAN SLOPE W X ; ell LU was. -i�. ;mod �• ' �. M CLEAN, CRUSHED, INSTALL INTERLOCKING PAVERS PER INTERLOCKING CONCRETE PAVEMENT INSTITUTE GUIDELINES & SPECIFICATION CONTRACTOR TO CONSTRUCT PAVER SECTION AT THE END OF ALL MAJOR GRADING AND CONSTRUCTION OPERATIONS OR PROTECT WITH FENCING TO KEEP FROM COMPACTION OF SOILS IN & AROUND AREA PERMEABLE PAVERS SCALE = NONE Westmont Subdivision Project 21146 Attachment B Page 15 www.abovegradeengineering.com A California Corporation . Scott Stokes PE # 58256 j' 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: John Rourke Application No: Project Name: Westmont Subdivision Location Address: 468 Westmont Ave., San Luis Obispo, CA 93405 Location APN: 052-496-001 Site Zoning: R-1 Project Type: ✓ Commercial f Detached Single Family Residential Industrial Multi -unit Residential Mixed Use Public Project Phase: Project Description: 41 Lot Single -Family Subdivision with affordable housing Total Project Site Area 1216,870 (a) Total New Impervious Surface Area = 53,325 sqft (b) Total Replaced Impervious Surface Area = 20,021 sgft (c) Total Existing Impervious Area = 21,897 sgft (d) Total Impervious Area of Completed Project = 73,346 sqft 73 346 sqft q (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 gonArea (a+b) of project is < 2,500 square feet — e OR Area (a+b) of project is > 2,500 square feet, and is a project type listed below (✓ type) — ie[] 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, ffer 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. Form revised: July 12, 2016 ALL~ 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: j wMz-i 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 ;tep 4 Project is > 2,500 square feet El Yes - Go To Requirement 1— Site Design & Runoff Reduction - Form 1 AND THEN Go To Step 5 itep 5 Detached single family residential project where Area (e) is > 15,000 square feet OR Project where Area (e) > 5,000 square feet III Yes - Go To Requirement 2 — Water Quality Treatment - Form 2 AND THEN Go To Step 6 ❑ No - Done tep 6 Detached single family residential project where Area (e) > 15,000 square feet OR Project where Area (a+b) > 15,000 square teet AND is in Watershed Management Zone 4,7,10 over a ground water basin OR in Zone 1,2,5,6,8,9 El Yes - Go To Requirement 3 — Runoff Retention - Form 3 AND THEN Go To Step 7 ❑ No - Done ;tep 7 Project where Area (a+b) > 22,000 square feet AND is in Watershed Management Zone 1,2,3,6,9 0 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. Disturbance to creeks and natural drainage features minimized. 2. Minimize compaction of highly permeable soils Compaction of permeable soils minimized where possible. 3. Minimize clearing of native vegetation and grading, conserving natural areas and maximizing undisturbed areas, and developing along natural landforms. Clearing of native vegetation and grading minimized where possible. 4. Minimize impervious surfaces including roadways and parking lots Impervious surfaces minimized where possible. 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 ✓ 0 On Site ❑ Off Site - Alternative Compliance Measure Used ✓ 111 1. Harvesting, infiltration, evapotranspiration L l 2. Bio-filtration I reatment (Document inability to use 1.) ❑ 3. Non -Retention Based Treatment (Document inability to use 1. or 2.) Description of structural controls: Permeable paver section Alternative compliance measures: Attachments Attach t Calment/54'Zing %ai%uiati0nI5, inClU'dirg any volume treated with of f_S;te �rnmrnlin„�nno • 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 Scott Stokes 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 ume tatio , Water Quality Treatment will be met through alternative compliance. 11-11-2021 Sign ure 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 I] 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 11 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 0 95" percentile event retained via infiltration ❑ Finding of technical infeasibility —Structural Stormwater Measure proposed Site in Zone 2 ❑ 95" percentile event retained via storage, harvesting, infiltration, and/or evapotranspiration ❑ Finding of technical infeasibility— Structural Stormwater Measure proposed Sit- in Zn r �wri/nr Si ❑ 85" percentile event retained via infiltration [I Finding of techniral 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 ­aCL. A+ -+,.,.a... --+ n .- 11111r,rinnc Fnr hvrirninnir• nnnlvcit and Stormwater control measure sizina MELUI.If "LLUl.III I ICIIL V 6ull. u.aw �. v....�v. ..r v........J... •••••—•,— — -"----------- - -- ■ 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 Scott Stokes 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 identiZehe a e documentation, Runoff Retention will be met through alternative compliance. 11-11-2021 Sign ture 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 0 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 Scott Stokes 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 cum tati n, Peak Management will be met through alternative compliance. 11-11-2021 Sig ture Date Stormwater Control Plan for Post Construction Requirements Exhibit 1 Watershed Management Zones T C LV E N y N c C3 Q O �N ®3 O t LCl 0 CO 01 - 1