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1.3 Design Criteria
The stormwater facilities were designed in accordance with the ACHD Policy Manual Section
8000 and Section 8200 relating to stormwater. See appendix B for calculations.
The design criteria for this seepage beds are:
▪ Store 100-year storm event at a 1-hour duration
▪ Infiltrate the 90% storm in 48 hours
▪ Minimum cover of 1.5-feet, unless under roadway or sidewalk then the
minimum cover is 1-foot
▪ Washed Drain Rock is 1.5-2 inches
▪ 1.5 -feet of ASTM C33 filter sand
▪ Sediment increase factor of 25%
▪ 3-feet minimum from groundwater
1.4 Calculations
1.4.1 Design Inputs
▪ Total acreage included in calculations is 0.97 acres.
▪ The runoff coefficient for the future building is 0.95 with a total area of 10,194 square
feet.
▪ The runoff coefficient for concrete and asphalt is 0.90, with a total area of 32,663 square
feet.
▪ The runoff coefficient for landscaping is 0.20, with a total area of 12,526 square feet.
▪ Time of concentration is 5 minutes.
▪ Sedimentation Increase Factor is 1.25
1.4.2 Summary of Calculations
▪ Seepage bed dimensions – 65’ X 25’ X 6’
▪ Seepage bed storage volume – 3900 CF
▪ Storm drainpipe sized for 100-year event – 12” Diameter
▪ Sand and grease trap size sized for 25-year event
See appendix B for calculations.
12/27/21
A-2021-0223