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Design Standards for Stormwater Detention and Retention for Pima County

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Design Standards for Stormwater Detention and Retention for Pima County Pima County Regional Flood Control District Design Standards for Stormwater Detention and Retention Supplement to Title 16, Chapter 16.48, Runoff Detention Systems Floodplain and Erosion Hazard Management Ordinance Pima County Regional Flood Control District 97 E. Congress St., 3rd Floor Tucson, AZ 85701-1791 (520) 724-4600 May 2014 Design Standards for Stormwater Detention and Retention for Pima County TABLE OF CONTENTS 1. INTRODUCTION ............................................................................................................................ 1 1.1. Purpose ................................................................................................................................................... 1 1.2. Ordinance Overview and Detention Requirements .................................................................................... 2 1.3. Applicability ................................................................................................................................................. 3 1.4. Conflicting Requirements and Use of Alternative Requirements ............................................................... 3 1.5. Low Impact Development Practices ............................................................................................................ 3 1.6. Site Planning ................................................................................................................................................ 6 1.7. Design Standards ......................................................................................................................................... 6 2. FIRST‐FLUSH RUNOFF VOLUME REDUCTION ................................................................................. 7 2.1. Applicability of First‐flush Retention and First‐flush Runoff Volume Calculation ....................................... 7 2.2. First‐flush Runoff Volume Calculation ......................................................................................................... 8 2.3. Site Planning and Preliminary Design of LID Practices to Minimize First‐flush Volume ........................... 12 2.3.1 Minimize Disturbed, Compacted and Connected Impervious Surfaces ............................................... 12 2.3.2 Protect/Enhance Riparian Habitat and Other High Permeability Areas ............................................... 14 2.4. Use of LID Practices to Retain the First‐flush Runoff Volume ................................................................... 15 2.4.1 Stormwater Harvesting Basins .......................................................................................................... 15 2.4.2 Roadside Stormwater Harvesting Basins ........................................................................................... 16 2.4.3 Non‐contributing Basins ................................................................................................................... 17 2.4.4 Bioretention Basins ........................................................................................................................ 18 2.5. Retention within Detention Basins ...................................................................................................... 19 3. PEAK DISCHARGE RATE REDUCTION ........................................................................................... 20 3.1. Peak Discharge Rate Reduction Requirements ......................................................................................... 20 3.2. Peak Discharge Rate Determination ......................................................................................................... 20 3.2.1 Pre‐Developed Conditions ................................................................................................................ 20 3.2.2 Post‐Developed Conditions .............................................................................................................. 20 3.3. Peak Discharge Rate Reduction by Stormwater Harvesting Basins .......................................................... 20 4. DETENTION BASIN DESIGN STANDARDS ..................................................................................... 25 4.1. Detention Basin General Requirements .................................................................................................... 25 4.2. Detention Basin General Prohibitions ....................................................................................................... 26 4.3. Detention Basin Location and Collection .................................................................................................. 26 4.3.1. Detention Basin Location and Collection Standards .......................................................................... 26 4.3.2. Detention Basin Location and Collection Prohibitions ....................................................................... 27 4.4. Detention Basin Depth and Freeboard ..................................................................................................... 27 4.4.1. Detention Basin Depth and Freeboard Standards ............................................................................. 27 4.4.2. Detention Basin Depth Prohibition ................................................................................................... 28 4.5. Storage Time ............................................................................................................................................. 28 4.5.1. Storage Time Standards ................................................................................................................... 28 4.6. Basin Floor 29 4.6.1. Basin Floor Standards ...................................................................................................................... 29 4.6.2. Basin Floor Prohibitions ................................................................................................................... 29 4.7. Side Slopes ................................................................................................................................................. 29 4.7.1. Side Slope Standards ....................................................................................................................... 29 4.7.2. Side Slope Prohibitions .................................................................................................................... 31 4.8. Inlet Structures .......................................................................................................................................... 32 4.8.1. Inlet Structure Standards ................................................................................................................. 32 i Design Standards for Stormwater Detention and Retention for Pima County 4.8.2. Inlet Structure Prohibitions .............................................................................................................. 34 4.9. Outlet Structures ....................................................................................................................................... 34 4.9.1. Outlet Structure Standards ...................................................................................................................... 34 4.9.2. Outlet Structure Prohibition ............................................................................................................ 37 4.10. Embankments ............................................................................................................................................ 37 4.10.1. Embankment Standards ................................................................................................................. 37 4.10.2. Embankment Prohibition ............................................................................................................... 39 4.11. Security Barrier .......................................................................................................................................... 39 4.11.1. Security Barrier Standards .............................................................................................................. 39 4.11.2. Security Barrier Prohibitions ........................................................................................................... 40 4.12. Perimeter Walls ......................................................................................................................................... 40 4.12.1. Perimeter Wall Standards .............................................................................................................. 40 4.12.2. Perimeter Wall Prohibitions ........................................................................................................... 40 4.13. Underground Storage ................................................................................................................................ 40 4.13.1. Underground Storage Standards .................................................................................................... 40 4.14. Setbacks ................................................................................................................................................. 42 4.14.1. Setback Standards ........................................................................................................................ 42 4.14.2. Setback Prohibition .......................................................................................................................
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