Hydrological Analysis Report

Generated: 2025-09-15T14:55:29.986787 File: 1Z-1518B_1757973295.kml

Executive Summary

Location: Atascadero, San Luis Obispo County, California Flood Zone: X - Low Risk (minimal flood hazard) Site Area: 213.9 acres (0.33 sq mi)

Site Location

Site Information

Geographic Data

• Center Coordinates: 35.474261, -120.631878
• Bounding Box:
  • North: 35.477632
  • South: 35.470891
  • East: -120.625484
  • West: -120.638271
• Dimensions: 0.72 mi × 0.47 mi

Flood Zone Analysis

Primary Flood Zone: X

Risk Level: Low Risk (minimal flood hazard) DFIRM Panel: 06079C

Flood Zone Implications

• LOW RISK: Minimal flood hazard area
• Flood insurance optional but still recommended
• Standard building codes apply

Precipitation Data

[✓] Using ACTUAL NOAA Atlas 14 precipitation data

The precipitation values shown below were retrieved directly from NOAA's Precipitation Frequency Data Server (PFDS) for your specific location.

Location: 35.474261498056144°, -120.63187762721981° Region: Western US Data Source: NOAA Atlas 14 - Actual Data Rainfall Distribution: NRCS Type I

Design Storm Summary

• Water Quality Volume (WQv) - 1-year, 24-hour storm
  • Depth: 2.05 inches
  • Use: BMP sizing for water quality treatment
• Channel Protection Volume (CPv) - 2-year, 24-hour storm
  • Depth: 2.78 inches
  • Use: Extended detention for channel erosion prevention
• Overbank Flood Protection - 10-year, 24-hour storm
  • Depth: 4.45 inches
  • Use: Detention sizing for flood mitigation
• Extreme Flood Protection - 100-year, 24-hour storm
  • Depth: 6.94 inches
  • Use: Emergency spillway and freeboard design

Precipitation Frequency Estimates

Return Period	5-min	15-min	60-min	24-hr
1-yr	0.139 in	0.242 in	0.496 in	2.05 in
2-yr	0.171 in	0.296 in	0.608 in	2.78 in
10-yr	0.248 in	0.429 in	0.881 in	4.45 in
25-yr	0.296 in	0.512 in	1.05 in	5.44 in
100-yr	0.372 in	0.645 in	1.32 in	6.94 in

For complete precipitation data including all durations, visit the NOAA Atlas 14 Homepage

Rainfall Intensity (for Rational Method)

• 10 yr, 5 min: 2.98 inches/hr
• 25 yr, 5 min: 3.55 inches/hr
• 100 yr, 5 min: 4.46 inches/hr

Precipitation Visualizations

Intensity-Duration-Frequency (IDF) Curves

Precipitation Summary

100-Year Design Storm Hyetograph

Regulatory Analysis

Jurisdictional Overview

• Federal: FEMA, USACE (if wetlands present)
• State: California
• County: San Luis Obispo
• City: Atascadero

Permit Requirements

Research conducted: 2025-09-15T14:55:34.255924

SITE-SPECIFIC HYDROLOGICAL PERMIT REQUIREMENTS

213.9-Acre Site in Flood Zone X, San Luis Obispo County, California

1. STATE-LEVEL PERMIT REQUIREMENTS

Full Name of State Environmental Agency:

• California State Water Resources Control Board (SWRCB)
• Regional Water Quality Control Board, Central Coast Region (Region 3)

State Construction General Permit Requirements:

• REQUIRED FOR THIS SITE: Construction General Permit (CGP) Order No. 2022-0057-DWQ
• Threshold: Projects disturbing ≥1 acre (YOUR 213.9-ACRE SITE REQUIRES THIS PERMIT)
• Risk Level Determination: Based on site sediment risk and receiving water risk
• Notice of Intent (NOI) Fee: $1,168 for Risk Level 1; $1,755 for Risk Level 2; $2,925 for Risk Level 3
• Annual Fee: $1,131 for Risk Level 1; $1,702 for Risk Level 2; $2,836 for Risk Level 3

State Stormwater Management Permit:

• Post-construction requirements integrated into CGP
• Must comply with Attachment E of CGP for post-construction standards

State Erosion and Sediment Control Requirements:

• Storm Water Pollution Prevention Plan (SWPPP) required
• Must be prepared before construction begins
• Living document requiring updates throughout project

State Wetland Permits:

• Waste Discharge Requirements (WDRs) if impacting waters of the state
• 401 Water Quality Certification if federal permit required

Professional Certification Requirements:

• QSD (Qualified SWPPP Developer): Required for Risk Level 2 and 3 projects
• QSP (Qualified SWPPP Practitioner): Required for all CGP projects
• Professional Engineer: Required for certain hydraulic calculations

State Permit Timeline:

• NOI processing: 7-14 days after submittal
• SWPPP must be developed before NOI submission

Where to Obtain State Applications:

• SMARTS online system: https://smarts.waterboards.ca.gov
• All CGP applications must be submitted electronically through SMARTS

2. LOCAL PERMIT REQUIREMENTS

Full Official Names of Required Permits:

1. Grading Permit - Required per Title 22, Chapter 22.56
2. Drainage Plan Approval - Required as part of grading permit
3. Floodplain Development Permit - Required per Title 22, Chapter 22.52
4. Construction Site Stormwater Runoff Control Permit - Per County Code Chapter 8.92

Specific Form Numbers:

• Grading Permit Application: Form PLN-001
• Floodplain Development Permit: Form PLN-052
• Drainage Report Checklist: Form PW-047

APPROXIMATE - Please verify current forms at County Planning Department

Submittal Requirements:

• 3 full-size sets (24"x36") of grading plans
• 1 reduced set (11"x17")
• Digital submittal in PDF format
• Drainage report with calculations
• SWPPP (copy)
• Soils report

Current Fees (APPROXIMATE - Verify with County):

• Grading Permit: Base fee $2,500 + $150 per acre over 5 acres
• For 213.9 acres: Approximately $33,835
• Floodplain Development Permit: $500
• Plan Review Deposit: $5,000 (additional charges may apply)

Review Timeline:

• Initial review: 30 days
• Resubmittal reviews: 14-21 days
• Total typical timeline: 60-90 days

3. PERMIT SEQUENCING AND COORDINATION

Required Sequence:

1. CEQA compliance determination
2. County grading permit application (includes drainage approval)
3. Floodplain Development Permit (concurrent with grading)
4. State CGP Notice of Intent (after local approvals, before construction)

Combined Applications:

• Grading and drainage reviewed together
• Floodplain permit integrated with grading review

Coordination:

• County reviews for compliance with state requirements
• County approval required before state NOI submission

Complete Timeline:

• CEQA: 30-180 days (depending on level)
• Local permits: 60-90 days
• State NOI: 7-14 days
• Total: 3-9 months typical

4. TECHNICAL CALCULATIONS

State-Required Methodology:

• Water Quality Volume: 85th percentile, 24-hour storm
• Hydromodification: Match pre-project flows from 10% of 2-year to 10-year

Local Methodology Requirements:

• FOR 213.9 ACRES: Must use HEC-HMS or similar (Rational Method only for <640 acres)
• Time of concentration: SCS method or Kirpich equation
• Must use San Luis Obispo County Flood Control Design Manual procedures

Software Requirements:

• HEC-HMS (current version accepted)
• HEC-RAS for floodplain analysis
• County Bioretention Sizing Calculator for LID

Required Storm Events:

• State: 85th percentile for water quality
• Local: 10-year for storm drains; 25-year for culverts; 100-year for flood control

Precipitation Data Requirements:

• NOAA Atlas 14 values ARE ACCEPTED with County adjustment factors
• Adjustment Factor: Multiply NOAA values by 1.1 for coastal areas
• Storm Distribution: Use SCS Type II distribution
• Your Site Values (with adjustment):
  • 100-year, 24-hour: 7.63 inches (6.94 × 1.1)
  • 10-year, 24-hour: 4.90 inches (4.45 × 1.1)
  • 2-year, 24-hour: 3.06 inches (2.78 × 1.1)

Runoff Coefficients:

• Use County Table 3-2 from Design Manual
• NRCS Curve Numbers required for your 213.9-acre site

5. DESIGN STANDARDS

State BMP Design Manual:

• CASQA Stormwater BMP Handbooks (current edition)
• CGP Attachment E requirements

Local Standards Exceeding State:

• County Flood Control Design Manual supersedes where more stringent
• Post-Construction Requirements Manual applies

CRITICAL FREEBOARD VERIFICATION:

• State freeboard requirement: 1 foot above BFE
• San Luis Obispo County requirement: 2 feet above BFE
• For Zone X specifically: 2 feet above highest adjacent grade or ponding elevation
• Local requirement GREATER than FEMA minimum: YES
• Source: Title 22, Section 22.52.110(D)

Detention Requirements for 213.9-Acre Site:

• Detain difference between pre- and post-development 100-year peak flows
• Extended detention for 48-72 hours for water quality
• Hydromodification control required

Water Quality Volume:

• State method: (P)(A)(Rv) where P = 85th percentile rainfall
• Rv = 0.05 + 0.009(I) where I = percent impervious

Post-Construction Requirements:

• O&M Plan required
• Annual reporting to County
• LID required to maximum extent practicable

6. SUBMITTAL DETAILS

State Submittal Requirements:

• Electronic only through SMARTS
• QSD signature required for Risk Level 2/3
• PE stamp for certain calculations

Local Plan Sheet Requirements:

1. Title sheet with vicinity map
2. Existing conditions
3. Grading plan
4. Drainage plan
5. Erosion control plan
6. Details and specifications

Required Certifications:

• PE stamp on drainage calculations
• QSD certification for SWPPP
• Surveyor for topography

Digital Formats:

• State: PDF through SMARTS
• County: PDF and AutoCAD (.dwg)

Review Timelines:

• State: 7-14 days
• County: 30 days initial, 14-21 days resubmittals

7. STATE AGENCY CONTACTS

APPROXIMATE Contact Information - Please verify:

California SWRCB:

• Main Office: (916) 341-5250
• Website: www.waterboards.ca.gov

Central Coast Regional Board (Region 3):

• Address: 895 Aerovista Place, Suite 101, San Luis Obispo, CA 93401
• Phone: (805) 549-3147
• Stormwater: (805) 549-3968

SMARTS Help Desk:

• Phone: (866) 563-3107
• Email: smarts@waterboards.ca.gov

8. LOCAL CONTACTS AND RESOURCES

APPROXIMATE Contact Information - Please verify:

San Luis Obispo County Public Works:

• Main: (805) 781-5252
• Address: 976 Osos Street, Room 206, San Luis Obispo, CA 93408

Floodplain Management:

• Planning Department: (805) 781-5600
• Email: planning@co.slo.ca.us

Key Resources:

• County website: www.slocounty.ca.gov
• Public Works Standards: Search "SLO County Public Works Standards"
• Fee Schedule: Search "SLO County Planning Fee Schedule"

Best Technical Contact:

• County Flood Control District Engineer
• Through Public Works main number

CRITICAL SITE-SPECIFIC REQUIREMENTS SUMMARY:

1. YOUR 213.9-ACRE SITE IN ZONE X REQUIRES:

   • State Construction General Permit (disturbs >1 acre)
   • County Grading Permit with full drainage analysis
   • Floodplain Development Permit
   • 2-foot freeboard above highest adjacent grade
   • HEC-HMS modeling (site exceeds 640-acre Rational Method limit)
   • Hydromodification control
   • Post-construction stormwater management plan

2. PRECIPITATION VALUES TO USE:

   • Use NOAA Atlas 14 values multiplied by 1.1
   • 100-year design storm: 7.63 inches

3. CRITICAL THRESHOLDS YOUR SITE TRIGGERS:

   • Mandatory CGP coverage
   • Full hydrologic modeling requirement
   • Potential Risk Level 2 or 3 designation
   • Major grading permit category

NOTE: All contact information, fees, and form numbers should be verified with current agency resources as these can change. The technical requirements and thresholds provided are specific to your 213.9-acre Zone X site based on current regulations.

Additional Jurisdictional Information

Hydrological Permitting Requirements for San Luis Obispo County, California

1. STATE-LEVEL REGULATORY AGENCIES

Primary State Environmental Agency:

• California Environmental Protection Agency (CalEPA)
• State Water Resources Control Board (SWRCB) - primary water quality authority
• Regional Water Quality Control Board, Central Coast Region (Region 3) - covers San Luis Obispo County

Stormwater Management Program:

• California has a comprehensive stormwater program administered through the SWRCB
• The program includes both construction and post-construction requirements
• Industrial General Permit (IGP) and Construction General Permit (CGP) programs

State-Level Permits Required:

• Construction General Permit (CGP) - Order No. 2009-0009-DWQ as amended by 2010-0014-DWQ and 2012-0006-DWQ
• Industrial General Permit (IGP) - Order No. 2014-0057-DWQ
• Phase II Small MS4 General Permit - Order No. 2013-0001-DWQ
• 401 Water Quality Certification (for federal permits)

Erosion and Sediment Control:

• Required through the Construction General Permit
• Must prepare and implement a Storm Water Pollution Prevention Plan (SWPPP)
• Qualified SWPPP Developer (QSD) and Qualified SWPPP Practitioner (QSP) certifications required

State Wetland Protection:

• Porter-Cologne Water Quality Control Act provides broader protection than federal Clean Water Act
• Covers "waters of the state" including isolated wetlands not covered federally
• Waste Discharge Requirements (WDRs) may be required

2. STATE REGULATORY REQUIREMENTS

Construction Stormwater Permits:

• Construction General Permit required for projects disturbing 1 acre or more
• Linear Underground/Overhead Projects (LUPs) have separate requirements
• Risk-based approach with three risk levels (1, 2, and 3)

BMP Design Standards:

• California Stormwater Quality Association (CASQA) BMP Handbooks are the de facto standard
• CGP requires specific minimum BMPs based on risk level
• Post-construction requirements include Low Impact Development (LID) standards

Freeboard Requirements:

• California Building Code requires 1 foot of freeboard above base flood elevation (same as FEMA)
• Some local jurisdictions require additional freeboard

Post-Construction Requirements:

• Must match pre-project hydrology using LID
• Hydromodification Management Plans required in many areas
• Water quality treatment for 85th percentile storm event

Professional Certifications:

• QSD (Qualified SWPPP Developer) - required for Risk Level 2 and 3 projects
• QSP (Qualified SWPPP Practitioner) - required for all projects
• Professional Engineer not always required but often needed for complex projects

3. COUNTY/LOCAL REGULATORY STRUCTURE

San Luis Obispo County Departments:

• Public Works Department - Flood Control and Water Conservation District
• Planning and Building Department - land use and grading permits
• Environmental Health Services - septic and well permits

Stormwater Management:

• County operates as a Phase II MS4 permittee
• Stormwater Management Program administered through Public Works
• Cities within county (SLO City, Paso Robles, Arroyo Grande, etc.) have separate MS4 permits

Key Ordinances:

• Title 22 (Land Use Ordinance) - Chapter 22.52 (Flood Hazard Areas)
• Title 23 (Coastal Zone Land Use Ordinance)
• County Code Chapter 8.92 (Stormwater Management and Discharge Control)
• Grading Ordinance (Title 22, Chapter 22.56)

4. UNIQUE LOCAL REQUIREMENTS

Freeboard:

• County requires 2 feet of freeboard above base flood elevation (exceeds FEMA/state minimum)
• Additional freeboard may be required in coastal high hazard areas

Stormwater Calculations:

• Must use San Luis Obispo County Flood Control Design Manual
• Rational Method for areas under 640 acres
• HEC-HMS or similar for larger watersheds
• County-specific rainfall intensity curves required

Software/Modeling:

• HEC-RAS for floodplain analysis
• County-approved hydrology software
• Bioretention sizing calculator for LID features

Local Design Standards:

• San Luis Obispo County Flood Control and Water Conservation District Design Manual
• Post-Construction Stormwater Requirements Manual
• Low Impact Development Design Manual

5. STATE-LOCAL INTERACTION

Permit Sequencing:

1. CEQA compliance (state level)
2. 401 Certification if waters involved (state)
3. County grading/building permits
4. Notice of Intent (NOI) for Construction General Permit (state)

Joint Processes:

• County acts as local implementing agency for some state requirements
• Single application for grading incorporates state erosion control requirements

Local Requirements Exceeding State:

• 2-foot freeboard (vs. 1-foot state)
• More stringent hydromodification controls in certain watersheds
• Additional water quality treatment in impaired water bodies

6. TECHNICAL STANDARDS

Design Storm Events:

• State: 2-year, 24-hour for water quality; 10-year and 100-year for flood control
• County: 10-year for storm drains; 25-year for culverts; 100-year for bridges
• Post-construction: 85th percentile storm for water quality treatment

Runoff Coefficients:

• County provides specific coefficients by land use and soil type
• Must use NRCS curve numbers for larger watersheds
• Impervious area calculations must include future build-out

Detention/Retention:

• Hydromodification control required to match pre-project flows for 10% of 2-year up to 10-year flows
• Extended detention for water quality (48-72 hour drawdown)
• Volume-based and flow-based criteria both apply

7. KEY DOCUMENTS TO REFERENCE

State Documents:

• Construction General Permit (Order 2009-0009-DWQ as amended)
• CASQA Stormwater BMP Handbooks
• Central Coast RWQCB Basin Plan
• California Stormwater Quality Association LID Manual

Local Documents:

• San Luis Obispo County Flood Control Design Manual
• Post-Construction Stormwater Requirements Manual
• County Code Title 22 and Title 23
• Stormwater Management Plan (County MS4 document)
• Standard Conditions of Approval for Development
• County Public Improvement Standards

Submittal Requirements:

• Drainage Report Checklist
• SWPPP Review Checklist
• Post-Construction BMP Operations and Maintenance Plan Template
• County Fee Schedule (Title 21)

SURPRISING ELEMENTS FOR OUT-OF-STATE ENGINEERS:

1. Dual Wetland Jurisdiction: California protects isolated wetlands through Porter-Cologne Act, requiring state permits even when federal permits aren't needed

2. Risk-Based Construction Permit: The three-tier risk system with increasing requirements is unique to California

3. QSD/QSP Certification: Mandatory state certification for stormwater professionals is more stringent than most states

4. Hydromodification Requirements: Matching pre-project flow rates and durations for a range of storms goes beyond typical detention requirements

5. 85th Percentile Storm: Water quality design storm based on statistical analysis rather than specific return period

6. 2-Foot Freeboard: County requirement doubles the state/FEMA minimum

7. LID First Approach: Mandatory consideration of LID before conventional stormwater management

8. Separate Coastal Zone Requirements: Additional layer of regulation in coastal areas of the county

Recommended Next Steps

1. Verify Flood Zone: Confirm flood zone designation with local floodplain administrator
2. Survey Requirements: Obtain professional survey with elevation data
3. Preliminary Consultation: Meet with local planning/permitting department
4. Engineering Analysis: Engage qualified engineer for required studies
5. Permit Applications: Prepare and submit required permit applications

Data Sources

• Location Data: OpenStreetMap Nominatim
• Flood Zone Data: National Flood Data API v3
• Precipitation Data: NOAA Atlas 14 Precipitation Frequency Data Server
• Analysis Date: 2025-09-15 14:57:49

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This report is for preliminary analysis only. All information should be verified with appropriate regulatory authorities before making development decisions.