RES 86171 - Laserfiche WebLink

point of discharge to each most upstream inlet. The calculations must use the Rational Method and must use the most current Alameda County Flood Control District Zone 7 rainfall data for determining runoff intensity. The frequency of storm to be used in design shall be a 10 year storm for drainage basins containing up to 50 acres and a 15 year storm for drainage basins containing over 50 acres. The elevation of the hydraulic grade line must be calculated at each structure. Runoff coefficients and initial times of concentration shall be as specified in the Alameda County Flood Control and Water Conservation District Hydroloqy and Hydraulics Criteria Summary. For flat sites an assumed initial time of concentration of 5 minutes from roof to gutter may be used. If the project is within a FEMA designated Flood Zone A, City of Pleasanton and FEMA criteria must be met. If the beginning water surface in the storm system at its discharge point is a water surface which has been "backed up" from a 100 year water .surface in the main public storm system for the area, then the onsite drainage structures may be as much as 1/2 foot below the 10 year water surface calculated at the onsite structure itself. If the water surface at the discharge point has been "backed up" from a 10 or 15 year water surface in the main public storm system for the area, then the rim elevations of the onsite drainage structures must be at or above the 10 year water surface calculated at the onsite structure itself. The Manning's formula shall be used to calculate hydraulic profiles. The friction value "n" shall be as follows: Concrete, asbestos cement, or plastic pipe 0.013 Corrugated metal pipe 0.024 Earth channels - smooth geometric 0.~30 Earth channels - Irregular or natural See ACFC Manual Junction losses shall be calculated as follows: Deflection Angle of Flow 0 HGL = 0.10 V2avg 2g 45 HGL = 0.50 V2avg 2g 90 HGL = V2avg 2g 13