specs

STORMWATER POND MODIFICATION
SOUTH NAPA MARKETPLACE
JUNE 24, 2011
PAGE 1
INTRODUCTION
This report includes a modified stormwater runoff management plan and a hydraulic design
report for the proposed modifications to the existing stormwater runoff system for the South
Napa Marketplace shopping center. The modifications will replace the existing South Pond
system, which collects runoff from the southern portion of the center, with an upgraded
stormwater collection and treatment system in an underground structural BMP chamber system.
This modification structural BMP system is located in the development of part of the planned
South Napa Century Center project which calls for the construction of a theatre complex and
related retail on the parcel adjacent to the South Napa Marketplace center.
EXISTING CONDITIONS
The South Napa Marketplace site includes separate pond systems that collect drainage from the
northern and southern portions of the shopping center. The southern portion of the site (25.3
acres) is served by the South Pond drainage system and the northern portion (5.4 acres) is served
by the North Pond drainage system. Only the South Pond system will be affected by this
proposed stormwater system modifications. The South Pond system is located on the parcel
adjacent to the shopping center on the proposed site for the South Napa Century Center.
The South Pond stormwater treatment system includes a two-stage vegetated wet pond consisting
of a smaller settling pond (forebay) and a large retention basin (afterbay), followed by a linear
riprap channel. Collected runoff from 25.3 acres enters the forebay via a 48” concrete pipe that
crosses under Gasser Drive from South Napa Marketplace. The forebay has a detention capacity
of approximately 0.57 acre-feet, and the detention basin has an estimated nominal capacity of
5.07 acre-feet, with a total system volume of 5.6 acre-feet. Treated stormwater is then
discharged via the riprap lined channel through two 66” CIP concrete storm drains to the
remnant Tulocay Creek Channel on the south side of Imola Avenue, and ultimately to the Napa
River. The riprap lined channel and twin 66” CIP pipes were originally installed by the Napa
Flood Control District.
The existing ponds lie at a bottom elevation of 3.5 ft with surrounding ground at approximately
elevation 10. The forebay and detention ponds are separated by a weir at elevation 7.5 and the
detention pond has a discharge via a dock weir at elevation 7.5. Hydraulic records for this
system were not found in City of Napa files, with the best flow information available through the
stormwater treatment system report by Woodward-Clyde that discusses the 10 and 100 year flow
rates noted herein. Further refinement of design flows are noted in this report. We estimate the
existing pond water elevation at approximately elevation 8 at the 100 year storm, backwatering
into the South Napa Marketplace discharge piping. The proposed structural BMP modification
hydraulically adheres to the proposed estimated existing condition.