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TOWN OF Los ALTOS HILLS April 26, 1995
Staff Report to the Planning Commission
RE: REQUEST FOR TENTATIVE SUBDIVISION MAP APPROVAL, GENERAL
PLAN AMENDMENT, PREZONE, ANNEXATION, AND DRAFT
SUPPLEMENTAL EIR REGARDING THESE APPROVALS; LANDS OF
VIDOVICH, 11920 STONEBROOK DRIVE. (254-93-TM and 257-93-EIR)
FROM: Debbie Pollart and Mike Porto,Interim Town Planners i ad_
•
RECOMMENDATION That the Planning Commission:
Re-open the Public Hearing and take comments on the adequacy of the Draft
Supplemental Environmental Impact Report (SEIR) and comments on the tentative
subdivision map, and continue public hearing.
DISCUSSION:
At the April 12, 1995 Planning Commission hearing, public testimony was given
regarding the adequacy of the Draft SEIR and issues of the tentative subdivision map.
These issues are summarized below: (Please note that this list only includes written and
oral comments made specifically on the adequacy of the Draft SEIR and/or comments
on the teritative map. Comments made regarding the merits of the project are not
included here as they do not pertain to the two actions at hand. A more complete listing
of public comments can be found in the minutes of the April 12, 1995 meeting. Please
also note that comments have been grouped according to topic and may represent more
than one person's comment/opinion.)
• Sanitary sewer system and the related damage to Juan Prado Mesa Preserve should
the proposed project (rather than one of the alternatives) be implemented; damage
to oak and redwood trees; construction within dripline; damage to Preserve caused
if sewer line were to leak. Concern as to whether construction vehicles have legal
right to use pathway easement on 11666 Dawson Drive. Also potential impacts to
owner of 11665 Dawson Drive due to construction of line. The proposed sewer line
should be continued up Stonebrook to near Magdalena, and that the existing houses
on Stonebrook, the lower section of Northcrest and at 10955 Magdalena Road be
connected to this main.
• Disagreement that proposed traffic on Stonebrook Avenue considered insignificant .
(as stated in Draft SEIR). Issue of safety and how much of unpaved road would be
paved/widened. How many oak/other trees would be removed due to widening?
Concerns of stability and safety of Stonebrook Avenue after construction.
Emergency access not adequate. Additional 220 trips daily on Stonebrook onto El
Monte not adequately addressed with regard to safety. Concern with possible
problem on the section of Stonebrook with handling of parked cars if this area
Planning Commission • '
• April 26, 1995
Lands of Vidovich •
Page 2
becomes a major access to the open space preserve. Does the Town have the right to
abandon the 40-foot right-of-way making it a drainage channel (who owns the
property)?
• Concerns regarding trespassing, both related to the quarry itself and the public
access to JPMP. Should allow residents and guests of residents to park along
Stonebrook,but to exclude guests to the open space preserve.
• Impacts to wildlife and increased noise levels with removal of mature oak trees.
• Noise associated with natural acoustics of the quarry.
• Traffic - Disagreement over EIR's discussion of Level of Service "A"' at 1,450
trips/day.
• Address the(runoff from the La Loma hillside when Stonebrook is closed (where
and how will,
the runoff be diverted).
• Lake water level may need to be updated after recent fills. Also issue of public safety
(lave level now and in the future). What happens if the lake flow does not reach
expected levels? Issue of water quality. Maintenance and monitoring of Lake
Management Plan (who in control? Homeowners Association?). Commission should
have the application of increased diameter of the outfield pipe that was previously
required. Lake has not been filled with excess water from Hale Creek as proposed.
Need monitoring reports from years 1989-1994 and wants future reports up through
2001. Issue of private lake (proposed) and public access to it (recommended by
Pathways Committee).
• Hazardous materials/waste on-site. Can't mitigate an unknown.
• More information desired for Hillside lots (18-21). Driveways to lots 19-21 are not
adequate. Could the three eight-acre lots be further subdivided, thereby resulting in
additional lots?
• Not enough information on the Reclamation Plan.
• Fencing - desire aesthetically pleasing barrier fence with locked gated at end of
Stonebrook Avenue to prevent children/dogs from entering. Would like to see
perimeter fence with copies of approval from Department of Fish and Game and
other environmental agencies.
Planning Commission
April 26, 1995
Lands of Vidovich
Page 3
• Concerns over utilities and urban services. Suggest that utilities that are currently
elevated along Stonebrook be buried and upgraded to permit all wire services likely
to be available in the foreseeable future, including copper wire and optical fiber for
phone/ISDN and cable TV. Don't want additional electrical utilities servicing the
new development to along Stonebrook Avenue (above ground).
• Concerns over cumulative hydrological water quality.
• Concerns with visual impacts associated with skylights in the basin. Consideration
of view corridors of the lake.
• Concerns with air quality as it relates to wood burning stoves/fireplaces in basin.
• Concerns with paths and circulation elements.
• Concerns with erosion of the area between the lake and Hale Creek and the
potential damage from flooding as a result of an earthquake (stability of fill
material). Need map to show depths of fill areas.
• Show all conservation easements on areas over 30% slope including an easement
shown over the banks of the lake and around the entire lake.
• The seasonal creek currently running through 10925 Stonebrook should be rerouted
directly under Stonebrook into the lake, rather than being allowed to pass over
Northcrest and continuing westward along Stonebrook.
Since the previous staff report focused primarily on the Draft SEIR issues, this report
will give more detail regarding the tentative map.
The EIR discusses a potential project. Although the Planning Commission is required to
consider the EIR prior to reviewing the project, the issues involved with the project
itself are in fact the main focus of the EIR. The tentative tract map before the Planning
Commission represents "the project." It has been difficult to separate the EIR issues from
the tentative tract map issues, so for the purposes of discussion the major map issues
will be presented for review based on project review and previous comments. The
actual review of the tentative tract map including the placement of Conditions of
Approval will be done later in the process once we receive additional information
previously requested from the applicant.
Included for review by the Planning Commission are letters from two of the Town's
consultants regarding the issues that they have identified. The Civil Engineering firm of
Wilsey & Ham and the Geotechnical firm of William Cotton and Associates have
reviewed the proposed tentative tract map and their comments have been provided to
the applicant. Their issues will be included in the Issues Analysis section of this staff
Planning Commission •
April 26, 1995
Lands of Vidovich
Page 4
report referenced below, along with other pertinent issues which have recently come to
our attention. The listing of the pertinent issues, as we currently know them, are
presented below along with a narrative of the concern that created the issue.
TENTATIVE TRACT MAP ISSUE ANALYSIS:
DRAINAGE:
The Quarry site is topographically the lowest property in this general area.
Consequently, most drainage in the vicinity of this property is directed onto this site.
The drainage flows typically end up in the lake. There are several issues and concerns
with regard to the impact to the property from the overland flow of the water, the
water's potential for erosion and the ability to convey water to the lake without the
associated silt and debris. The issue of the maintenance and the clarity of the water is
also a significant issue.
The applicant has proposed to convey water to the lake primarily in a series of
"roadside ditches" with the exception of the cross-lot by-pass channel long the back of
lots 5 through 10, the outlet channel adjacent to lots 12, 10 and 11 and a storm drain line
coming from Stonebrook Drive. This storm drain line is proposed "to relieve the
Prospect Avenue drainage" and appears to outlet the drainage into the roadside ditch in
front of lot 14 where it then flows along the common boundary between lots 13 and 14
to the lake. Small sections of storm drain are also proposed to carry water under the
intersection of Streets C and B but the water outlets into the roadside ditch in the
vicinity of lot 17,and a storm drain is proposed under Stonebrook Drive at the entrance
to the site. Again water enters the pipe from an earthen ditch and exits into a earthen
ditch.
The potential for erosion and siltation in the roadway ditches is significant. The civil
engineering consultant has suggested that the roadside ditches be used to handle only
"local drainage" and not as a method to channel large amounts of off-site flows through
the site to the lake. Additionally, the issue of the capacities of all of the open drainage
channels proposed, raises some concern especially in the areas where the open channel
is above a proposed residential site. The Wilsey & Ham letter dated March 24, 1995
adequately states the issue and requests additional information from the applicant
regarding the drainage concerns. This information will be evaluated prior to any formal
hearing on the tentative tract map.
There are still some issues regarding the ability of a privately maintained system to
accept public water. Basically, water from off-site, primarily from existing Stonebrook
Drive to the north and the area northwesterly of lots 1 and 23 will be accepted onto the
site. These flows have the potential for bringing pollutants into the private lake.
Should an on-site storm drain system be developed, the inclusion of oil and grease traps
would assist in 'reducing the amount of pollutants that reach the lake, but it is highly
Planning Commission
April 26, 1995
Lands of Vidovich
Page 5
likely that items such as fertilizers and pesticides will pass right through these systems.
The resultant concern would be the viability of the lake to support an ecosystem
without the ability to filter out the objectionable material.
As the lake will become the low point for drainage in the general vicinity, the ability to
assure that an overflow will be in place to channel water out of the lake during extreme
flows will be important. During periods of excessive rainfall and at times when the lake
has reached its design capacity, a provision needs to be made to guarantee that there
will be no inundation of the building sites. The applicant is currently showing an outlet
channel from the lake, skirting the building pads on lots 12 and 10. This overflow
channel joins with the by-pass channel and together they carry the overflow and
drainage water to a point at the north east corner of the property. At this point the
combined water flows will enter the Juan Prado Mesa Reserve. The applicant will need
to demonstrate that these concentrations of drainage and overflow will not impact the
Reserve. Additionally, water flows through the Reserve and enters the Hale Creek
Drainage Basin, a County maintained facility. The applicant will need to assure Santa
Clara Valley Water District that the flow that will emanate from this site will not
overburden their facilities. Through the tentative tract map process, concerns of this
type will be addressed and planned for with respect to the design of the improvements.
ACCESS:
Public access to the individual lots is of prime concern. Currently, the proposed method
of access is from Stonebrook Drive southerly of its intersection with Prospect Avenue.
"A" Street becomes the prime access into the subdivision with cul-de-sacs "B" and "C"
serving the lots. Reference has been made to the need for emergency vehicle access to
provide a second means to get to and from the site should a medical emergency or
natural disaster occur. The extension of Stonebrook Avenue from Magdalena Road in
the southeast has always been referenced as the most logical emergency access point.
This portion of roadway runs dangerously close to the top of the shear vertical cut that
became the southerly quarry wall. The fact that the current access is substandard in
width, and will result in alteration of the existing mature vegetation, creates a further
problem with the ability to provide an adequate emergency vehicle access.
Considerable attention needs to be paid to this aspect of the project and this method of
providing the necessary emergency vehicle access.
Presently there is a 40' right of way that provides access to the parcels adjacent to the
northerly and westerly boundaries of the property. This right of way gives access to
Stonebrook Drive (between the cul-de-sac end of "C" Street and Magdalena) in the
vicinity of the entrance to this project. While those properties westerly will be served by
new streets "C, B" and "A, "the properties northerly of lots 1 through 4 may be
precluded access as the applicant is proposing to abandon this right of way. This access
issue should be resolved before any final action on the tentative map.
Planning Commission
April 26, 1995
Lands of Vidovich
Page 6
SAFETY:
In conjunction with the access to the property, there is a separate concern regarding the
safety associated with the property. The shear quarry wall adjacent to Stonebrook
Avenue along the southerly boundary of the property is an issue. Protective measures
should be included in the project to assure that pedestrian, animals and automobiles are
precluded from falling down this steep cut made to accommodate the quarry. This
remedial repair,could be completed in conjunction with the widening of Stonebrook
Drive to provide the need emergency vehicle access to the site from Magdalena.
Wilsey & Ham have referenced a concern regarding the provision of access to the lake
and the need to provide rescue abilities. The plan, as currently proposed, significantly
limits the ability of emergency personnel to access the lake. The only roadway adjacent
to the lake itself is the cul-de-sac end of "C" Street. The slope from "C" Street down to
the lake edge is too steep to accommodate motorized vehicles. There is an existing road
down to the water surface which could be utilized to supply access for emergency
personnel and their vehicles. This same access would provide a method for
maintenance vehicles to also gain passage to the water surface. The existing roadway
that has been cult into the side of the quarry along the southerly wall begins its decent
from the area within Lot 17. It would seem logical to utilize this existing access point
for the maintenance and safety of the lake.
LANDFORM DESIGN:
Generally, the land contained within the development area of lots 1 through 17, is
considered reasonable for development given the slope and the possible mitigation
issues referenced in the Wilsey &Ham and Cotton letters. The development pattern that
would be possible given the layout of the streets and the resultant lots could create a
random subdivision look commensurate with standard subdivisions found in
communities not noted for hillside development. Special care should be taken when
considering design of the homes to be located on these lots to assure that setbacks are
provided to allow for a more random placement of the homes rather than having all of
the homes at their minimum front setbacks.
Lots 18, 19, 22 arid 23 have significant issues such as access and debris flow which will
constrain development, but through mitigation, may leave most of the development
potential of these lots in tact. Lots 20 and 21, however, present unique design issues that
cannot be mitigated without significant redesign. The primary concern is with regard to
the steepness of the drive. Lot 19 can be accessed from Street "C" directly and does not
need to take access from the joint driveway with lots 20 and 21. This eliminates the
need to provide ,a 50' right of way for street purposes. The requirement that each lot
have 25' of frontage is still valid and so the configuration of lots 20 and 21 would still
require this frontage of the.panhandle. Attempting to flatten the grade of the driveway
serving these two lots would require significant grading and ultimately retaining walls.
` s Planning Commission
April 26, 1995
Lands of Vidovich
Page 7
That would not be in keeping with development theory in Los Altos Hills. The
steepness of the grade when combined with the driveway intersection with "C" Street
could make for unsafe stopping. Additionally, vehicles starting up the driveway from
"C" Street would be forced to start the steep climb up the driveway without any initial
transition from the relatively flat Street. The driveway area is heavily protected by trees
which, when they drop their leaves and during wet weather would create a situation
where braking and stopping would be a serious concern.
Wilsey & Ham reference a maximum driveway slope of 6% for a distance of 40' where
the driveway approaches "C" Street. This can only be accomplished if "C" Street is
moved northeasterly constraining Lot 17. The viability of lot 17 comes to question when
consideration is given to vehicular and boat access to the lake and proper transitioning
of the driveway for lots 20 and 21.
Additionally, lots 20 and 21 are located in terrain that is topographically steep and
dotted with substantial trees of different variety. The may be an issue as to the
feasibility of the development of these lots. The Wilsey & Ham letter requests that the
applicant demonstrate the development feasibility of these lots. Prior to any final
action on the tentative tract map, the applicant will be providing plans showing that
these lots can be developed.
SEWER:
The applicant is proposing to provide sewer service to the lots by connection to the
existing sewer located within the Juan Prado Mesa Preserve. Local residents are
concerned that future maintenance work for the proposed sewer would be required
from Dawson Drive. Care should be given when considering sewer alternatives as to
location of connection points and methods of maintenance and accessibility.
CONCLUSIONS:
Staff is not making any formal recommendations regarding any necessary action on this
project. The issues presented herein are introductory and by no means totally
inclusionary. It is envisioned that many more issues will be presented before the
tentative map is finally heard by the Planning Commission. Undoubtedly, more issues
will be resolved once we receive the additional information requested from the
applicant.
The purpose of the Planning Commission hearings on the Draft Supplemental
Environmental Impact Report and tentative map is to take public testimony and to gain
additional information so that an informed decision can be made. Discussions relative
to the tentative tract map help to determine concerns of the Planning Commission so
that answers can be provided.
iI
Planning Commission
April 26, 1995
Lands of Vidovich
Page 8
1
The public review period for comments on the Draft SEIR will remain open through
May 1, 1995. At that time, staff will instruct the EIR consultants to begin preparation of
the Response to Comments document, which together with the Draft SEIR will
constitute the Final EIR. Upon completion of the Response to Comments, the Final EIR
(and Draft Mitigation Monitoring and Reporting Program) will come before you for
recommendation on certification. The public will be allowed to give oral comments
during the subsequent hearing(s) on the Final EIR. Once recommendation on the Final
EIR is given, the Commission will discuss and make recommendations regarding the
additional discretionary actions (tentative map approval, annexation, etc.) related to the
proposed project.
Staff is available to respond to questions you may have.
ATTACHMENTS:
1. Wilsey &Ham letter dated March 24, 1995
2. William Cotton & Associates letter dated February 16, 1995
cc: John Vidovich
Jim Sisk
De Anza Properties
it
WIIASEY &
ENGINEERING AND PLANNING SERVICES
156-20-57 331 Lakeside Drive,Suite 8
Foster City,California 944(14
(415)349-2151
Fax: (415)34R-4021
March 24, 1995
Jeff Peterson
City Manager
Town of Los Altos Hills
26379 Fremont Road
Los Altos Hills, CA 94022
Re: Quarry Hills Tentative Map
Dear Jeff
We have reviewed the Quarry Hills Tentative Map (Rev. 5-2-94)and the following related
documents.
■ Quarry Hills Tentative Map Rev. 5/2/94 Sheet 1&2 of 2.
■ Quarry Hills Tentative Map 10/18/94 Sheet 1.of 1.
✓ Quarry Hills Tentative Map -Profile Rev. 5/2/94 Sheet 1 of 1.
• Quarry Hills Tentative Map - Slope Classification Map Rev. 5/2/94 Sheet 1.of 1.
• Quarry Hills Tentative Map-Roadway&Driveway Slope Classification Map Rev.
5/2/94 Sheet 1 of 1.
■ Quarry Hills Tentative Map-Probable Maximum Subdivision 10/18/93 Sheet 1 of 1.
■ Quarry Hills Tentative Map- Channel Routes- 5/2/94 Sheet 1 of 1..
■ Quarry Hiills Tentative Map-Proposed Abandonment- 8/15/94 Sheet 1 of 1.
a Quarry Hills Tentative Map-Lake Bottom`20' Contours- 8/15/94 Sheet 1 of 1.
• Quarry Hills Tentative Map-Possible Corrective Grading- 11/7/94 Sheet 1 of 1.
• Aerial Photo - 1" = 100' -.Photo Date 6/7/94.
■ Recent Correspondence:
From DALC Subject
- S&A Engineering Inc. May 2, 1994 Emergency Access
S&A Engineering Inc. May 2, 1994 Driveway Slope
- S&A Engineering Inc. May 2, 1994 Emergency Access
- James H. Sisk-Project Manager May 26, 1994 Response to previous comments
- James H. Sisk-Project Manager Aug. 24, 1994 Final Submittal
• Statement Regarding Management of Quarry Hills Lake-Nov. 12, 1993.
• Preliminary Hydrology&:Hydraulic Calculations- S&A Engineering 10/18/93.
DAWFIVosa sao5,,wro
156-20-57
March 24, 1995
Page Two'
•
• Quarry Hills Lake-Modifications to Quantitative Analysis- S&A Engineering May 2,
1994.
• Letter Report-Engineering Science-Quarry Hills Lake Management Plan Update
6/21/94. 1988.
I . Lake Management Plan-Engineering Science January
e Slope Density Calculations- S&A Engineering 10/11/93
• Preliminary,Title Report-April 13, 1994.
Our review included Comparing the submitted documents with the items and requirements listed
in the five page document entitled "Quarry Hills Tentative Map Application Requirements"
dated 8/5/93 and a two page document entitled, "Vidovich Application Submittal Requirements"
dated 8/5/93, our previous comments, and the requirements of the Town.
Following are our comments and recommendations based on our review of the referenced
documents. We recommend that a revised Tentative Map and supporting documents be
resubmitted incorporating the below noted changes and additions.
1. Subdivision Boundary-The exterior boundary of the proposed subdivision is not shown.
along the rear of lots 1 -7 and at each end of Street C. Correct the Tentative Map to
correctly and'clearly show the exterior boundary of the Tentative Map. The Tentative
Map boundary to be shown along the present right-of-way of Stonebrook unless evidence
can be provided showing that present property boundary extends to the centerline of
Stonebrook.
2. Enlarge the proposed Trail Easement at the corner of lot 11 to a triangular easement 50
feet along the east boundary and 150 feet along the north boundary.
3. Drainage
The Tentative Map proposes to convey most of the upland storm water flows in open
channels adjacent to the road way and along the rear or through lots 1 - 10. -S&A
Engineering, May 2, 1994, has calculated 10 year flows in the major channels as follows:
10 yr. Reported Adjacent Adj
Basica #low.cf5 slope YALU * Yom`
A Through Lot 23 100.8 5% 11 8.5% 14
A+B Roadway in front of Lots 17-14 132.4 3% 10 7.5% 13
C Rear of Lots 1 -4 44.4 5% 8.5 same same
C+D Rear of Lots 5 -9 .61.8 1% 5 3% 8
* Added by Wilsey& Ham
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156-20-57
March 24, 1995
Page Three
The proposed plan for handling storm water flows raises several issues and concerns.
These include: Protection of residential development from potential overtopping of the
•• ' bypass channel;the proposed conveyance in roadside ditches large volumes of upland
flows; the proposed use of a 10 year return frequency storm for design of the channels
from the major watersheds;velocities in earthen channels, and need for and means and
methods of control of silt in storm water flows.
Accordingly, the following items are to be incorporated into a revised Tentative Map
submittal.
• , The major storm water ways noted in Table 5 of the May 2, 1994 report are to be
designed to carry a 25 year storm with provisions to safely pass a 100 year event
without flooding residential structures or significantly damaging publicly maintained
improvements.
• Resubmit the May 2, 1995 Modifications to Quantitative Analysis report showing
estimated flows for 25 year and 100 year return frequency for the basins shown on
Table 5.
• In all cases, using our adjusted velocities,the flows will exceed erodible velocities
thus requiring some form of protective lining or other means of eliminating erodible
velocities. Submit proposal for reducing velocities for design flows to below erodible .
velocities.
• With the present design proposal,Lots 1-10 and 14-17 each will have a channel
above residential structures. We recommend that the Town Geologist evaluate the
likehood of migration of ground water to building structures. An apparent similar
condition was noted during our field trip on February 9,.1995 where subterranean
water was escaping to the surface in the vicinity of lots 5 - S.
• Road side ditches/channels should only be used to accommodate local drainage and
not to convey large volumes of off-site flows through the site. Redesign the drainage.
plan to convey by pipe Basin A flows directly from Lot 23 to the Lake in the vicinity.
of or through Lot 17.
• Portions of the drainage system will drain into the lake with the potential of carrying
•
silt and other elements. Provide an analysis and plan for controlling silt before it
enters lake as an expansion of previously submitted reports which do not provide
sufficient detail on this subject. Specifically address in the analysis the need.foe
detention/stilling basins: Prepare draft of NOI and Storm Water Pollution Prevention
Plan in accordance with NPDES requirements.
156-20-57
March 24, 1995
Page Four
• Clearly show that there is to be no cross-connection between the Prospect drainage
system and the bypass channel.
• Since the lake is to be privately maintained, the Town will need an paieniencjor other
form of agreement to discharge flows from the public roadway ditches into the lake.
Provide a statement on the Tentative Map as to how this will be accomplished!
• Provide proposed agreement for indemnifying the Town from any consequences from
its discharging.drainage from public roadways into the lake.
• All improved drainage systems to be privately owned and maintained, except
incidental roadside swales carrying local drainage. Provide statement on Tentative:;''
Map noting limits of public and private drainage systems.
• Other issues and concerns: pie
- The Tentative Map shows the Prospect storm drain pipe going the middle of the
out parcel between Lots 7& 8. The applicant to provide written approval in a
form acceptable to the Town to construct the storm drain pipe within the existing
10'right of way/easement and through the out parcel between lots 7 and 8, or
redesign system if such permission has not been obtained at time of next
Tentative Map submittal.
- Lots 1 - 10 and 14- 17 should all contain a 10 foot side lot line drainage easement
and swale acceptable to the Town to provide a corridor for drainage overflow
across the respective lots in the event flows exceed the installed capacity of the
upstream system
- Desilturg basins if required to be shown on;-Tentative Map upstream of drainage
discharge into the lake.
- The planned use of the existing RCP noted on the site is unknown. This pipe
would not be acceptable for use in a publicly maintained systems.
- The engineering features of the spillway at the NW corner of the lake are
unknown and should be documented as to design, construction and function.
The applicant to prepare a revised drainage plan and calculations addressing the
foregoing concerns and submit to the Town for evaluation and comment. Stonebrook
Drive right-of-way adjacent to Lots 1 -4 to be converted to a drainage easement if
roadway and access rights are abandoned.
4. Utilities
Provide a "will serve" letter for electric, gas, water, sanitary sewer including collection
and disposal, CATV, and telephone services from the appropriate agencies.
156-20-57
March 24, 1995
Page Five
5. mss.. ! . .
Improve Stonebrook Avenue:$om Magdalena to Lot ke with a 20'wide(A.CJCIass 2
• . AB)travelled way. Identify{any heritage trees that would be affected by such a
widening. Provide documentation of right to use and improve for emergency access the
existing right-of-way between subdivision boundary and Magdalena Road. :!!!*1-401
evidence that the existing roadway is publicly maintained as stated on the Tentative Map.
6. Street B, east of Street A, and Street C north of Street B,to have a 22 foot traveled way.
Stonebrook Drive, Street A, Street B between A and C Streets, and Street C south of
Street B to have a 24'paved traveled way. Property line radius at the SE corner of Lot 4x'
and NW corner of Lot 17 to be 50 feet.' In corporate the foregoing into the revised
Tentative Map.:
7. L
In our opinion the lake has the opportunity of being a wonderful amenity and at the same
time a potential hazard and/or liability to the Town and others. Accordingly the applicant
needs to show on the Tentative Map or by other documentation the following
information.
• Proposed protective measures along the shoreline from erosion and wave action for
the present lake level and for the design lake level.
• Demonstrate the financial feasibility of the HOA to fund and maintain the required
water quality and safety provisions.
• Show the proposed public and wild life safety protective measures that will be
provided along the edge of the steeper portions of the shoreline at various lake levels.
• Show method of vehicle and boat access to the lake for rescue and maintenance
activities.
• Provide statement from the selected water agency as to conditions under which it will
or will not provide make-up water to fill the lake'or maintain the desired lake level.
Provide design concepts for shore line treatment and operating plan if make-up water
cannot or will not be provided by the selected water agency.
• Provide statement as to can or should provisions be made to permit use of the lake for
emergency water supply for fire fighting, either pumped or by helicopter.
•
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157-20-57
March 24, 1995
Page Six PSE
8. Existing Stonebrook Drive from the existing improved section near Prospect to Street A
to be fully improved with 24 foot paved section.
•
9. Add a legend to the Lot Design and Development Plan showing features not otherwise 4
identified by notes. .
10. Lots 19, 20,21
• Grades and construction materials/standard for driveways to be approved by the Fire
Marshall.
• Will the HOA maintain the common use portions of the driveway? If not how will
responsibility be defined.
• Town standards require that no more than two lots can be served by a common
driveway. Redesign driveway access to conform to standards or provide for public
road to Lot 19 driveway intersection.
• Provide�a Tentative Lot Design and Development Plan as required by Section 9-1.603
of the Subdivision Ordinance, to demonstrate feasibility of development for Lots 19,
20 &21.
• Driveway slope as it approaches Street C to be a maximum of 6%for at least 40 feet.
Adjust profile and provide grading plan to demonstrate feasibility of meeting this
criteria. This may require moving Street C to the northeast to meet driveway slope
requirements. •
11. Corrective Gradin
The feasibility of constructing retaining walls shown on the Corrective Grading Plan
needs to be demonstrated by the applicant. Alternate methods of mitigating the slide
potential needs to be discussed as well as alternate structural solutions.
12. Provide written approval from all parcel owners now having access rights to the portion
of Stonebrook proposed for abandonment. Provide evidence that the abandon right-of-
way will revert to the adjacent property owners. Provide a statement as to the effect on
the subdivision if the proposed abandonment is not granted.
13. A 10' publics utility easement adjacent to all roadways to be shown or indicated by note
on Tentative Map.
IMIMINSIMPLOCSVIUXIMMILTCUM3057.11PD
156-20-57
March 24, 1995
Page Seven
14. PSC
Provide for Town Standard pathway along a street right-of-way-shown on Tentative Map!
• - and roadway cross-sections. All new trails to be less than 10%. Realign new train
through Lot 21 and 23 to meet this standard.
15. The latest title repodis dated April 13,1994. As this is more than 6 months since the
latest report provide an updated report. Show all existing easements and parcels on they
Tentative Map. 9
16. Stake the approximate alignment of the off-site sanitary sewer for inspection by ourselves
and the City of Los Altos as well as the consultant preparing the supplemental of EIR so
that feasibility can be asserted prior to acting on the Tentative Map.
17. The Town Geologists reports that there is a considerable variation in fill thickness along
Street B and across building sites. Applicant to provide an analysis and
recommendations to address this issue relative roadway utility design building
foundations, and utility connections at buildings considering differential settlement.
18. Show on the Tentative Map street sections- Sheet 2, that the streets are to be paved with
asphaltic concrete(A.C.) and Class 2 aggregate base(CL2AB) based on soils engineers
recommendations and an acceptable Traffic Index.
19. The applicant has not proposed a method of mitigating the debris flow potential
previously identified. A feasible proposal needs to be presented for mitigating these
possible flows. The proposal to include structural improvements, desilting basins and
provisions for on-going maintenance.
20. For next Tentative Map submittal, sequentially number all drawings and combine into
one bound set.
We understand that a Supplement EIR is being prepared to address Transportation,Land Use,
Hydrology and Watershed, Geology and Soils, Sanitary Sewer and a New Lake Alternative.
After review of this completed document we may have additional comments on the current
Tentative Map.
We have reviewed the slope density calculations and find them to accurately represent existing
conditions, based on the available topographic map.
a,wrer KO x16rr,arn
156-20-57
March 24, 1995
Page Eight
After you have considered the foregoing items, and if you find them consistent with your
requirements, we recommend that the applicant respond to our comments by submittal of a
. revised Tentative Map and related documents for further consideration.
Please give me a call if you have any questions on the foregoing.
Very truly yours,
WILSEY&HAM
R. T. Calhoun, P.E.
Principal
•
rearIVINIONPDOMPLODUCITKIZIS 3OS7,9/10
WilliamCotton Cotton 330 Village Lane
Los Gatos, California 95030
`�' and Associates (408) 354-5542
February 16, 1995
1117N
REclitt '�11
FEB 2 1 1995
TO : Mr.Jeff Peterson
• Director of Public Works/Town Engineer TOWN OF LOS,ALT:(1SiHlla
TOWN OF LOS ALTOS HILLS
26379 Fremont Road
Los Altos Hills, California 94022
SUBJECT : Geologic and Geotechnical Review
RE : Vidovich/Quarry Hills Subdivision
We have completed a geologic and geotechnical review of the Tentative Map
application using:
• Progress Report #5 - Quarry Hills Reclamation Project, prepared by
Alan Kropp Associates (AKA) dated December 27, 1994;
• Possible Corrective Grading Plan (1 sheet, 100-scale) preparer not
indicated, dated November 7, 1994;
• Second Response to Reviewer Comments (report) prepared by Alan
Kropp and Associates, dated September 13, 1994;
• Tentative Map - Lot Design and Development Plan (1 sheet, 100-
scale) preparer not indicated, dated October 18, 1993;
• Study Plan #4 (1 sheet, 40-scale) prepared by S&A Engineering,
dated September 24, 1993;
• Geotechnical Investigation - Quarry Hills Subdivision (report)
prepared by Alan Kropp and Associates, dated June 7, 1991; and
• Reclamation Plan Approval and Exhibit A Conditions (letter, 14
pages) prepared by the County of Santa Clara, dated March 2, 1989.
In addition,we have reviewed pertinent technical documents from our office files
and completed a recent site reconnaissance (on February 3, 1995) to inspect reported
seepage in the vicinity of proposed building sites.
DISCUSSION
The applicant proposes to subdivide the subject property into 23 lots for
residential development. In our review report of October 14, 1994, we outlined various
geotechnical issues to be satisfactorily addressed either prior to deeming the application
complete, or prior to approval of the Tentative Map. These issues dealt with
modifications of proposed, building envelopes (lots 21, 22 and 23), ongoing reclamation
grading, mitigation of quatry rim instability, and off site debris flow hazards. Several of
these issues have not yet been addressed by documents submitted to date for review by
the Town.
ENGINEERING GEOLOGY o ENVIRONMENTAL EARTH SCIENCES • FOUNDATION ENGINEERING
Mr. Jeff Peterson February 16, 1995
Page 2 L1017N
In addition, our recent site reconnaissance confirmed reports of significant slope
seepage emanating along an approximate east-west trend through the vicinity of
proposed lots 5 to l 10. Photographs of the observed seepage are available for review, as
needed, from our;project file. The seepage, as observed, will negatively impact the
residential feasibility of several lots and should be addressed by the applicant's
• geotechnical consultant.
CONCLUSIONS AND RECOMMENDED ACTION
Proposed site development is constrained by several adverse geotechnical factors
that include the following:
• Adverse seepage within proposed building envelopes;
• Significant differential fill thickness beneath proposed building sites
that may result in the need for special (non-standard) residential
foundation design;
• Potential instability of the southwestern quarry rim that impacts the
proposed Stonebrook Avenue emergency access route;
•
Potential instability of the southeastern quarry rim that may impact
the adjacent neighbors'backyards;
• Potential for debris flows to originate within or upslope of the
subject property that may travel downslope and adversely impact
existing off site residences;
• Potential debris flow hazards on three proposed lots resulting in the
need to carefully define safe building envelopes;
• Future anticipated shallow landslide failure of the precipitous,
benched quarry walls; and
• Various ongoing site grading activities not fully inspected or
approved by the applicant's geotechnical consultant.
In addition,idetails of the subdivision level improvements are limited to the single
referenced Tentative Map sheet and must be defined in greater detail. A detailed site
drainage plan should be prepared including all existing/proposed water inlet and outlet
facilities for the reservoir.
We recommend that the following issues be satisfactorily addressed prior to
geotechnical approval of the Tentative Map:
1. Seepage - The Project Geotechnical Consultant should inspect the
property for signs of seepage in the vicinity of lots 5-10 and review
photographs from the file of the Town Geotechnical Consultant as
necessary. The cause of the observed seepage should be
investigated and appropriate mitigation measures should be
recommended..
William Cotton and Associates
•Mr.Jeff Peterson February 16, 1995
Page 3 L1017N
2. Preliminary Foundation Recommendations - Preliminary
recommendations should be prepared by the Project Geotechnical
Consultant to define the type of residential foundation deemed
appropriate for each proposed lot. The potential need for any
unusually deep or rigid foundation systems should be addressed.
The range of differential fill thickness across proposed building
• envelopes should be quantified as part of this evaluation. Any
• requirements for fill settlement monitoring prior to construction
should be addressed.
3. Southwestern Quarry Rim Stability Improvement Measures - We
understand that Alan Kropp and Associates (AKA) has previously
concluded that corrective grading is appropriate adjacent to
Stonebrook Avenue to provide adequate protection for this
roadway as an emergency route. A specific stability improvement
plan should be developed and proposed for this area consistent
with AKA recommendations. We understand that the referenced
"Possible Corrective Grading Plan" is not proposed by the
applicant.
4. Potential Southeastern Quarry Rim Instability - AKA has
previously identified the potential for failure and retreat of the
quarry rim that may encroach into the back yards of adjacent
neighbors to the southeast. While AKA does not anticipate a
significant hazard to any existing homes, the consultant has
indicated that corrective grading and/or wall construction along
this portion of the quarry rim could reduce the potential for
encroachment of instability into adjacent back yard areas. The
applicant and potentially impacted property owners should
attempt to arrive at a mutually agreeable solution to this issue and
any proposed mitigation measures should become part of the
Tentative Map application.
5. Off Site Debris Flow Hazards - We have previously presented our
opinion that potential future debris flows originating within swales
#1, #4 and #5 (i.e., locations as defined in the Harlan Tait
Associates "HTA" Report of May 10, 1991) have a moderate to high
potential for adversely impacting existing residential parcels
adjacent to the subject subdivision. Mitigation measures to address
these hazards, previously detailed by AKA, could include
construction of debris flow retention basins within the subdivision
property. We recommend that specific mitigation measures be
proposed to address off site debris flow hazards prior to Tentative
Map approval. As indicated in our previous review reports
(8/2/94, 10/14/94 and 12/12/94), the applicant should also
present written notification to the identified, potentially impacted
adjacent property owners.
6. On Site Debris Flow Hazard Mitigation - As discussed and
recommended in our review report of October 14, 1994, the building
envelope on lot 23 should be revised in order to make it consistent
with AKA recommendations. The revised building envelope, along
with a debris flow building exclusion zone (i.e., zone including the
Qc fan across lot 23 previously mapped by HTA), should be
William Cotton and Associates
Mr.Jeff Peterson February 16, 1995
Page 4 L1017N
indicated on the Tentative Map. No residential construction should
be allowed within the building exclusion zone unless detailed lot-
specific geotechnical investigation is performed to demonstrate the
feasibility of construction with appropriate mitigation measures.
Lot 21 is also constrained by debris flow hazards. As noted in our
October 1994 review, a significant portion of the proposed,
relocated building envelope is still situated within the debris
flow/colluvial fan deposits (Qc) mapped by HTA. Specific site
grading measures, resulting in elevation of the building pad as
recommended by AKA, are an integral part of the safe development
of this lot. An appropriate final pad elevation and proposed site
grading plans should be prepared and accepted by AKA prior to
Tentative Map approval.
The application should clarify that the referenced Study Plan #4,
with the modifications stated above, is to become part of the
Tentative Map application. It should be understood that specific
final building envelopes and driveway alignments are to be defined
by the modified Study Plan#4.
7. Quarry Wall Landsliding - AKA has indicated that the benched
quarry walls have a high potential for ongoing shallow landsliding.
The consultant has concluded that corrective grading of these slopes
(i.e.1,removing existing benches,landslide deposits and loose debris)
to form a smooth slope would significantly reduce the potential for
future shallow failures. Based on our review of the County
Conditions of Reclamation Plan approval, it appears that such
corrective grading has already been required. The applicant should
clarify his proposal for reclamation of the quarry walls.
8. Geotechnically Undocumented Site Grading - The referenced
Progress Report #5, prepared by AKA regarding site grading
operations, contains a list of 11 site reclamation/grading operations
that are not within their scope of inspection services. This list
includes items such as "placement of non-engineered fill in the
southeastern end of the quarry terraces", "removal of contaminated
materials to a different location on-site. . .", "cutting and removal of
trees from the quarry area", and "construction of a temporary haul
road". It is important that all portions of the property are
ultimately presented for public or private use in a safe condition. In
order to develop a punch list of areas that must still be properly
engineered to meet currently acceptable standards, we recommend
that a site meeting be convened between the project geotechnical
consultant and appropriate Town technical staff and consultants.
The] extent of necessary corrective grading activities should be
defined and made a condition of Tentative Map approval.
9. Supplemental Subdivision-Level In_provement Plans -
Supplemental plans should be prepared to illustrate all proposed
subdivision level improvements. A detailed site drainage plan
should be prepared including all existing/proposed final water inlet
and outlet facilities for the reservoir. Items such as proposed street
pavement sections, fire hydrant locations, details for lined surface
William Cotton and Associates
• Mr. Jeff Peterson February 16, 1995
Page 5 L1017N
drainage facilities, and other specific items to be further defined by
the Town Engineer, should be included with the Tentative Map
submittal.
Appropriate documentation to satisfactorily address the items listed above
should be submitted to the Town for review by the Town Engineer and Town
• Geotechnical Consultant prior to approval of the Tentative Map.
Respectfully submitted,
WILLIAM COTTON AND ASSOCIATES,.INC.
TOWN GEOTECHNICAL CONSULTANT
t(M6q.wc. d2.
William R. Cotton
Principal Engineering Geologist
CEG 882
Patrick O. Shires
Principal Geotechnical Engineer
GE 770
Ted Sayre
Senior Engineering Geologist
WRC:POS:TS:rb
William Cotton and Associates