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. 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.

2 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. 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 ..16. Stormwater Harvesting Basins ..16. Roadside Stormwater Harvesting Basins ..17. Non-contributing Basins.

3 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 ..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 .

4 30. Basin Floor Prohibitions ..31. Side Slopes ..31. Side Slope Side Slope Prohibitions ..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 ..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.

5 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 ..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 ..68. Public Health Benefits of Shaded Transportation Benefits of Shaded Basins.

6 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 ..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 .

7 76. Additional Inspection and Maintenance Requirements for Multi-Use Basins ..77. 8. COVENANTS .. 78. General Requirements ..78. 9. Detention WAIVER REQUEST AND PAYMENT OF IN-LIEU FEE .. 79. Waiver Request Requirements ..79. Tributary Location ..80. Waiver Application Requirements ..83. Waiver Request Response and Approval ..83. 10. DRAINAGE REPORT CONTENT .. 85. Content for an Approved Detention Waiver ..85. Content for LID Practices ..85. Content for Detention Basins ..86. Content for Dry Wells ..88. Required Summary Table for All Projects ..89. V. _____ Design Standards for Stormwater Detention and Retention for Pima County No. Description Page No. 11. REQUIREMENTS FOR PLATS, DEVELOPMENT PLANS AND. CONSTRUCTION PLANS .. 90. Required Content for Tentative Plats and Development Plans ..90. Detention Waiver ..90. General Notes and Permitting Required Descriptions for Detention Basins and Stormwater Harvesting Basins ..91. Requirements for LID Practices other than Stormwater Harvesting Required Detention Basin and LID Practices Content for Final Plats.

8 93. Detention Waiver ..93. General and Permitting Notes ..94. Requirements for Detention Basins and Stormwater Harvesting Basins ..95. Required Detention Basin and LID Practices Content for Construction Plans ..95. Requirements for LID Practices other than Stormwater Harvesting 12. REQUIRED CONTENT FOR AS-BUILT CERTIFICATION AND 98. General Requirements ..98. Detention Basins ..98. Stormwater Harvesting Basins ..99. Other LID 1. GLOSSARY OF TERMS .. 100. 2. REFERENCES .. 105. VI. _____ Design Standards for Stormwater Detention and Retention for Pima County LIST OF FIGURES. Figure No. Description Page No. Figure An Example LID Site Figure An Example LID Landscape Concept Plan ..5. Figure Example Commercial Site with First-flush Retention within Stormwater Harvesting Basins ..11. Figure Traditional Layout Compared to Alternative Compact Development Footprint ..14. Figure Illustration of Protection of Riparian Habitat Adjacent to Figure Elevated Grate Inlet which Maximizes Stormwater Harvesting.

9 17. Figure Minimum Setback from Property Figure Basin Depth and Freeboard Requirements ..29. Figure Basin Floor Minimum Slope for Positive Drainage ..30. Figure Riprap Blanket with Filter Fabric End Treatment ..32. Figure Multi-Level Weir Outlet for the 2-, 10-, and 100-year Storms ..36. Figure Combination Weir Culvert Outlet ..36. Figure Embankment Requirements ..39. Figure Typical Dry Well Installation ..50. Figure Sediment Trap ..54. Figure Roadside Stormwater Harvesting Basin ..57. Figure Expanded Area Non-Contributing Area Basin in a Parking Lot Median ..58. Figure Bioretention Basin ..59. Figure Terraced Detention Basin with Retention ..61. Figure Porous Gravel Cross Section ..62. Figure Concrete Grid Cross Section ..63. Figure PICP Cross-Section ..64. Figure Pervious Concrete Cross-Section ..65. Figure Sample Watershed for Example ..80. VII. _____ Design Standards for Stormwater Detention and Retention for Pima County LIST OF TABLES. Table No. Description Page No.

10 Table First-flush Retention Volume Required for Each Acre of Impervious or Disturbed Area by Type of Area ..9. Table Example Site First-flush Volume Required ..12. Table Example Site First-flush Volume Provided ..13. Table Acceptable Techniques to Minimize Disturbed, Compacted and Impervious Surfaces ..14. Table Acceptable Techniques to Protect/Enhance Regulated Riparian Habitat and Regulatory Floodplains ..15. Table Side Slope Stabilization ..31. Table Dumped or Hand Placed Riprap Gradation ..32. Table Pervious Pavement System Filter Material Gradation ..66. VIII. _____ Design Standards for Stormwater Detention and Retention for Pima County LIST OF PHOTOS. Photo No. Description Page No. Photo Roadside Stormwater Harvesting Basin at Full Depth ..18. Photo Rainfall Only Non-contributing Photo Basin Inlet with Riprap Protection ..34. Photo Basin Inlet with Inadequate Riprap Placement ..35. Photo Combination Weir Box ..37. Photo Residential Subdivision Basin with Embankment.