Agenda Item # 6.1 - Benjamin Hwang, P.E., Valley Water | Received 12/04/2023 (2)
Santa Clara Valley Water District | 5750 Almaden Expressway, San Jose, CA 95118-3686 | (408) 265-2600 | www.valleywater.org ♺
Clean Water • Healthy Environment • Flood Protection
File: 33611
West Branch Llagas Creek
December 4, 2023
Cindy McCormick
Senior Planner
City of Gilroy
7351 Rosanna Street
Gilroy, CA 95020
Cindy.McCormick@cityofgilroy.org
Subject: Initial Study/Mitigated Negative Declaration for Storquest Self-Storage (1000 Gilman
Avenue)
Dear Cindy:
The Santa Clara Valley Water District (Valley Water) has reviewed the Initial Study/Mitigated Negative
Declaration (IS/MND) for the proposed Storquest Self-Storage Project at the intersection of Gilman
Avenue and Camino Arroyo in Gilroy. Upon review of the IS/MND, Valley Water staff has the following
comments:
1. Section 2.24.10 – Hydrology and Water Quality, Environmental Setting: Ronan
Channel/Miller Slough is described as bounding the project site to the south; the adjacent
channel is West Branch Llagas Creek and should be specified as such in the summary of the
project site. Furthermore, it is noted that “The project is located in the Ulvas [sic]-Llagas
Watershed;” this should be corrected to Uvas-Llagas Watershed.
2. Section 2.4.10 – Hydrology and Water Quality, Impact Analysis (a): it is noted under the
‘Operation’ section of impact ‘a’, that “the project incorporates two bioretention ponds and
stormwater basin that were designed and sized to meet the requirements outlined in the
Stormwater Management Guidance Manual….to treat runoff before it leaves the project site.”
Subsequently, under the discussion for impact ‘b’, (Page 56) it is stated that “the project would
include two bioretention ponds and a stormwater basin to collect and absorb runoff from the
project site, which could result in some potential recharge of groundwater.” The proposed
operation of the bioretention ponds need to be clarified in the IS/MND as it is not clear whether
the bioretention ponds will be used to treat runoff and delay discharges into a nearby storm
drain system, or to promote infiltration of stormwater to groundwater. If the intended purpose of
the bioretention ponds is for infiltration, the following should be considered:
a) There may be conflicts with Valley Water’s guidelines for infiltration devices, which is also
referenced in the Stormwater Management Guidance Manual and has been adopted by the
City of Gilroy. It should be noted that stormwater infiltration devices are subject to minimum
horizontal setbacks from drinking water wells and vertical separation from seasonally high
groundwater levels. Valley Water groundwater level readings, at a location approximately
1,000 feet southwest of the subject parcel, indicate a minimum depth to groundwater of 8.5
feet. Considering the proposed depth of the bioretention ponds, the vertical separation may
be less than 10 feet. Moreover, infiltration devices require a minimum horizontal clearance
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December 4, 2023
Initial Study/Mitigated Negative Declaration for Storquest Self -Storage (1000 Gilman Avenue)
of 1,500 feet from drinking water wells. It should be noted that there are several domestic
water supply wells within 1,500 feet of the proposed detention basins. It is also stated under
the ‘Operation’ section of impact ‘a’ that the project “would introduce the potential for
pollutants such as chemicals from household cleaners, nutrients from fertilizer, pesticides
and sediment from landscaping, trash and debris, and oil and grease from vehicles.” The
IS/MND needs to provide a discussion of how groundwater will be protected from these
pollutants if the bioretention ponds will be used for infiltration.
b) Valley Water recommends a soil analysis to measure infiltration rates to determine whether
the proposed bioretention ponds would be feasible at this location.
3. Section 2.4.10 – Hydrology and Water Quality, Impact Analysis (c): under the ‘Operation’
section of impact ‘c’, it is noted “Under the existing conditions, runoff generally drains southeast
via sheet flow and enters the drainage ditch along the eastern boundary of the property before
flowing into the City storm drain infrastructure at the southeast corner of the parcel.” The
existing drainage ditch drains directly into West Branch Llagas Creek, which is a flood protection
facility constructed by the Natural Resource Conservation Service’s (NRCS) and currently
operated and maintained by Valley Water. The discussion of the existing drainage swale
throughout the IS/MND should acknowledge this channel as a flood protection facility instead of
referencing it as the “City’s storm drain infrastructure” or “storm drain system.”
4. Section 2.4.10 – Hydrology and Water Quality, Impact Analysis (d): under the ‘Less than
Significant Impact’ section of impact ‘d,’ it is noted that “The proposed project will encroach into
the floodplain, but only along the fringe of the mapped inundation area where flood depths are
minimal.” While it is true that the project site is located on the fringe of the floodplain, this does
not necessarily mean the project site is subject to “minimal” flood depths since it can also be
subject to events larger than the 100-year event. Having the project site located on the fringe of
the floodplain means it was assumed as part of the assumed encroachment into FEMA’s 100-
year floodplain when the limits of the regulatory floodway was determined. Claims that the
project location would have minimal flood depths should avoided as this can be misconstrued as
having minimal risk, which is not true. Rather, the IS/MND should state that the project site is
subject to water surface elevations approximately between 188.2-ft (NAVD 88) and 189.3
(NAVD 88) during the 100-year flood or 1% annual exceedance event.
5. Section 2.4.10 – Hydrology and Water Quality, Impact Analysis (e): under the ‘Less than
Significant Impact’ section of impact ‘e,’ it is stated that “Valley Water prepared a Groundwater
Management Plan report published in 2016…” It should be noted Valley Water has updated its
Groundwater Management Plan since this time and has released the 2021 Groundwater
Management Plan for the Santa Clara and Llagas Subbasin.
If you have any further questions regarding Valley Water’s comments on the NOP, you may contact me
at (408) 630-3066, or reach me via email at bhwang@valleywater.org.
Thank you,
BENJAMIN HWANG, P.E.
Associate Engineer - Civil
Community Projects Review Unit
Cc: Y. Arroyo, B. Hwang, V. De La Piedra, G. Tick, J. Larabee, File
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