Design Standards for Stormwater Detention and Retention

1 Pima County Regional Flood Control District Design Standards for Stormwater Detention and Retention Supplement to Title 16, Chapter , 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. June 2014. _____ Design Standards for Stormwater Detention and Retention for Pima County REVISIONS. Because of ongoing regulatory and technical changes in the fields of floodplain and Stormwater management, revisions to this manual will be required from time to time.

2 Such revisions will be approved by the Floodplain Administrator. Hard copy (printed) revisions will not be distributed. It is the holder's responsibility to keep the document current by periodically checking the Regional Flood Control District's web page for new digital versions. The revision history of the document is listed below. Chronology of Publication, Updates and Revisions Description Date First Edition June 2014. Chapter 6 Revised to Include Benefits of February 2015. Multiple-Use Basins I. _____ Design Standards for Stormwater Detention and Retention for Pima County TABLE OF CONTENTS.

3 No. Description Page No. 1. INTRODUCTION .. 1. Purpose ..1. Ordinance Overview and Detention Requirements ..2. Conflicting Requirements and Use of Alternative Requirements ..3. Low Impact Development Practices ..3. Site Design Standards ..7. 2. FIRST-FLUSH RUNOFF VOLUME REDUCTION .. 8. Applicability of First-flush Retention and First-flush Runoff Volume Calculation ..8. First-flush Runoff Volume Calculation ..9. Site Planning and Preliminary Design of LID Practices to Minimize First-flush Volume ..13. Minimize Disturbed, Compacted and Connected Impervious Surfaces ..14. Protect/Enhance Riparian Habitat and Other High Permeability Use of LID Practices to Retain the First-flush Runoff Volume.

4 16. Stormwater Harvesting Basins ..16. Roadside Stormwater Harvesting Basins ..17. Non-contributing Basins ..18. Bioretention Basins ..19. Retention within Detention Basins ..20. 3. PEAK DISCHARGE RATE REDUCTION .. 21. Peak Discharge Rate Reduction Requirements ..21. Peak Discharge Rate Determination ..21. Pre-Developed Conditions ..21. Post-Developed Conditions ..21. Peak Discharge Rate Reduction by Stormwater Harvesting Methods to Calculate the Peak Discharge Rate Reduction by Detention Basins ..24. 4. Detention BASIN Design Standards .. 26. Detention Basin General Requirements.

5 26. Detention Basin General Prohibitions ..27. Detention Basin Location and Detention Basin Location and Collection Standards ..27. Detention Basin Location and Collection Prohibitions ..28. II. _____ Design Standards for Stormwater Detention and Retention for Pima County No. Description Page No. Detention Basin Depth and Freeboard ..29. Detention Basin Depth and Freeboard Standards ..29. Detention Basin Depth Prohibition ..29. Storage Time ..30. Storage Time Standards ..30. Storage Time Basin Basin Floor Standards ..30. Basin Floor Prohibitions ..31. Side Slopes ..31. Side Slope Side Slope Prohibitions.

6 33. Inlet Inlet Structure Standards ..33. Inlet Structure Outlet Outlet Structure Standards ..35. Outlet Structure Prohibition ..38. Embankments ..38. Embankment Embankment Prohibition ..40. Security Barrier ..40. Security Barrier Standards ..40. Security Barrier Prohibitions ..41. Perimeter Walls ..41. Perimeter Wall Standards ..41. Perimeter Wall Prohibitions ..41. Underground Storage ..42. Underground Storage Standards ..42. Underground Storage Prohibitions ..43. Setbacks ..43. Setback Setback Prohibition ..43. Elevation Requirements ..43. Maintenance Access ..44. Maintenance Access Standards .

7 44. Maintenance Access Prohibition ..44. Landscaping Other than Riparian Habitat Mitigation ..44. III. _____ Design Standards for Stormwater Detention and Retention for Pima County No. Description Page No. Landscaping Prohibitions ..44. Pumps ..45. Pump Standards ..45. Pump Dry Wells ..47. Dry Well Standards ..47. Dry Well Prohibitions ..51. 5. LOW IMPACT DEVELOPMENT Design Standards .. 52. Low Impact Development General Requirements ..52. Low Impact Development General Stormwater Harvesting Basins ..53. General Standards for Stormwater Harvesting Basins ..53. Stormwater Harvesting Basin Prohibition.

8 56. Roadside Stormwater Harvesting Basins ..56. Stormwater Harvesting Basins Adjacent to Parking Areas ..57. Non-contributing Basin Standards ..57. Bioretention Basin ..58. Retention within a Detention Basin ..60. Pervious Porous Gravel ..62. Concrete Grid Permeable Interlocking Concrete Pavement ..64. Pervious Concrete ..65. General Standards for Pervious Pavements ..65. Practices that Increase the Time of Concentration ..67. Lengthened Flow Path Standards ..67. 6. MULTIPLE-USE AND MULTIPLE BENEFIT CONCEPTS .. 68. Multiple Benefit Concepts ..68. Resource Benefits of Shaded Basins.

9 68. Public Health Benefits of Shaded Transportation Benefits of Shaded Basins ..69. Benefits of Basins to Habitat ..69. Property Value Benefits ..69. Multiple Use Concepts ..70. Human Activity Zones ..70. Basins Containing Landscape Bufferyards ..70. IV. _____ Design Standards for Stormwater Detention and Retention for Pima County No. Description Page No. 7. INSPECTION AND MAINTENANCE REQUIREMENTS .. 71. Inspection and Maintenance General Inspection and Maintenance Requirements for All Basin Location and Collection ..71. Basin Depth and Freeboard ..72. Basin Storage Basin Basin Side Slope.

10 73. Basin Inlet Structure ..73. Basin Outlet Basin Maintenance Access ..74. Basin Landscaping ..74. Additional Inspection and Maintenance Requirements for Detention Basins ..74. Detention Basin Embankment ..74. Detention Basin Security Barrier ..74. Detention Basin Perimeter Wall ..75. Detention Basin Underground Storage System ..75. Detention Basin Equipment Elevation ..75. Detention Basin Pump ..75. Detention Basin Dry Well(s) ..76. Additional Inspection and Maintenance Requirements for Bioretention Additional Inspection and Maintenance Requirements for Detention Basins with Retention .