IV. WATER
- The proposed project would
require paving a substantial portion of the substation
site, which would reduce infiltration and slightly
increase the amount and rate of runoff. The existing
impermeable surface area at the site is 6,620 square
feet. PG&E would add 34,205 square feet of
impermeable surfaces including a 20-foot wide asphalt
road, gunite swale to direct surface runoff, concrete
pads for electrical equipment, the oil containment berm
around the transformer pads and the concrete oil
containment structure and skimmer structure. Following
development, the total impermeable surface area would be
40,825 square feet (PG&E, 1997b). The increased
impermeable surface area would increase runoff from the
site. As the existing site is partially covered with
impermeable surfaces and most of the soils are compacted,
the increase in runoff at the site from the above noted
added impermeable surfaces would not be directly
proportional to increase in impermeable surface area.
However, the anticipated runoff would increase
sufficiently to require construction of a detention basin
on the site to prevent exceedance of the capacity of the
Town's storm drain into which site runoff would be
directed.
PG&E's design guidelines for runoff are for a storm
with a ten-year recurrence interval and five minute
duration. At the site this equates to rainfall of 4.32
inches per hour (PG&E, 1997b). Using this
precipitation intensity factor applied to the increased
impermeable surface area, runoff at the site following
development is estimated to be 9.5 cubic feet per second
(cfs). This rate of runoff would exceed the capacity of
the Town's storm drain, which is designed to accept 7.1
cfs from the Vasona site for the same design storm.
Therefore, following its design guidelines, PG&E has
proposed construction of a 20708 square foot detention
storage area at the north end of the site. The drainage
swales, if filled, would overflow into that area to
prevent excessive discharges to the storm drain provides
storage that would allow PG&E to control runoff for
the design storm such that a discharge of 2.5 cfs would
be directed into the Town's stormwater collection system
(PG&E, 1997b). Because of the proposed stormwater
collection system and detention pond, the impact of the
project would be less than significant. Additional
mitigation is not required.
- The project site is not
within either (1) a zone of potential flood hazard as
defined by the Federal Insurance Program and illustrated
in the Safety Element of the Town General Plan
(Los Gatos, 1994), or (2) the inundation boundaries of
the Vasona or Lexington Reservoir failure inundation maps
(PG&E, 1997a); thus, no impacts related to flooding
are present.
- Stormwater discharges during
construction may contain high concentrations of
pollutants, including total suspended solids. Erosion is
expected to be minor because the site is flat. Sediments
entrained in runoff during construction on the site would
be captured by the existing runoff control ditches and
existing runoff catchment pond on-site (which would also
be modified as part of the project). Sediment would be
generated by ditch excavation for laying the distribution
feeder line, particularly if the excavated soil is piled
adjacent to the ditch where it would be exposed to
rainfall. While the construction period for the open
trenching is short, the construction is proposed to occur
in the winter when rain is a common event and there is a
potential hazard of generating large amounts of silt that
would be entrained in runoff and discharged into storm
drains. Were a storm to occur during the construction,
substantial sediment could be directed into the storm
drains, clogging them. Mitigation of the impact is
possible through application of "best construction
practices", which at a minimum should include the
mitigation measures noted below. As a result of these
measures, little sediment is likely to be discharged into
the Town's storm drains. As the project includes proposed
construction activity that would disturb less than five
acres of land, PG&E need not obtain a permit from the
Regional Water Quality Control Board.
Surface water runoff from the site after construction is
expected to contain minor concentrations of a variety of
pollutants typical of electrical substations (e.g.,
automobile fluids, suspended solids, metals, and
organics), but it is not expected to occur in
concentrations that would be acutely toxic to aquatic
life because the volume of construction-related traffic
would be small.
The proposed electrical transformer banks would contain
up to 12,280 gallons of inert mineral oil. The
transformer would be installed on sealed concrete
foundations, and the surface of the substation would be
constructed of hard materials that would direct any leaks
into an on-site Spill Prevention Control and
Countermeasure Pond (SPCC). The gunite-lined SPCC pond
would be designed to contain all of the oil from the
largest piece of equipment plus the amount of rainwater
associated with a two-year storm. Gunite is a
construction material composed of cement, sand, crushed
slag and water, mixed and pressure sprayed onto a
surface. It forms a cement-like surface. An over/under
weir system (essentially a dam-like structure that
controls water flow and allows passage of flows from a
lower level in the ponded water while restricting passage
of the upper water) with a manually operated gate valve
is proposed to ensure separation of the oil and water and
any oil released would be retained in the SPCC pond until
it can be collected and transported to an approved
disposal site. The skimmer would separate oil from the
runoff water; the latter would be discharged into the
Town's storm sewer through a 15-inch reinforced concrete
pipe. In heavy storm periods, the SPCC pond would be
monitored by PG&E staff inspections for containment
and proper transport of storm discharge. This should be
adequate to prevent unplanned releases and overflows.
Pursuant to Environmental Protection Agency (EPA)
requirements, the equipment and spill containment area
are inspected by PG&E staff on a monthly basis. Storm
water runoff from other portions of the yard which are
not included within the drainage ditch that would receive
runoff from the transformer pad would be separately
discharged to a storm drainage pipe system. This storm
drainage pipe system would discharge to an existing storm
water collection system (in Winchester Boulevard).
Because of the proposed runoff containment and discharge
design, no developed portion of the site would discharge
directly to Los Gatos Creek. All discharges would be
directed into existing Town storm drainage facilities.
Mitigation
The following mitigation measure
would reduce the potential impact of sediment generation
discharged into storm drains to a less-than-significant level.
Mitigation Measure IV.c-1:
During grading and construction of the site, PG&E shall
require the construction crew or contractor to:
- install a silt barrier in
the Sill Prevention Control and Countermeasure (SPCC)
pond to prevent silt from discharging into the storm
drain;
- remove silt from the SPCC
pond when it collects to a depth of one-inch in the
pond and remove all silt upon completion of site
grading and covering; and
- consistent with
Mitigation Measure V.a-1, sweep Winchester Boulevard
within one block of the site daily until site grading
and covering is completed.
Mitigation Measure IV.c-2:
During construction of the distribution feeder line, PG&E
shall require the construction crew or contractor to:
- place silt barriers at
all storm drain inlets along Winchester Boulevard and
Lark Avenue that are within 500 feet of, and
down-gradient of the open trench or soil stockpile;
remove silt filters upon completion of construction;
- place excavated or
stockpiled soil no closer than ten feet from storm
drain inlets; and
- consistent with
Mitigation Measure V.a-1, sweep Winchester Boulevard
and Lark Avenue within one block of the trench each
day until the trench is closed.
- There are no water bodies
present on the site. The project would result in creation
of a storm drainage system that would contain water only
intermittently. The level of increase in offsite
discharge from the project would be within the design
requirement for the Town's storm drains and would not
result in a significant change in the amount of water in
any water body (see {a}, above).
- No water course is present on
site. The proposed project would have no effect on the
course or direction of surface waters.
- The proposed project is
located in the Santa Clara County Groundwater Basin,
which is managed by the Santa Clara Valley Water
District. The ground water basin has experienced
significant draw down historically. The project site is
located on an alluvial terrace at an elevation of between
288 and 295 feet above sea level. The groundwater table
in this area is not known, but the elevation and
surrounding topography indicate that it is well below the
ground surface. Neither the site nor the slope at the
eastern side of the site have any springs or indications
of conditions that suggest high groundwater conditions on
the site.
Ground water recharge facilities are located along Los
Gatos Creek approximately 5,000 feet downstream of the
project site. The recharge facilities are located at a
maximum elevation of approximately 250 feet,
approximately 38 feet lower than the proposed project
site surface. The project would not require any removal
of groundwater, either during construction or operation.
The project would increase the impervious surface area of
the site which would produce some decrease in
infiltration of precipitation. However, the site is
unlikely to contribute significantly to groundwater
recharge because of its small area, the absence of a
creek or water body on site and the existing, relatively
compacted condition of the surface soil. As a result, the
project would reduce the potential for groundwater
recharge by a negligible amount. Therefore, there would
be no impact related to any change in the quantity of
groundwater.
- The project would not require
any removal of groundwater, either during construction or
operation, nor would it intercept a groundwater aquifer
during construction (see discussion under checklist item
IV.f). The project would not include any deep cuts or
other features which would intercept or impede the flow
of groundwater. The shallow trenching required for laying
the distribution line is not likely to affect groundwater
conditions. Therefore, the project would have no impact
on the direction or rate of flow of groundwater.
- The compacted fills and
impervious surface areas would prevent infiltration of
contaminants into the soils. The roposed SPCC pond would
be lined to prevent infiltration of contaminants from the
pond into the subsurface soils. Run off or percolation
from the proposed project would not be expected to affect
groundwater quality in the area. Minor pollutants from
construction would be entrained in runoff and would not
pose a hazard to groundwater conditions (See also the
discussion under checklist item IV.c).
- The proposed project would
not use groundwater or affect a local aquifer. Therefore
the project would have no impact on the availability of
groundwater for public water supply.
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