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POST-CONSTRUCTION REQUIREMENTS from THE JOINT EFFORT for Hydromodification Control & LID

POST-CONSTRUCTION REQUIREMENTS from THE JOINT EFFORT for Hydromodification Control & LID. Presented by: Dominic Roques, P.G. Central Coast Water Board. Annual ASCE Seminar February 28, 2013. Today’s Presentation. New Stormwater Permit Requirements Watershed Processes

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POST-CONSTRUCTION REQUIREMENTS from THE JOINT EFFORT for Hydromodification Control & LID

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  1. POST-CONSTRUCTION REQUIREMENTSfromTHE JOINT EFFORT for Hydromodification Control & LID Presented by: Dominic Roques, P.G. Central Coast Water Board Annual ASCE Seminar February 28, 2013

  2. Today’s Presentation • New Stormwater Permit Requirements • Watershed Processes • Post-Construction Requirements • Runoff Retention • LID Standards • Water Board Guidance

  3. NPDES Stormwater Permits General State-wide Municipal Permit adopted Feb. 5th (effective July 1) Cities and Counties are Permittees Includes Post-Construction Requirements Central Coast Permittees will use PCRs developed for this region September 6, 2013 begin implementing

  4. Post-Construction Stormwater Requirements Protect watershed processes potentially impacted by how stormwater runoff is managed once project is built

  5. Watershed Processesin the Central Coast • Overland flow • Infiltration and groundwater recharge • Interflow • Rillingand gullying • Evapotranspiration • Delivery of sediment to receiving waters • Delivery of organics to receiving waters • Chemical and biological transformations: the suite of processes that alter the chemical composition of water as it passes through the soil column on its path to a receiving water

  6. Dominant Watershed Processes Shallow subsurface flow and sediment creep

  7. Dominant Watershed Processes Groundwater infiltration

  8. Map of Watershed Management Zones

  9. Methods and Findings of the Joint Effort for Hydromodification Control in the Central Coast Region of California Prepared by: Stillwater Sciences Tetra Tech

  10. Watershed Management Zones and Selection of Regulatory Criteria Tailor management strategies to Watershed Management Zones: Sites that drain to small streams need to maintain the watershed processes that regulate flows, maintain water quality, and ensure continued delivery of sediment and organics Sites that support groundwater aquifers need to maintain high rates of infiltration

  11. Post-Construction Requirements > 2,500 ft2 Site Design / Runoff Reduction > 5,000 ft2 Water Quality Treatment > 15,000 ft2 Runoff Retention > 22,500 ft2 Peak Management

  12. Post-Construction Requirements > 2,500 ft2Site Design / Runoff Reduction Use following where feasible: • Prevent disturbance of creeks • Minimize compaction of native soils • Limit clearing and grading of native vegetation • Minimize impervious surfaces • Direct runoff

  13. Post-Construction Requirements 5,000 – 15,000 ft2Water Quality Treatment LID Treatment Systems (retain) 85th Percentile 24-hr event Biofiltration Treatment System Flow-through Treatment (SUSMP)

  14. Post-Construction Requirements > 15,000 ft2Runoff Retention Depending on Watershed Management Zone: Retain 85th or 95th Percentile via infiltration or storage

  15. Post-Construction Requirements > 22,500 ft2Peak Management Match Post-Project to Pre-Project peak flows for the 2- through 10-year storm events

  16. Meet Retention Requirements viaLID Development Standards • Site Assessment • Site Design • Site Runoff Reduction • Structural Measures

  17. Meet Retention Requirements viaLID Development Standards Site Assessment Measures – identify opportunities and constraints Document : • Site topography • Hydrologic features • Depth to seasonal high groundwater • Depth to an impervious layer such as bedrock • Presence of unique geology (e.g., karst) • Geotechnical hazards • Documented soil and/or groundwater contamination • Soil types and hydrologic soil groups • Vegetative cover/trees

  18. Meet Retention Requirements via LID Development Standards Site Design Measures – optimize the use of LID site design measures, as feasible and appropriate at the project site: • Define development envelope, protected areas, areas most suitable for development, areas to be left undisturbed • Conserve natural areas, existing trees, other vegetation, and soils • Concentrate development on portions of the site with less permeable soils, preserve areas that can promote infiltration

  19. Meet Retention Requirements via LID Development Standards Site Runoff Reduction Measures • Reduce amount of runoff for which retention and treatment is required • Direct runoff from impervious surfaces to undisturbed or natural landscaped areas • Any remaining volume must be addressed using Structural Stormwater Control Measures

  20. Meet Retention Requirements via LID Development Standards Structural Stormwater Control Measures Priorities: • Bioretention • Rainwater harvesting • Pervious Pavement • Vegetated Roofs • Soil Amendments Soil Retention Products, Inc.

  21. Post-Construction Requirements Stormwater Control Plan • Project name, type, application number… • Total project site area • Total new and/or replaced impervious surface area • Site assessment summary • Summary of Runoff Reduction Measures and Structural Stormwater Control Measures • Supporting calculations used to meet Water Quality Treatment and Runoff Retention Requirements • Documentation of infeasibility where on-site compliance can’t be achieved

  22. Adjustments and Off-Ramps Redevelopment Special Circumstances Alternative Compliance

  23. Redevelopment Projects Mitigation Required for Less than Full Area Replaced Impervious Surface X 0.5

  24. Special Circumstances Highly Altered Channels Intermediate Flow Control Facilities Historic Lake or Wetland

  25. Alternative Compliance Technical Infeasibility Urban Sustainability Area Watershed or Regional Plan

  26. Water Board Guidance For Project Applicants: Rainfall statistics – what’s my 95th percentile? Attachment D of the PCRs – why multiply? Bioretention Specs Infiltration Testing Methods For Permittees: User Guide to Municipal Implementation Guidance for Stormwater Control Plans Landscape Code revisions

  27. 95th Percentile Rain Event Variable Source:TetraTech

  28. 95th and 85thIsohyetal Maps

  29. 95th Percentile 24-hr depth = 2.1 – 2.4″

  30. Attachment DWhy multiply? Rain events do not occur in convenient 24-hr increments Facilities sized to prevent runoff from a design storm must account for this An easy, conservative approach is to use a multiplier Attachment D was included in PCRs to provide a simple, conservative way to comply

  31. Rain events do not occur in convenient 24-hr increments

  32. Runoff volume from 95th percentile 24-hr event Day 1 40th Percentile Event Day 2 45th Percentile Event Day 3 60th Percentile Event

  33. Facilities must be sized to account for Back-to-back rain events • The multiplier of 1.963 is being reconsidered based on hydrograph analysis for region • Proposed revision available next month Runoff volume from 95th percentile 24-hr event Day 1 Day 2 Day 3

  34. Guidance for Permittees

  35. Phase II MS4 General Permit Permit Workshop Central Coast Water Board Office May 16, 2013 9 AM – 12 PM

  36. Joint Effort Docs

  37. www.waterboards.ca.gov/centralcoast/ droques@waterboards.ca.gov 805.542.4780

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