specs

STORMWATER POND MODIFICATION
SOUTH NAPA MARKETPLACE
JUNE 24, 2011
PAGE 3
PROPOSED CONDITIONS
Refer to the Carlile Macy Stormwater Pond Modification plans on (pages C8 – C11 of the South
Napa Century Center plans by Foulk Gomez and Associates) for the layout of the proposed
system, including details and specifications. Modifications to the storm drain system will begin
at Gasser Drive; existing storm drains and flow controls located on the South Napa Marketplace
site will not be modified.
Drainage System
The existing 18 and 15 inch storm drains that drain into the 30 inch concrete discharge pipe to
the South Pond will be rerouted to a 60” manhole (MH 405) through a 48 inch pipe (all
following storm drains will be 48 inches unless specified). MH 404 will connect to another 60”
manhole (MH 404). The other existing 15 inch storm drain and the 24 inch storm drain will be
rerouted to MH 404. From here a storm drain will direct stormwater west to a 60” manhole (MH
402), make a 90-degree turn and enter the 8’x 8’ rectangular Kristar Separator (Sructure 403).
The flow will make a 90-degree turn through the Kristar unit and enter a 84” manhole, MH 401,
at the entrance to the StormTech chamber system. (Refer to page C8 for a layout of the proposed
drainage system and page C10 for details of the Kristar unit).
StormTech Chamber System
MH 401 (see detail 2 on page C10) will control the flow into the Isolator Row and gravel
chambers of the StormTech chamber system (see details on page C9 for the StormTech system).
Flows up to 7.75 cfs will be directed through the 24 inch inlet stub to the Isolator Row where the
stormwater will flow through the Isolator filter and subsurface gravel. The stormwater will
continue to flow through the following three gravel chambers and discharge to a 96 inch
manhole, MH 400 (see detail 1 on page C10). During high flows over 7.75 cfs, the stormwater
will fill up the Isolator Row, over top of a weir at elevation 6.35 in MH 401, and bypass the
Isolator Row by entering the 36 inch gravel chamber manifold. This weir will force most
stormwaters through all chambers, enhancing filtration treatment and preventing short circuiting
with a resulting backwater similar to existing hydraulics. The manifold will direct the high flow
into the three gravel chambers through 30” inlet stubs. All collected and detained stormwater in
the Isolator Row and the gravel chambers will flow through the subsurface gravel and discharge
to MH 400 through a 48 inch pipe. The StormTech chambers and surrounding gravel will remain
full of water controlled by the elevated discharge stormdrains at MH 401. Water surface in the
structural BMP will vary from elevation 5.73 to approximately 8.0. A 6 inch perforated
drainpipe will be placed at the bottom the StormTech chambers and will be directed into a
concrete manhole pump sump. During the dry season an irrigation pump can draw the stored
treated water from the sump for landscape irrigation, providing a maximum of 501,800 gallons
of reclaimed water if the StormTech system is full to capacity.
Bioswale
The StormTech system will drain into MH 400 and eventually discharge into an engineered
bioswale (see details page C11). Until the StormTech system has reached its overall retention
volume 1.54 acre-feet, the stormwater will be stored and treated in the subsurface chambers and