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HomeMy WebLinkAboutAppendix F - Stormwater Report Preliminary Waterway Management Plan and Post-Construction Requirements Report for Gas Works Mixed-Use 1390 Walker Street, 280 Pismo Street, 251 Pismo Street APNs: 002-505-005, 002-505-006, 002-505-001 Prepared: July 24, 2024 Updated: September 17, 2024 Owner: Covelop 1135 Santa Rosa Street, Suite 210 San Luis Obispo, CA 93401 Attn: Damien Mavis Phone: 805.781.3133 Email: dmavis@covelop.net Prepared by: Omni Design, Inc. Brandon L. Marchell, PE 1326 Chorro Street San Luis Obispo, CA 93401 805.544.9700 PRELIMINARY N OT F OR CONSTRU CTI O N GAS WORKS PAGE 2 OF 8 SEPTEMBER 17, 2024 I. Introduction and Pre-Development Conditions The Gas Works development is proposed on a 1.1-acre property located at the northeast corner of Walker and Pismo Streets in San Luis Obispo, California, situated in a mixed-use commercial and residential area. The property, which was previously owned by Pacific Gas and Electric Company (PG&E) is fenced and included three buildings and a shed, all of which were vacant and unoccupied. The City of San Luis Obispo has designated one of these buildings as a historical structure, which currently remains on-site. The other two buildings were demolished by PG&E in the summer of 2016 and the site was prepared for environmental cleanup. This voluntary remediation work was carried out with oversight from the California Department of Toxic Substances Control (DTSC) and the City of San Luis Obispo in anticipation of its future redevelopment. From 1905 to 1918, the San Luis Gas and Electric Company operated a manufactured gas plant on this site, using crude oil to produce gas. The gas production process also yielded by-products such as tar, light oils, sludge, and a fine black soot-like substance known as lampblack. In 1918, the availability of natural gas in the San Luis Obispo area led to the plant’s closure. PG&E acquired the site and its surrounding parcels in 1938, eventually selling all parcels by 1959. Various commercial enterprises occupied the site until 2009, when PG&E repurchased the property to facilitate investigation and remediation activities. The northern parcel falls within the Mid-Higuera Specific Plan Area and has been designated as a parking lot to avoid modifying the floodplain. Buildings will be constructed on the southern parcel, with finished-floor elevations at least one foot above the 100-year base flood elevation (BFE) and flood proofing per FEMA standards and guidelines. Figure 1: Site Vicinity Map with Watershed GAS WORKS PAGE 3 OF 8 SEPTEMBER 17, 2024 II. Methodology Runoff analysis was performed in HydroCAD—version 10.20-3g—using the following parameters: Design Storm 100 year, 24-hour Runoff Method Santa Barbara Urban Hydrograph (SBUH) Rainfall Distribution SCS 24-hour, Type II Distribution Antecedent Moisture Condition AMC II (Normal) Hydrograph Ordinate Time Increment 0.05 hour Design Infiltration Rate Negligible, HSG ‘C/D’ Time of Concentration 10.0 minutes III. Precipitation Data Per NOAA’s PFDS, the 24-hour rain event depths are as follows (see also Attachment 1): 1-year 2.13 inches 2-year 2.88 inches 5-year 3.83 inches 10-year 4.58 inches 25-year 5.57 inches 50-year 6.30 inches 100-year 7.03 inches IV. Post-Construction Requirements – Compliance Strategy Post-Development: New and Replaced Impervious Area 36,476 square feet Reduced Impervious Area Credit 4,150 square feet (based on 40,626 square feet of existing impervious surfaces) Net Impervious Area 32,326 square feet Tier 1 – Site Design & Runoff Reduction: Limit disturbance of creeks and natural drainage features and setback development from these features. No on-site natural drainage features (Redevelopment) GAS WORKS PAGE 4 OF 8 SEPTEMBER 17, 2024 Minimize compaction of highly permeable soils. N/A (Redevelopment) Minimize clearing of native vegetation and grading, conserving natural areas and maximizing undisturbed areas, and developing along natural landforms. N/A (Redevelopment) Minimize impervious surfaces including roadways and parking lots. Pervious concrete will be provided in the northern parking lot. Other Strategies: Direct roof run-off into vegetated area Direct driveway and/or parking area into vegetated area Tier 2 – Water Quality Treatment: Watershed Management Zone 1 85th Percentile 24-hour Rainfall 1.24 inches Required Infiltrative Treatment Area* 1,293 square feet Provided Infiltrative Treatment Area 7,646 square feet Treatment Structural Control Measures Landscaping *Note: The sizing factor is the ratio of the design intensity of rainfall on tributary impervious surfaces (0.2 inches/hour) to the design percolation rate in the facility (5 inches/hour), or 0.04 (dimensionless). Tier 3 – Runoff Retention: 95th Percentile 24-hour Rainfall 1.99 inches Total Runoff Generated by 95th Percentile 24-hour Rainfall Event 6,076 cubic feet Redevelopment Project Volume Reduction Factor per Resolution R3-2013-0032 §4.b.i 0.5 Required Retention Volume 0.5 (6,076) = 3,038 cubic feet Provided On-Site Retention Volume* 3,617 cubic feet Retention Structural Control Measures Bioretention Basin, Pervious Pavement *Note: Assumes 24” thick effective pervious pavement section at 35% void ratio and 6” bioretention basin ponding depth Tier 4 – Peak Management: Per City of San Luis Obispo requirements, post-development runoff shall not exceed pre-development flows for rain events up to the 100-year, 24-hour storm event, as defined by NOAA’s Precipitation GAS WORKS PAGE 5 OF 8 SEPTEMBER 17, 2024 Frequency Data Server (PFDS). Being the worst-case scenario, the 100-year, 24-hour event was chosen for design purposes. Pre-Development: Rooftops & Structures, CN = 98 10,122 square feet Asphalt & Concrete Pavement, CN = 98 30,504 square feet Dirt Parking Area, CN = 89 10,114 square feet Total Project Area, weighted CN = 96 50,740 square feet Peak Runoff 8.03 cubic feet per second at 12 hours 100-year, 24-hour Runoff Volume 27,826 cubic feet Post-Development: Existing Building Rooftop, CN = 98 1,375 square feet Proposed Building Rooftops, CN = 98 11,957 square feet Asphalt Pavement, CN = 98 16,614 square feet Hardscape, CN = 98 7,905 square feet Bioretention Basin Water Surface, CN = 98 318 square feet Pervious Pavement, CN = 82 4,925 square feet Landscaping, CN = 74 7,646 square feet Total Project Area, weighted CN = 93 50,740 square feet Peak Runoff 7.63 cubic feet per second at 12 hours 100-year, 24-hour Runoff Volume 26,202 cubic feet Required Storage = (Post-Development Runoff Volume) – (Pre-Development Runoff Volume) = = 26,202 cubic feet – 27,826 cubic feet = -1,624 cubic feet = No peak management volume needed V. Design Narrative The existing site slopes gently—1 to 3 percent—to the south. Per topographic survey and City GIS data, there are no existing public storm drain main lines to tie into along the fronting streets. The proposed site will be graded to mimic existing drainage patterns. Roof downspouts will be disconnected and directed into landscaped areas with splash blocks where feasible. Area drains will be piped into a bioretention basin and/or thickened drain rock subsection beneath pervious concrete on the northern parcel’s parking lot for infiltration. Curb cuts and surface gradient will provide overland escape into adjacent planters and ultimately the streets in larger storm events. GAS WORKS PAGE 6 OF 8 SEPTEMBER 17, 2024 V.A. Watershed Description The project lies in the northern end of the Lower San Luis Obispo Creek watershed, which ultimately drains to the Pacific Ocean at Port San Luis. Per Central Coast Regional Water Quality Control Board, this watershed is designated high-risk per 2022 dataset, though not classified as impaired. Figure 2: Overall Watershed Pollutant Status Benthic Community Effects TMDL required Chloride TMDL required Escherichia coli (E. coli) TMDL required Nitrate Being addressed with USEPA approved TMDL Oxygen, Dissolved TMDL required Sodium TMDL required Toxicity TMDL required Figure 3: Overview of Total Maximum Daily Loads (TMDLs) GAS WORKS PAGE 7 OF 8 SEPTEMBER 17, 2024 V.B. Off-Site Run-On Precipitation run-on from the adjacent 265 Pacific Street development will be intercepted by a proposed concrete v-gutter and conveyed to the existing v-gutter in Railroad Alley, for discharge onto Walker Street via an existing sidewalk underdrain. V.C. Hydrologic and Hydraulic Analysis Refer to § IV. Post-Construction Requirements – Compliance Strategy. Additionally, peak flow rates for the various 24-hour design storms have been calculated according to the criteria outlined in §II - III and are summarized below: 24-hour Design Storm (SBUH) Pre-Development Runoff Post-Development Runoff 2-year 10,478 cf 9,461 cf 10-year 17,548 cf 16,201 cf 25-year 21,694 cf 20,215 cf 100-year 27,826 cf 26,202 cf V.D. Existing and Proposed Drainage Facilities Railroad Alley possesses a concrete v-gutter which terminates in a shallow catch basin. This catch basin outlets to a sidewalk underdrain pipe into Walker Street. The v-gutter will be extended east and subsequently north, to intercept flows from the 265 Pacific Street parcel—see § V.B. above—so as not to overburden the permeable pavement section with off-site run-on. Based on field survey and record drawing information, no City storm drain mainlines exist along any of the project’s fronting streets, therefore a fill-and-spill design approach has been used, with on-site run-off being piped to permeable pavement subsections to be retained for smaller storm events and safely conveyed off-site via overland sheet flow for larger storm events. V.E. Risk Assessment/Impacts Discussion Due to the fact that the post-development project reduces the historic impervious surface quantities of the site—see attached Pre-Development Drainage Conditions map—while also removing vertical obstructions from the northern parcel within the floodway, the risk and impacts to downstream facilities and the overall watershed is deemed low. V.F. Enhancement/Mitigation The project proposes permeable pavements and ample planters and landscaped pockets to mitigate up to the 95th percentile 24-hour storm events and enhance downstream stormwater facility function by reducing run-off—see § V.C above. V.G. Unusual or Special Conditions The northern project parcel—which falls within the 100-year floodplain of the Mid-Higuera Special Zone #1—conists of replacing an existing electrical substation with a parking lot consisting of permeable pavers and asphalt. This will improve the floodway characteristics, given the reduction in vertical obstructions and fencing—see §3.5.2 of the Drainage Design Manual. GAS WORKS PAGE 8 OF 8 SEPTEMBER 17, 2024 Figure 4: Project Location within Mid-Higuera Specific Plan Area VI. Conclusions The Gas Works development serves as an integration of urban redevelopment and low-impact development practices. Situated on a historically contaminated site, the project has undergone extensive remediation and redesign to accommodate both commercial and residential use. The project seeks to meet or exceed State and City requirements for stormwater management, flood mitigation, and pollutant reduction. Post-development conditions show a notable improvement in the site’s ability to manage stormwater and reduce runoff. The implementation of bioretention basins, permeable pavement, and landscaped areas will meaningfully decrease the impervious surface area. This, in turn, reduces peak runoff flow rates during storm events and enhances water quality by promoting natural filtration and infiltration. Additionally, the removal of vertical obstructions from the northern parcel will improve floodplain dynamics, further mitigating flood risk in the surrounding area. VII. Technical Appendix of Supporting Documentation See following pages for: Pre- and Post-Development Drainage Maps, HydroCAD report, NOAA PFDS precipitation depths, 85th & 95th rainfall depths, FEMA flood map, and USDA web soil survey. BUILDING AP4: 818 SF BUILDING TS1: 1,375 BUILDING AP5: 2,266 SF BUILDING AP2: 176 SF CONCRETE P2: 760 SF UTILITY P13: 13 SF CONCRETE P10: 796 SF UTILITY P9: 117 SF CONCRETE P6: 576 SF CONCRETE P3: 509 SF UTILITY P4: 18 SF CONCRETE P11: 143 SF ASPHALT AP1: 27,505 SF BUILDING AP3: 4,399 SF STEEL P8: 21 SF STEEL P7: 26 SF STEEL P5: 26 SF STEEL P1: 15 SF BUILDING P14: 852 SF CONCRETE P12: 215 SF 170169169 168 170 171171171 172 171PACIFIC STREET RAILROAD ALLEY WASLKER STREETARCHER STREETPISMO STREET SOURCE: TOPOGRAPHIC SURVEY (TS) TYPE IMPERVIOUS AREA (SF) EXISTING BUILDING 1,375 EXISTING ASPHALT PAVEMENT 0 EXISTING CONCRETE FLATWORK 0 EXISTING STEEL TOWERS 0 EXISTING SURFACE UTILITY 0 TOTAL 1,375 SOURCE: FIGURE 2-7 CURRENT LAYOUT OF PG&E (P) TYPE IMPERVIOUS AREA (SF) EXISTING BUILDING 852 EXISTING ASPHALT PAVEMENT 0 EXISTING CONCRETE FLATWORK 2,999 EXISTING STEEL TOWERS 88 EXISTING SURFACE UTILITY 148 TOTAL 4,087 SOURCE: AERIAL PHOTO (AP) TYPE IMPERVIOUS AREA (SF) EXISTING BUILDING 7,659 EXISTING ASPHALT PAVEMENT 27,505 EXISTING CONCRETE FLATWORK 0 EXISTING STEEL TOWERS 0 EXISTING SURFACE UTILITY 0 TOTAL 35,164 AREA SUMMARY TYPE AREA (SF) EXISTING BUILDING 9,886 EXISTING ASPHALT PAVEMENT 27,505 EXISTING CONCRETE FLATWORK 2,999 EXISTING STEEL TOWERS 88 EXISTING SURFACE UTILITIES 148 EXISTING DIRT PARKING AREA 10,114 TOTAL 50,740 Project: LITTLE ALAMO MIXED-USE PARCEL 1: 1390 WALKER STREET PARCEL 2: 280 PISMO STREET PARCEL 3: 251 PISMO STREET SAN LUIS OBISPO | CALIFORNIA | 93401 1326 CHORRO STREET SAN LUIS OBISPO, CALIFORNIA 93401 C:\Users\Brandon\OneDrive - Cal y Mayor\OMNI\Projects - Data OMNI's files\1360-01 Little Alamo\_CIVIL\Sheets\EXHIBIT_STORM.dwg, PRE, Sep 11, 2024 8:51am, BrandonClient: COVELOP 1135 SANTA ROSA STREET, SUITE 210 SAN LUIS OBISPO | CALIFORNIA | 93401 THE INFORMATION CONTAINED IN THIS DOCUMENT IS PROPRIETARY TO OMNI DESIGN, INCORPORATED. THIS DOCUMENT IS PREPARED FOR A SPECIFIC SITE AND I NC OR P O RA T ES C A LCULA T I ON S BA S ED ON DA T A A VA I LA BLE FR O M T HE C LI EN T A T T HIS TIME. BY ACCEPTING AND USING THIS DOCUMENT, THE RECIPIENT AGREES TO PROTECT ITS CONTENTS FROM FURTHER D ISS EM IN ATION, (OTHER THAN T HAT WIT HI N THE O RGA N IZ A TIO N NE CESS ARY TO EV A LUA T E SU CH SPECIFICATION) WITHOUT THE WRITTEN PERMISSION OF OMNI DESIGN, INCORPORATED. THE CONTENTS OF THIS DOCUMENT ARE NOT TO BE REPRODUCED OR COPIED IN WHOLE OR IN PART WITHOUT THE WRITTEN PERMISSION O F O M N I D E S I G N , I N C O R P O R A T E D . SHEET TITLE: SHEET NUMBER: PROJECT NUMBER: 1360-01 DATE: 2024.09.11 FOR GOVERNING AGENCY USE: PRE LI MI N A R Y No.Revision Date By 40% DESIGN DEVELOPMENT DESIGN: BLM DRAWN: SA SW1.0 PRE-DEVELOPMENT DRAINAGE CONDITIONS N 0'20'20' PRE CONSTRUCTION STORM WATER CONTROL EXHIBIT SCALE: 1" = 20'24 PRE-DEVELOPMENT SUMMARY 14 LIMIT OF TRIBUTARY AREA EXISTING IMPERVIOUS AREA OBTAIN FROM TOPOGRAPHIC SURVEY EXISTING IMPERVIOUS AREA OBTAIN FROM FIGURE 2-7 CURRENT LAYOUT OF PG&E EXISTING IMPERVIOUS AREA OBTAIN FROM AERIAL PHOTO SYMBOL AND LINE TYPE LEGEND 12 EXISTING BUILDING EXISTING CONCRETE FLATWORK EXISTING ASPHALT PAVEMENT EXISTING STEEL TOWERS EXISTING SURFACE UTILITY ASPHALT 1: 7,688 SF170 169169 168 170 171171171 172 171LANDSCAPE AREAS: 8,547 SF (CUMULATIVE) PROJECT LIMITS PROJECT LIMITS PACIFIC STREET RAILROAD ALLEY WASLKER STREETARCHER STREETPISMO STREET BIORETENTION BASIN TOP = 169.5 WSEL = 169.0 BOT = 168.5 VOLUME = 130 CF ASPHALT 2: 8,498 SF PERMEABLE PAVERS 2: 2,625 SF BUILDING 1: 3,673 SFBUILDING 2: 3,387 SF BUILDING 3: 1,442 SF BUILDING 4: 1,558 SF BUILDING 5: 773 SFEXISTING BUILDING: 1,375 SFHARDSCAPE: 9,604 SF(CUMULATIVE)PERMEABLE PAVERS 1:1,851 SFTYPE AREA (SF) EXISTING BUILDING 1,375 PROPOSED BUILDING 10,833 PROPOSED ASPHALT PAVEMENT 16,186 PROPOSED HARDSCAPE 9,604 BIORETENTION BASIN WATER SURFACE 318 PROPOSED PERVIOUS PAVEMENT 4,476 LANDSCAPE 8,183 TOTAL 50,657 Xref C:\Users\Brandon\OneDrive - Cal y Mayor\CAD\BLM Stamp.dwg Project: GAS WORKS PARCEL 1: 1390 WALKER STREET PARCEL 2: 280 PISMO STREET PARCEL 3: 251 PISMO STREET SAN LUIS OBISPO | CALIFORNIA | 93401 1326 CHORRO STREET SAN LUIS OBISPO, CALIFORNIA 93401 C:\Users\Sebastian Atilano\OneDrive - Cal y Mayor\1360-01 Little Alamo\_CIVIL\Sheets\EXHIBIT_STORM.dwg, POST, Sep 16, 2024 7:38am, Sebastian AtilanoClient: COVELOP 1135 SANTA ROSA STREET, SUITE 210 SAN LUIS OBISPO | CALIFORNIA | 93401 THE INFORMATION CONTAINED IN THIS DOCUMENT IS PROPRIETARY TO OMNI DESIGN, INCORPORATED. THIS DOCUMENT IS PREPARED FOR A SPECIFIC SITE AND I NC OR P O RA T ES C A LCULA T I ON S BA S ED ON DA T A A VA I LA BLE FR O M T HE C LI EN T A T T HIS TIME. BY ACCEPTING AND USING THIS DOCUMENT, THE RECIPIENT AGREES TO PROTECT ITS CONTENTS FROM FURTHER D ISS EM IN ATION, (OTHER THAN T HAT WIT HI N THE O RGA N IZ A TIO N NE CESS ARY TO EV A LUA T E SU CH SPECIFICATION) WITHOUT THE WRITTEN PERMISSION OF OMNI DESIGN, INCORPORATED. THE CONTENTS OF THIS DOCUMENT ARE NOT TO BE REPRODUCED OR COPIED IN WHOLE OR IN PART WITHOUT THE WRITTEN PERMISSION O F O M N I D E S I G N , I N C O R P O R A T E D . SHEET TITLE: SHEET NUMBER: PROJECT NUMBER: 1360-01 DATE: 2024.09.16 FOR GOVERNING AGENCY USE: PRE LI MI N A R Y No.Revision Date By 40% DESIGN DEVELOPMENT DESIGN: BLM DRAWN: SA SW2.0 POST-DEVELOPMENT STORMWATER MANAGEMENT PLAN N 0'20'20' POST CONSTRUCTION STORM WATER CONTROL EXHIBIT SCALE: 1" = 20'24 POST-DEVELOPMENT SUMMARY 12 LIMIT OF TRIBUTARY AREA HARDSCAPE ASPHALT PAVEMENT BUILDING SYMBOL AND LINE TYPE LEGEND 11 PERVIOUS CONCRETE LANDSCAPE STABILIZED DG Type II 24-hr 95th Percentile Rainfall=1.99"Little Alamo Mixed-Use Development Printed 9/10/2024Prepared by Omni Design, Inc Page 1HydroCAD® 10.20-3g s/n 04452 © 2023 HydroCAD Software Solutions LLC Subcatchment 1S: Pre-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)210Type II 24-hr 95th Percentile Rainfall=1.99" Runoff Area=50,740 sf Runoff Volume=6,833 cf Runoff Depth=1.62" Tc=10.0 min CN=89/98 2.07 cfs Subcatchment 2S: Post-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)210Type II 24-hr 95th Percentile Rainfall=1.99" Runoff Area=50,740 sf Runoff Volume=6,076 cf Runoff Depth=1.44" Tc=10.0 min CN=77/98 1.81 cfs Type II 24-hr 2-yr Rainfall=2.88"Little Alamo Mixed-Use Development Printed 9/10/2024Prepared by Omni Design, Inc Page 2HydroCAD® 10.20-3g s/n 04452 © 2023 HydroCAD Software Solutions LLC Subcatchment 1S: Pre-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)3210Type II 24-hr 2-yr Rainfall=2.88" Runoff Area=50,740 sf Runoff Volume=10,478 cf Runoff Depth=2.48" Tc=10.0 min CN=89/98 3.13 cfs Subcatchment 2S: Post-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)3210Type II 24-hr 2-yr Rainfall=2.88" Runoff Area=50,740 sf Runoff Volume=9,461 cf Runoff Depth=2.24" Tc=10.0 min CN=77/98 2.79 cfs Type II 24-hr 10-yr Rainfall=4.58"Little Alamo Mixed-Use Development Printed 9/10/2024Prepared by Omni Design, Inc Page 3HydroCAD® 10.20-3g s/n 04452 © 2023 HydroCAD Software Solutions LLC Subcatchment 1S: Pre-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)543210Type II 24-hr 10-yr Rainfall=4.58" Runoff Area=50,740 sf Runoff Volume=17,548 cf Runoff Depth=4.15" Tc=10.0 min CN=89/98 5.14 cfs Subcatchment 2S: Post-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)543210Type II 24-hr 10-yr Rainfall=4.58" Runoff Area=50,740 sf Runoff Volume=16,201 cf Runoff Depth=3.83" Tc=10.0 min CN=77/98 4.74 cfs Type II 24-hr 25-yr Rainfall=5.57"Little Alamo Mixed-Use Development Printed 9/10/2024Prepared by Omni Design, Inc Page 4HydroCAD® 10.20-3g s/n 04452 © 2023 HydroCAD Software Solutions LLC Subcatchment 1S: Pre-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)76543210Type II 24-hr 25-yr Rainfall=5.57" Runoff Area=50,740 sf Runoff Volume=21,694 cf Runoff Depth=5.13" Tc=10.0 min CN=89/98 6.31 cfs Subcatchment 2S: Post-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)6543210Type II 24-hr 25-yr Rainfall=5.57" Runoff Area=50,740 sf Runoff Volume=20,215 cf Runoff Depth=4.78" Tc=10.0 min CN=77/98 5.90 cfs Type II 24-hr 100-yr Rainfall=7.03"Little Alamo Mixed-Use Development Printed 9/10/2024Prepared by Omni Design, Inc Page 5HydroCAD® 10.20-3g s/n 04452 © 2023 HydroCAD Software Solutions LLC Subcatchment 1S: Pre-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)876543210Type II 24-hr 100-yr Rainfall=7.03" Runoff Area=50,740 sf Runoff Volume=27,826 cf Runoff Depth=6.58" Tc=10.0 min CN=89/98 8.03 cfs Subcatchment 2S: Post-Development Site Runoff Hydrograph Time (hours) 484644424038363432302826242220181614121086420Flow (cfs)876543210Type II 24-hr 100-yr Rainfall=7.03" Runoff Area=50,740 sf Runoff Volume=26,202 cf Runoff Depth=6.20" Tc=10.0 min CN=77/98 7.63 cfs NOAA Atlas 14, Volume 6, Version 2 Location name: San Luis Obispo, California, USA* Latitude: 35.2731°, Longitude: -120.6687° Elevation: 170 ft** * source: ESRI Maps ** source: USGS POINT PRECIPITATION FREQUENCY ESTIMATES Sanja Perica, Sarah Dietz, Sarah Heim, Lillian Hiner, Kazungu Maitaria, Deborah Martin, Sandra Pavlovic, Ishani Roy, Carl Trypaluk, Dale Unruh, Fenglin Yan, Michael Yekta, Tan Zhao, Geoffrey Bonnin, Daniel Brewer, Li-Chuan Chen, Tye Parzybok, John Yarchoan NOAA, National Weather Service, Silver Spring, Maryland PF_tabular | PF_graphical | Maps_&_aerials PF tabular PDS-based point precipitation frequency estimates with 90% confidence intervals (in inches)1 Duration Average recurrence interval (years) 1 2 5 10 25 50 100 200 500 1000 5-min 0.160 (0.140‑0.185) 0.195 (0.170‑0.226) 0.241 (0.209‑0.279) 0.278 (0.239‑0.326) 0.328 (0.270‑0.403) 0.367 (0.294‑0.463) 0.406 (0.315‑0.529) 0.446 (0.333‑0.603) 0.500 (0.354‑0.713) 0.541 (0.367‑0.807) 10-min 0.229 (0.201‑0.265) 0.279 (0.244‑0.323) 0.345 (0.300‑0.400) 0.398 (0.343‑0.467) 0.470 (0.387‑0.577) 0.526 (0.421‑0.663) 0.582 (0.451‑0.758) 0.639 (0.478‑0.864) 0.717 (0.508‑1.02) 0.776 (0.526‑1.16) 15-min 0.277 (0.243‑0.320) 0.338 (0.295‑0.391) 0.417 (0.363‑0.484) 0.481 (0.414‑0.565) 0.569 (0.468‑0.698) 0.636 (0.509‑0.802) 0.703 (0.545‑0.917) 0.773 (0.578‑1.04) 0.867 (0.614‑1.24) 0.938 (0.636‑1.40) 30-min 0.382 (0.334‑0.442) 0.466 (0.407‑0.539) 0.575 (0.500‑0.667) 0.663 (0.571‑0.779) 0.784 (0.645‑0.962) 0.876 (0.701‑1.10) 0.969 (0.751‑1.26) 1.06 (0.796‑1.44) 1.19 (0.846‑1.70) 1.29 (0.876‑1.93) 60-min 0.539 (0.472‑0.623) 0.657 (0.573‑0.760) 0.810 (0.705‑0.941) 0.935 (0.805‑1.10) 1.10 (0.909‑1.36) 1.24 (0.989‑1.56) 1.37 (1.06‑1.78) 1.50 (1.12‑2.03) 1.68 (1.19‑2.40) 1.82 (1.24‑2.72) 2-hr 0.814 (0.712‑0.941) 1.01 (0.881‑1.17) 1.26 (1.09‑1.46) 1.46 (1.25‑1.71) 1.72 (1.42‑2.11) 1.92 (1.54‑2.42) 2.12 (1.64‑2.76) 2.32 (1.74‑3.14) 2.59 (1.83‑3.69) 2.79 (1.89‑4.16) 3-hr 1.03 (0.903‑1.19) 1.29 (1.13‑1.50) 1.62 (1.41‑1.89) 1.89 (1.62‑2.22) 2.24 (1.84‑2.74) 2.50 (2.00‑3.15) 2.75 (2.13‑3.59) 3.01 (2.25‑4.07) 3.35 (2.37‑4.78) 3.61 (2.45‑5.38) 6-hr 1.39 (1.21‑1.60) 1.78 (1.55‑2.05) 2.26 (1.97‑2.63) 2.65 (2.28‑3.11) 3.16 (2.60‑3.87) 3.53 (2.83‑4.46) 3.90 (3.02‑5.08) 4.27 (3.19‑5.77) 4.75 (3.36‑6.77) 5.11 (3.46‑7.62) 12-hr 1.72 (1.50‑1.98) 2.26 (1.97‑2.61) 2.94 (2.56‑3.41) 3.47 (2.99‑4.08) 4.18 (3.44‑5.12) 4.69 (3.76‑5.92) 5.21 (4.04‑6.78) 5.72 (4.27‑7.72) 6.38 (4.52‑9.10) 6.88 (4.66‑10.2) 24-hr 2.13 (1.95‑2.38) 2.88 (2.62‑3.22) 3.83 (3.48‑4.30) 4.58 (4.12‑5.19) 5.57 (4.83‑6.55) 6.30 (5.34‑7.60) 7.03 (5.79‑8.72) 7.76 (6.19‑9.93) 8.72 (6.64‑11.7) 9.44 (6.92‑13.2) 2-day 2.62 (2.39‑2.93) 3.61 (3.30‑4.04) 4.89 (4.44‑5.49) 5.91 (5.32‑6.69) 7.26 (6.30‑8.55) 8.28 (7.01‑9.98) 9.30 (7.66‑11.5) 10.3 (8.24‑13.2) 11.7 (8.90‑15.7) 12.7 (9.33‑17.7) 3-day 2.96 (2.70‑3.30) 4.11 (3.75‑4.60) 5.61 (5.10‑6.30) 6.81 (6.14‑7.72) 8.43 (7.31‑9.92) 9.66 (8.18‑11.6) 10.9 (8.97‑13.5) 12.2 (9.69‑15.5) 13.8 (10.5‑18.5) 15.1 (11.1‑21.1) 4-day 3.21 (2.94‑3.59) 4.48 (4.09‑5.02) 6.14 (5.58‑6.89) 7.49 (6.74‑8.48) 9.30 (8.06‑10.9) 10.7 (9.05‑12.9) 12.1 (9.96‑15.0) 13.5 (10.8‑17.3) 15.5 (11.8‑20.7) 17.0 (12.4‑23.7) 7-day 3.84 (3.51‑4.29) 5.32 (4.85‑5.96) 7.30 (6.63‑8.19) 8.92 (8.04‑10.1) 11.2 (9.68‑13.1) 12.9 (10.9‑15.6) 14.7 (12.1‑18.2) 16.6 (13.2‑21.2) 19.1 (14.6‑25.6) 21.1 (15.5‑29.4) 10-day 4.30 (3.93‑4.81) 5.94 (5.41‑6.64) 8.13 (7.39‑9.13) 9.96 (8.97‑11.3) 12.5 (10.8‑14.7) 14.5 (12.3‑17.5) 16.6 (13.6‑20.5) 18.7 (14.9‑24.0) 21.7 (16.5‑29.1) 24.1 (17.7‑33.6) 20-day 5.49 (5.01‑6.13) 7.54 (6.88‑8.44) 10.3 (9.39‑11.6) 12.7 (11.4‑14.4) 15.9 (13.8‑18.8) 18.5 (15.7‑22.4) 21.3 (17.5‑26.3) 24.1 (19.2‑30.8) 28.1 (21.4‑37.7) 31.3 (22.9‑43.6) 30-day 6.64 (6.06‑7.41) 9.08 (8.28‑10.2) 12.4 (11.3‑13.9) 15.2 (13.7‑17.2) 19.1 (16.5‑22.4) 22.2 (18.8‑26.7) 25.4 (20.9‑31.4) 28.7 (22.9‑36.8) 33.5 (25.5‑44.8) 37.2 (27.2‑51.8) 45-day 8.14 (7.43‑9.09) 11.0 (10.1‑12.4) 15.0 (13.6‑16.8) 18.2 (16.4‑20.7) 22.8 (19.8‑26.8) 26.4 (22.3‑31.8) 30.1 (24.8‑37.3) 34.0 (27.2‑43.6) 39.5 (30.0‑52.9) 43.8 (32.0‑61.0) 60-day 9.61 (8.78‑10.7) 12.9 (11.8‑14.4) 17.3 (15.8‑19.5) 21.0 (18.9‑23.8) 26.1 (22.7‑30.8) 30.1 (25.5‑36.3) 34.3 (28.2‑42.5) 38.6 (30.8‑49.3) 44.5 (33.9‑59.6) 49.2 (36.0‑68.5) 1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS). Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates (for a given duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds are not checked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values. Please refer to NOAA Atlas 14 document for more information. Back to Top PF graphical 7/11/24, 11:09 AM Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=35.2731&lon=-120.6687&data=depth&units=english&series=pds 1/4 Back to Top Maps & aerials Small scale terrain 7/11/24, 11:09 AM Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=35.2731&lon=-120.6687&data=depth&units=english&series=pds 2/4 Large scale terrain Large scale map Large scale aerial + – 3km 2mi + – 100km 60mi + – 100km 60mi 7/11/24, 11:09 AM Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=35.2731&lon=-120.6687&data=depth&units=english&series=pds 3/4 Back to Top US Department of Commerce National Oceanic and Atmospheric Administration National Weather Service National Water Center 1325 East West Highway Silver Spring, MD 20910 Questions?: HDSC.Questions@noaa.gov Disclaimer + – 100km 60mi 7/11/24, 11:09 AM Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=35.2731&lon=-120.6687&data=depth&units=english&series=pds 4/4 National Stormwater Calculator Report Results Little Alamo Rainfall Retention Frequency Runoff Contribution by Rainfall Percentile National Flood Hazard Layer FIRMette 0 500 1,000 1,500 2,000250 Feet Ü SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT SPECIAL FLOOD HAZARD AREAS Without Base Flood Elevation (BFE) Zone A, V, A99 With BFE or DepthZone AE, AO, AH, VE, AR Regulatory Floodway 0.2% Annual Chance Flood Hazard, Areas of 1% annual chance flood with average depth less than one foot or with drainage areas of less than one square mileZone X Future Conditions 1% Annual Chance Flood HazardZone X Area with Reduced Flood Risk due to Levee. See Notes.Zone X Area with Flood Risk due to LeveeZone D NO SCREEN Area of Minimal Flood Hazard Zone X Area of Undetermined Flood HazardZone D Channel, Culvert, or Storm Sewer Levee, Dike, or Floodwall Cross Sections with 1% Annual Chance 17.5 Water Surface Elevation Coastal Transect Coastal Transect Baseline Profile Baseline Hydrographic Feature Base Flood Elevation Line (BFE) Effective LOMRs Limit of Study Jurisdiction Boundary Digital Data Available No Digital Data Available Unmapped This map complies with FEMA's standards for the use of digital flood maps if it is not void as described below. The basemap shown complies with FEMA's basemap accuracy standards The flood hazard information is derived directly from the authoritative NFHL web services provided by FEMA. This map was exported on 5/10/2024 at 11:17 AM and does not reflect changes or amendments subsequent to this date and time. The NFHL and effective information may change or become superseded by new data over time. This map image is void if the one or more of the following map elements do not appear: basemap imagery, flood zone labels, legend, scale bar, map creation date, community identifiers, FIRM panel number, and FIRM effective date. Map images for unmapped and unmodernized areas cannot be used for regulatory purposes. Legend OTHER AREAS OF FLOOD HAZARD OTHER AREAS GENERAL STRUCTURES OTHER FEATURES MAP PANELS 8 B 20.2 The pin displayed on the map is an approximate point selected by the user and does not represent an authoritative property location. 1:6,000 120°40'27"W 35°16'40"N 120°39'50"W 35°16'11"N Basemap Imagery Source: USGS National Map 2023 PROJECT SITE Soil Map—San Luis Obispo County, California, Coastal Part (Little Alamo) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/11/2024 Page 1 of 33905780390580039058203905840390586039058803905900390580039058203905840390586039058803905900711930711950711970711990712010712030712050712070712090712110712130712150 711930 711950 711970 711990 712010 712030 712050 712070 712090 712110 712130 712150 35° 16' 25'' N 120° 40' 11'' W35° 16' 25'' N120° 40' 2'' W35° 16' 21'' N 120° 40' 11'' W35° 16' 21'' N 120° 40' 2'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 25 50 100 150 Feet 0 5 10 20 30 Meters Map Scale: 1:609 if printed on B landscape (17" x 11") sheet. Soil Map may not be valid at this scale. MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: San Luis Obispo County, California, Coastal Part Survey Area Data: Version 16, Sep 11, 2023 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Mar 12, 2022—Apr 12, 2022 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—San Luis Obispo County, California, Coastal Part (Little Alamo) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/11/2024 Page 2 of 3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI 198 Salinas silty clay loam, 2 to 9 percent slopes, MLRA 14 1.4 100.0% Totals for Area of Interest 1.4 100.0% Soil Map—San Luis Obispo County, California, Coastal Part Little Alamo Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/11/2024 Page 3 of 3 San Luis Obispo County, California, Coastal Part 198—Salinas silty clay loam, 2 to 9 percent slopes, MLRA 14 Map Unit Setting National map unit symbol: 2tyxy Elevation: 0 to 1,480 feet Mean annual precipitation: 16 to 26 inches Mean annual air temperature: 55 to 63 degrees F Frost-free period: 300 to 360 days Farmland classification: Prime farmland if irrigated Map Unit Composition Salinas and similar soils:85 percent Minor components:15 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Salinas Setting Landform:Flood plains, terraces, alluvial fans Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Parent material:Alluvium derived from sedimentary rock Typical profile H1 - 0 to 26 inches: silty clay loam H2 - 26 to 70 inches: silty clay loam Properties and qualities Slope:2 to 9 percent Depth to restrictive feature:More than 80 inches Drainage class:Well drained Runoff class: Negligible Capacity of the most limiting layer to transmit water (Ksat):Moderately high (0.20 to 0.57 in/hr) Depth to water table:More than 80 inches Frequency of flooding:None Frequency of ponding:None Calcium carbonate, maximum content:5 percent Maximum salinity:Nonsaline to very slightly saline (0.0 to 2.0 mmhos/cm) Available water supply, 0 to 60 inches: High (about 10.8 inches) Interpretive groups Land capability classification (irrigated): 2e Land capability classification (nonirrigated): 3e Hydrologic Soil Group: C Ecological site: R014XD109CA - FINE LOAMY BOTTOM Hydric soil rating: No Map Unit Description: Salinas silty clay loam, 2 to 9 percent slopes, MLRA 14---San Luis Obispo County, California, Coastal Part Little Alamo Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/11/2024 Page 1 of 2 Minor Components Pachic haploxerolls, fine Percent of map unit:6 percent Landform:Flood plains, terraces, alluvial fans Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Salinas, mod deep to deep to clay Percent of map unit:3 percent Landform:Flood plains, terraces, alluvial fans Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Marimel, silty clay loam Percent of map unit:3 percent Landform:Flood plains, terraces, alluvial fans Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Cropley, clay Percent of map unit:3 percent Landform:Flood plains, terraces, alluvial fans Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Data Source Information Soil Survey Area: San Luis Obispo County, California, Coastal Part Survey Area Data: Version 16, Sep 11, 2023 Map Unit Description: Salinas silty clay loam, 2 to 9 percent slopes, MLRA 14---San Luis Obispo County, California, Coastal Part Little Alamo Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 7/11/2024 Page 2 of 2