specs

STORMWATER POND MODIFICATION
SOUTH NAPA MARKETPLACE
JUNE 24, 2011
PAGE 5
Mannings n-value: An n-value of 0.014 for HDPE pipe was used for all pipes within the
stormwater system, except the pipes in the StormTech system which were given an n-value of
0.012. An n-value of 0.34 for a vegetated roughened channel was used for evaluating the
bioswale.
Kristar Separator
The Kristar DVS-96 unit was selected because it can perform under flows up to 57 cfs. Head
losses due to friction were accounted for including 0.81 ft during the 10-year storm flow, and
1.34 ft during the 100-year storm flow.
Subsurface Stormwater Detention and Treatment System
The detention and treatment system consisting of a StormTech chamber system includes multiple
elements that require separate hydraulic analyses. StormTech design spreadsheets and hydraulic
methodology provided by the manufacturer were followed to design the size of the chambers and
associated piping. To ensure a detention volume equal to or greater than the mean average storm
and to ensure sufficient water quality treatment, the system sizing was selected as one Isolator
Row consisting of 99 chambers (52 inches wide per chamber) and three gravel chambers of 99
chambers each (see Appendix B for the StormTech sizing spreadsheet). The maximum volume
capacity of the chamber system is 1.63 acre-feet. The inlet and outlet manifolds were designed to
pass the 100 year storm modeling existing backflow conditions upstream and to prevent scouring
in the chambers that could erode the gravel bed.
The inlet weir into the StormTech chambers was designed at an elevation of 6.35 in MH 401 to
bypass high flows into the gravel chambers. The 24 inch inlet pipe to the Isolator Row can accept
flows up to 7.75 cfs (see the Stormtech Isolator Row Capacity results in Appendix B) before
flow is diverted to the other gravel chamber. The subsurface flow through the gravel flows at a
maximum rate of 0.24 cfs per chamber. With a total of 99 chambers in the Isolator Row,
stormwater flows through the gravel at a rate of approximately 24 cfs. The gravel chamber
manifold (36 inches) and stubs (30 inches) into the three chambers were designed to accept the
full 100 year flow of 50 cfs. The outlet manifold (48 inchs) and stubs (42 inches) were designed
to accept the full 100 year flow and discharge to the bioswale through MH 400.
Bioswale
The two 30 inch pipes from MH 400 will outfall to the surface of the bioswale at an elevation of
5.73 feet. The bioswale was designed to have 12 foot base, side slopes of 2:1, a length of 225
and a slope of 0.24%. The bioswale channel will include a minimum of 10 inches of gravel
covered by a maximum of 9 inches of biofiltration soil mix. The bioswale was modeled as a
uniform channel to determine the depth of the water above the bioswale at different storm events
(see the Bioswale Design Calculations in Appendix B).
The present rock riprap swale has been encroached on by the recent Imola Avenue Caltrans
improvements. The rock swale will be modified, staying in South Napa Century Center’s
property, by adding a topping of biofiltration medium. The existing, minor swale waters from