City of Milpitas
Engineering Design Guidelines
Storm Drain

The information and guidelines contained herein are to be used whenever applicable. In the event a development is to be constructed in stages, master utility plans are required prior to submittal of the first stage improvement plans. The master utility plans shall show all improvements, including interim (phased) and ultimate improvements. For information regarding these guidelines contact the Land Development Section of the Engineering Division, at (408) 586-3329.

Note:

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Design Criteria

1. Storm Drain
   
   Show all storm drain lines necessary for the development, from point of interception of upstream drainage to point of adequate discharge.  Show sizes, type of materials, catch basins, manhole & lateral locations and all profiles (slope, invert elevations, hydraulic grade lines and surface elevations).
   
   
2. All drainage studies shall be in accordance to City’s Storm Master Plan. Pipes shall be sized to carry the 10-year discharge without surcharging the pipe. When downstream surcharge effects are included, upstream hydraulic grades shall be no higher than the top of curb elevation at any manholes and inlets. Hydraulic grade shall not exceed the street right-of-way at any location.
   
   
3. The Rational method Q= CIA may be used.

   Q   is the flow rate in cfs.
   C   is the runoff coefficient which represents the amount of rainfall occurring as runoff and is expressed as a value between from 0.0 to 1.0. 
   Typical Values of "C":

   Low density residential	0.40
   Medium density residential	0.50
   High density residential	0.70
   Commercial	0.90
   Industrial	0.90
   Park	0.20
   School	0.30
   Other	determined individually

   I  is the rainfall intensity in inches per hour.
   Values of "I" have been determined for the City of Milpitas
   See Rainfall Intensity Charts
   A is the drainage area in acres.

4. The minimum and maximum velocities are 2 fps and 10 fps, respectively.
   where V = 1.486 / n x R  raised to 2/3  x S raised to 1/2.          
   V  is the velocity.
   n  is coefficient of friction, Values of "n" for RCP are taken as n = 0.013
   R  is the hydraulic radius.
   S  is the slope

5. Time of flow in gutters can be determined from the City of Milpitas Gutter
   Flow Table. See Milpitas Gutter Flow Table

6. Material
   Reinforced Concrete Pipe (RCP), Class III
   
   
7. Easements
   10 feet wide (minimum)
   (No trees or deep-rooted plants are permitted within the easement)
   
   
8. Normal Location
   5 feet toward centerline from face of curb, on the opposite side of the water line.
   
   
9. Manhole Spacing
   500 feet (maximum)
   
   
10. Catch Basin Spacing
    400 feet (maximum)
    Catch basins shall be spaced to provide for a maximum width of gutter flow not to exceed 8 feet from face of curb, for a 10-year flood or entrance capacity of inlet for a 10-year flood
    
    
11. Cover Over Pipe
    3 feet (minimum)
    
    
12. Mainline Pipe Size
    15" diameter (minimum)
    
    
13. Pipe Crossing Clearance
    The minimum pipe crossing clearance with other utilities is one foot. Where the minimum clearance cannot be met, encasement or cap shall be designed by the developer's engineer and approved by the City Engineer.
    
    
14. Storm drains shall be constructed from the upstream properties through the development to a point of adequate discharge downstream.
    
    
15. Provide stubs for future extension of on-site development and public street extension.
    
    
16. Change of direction or size shall occur only at structures such as manholes.
    
    
17. Outfalls and other work within the SCVWD rights-of-way are subject to the approval and issuance of a permit by that agency.
    
    
18. Open ditches of a temporary nature may only be used if approved by the City Engineer.
    
    
19. See Federal Emergency Management Agency's (FEMA) Flood Insurance Rate Maps (FIRM) for special flood hazard zones.
    
    
20. Non-rainfall discharge requires approval from the appropriate agencies such as the Regional Water Quality Control Board.