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HomeMy WebLinkAbout3.1 Attachment 3 COTTON { CONSULTING, SHIRESENGINEERSND ASSOCIATES, INC. ATTACHMENT 3A AND AGEOLOGISTS July 22,2015 L5031B TO: Cynthia Richardson Consulting Planner TOWN OF LOS ALTOS HILLS 26379 Fremont Road Los Altos Hills,CA 94022 C,�Tv7BECT: Supplemental G eo.obic and Geotechnical Peer Review RE: Stirling Tentative Map 193-14-TM 28030 Natoma Road At your request, we have completed a supplemental geologic and geotechnical peer review of the proposed Tentative Map application using: • •Tentative Map Plan Set - Lands of Stirling (8 sheets, various .scales) prepared by Wilsey Ham, latest revision date of May 22, 2015; and • Engineering Geologic Hazard Investigation - Lands of Stirling (report) prepared by Romig Engineers, dated May 15,2014. In addition, we have reviewed pertinent technical documents from our office files. DISCUSSION The applicant has submitted a plan to subdivide the subject 17 acre property. The Tentative Map illustrates 9 proposed lots and an access roadway extending to Natoma Road. In our previous peer review letter (dated August 20, 2014), we recommended that Tentative Map documentation be clarified to address completion of corrective grading within Lot 7 as a subdivision-level improvement, that a note be added requiring that future residences (including basements) on Lots 1, 5, 6, 7, and 8 be located so as to avoid the Monta Vista Fault, and that a specific option to mitigate the upper landslide along the common Lot 5 and 6 property line be proposed. Northern California Office Central California Office Southern California Office 330 Village Lane 6417 Dogtown Road '2804 Camino Dos Rios,Suite 201 Los Gatos,CA 95030-7218 San Andreas,CA 95249-9640 Thousand Oaks,CA 91320-1170 (408)354-5542•Fax(408)354-1852 (209)736-4252•Fax(209)736-1212 (805)375-1050•Fax(805)375-1059 www.cottonshires.com Cynthia Richardson July 22,2015 Page 2 L5031B CONCLUSIONS AND RECOMMENDED ACTION Proposed site residential development is constrained by isolated areas of past landsliding,surficial soils with a moderate to high expansion potential,and the potential for secondary sympathetic displacement along the weak surface of the local Monta Vista Fault plane. We conclude that the Project Geotechnical Consultant has completed adequate site investigation and recommended appropriate measures to mitigate 'apparent site constraints. We recommend geotechnical approval of the Tentative Map Application with the following conditions: 1. Lot 7 Grading Stabilization -The proposed bioretention basin on Lot 7 is situated within a grading stabilization area recommended •by the Project Geotechnical Consultant. The grading stabilization on Lot 7 should be completed as a sub-division level improvement at the same time as the bioretention basin construction. 2. Tentative Map and Final Map Note - A note should be added to the Tentative Map and Final Map indicating that new habitable structures (including basements) should not be located across the surface trace or subsurface plane of the Monta Vista Fault. This condition may potentially impact structure placement on Lots 1,5, 6, 7, and 8. Final proposed development plans for these individual lots should be reviewed by the Town Geotechnical Consultant to verify compliance with this condition. 3. Lots 5 and 6 - The upper landslide located on the common Lot 5 and 6 property line should be fully encircled with a 20-foot building setback on Tentative Map documents per recommendations of the Project Geotechnical Consultant. In addition, the subdrain systems within this landslide should be installed as a subdivision-level improvement under the direction of the Project Geotechnical Consultant. 4. Geotechnical Review of the Tentative Map Plan Set - All sheets of the Tentative Map Plan Set should be formally reviewed by the Project Geotechnical Consultant for conformance with project design recommendations. In addition, the Consultant should evaluate geotechnical design aspects of the proposed bioretention basin on Lot 7 including the potential for any adverse interactions with the underlying stabilization repair area. Any appropriate supplemental geotechnical design recommendations should be provided. COTTON, SHIRES AND ASSOCIATES, INC. ,1 Cynthia Richardson July 22,2015 Page 3 L5031B Results of the Geotechnical Plan Review should be submitted to the Town, for review by Town Staff,prior to formal Tentative Map approval. LIMITATIONS This supplemental geologic and geotechnical peer review has been performed to provide technical advice to assist the Town with discretionary permit decisions. Our services have been limited to review of the documents previously identified, and a visual review of the property. Our opinions and conclusions are made in accordance with generally accepted principles and practices of the geotechnical profession. This warranty is in lieu of all other warranties,either expressed or implied. Respectfully submitted, COTTON, SHIRES AND ASSOCIATES,INC. TOWN GEOTECHNICAL CONSULTANT Ted Sayre Prinicpal Engineering Geologist CEG 1795 David T.Schrier Principal Geotechnical Engineer GE 2334 DTS:TS:kc COTTON, SHIRES AND ASSOCIATES, INC. rali COTTON, SHIRES AND ASSOCIATES, INC. CONSULTING ENGINEERS AND GEOLOGISTS August 20,2014 L5031A TO: Cynthia Richardson Consulting Planner TOWN OF LOS ALTOS HILLS 26379 Fremont Road Los Altos Hills,CA 94022 SUBJECT: Geologic and Geotechnical Peer Review RE: Stirling Tentative Map 193-14-TM 28030 Natoma Road At your request, we have completed a geologic and geotechnical peer review of the proposed Tentative Map using: • Tentative Map Plan Set - Lands of Stirling (5 sheets, various scales) prepared by Wilsey Ham,latest date of June 17,2014;and • Engineering Geologic Hazard Investigation Lands of Stirling (report) prepared by Romig Engineers, dated.May 15,2014. In addition, we have been in communication with the project geotechnical consultant, completed a reconnaissance of the subject property, reviewed aerial photographs, and observed several of the exploratory site excavations completed by the Project Geotechnical Consultant. DISCUSSION The applicant has submitted a plan for subdivision of the subject property consisting of approximately.17 acres. The Tentative Map illustrates 9 proposed lots and an access roadway extending to Natoma Road. In our previous peer review (dated August 28, 2012) addressing-the Preliminary Map, we recommended that additional subsurface exploration be performed to better characterize potential rupture hazards associated with the Monta Vista Fault extending through the site..Initially collected geologic data suggestedthat the fault trace crossing the site may not be capable of primary rupture. We also recommendedthat either a stabilisation plan be prepared or appropriate building setbacks be established to address Northern California Office Central California Office Southern California Office 330 Village Lane 6417 Dogtown Road 550 St.Charles Drive,Suite 108 Los Gatos,CA 95030-7218 San Andreas,CA 95249-9640 Thousand Oaks,CA 91360-3995 (408)354-5542 0 Fax(408)354-1852 (209)736-4252 G Fax(209)736-1212 (805)497-7999•Fax(805)497-7933 www.cottonshires.com i I Cynthia Richardson August 20, 2014 Page 2 L5031A a dormant landslide situated astride the current common property line between Lots 5 and 6. We interpreted the presence of a shallow Old Landslide in the central portion of Lot 7 and we have discussed possible grading mitigation of this feature with Romig Engineers. SITE CONDITIONS Proposed Lots 1-4 are located along the eastern edge of the property and are characterized by gently inclined (5 to 10 percent) west-facing slopes. Areas of gentle to moderately steep (10 to 17 percent inclination) slopes are located in the eastern portions of proposed Lots 5-8. The western portions of these lots generally contain moderately steep to precipitous slopes (25 to greater than 100 percent inclination). Lot 9 contains an existing residence (to remain) situated on relatively level to gently inclined slopes. The southwestern portion of Lot 9 contains precipitous slopes. Drainage is generally characterized by sheet flow to the west and southwest intercepted by Matadero Creek in the western portion of the property. The property is bisected by a mapped trace of the Monta Vista fault that separates Santa Clara Formation bedrock (to the west) from Franciscan Complex bedrock(to the east). Based on our review of historical aerial photographs from 1939 and 1948;we observed geomorphic features consistent with an Old Landslide in the eastern and central portion of Lot 7. The configuration of this apparent landslide suggests relatively shallow movement on the order of 10 feet in depth. We note that the local • slope profile (below the existing tanks and well) displays a subtle convex bulge. Based o i'..one of the borings in the vicinity,colluvial soils were found to be mixed with bedrock at a depth of 11 feet suggesting the presence of a potential weak zone and possible surface of previous slope displacement. Two younger (dormant) landslides have been identified along the common property line between Lots 5 and 6. The.upper landslide has a distinct headscarp of approximately 16 feet in height and a displaced landslide mass with a total length of approximately 150 feet. RECENT GEOLOGIC AND GEOTECHNICAL EVALUATIONS The referenced investigation prepared by Romig Engineers satisfactorily addresses the peer review comments prepared for the Preliminary Map. The Consultant has presented persuasive_ data to support the conclusion that the'Monta Vista Fault crossing the property is a remnant feature and not capable of primary surface fault rupture within the site. However, the identified fault trace may be subject to secondary, sympathetic movement and the Consultant has recommended that foundations and basements of future residences should not extend across the fault plane. Specific - considerations for each lot with respect to the fault plane location are addressed in the referenced report. COTTON,SHIRES AND ASSOCIATES, INC. { Cynthia Richardson August 20,2014 L5031A Page 3 Regarding the upper dormant landslide along the Lot 5 and 6 property line, the . Consultant recommends that a series of subdrainsb the flrom margin. Theinstalled centered on the local spring and a 20-foot building setback be observed not recommended mitigation for the lower dormant landslide due to its Consultant has , remote location from intended primary house sites. On Lot 7, Romig En ' eers recommends that the colluvial swale area be mitigated by removal and replacement of potentially unstable with eeered materials along with construction of an underlying in l of We d corrective gradingmay be on the order of 5,000 cubic yards. ieu td cr tan a hatldm.g final graded slopes could be returned to native contours or be modified pad. CONCLUSIONS AND RECOMMENDED ACTION past Proposed site residential development is constrained of ntial,by tand the potential landsliding, surficial soils with a moderate to high expansion for secondary sympatheticmpathetic displacement along the weak sfceofe local sto � ta Fault plane. We conclude that the Project Geotechnical haco ed measur adequate uate situ: investigation and recommended e pT opriate Map do ecum o mitigate e apparent site,'constrafollowing recommend items: that th clarified to address theg 1. Remedial grading as recommended by Romig`Engineers should •=�,be completed ot1 Lot 7 as a subdivisionimprovement.7 Corrective grading extends up to ghee develo development of the adja eture property line on tnt and should be completed prior to P Lots 6 and,8. 2. The Tentative Map should reflect that to red resisa ences avoid or basements on Lots 1,5,6,7, and 8 should be placement of house foundations or basements across the, surface trace or subsurface plane of the Monta Vista Fault. 3. On Lots 5 and 6, either the common property oe sf heuldpee rrelocated to avoid splitting the displaced massr Dormant Landslide or (if acceptable to the Town) the recommended subdrain system shouldbe obt access capabilitieed as s separate subdrain systems with clean setback provided separately to each lot. The building requirements from the edge of the landslide Map. ended by Romig Engineers) should be noted on theTentative COTTON,SHIRES AND ASSOCIATES, IN Cynthia Richardson August 20,2014 Page 4 L5031A LIMITATIONS This geologic and geotechnical peer review has been performed to provide technical advice to assist the Town with discretionary permit decisions. Our services have been limited to review of the documents previously identified, and a visual review of the property. Our opinions and conclusions are made in accordance with generally accepted principles and practices of the geotechnicalprofession. This warranty is in lieu of all other warranties,either expressed or implied. Respectfully submitted, COTTON, SHIRES AND ASSOCIATES,INC. TOWN GEOTECHNICAL CONSULTANT Ted Sayre Prinicpal Engineering Geologist CEG 1795 6.A..)-rie0_ .Q1cLutls..)\__ David T.Schrier - Principal Geotechnical Engineer GE 2334 DTS:TS:kd COTTON,SHIRES AND'ASSOCIATES, INC. ATTACHMENT 3B Pakpour Consulting Group,Inc. TOWN OF LOS ALTOS HILLS August 21, 2014 10002.06 Plan Check#2014-02 Cynthia Richardson Town of Los Altos Hills' 26379 Fremont Road Los Altos Hills,CA 94022 Subject: First Plan Check Stirling Subdivision-Tentative Map/ Preliminary Site Development Plan Review Los Altos Hills,California Dear Ms. Richardson, .' Pakpour Consulting Group serves as District Engineers for the Purissima Hills Water District. The above project was reviewed to insure compliance with District standards and procedures. 1. It is understood that these plans were prepared for a tentative map submittal. The.District will have further comments upon submission of the improvement plans. We encourage the developer's engineer to work closely with the District prior to submitting improvement plans. 2. The developer will be required to install a new District water:main from Natoma Road to South Fork Lane. The size of the water main will be determined during the preparation of the improvement plans;most likely it will be 8-inch. 3.•' System pressures at Natoma Road and Charles Lane are approximately 130 psi. 4. The water main will be designed and installed to current District standards. 5. The developer will be responsible for obtaining all necessary easements to loop the water main to South Fork Lane on behalf of the District. 6. The District will take over ownership of the water main upon successful completion of the construction and acceptance of the project. If you have any questions please do not hesitate to contact me at (925)224-7717. Very truly yours, Pakpour Consulting Group,Inc. Joubin Pakpour, P.E. District Engineer cc: Patrick Walter,Purissima Hills Water District 1:\ProJeets\PHWD=10002.00\06-Subdivision Review\2014 Plan Check\2014-02 Stirling Subdivision\1st Review-Tentative Map\2014-02-W&H•Stlrling Subdivlslon.doc 5ge Mall , 3 , Pln,C948 (925)776 224-7717 Stonerid41)'.‘ FAXRoad(925)Suite 224-772620easantowww.pcgengr,Aco5m 8 ZeroFootorint Certified 491 ATTACHMENT 3B 26375 Fremont Rt._:i Tel (650) 948-1217 Los Altos Hills, CA 94022 Fax (650) 948-0961 Purrissi W Hilgs / Water District REC11lED September 2, 2015 SEP 00 2015 Jeff Peterson Wilsey Ham TOWN OF LOS ALTOS HILLS '3130 La Selva Street, Suite 100 San Mateo, CA 94403 Re: Nine Lot Subdivision at 28030 Natoma Road, Los Altos Hills, Stirling Subdivision Dear Jeff: The District can provide water service to the nine lots at 28030 Natoma Road based on your fire sprinkler flow requirements upon application and payment of the following fees: 1" 1-1/2" 2" Storage fees (each lot) $6,996 $13,993 $22,389 Service Installation (by others) N/C N/C N/C Meters 400 750 1,000 Backflows (2") 1,800 1,800 1,800 Total per lot $9,196 $16,543 $25,189 Please note that reduced pressure backflow devices are required for all residences with fire sprinklers or wells and must be installed at the meter to District standards. Sijce r' 1-y, Patrick W I 'er General Manager cc. *ynat rdso , Town of Los Altos Hills liS afna'i Wj if Town of Los Altos Hills Service To The Hills Since 1955 tf I ATTACHMENT 3C Cynthia Richardson From: Tran, Loc <Loc.Tran@deh.sccgov.org> Sent: Tuesday, August 12, 2014 12:28 AM To: Cynthia Richardson Cc: John Chau; Suzanne Avila;Jorgensen, Nicole;Wien, Martha Subject: Re: Stirling Subdivision at Natoma Road The major concern for DEH with any subdivision with lots connecting to sewer in the Town of Los Altos is the septic system be abandonment properly. If the Town chooses to have DEH verify the proper septic tank abandonment,the owner can pull a septic tank abandonment permit from DEH. I'm guessing there is 1 or possibly 2 septic system on the original lot? Nicole may have received the maps for the subdivision. I'll check with her. Loc Sent from my iPhone On Aug 11, 2014, at 9:58 AM, "Cynthia Richardson" <crichardsonPlosaltoshills.ca.Rov>wrote: Hi Loc, It is a requirement of the Subdivision Ordinance if you have no issues please just send me an email saying just that. Thanks. Cl�wthia Rick aresow PLaMAI-KgCoIAS Vita wt TOWVL of Los Altos f-ti.LLs Phovte 650941-7222 Direct 650947-2505 cvtchardsov&,@Losa Ltoshi.LLs.ca.gov office H-ours: Movvda j, Tuesdatj cd T��crsda j From:John Chau Sent: Friday,August 08, 2014 4:53 PM To:Tran, Loc Cc:Suzanne Avila; Cynthia Richardson Subject:Stirling Subdivision at Natoma Road Loc, We may have sent the tentative map to you for review by mistake.The proposed 9-lot subdivision will be connected to the Town's sewer mainline in Natoma Road. Please don't review the plans. Thanks John ATTACHMENT 3D Comments made by th Emvit oRmen 11]e"s1,44 Pt oteetion Committee_ On the Stirling Subdivision formal submittal for a nine lot Tentative Subdivision Map 1. We are happy to see the detailed mapping of the trees on the entire property. This will enable their value as single trees or as visually important groves to be established. 2. We would like to see the grove of trees on Lot 1 preserved as a group,with an Open Space Easement over the group. 3. The acreages of Lots 2, 3and 4 are an important factor in determining the position of the road. We prefer the old "Clements" road because,by changing the configuration of Lots 2, 3 and 4, more trees would be saved and with them the rural atmosphere so important to the Town. 4. The fence along the eastern edge of the property will divert wildlife along the backs of Lots 1, 2, 3 and 4, and a way must be left for this wildlife passage. The "Clements" road configuration wouldallow more space for this. 5. The pathway along the proposed road is in the road right of way. There must be no irrigation within 5 feet of the pathway. 6. Three_coastal live oaks (85, 83 and 82) in good condition and with diameters of 26", 20"and 20",are scheduled for felling because they are in the proposed drive- way of Lot 7. If the "bulb" of the road were curled west, then their,destruction would be avoided and the driveways of Lots 5, 6 and 7 relocated to the "bulb". 7. Many of the Simons Lane residents have requested View Easements. Pat Ley, EDPC Chair August 21St, 2014 ATTACHMENT 3E Cynthia Richardson From: Roger Spreen <roger@spreen.com> Sent: Sunday, August 24, 2014 11:30 PM To: Cynthia Richardson Subject: fl,len 5 ace;.Co`mrniitee comments for Tuesday info meeting on Stirling subdivision Hi, Cynthia: Thank you again for attending our Open Space Committee meeting last week, I hope it was informative for getting more context on our comments on the Stirling Subdivision Proposal. In termsof official comments from our committee, our written document of January 20, 2014, is still our latest accepted commentary. The recent tentative map update from the Stirlings does address the issue of a more accurate tree inventory, but I believe all other concerns listed in our document(particularly the high density of development with no fencing setbacks to prevent complete gridding, and also oak grove protection) are still relevant and current. We have also turned up more historical information related to large LAH subdivisions,which we will include in our next update of our comments,which I will agendize for our next meeting in September. Just FYI, other issues related to the Stirling project that came up during last week's meeting were: - as a comparable precedent, the 10-lot Loo subdivision (1976) required 50 foot"scenic& pathway easements" along the border of immediately adjacent parcels. Furthermore,the 20-lot Matadero Creek subdivision (1982) required a large open space easement on every lot around the border of the subdivision, including a 20-foot wide equestrian, pedestrian, and bike easement. -the fact that the Tentative Map does not indicate watercourses as required. -our sense from our site visit that the feeder running down the steep slope into the creek is not accurately indicated on the topo map (plus it is not even indicated on the map, which is a requirement for all watercourses). Perhaps this may require engineering to take a look at it to get an objective opinion. -the question of why they deem that the road only removes 13 instead of 16 trees as they stated previously,with no changes to the actual road - a reminder that the OSCA land designation applies to this parcel,which requires special consideration to its conservation. Cheers, Roger 1 " I To: Cynthia Richardson,Debbie Pedro & Carl Cahill Date: January 20,2014 Subject: Open Space Committee Comments on Stirling subdivision proposal Introduction:LAH Responsibilities The LAH Town Municipal Code concerning Subdivisions states that its purpose includes: - To preserve and protect the natural beauty and the established character of the community and,through good design,to provide for a harmonious relationship between completed subdivisions and the natural environment.(9-1.102-d) More significantly,on the Town's Land Use map,the majority of this parcel's boundaries fall within a regions defined as Open Space Conservation Area: - Within these areas,special measures should be taken to conserve the natural quality of the area and to avoid environmental degradation. Residences should be placed on the most buildable portions of lands designated OSCA and carefully sited so as to preserve existing trees,vegetation and wildlife habitat. (LAH General Plan,p. LU-5) We,the Open Space Committee,feel that the preliminary plans do not accomplish the purpose of the Subdivision code,nor have sufficient special measures been taken to conserve these areas. To the contrary,this plan appears to maximize development and minimize mitigations and protections, and is thus inconsistent with Town policies,goals,and ordinances regarding protection of environmentally sensitive conservation areas. Overview The Preliminary Subdivision plan for the 18-acre Lands of Stirling is one of the largest subdivision projects in many years,and is one of the most environmentally valuable and sensitive and valuable areas of Town.This property: - Includes a major creek corridor. - Has extensive steep slopes. - Has mature oak woodlands. - Provides significant wildlife habitat. - Serves as a critical hub for wildlife movement between the major open space areas in this part of Town,linking Arastradero Preserve,Matadero Creek,Poor Clare's area, and Byrne Preserve. - Has a steep,heavily vegetated swale creating a creek tributary. The Town has an obligation to preserve these valuable natural features as outlined in the goals and policies of the LAH General Plan and Subdivision Ordinance. The proposed plan maximizes the building lots,with only the minimum required areas preserved through open space easements. Only minor changes have been made from the previous proposal,with limited new protections. Because it is by far the largest undeveloped parcel being developed in this area of the Town,this parcel represents the final opportunity for the Town to properly balance development and environment in this area, and to avoid a suburban grid that we have seen in other areas of the Town. 1 The developers have made only minor changes in the plans from the previous proposal (7/2012) and the revised subdivision (10/2013) is still very high density.Developers have moved the road to the east and made minor changes in the shapes of several lots,but the total number of lots (9) and overall configuration are the same.Further,the developers have not adequately addressed many of the OSC recommendations. The revised map includes an open space easement over part of the very steep southern and western slopes as recommended by the OSC. However,the revised map does not address other major conservation areas OSC recommended for other areas that the Town usually seeks to preserve and/or protect from development(e.g.,steep slopes,mature oak woodlands,and watercourses). Comparables To express how we believe this proposal is insufficient,we believe it is fair to compare this proposal to another recent project in Town,the Nicholson subdivision: Nicholson _`Stirling Size 12.6 acres 18.2 acres Lots 3 lots 9 lots Avg lot size 4.20 acres/lot 2.02 acres/lot Avg slope 25% 28% Open Space 49% 39.7% The Stirling proposal has double the density and a significantly smaller proportion of open space. Protecting the Rural Character:Preventing a walled-off grid The Subdivision Ordinance requires that significant thought needs to be given to creating a subdivision that maintains the open space characteristics of both the Town and the adjacent existing neighborhoods. This area has already seen a notable increase in "gridding" due to property line fencing,some of which was initiated by the extremely long continuous boundary fence installed along the entire north border of the Stirling property in 2005. By packing in the large number of lots with no separation,the proposed subdivision virtually guarantees a complete walling off of this neighborhood. This would be completely antithetical to the Town's goals of preserving rural,open space environments.We believe that some sort of permanent fencing provisions,preferably a small fencing setback of 10 feet along property lines, must be included in this proposal to prevent this from happening.The Town has the obligation to mitigate this development to preserve the character and natural beauty of the area, and doing so will not be a burden to the future lot owners. In addition,openings should be provided in the existing fence along the eastern and northern border as a condition of subdivision. Providing breaks will allow a safer alternate route to the east for wildlife rather than forcing them to follow the cul-de-sac entrance road that intersects Natoma Road at a very sharp curve with poor sight distance for cars. 2 • Preventing wildlife issues Because this property supports resident wildlife species and serves as an established and heavily used hub for wildlife movement through the Town,the impact of high-density development on wildlife must be considered,for the sake of the surrounding neighborhood. It is the Town's responsibility to preserve wildlife habitat(which includes its migratory routes) as much as possible in the face of permitted development, and it also needs to protect surrounding neighborhoods from permanent,significant changes to the flow of wildlife. Failing to protect the significant and established wildlife area in this proposed subdivision is inconsistent with LAH Subdivision Ordinance and with the values and goals explicitly described in the LAH General Plan: - Special concern and scrutiny will be paid to the relationship of the subject property to surrounding properties. (Municipal Code,Subdivisions,9-1.501) - Maintain and enhance the integrity of wildlife habitat. (LAH General Plan: Conservation Element. Wildlife Species and Habitat, Goal 3) - Conservation of this [wildlife] habitat is not only important for the protection of wildlife, but also for the conservation of the semi-rural atmosphere of the community. To protect areas of significant wildlife habitat,such as creeks and riparian corridors,the dedication of conservation/open space easements should be encouraged. (LAH General Plan: Conservation Element. Wildlife Species and Habitat 314) - .There is a need for planning to provide for effective protection and conservation of the Town's wildlife heritage,while continuing to allow appropriate development and land use. Planning for natural movement of wildlife can help to avoid,minimize and compensate for serious negative impacts on wildlife and humans. Areas that link wildlife habitat have become vital because native animals such as deer,fox, bobcat and coyote are prevented by roads,fences, homes,and other development from moving freely as they once did.(LAH General Plan:Conservation Element. Wildlife Species and Habitat 315) - To the degree that the natural amenities,characteristics,topography,vegetation and wildlife,are preserved,the probability of acceptance of a given subdivision will be enhanced. (LAH Subdivision Ordinance 9-1.501) Consistency with Town policies can easily be achieved by assuring that although the subdivision may reduce the existing routes,it should not eliminate any. The routes on the western side of the property will be maintained by the proposed easements over the creek and steep slopes. However, the eastern border of the property also currently serves as a critical,active route for wildlife to access lands to the east; as proposed,this subdivision would allow that route to be completely cut off by lot fencing. Allowing this to happen will cause significant disturbance of wildlife flow,and risks causing negative effects to other houses and roads in the neighborhood. To access established foraging lands to the east,wildlife would be forced to travel along the cul-de-sac and the subdivision entrance road and cross Natoma Road at the sharp curve where there is poor sight distance for cars.The Town has a responsibility to protect the surrounding neighborhood by preserving this route,even if only in a minimal fashion. The Town's goal can minimize the disturbance to 3 wildlife patterns by preserving a 25-foot open space easement along the eastern border. In this way,the Town could insure that unintended consequences do not negatively impact the surrounding neighborhood. This proposed eastern border easement represents only 2.5% of the total area of the parcel; note the much smaller amount of open space currently proposed by the Stirling subdivision compared to the Nicholson subdivision above. Furthermore,this same easement also directly addresses the suburban gridding problem of boundary fencing mentioned previously. This one easement strip would solve both problems permanently, and would clearly not be an undue burden on the subdivision. Requiring these easements will not reduce the number of lots,the MDA or MFA of the lots,or the general configuration the developers have proposed. We strongly recommend the Town require this 25-foot easement along the entire eastern border. Oak Trees: Oak Grove& Road Alignment Failing to protect the mature oak grove near the entrance to the property and in the proposed road realignment is inconsistent with Town policies to preserve and protect heritage oaks.Further,the developers have not accurately shown all the trees on the Preliminary map as is required by the LAH Subdivision Ordinance. - ,The preliminary map shall contain the following information: Location of existing native and ornamental trees,including outline areas of species of orchard trees; ... Trees in concentrated areas shall be appropriately marked on the said map, and shall be matched with a corresponding legend indicator.Trees to be removed shall be appropriately marked by a symbol set forth in the legend. (LAH Subdivision Ordinance 9-1.406 c 26) The revised map does not even show all the oaks that are shown on the developer's previous map. For example,the revised map omits 19 oaks on Lots 1 and 2 from the Tree Table, 17 of which are heritage oaks.These oaks are shown as dots on the revised map,but have no diameters indicated and are not included in the Tree Table. This grove requires permanent protection in the form of an open space easement. This grove has a defined border,and is contiguous with the border of the property. It should not,be left to the individual lot owner to decide how to impact this grove. It is a significant stand of heritage oaks,and helps define the open space characteristics of the neighborhood; the Town must protect it with an open space easement for the sake of the neighborhood. This will have no burden on future developers. The road has been re-aligned in a way that removes significantly more oak trees than the previous alignment. In the previous plan,developers reported 5 trees would have to be removed for the road (not including additional trees in the two required emergency pull-outs).Page 3 of the revised map shows 16 trees to be removed for the realigned road. An additional 7 trees will be removed for the access road to Natoma (plan not previously submitted). Additional trees will also have to be removed for the two required 50-foot emergency pull-outs alongside the road. These trees need to be indicated (for removal) and included in the total impact of the subdivision. Also, trees that are shown within proposed driveway locations and building footprints are marked as "preserved," which is not accurate. This confusing notation lets them over-report the 4 1 • number of trees preserved, and simultaneously request implicit permission to remove them. This is all the more reason why the aforementioned oak grove must be preserved by easement: - Policy 2.2: Preserve and protect native and naturalized plants,with special attention to preservation of unique, rare or endangered species and plant communities such as oak woodlands. - Policy 2.3 Preserve and protect Heritage Trees, including native oaks and other significant trees, on public and private property. LAH General Plan. Conservation Element Matadero Creek Feeder/Swale In response to the developers' previous proposal,we recommended that the open space easement protecting the steep slopes along the western border should extend up to include the steep swale that forms a feeder of Matadero Creek. It is clearly an active waterway,and requires the normal easement protections of creeks & swales. In addition,this area is steeply sloped,heavily wooded, and includes large oaks. When we walked the property with the developers' engineer,we pointed out that the developers' map did not appear to accurately reflect the terrain in the area of the swale. The revised map does not even shown the feeder(as required by the Town and state subdivision ordinances),and the map under-reports the severity of the swale. - The preliminary map shall contain the following information: Unusual features of terrain, such as rock outcrops,tree masses,watercourses,in and adjacent to the proposed subdivision. (LAH Subdivision Ordinance 9-1.406, c 12) It is evident from walking the site,from overhead photos,and from the Town topo map that this area of terrain is well-defined by the significant vegetation around it. We continue to recommend that this entire area be protected by open space easement to protect the slope,the watercourse,and the mature oak woodland vegetation. This would be no burden on the developer. - Steep slopes, canyons and ravines in excess of 30%slope, as well as natural swales and drainage channels, and geologic hazard areas within the subdivision should be undisturbed and preserved in their natural condition to the maximum extent feasible. (LAN General Plan Land Use Element Goal 2, Policy 2.5) Connecting Adjacent Open Space Easements It is always the Town's goal to connect open space easements on adjacent properties,to form additive contiguous areas of protection. The existing adjacent open space easements (OSE's) are notated as part of this proposal to assure they are connected; several of these adjacent areas were specifically designed with the intention to ultimately connect with the terrain on the Stirling side of the property line. These easements include: - 28025 Natoma Road (Lands of Sloss,March 2006). The Town required an OSE all along the western border from Charles Ave to the adjacent lot on Natoma. The western border near Charles Ave abuts the proposed Stirling subdivision. 5 ( - 12400 Melody Lane(Lands of Wood),Jan 2009). The Town required a 1.79- acre OSE along the full length of the northern and southwestern border.This easement comprises 1.79 acres,representing 49% of the parcel. (Note comparison of this easement percentage with the Stirling proposal and Nicholson subdivision in the previous table.) - 13458 South Fork Lane (Lands of Yong).The Town required an OSE be placed over the northwestern slope of the property to preserve the steep slopes and oak grove. This OSE connects to the proposed Stirling subdivision and to lands to the north. - 13300 and 13311 Country Lane.Large OSEs were created on the eastern portions of these lots when the Matadero Creek Subdivison was put in.These provide connections to the north along Matadero Creek and to the west. CEQA Considerations CEQA (California Environmental Quality Act) requires the Town, as the Lead Agency,to identify significant environmental impacts,whether direct,indirect,or cumulative,and to avoid or mitigate those impacts to less-than-significant levels,if feasible. The significant size of this property, combined with its environmental value (as discussed at length above),means that correspondingly large efforts must be taken to address this proposed subdivision's overall impacts. This proposal cannot be granted a Categorical Exemption from CEQA, as this subdivision proposal does not fit into the Categorical Exemption Classification of"Minor Land Divisions," since there are more than 4 parcels being created,and the average slope of the property is greater than 20 percent. (California Code of Regulations, Title 14,Division 6, Chapter 3,Article 19, Section 15315). We anticipate that environmental review of this proposed subdivision and the anticipated residential development will require a formal biological assessment,including surveys for listed species known to occur within a few kilometers of the site. Much of this property falls within lands that the Town has specifically designated as Open Space Conservation Area, which require special measures to avoid environmental degradation.At minimum,environmental review will require a Mitigated Negative Declaration based on the potential of the project to have significant impacts to biological resources,including extensive tree removal,loss of oak woodlands,a pathway bridge over Matadero Creek,and adverse effects on resident wildlife and established wildlife movement. Mitigations as currently proposed by the developers are not sufficient to reduce these impacts to the required less-than-significant levels. The OSC looks forward to reviewing the biological assessment report and to working with the developers to assure that the additional necessary mitigation measures are provided. 6 Recommendations To be consistent with Town policies and goals stated in the LAH General Plan and Subdivision Ordinance,the OSC recommends the following: • Require fencing setbacks along all property lines,to preserve the open space character and prevent suburban-type gridding. • Breaks should be provided in the existing eastern & northern boundary fence to allow an alternate route to the East for wildlife that is away from the entrance road that exits at the sharp curve on Natoma. • The oak grove near the entrance must be preserved with an open space easement. • The entire western and southern portions of the parcel,which consist of steeply sloped, dense oak woodland should be placed in open space easement. This easement should be made to connect with easements on adjacent properties and: 1) extend all the way to the northern border(i.e.,include the area of the pathway easement); and 2) extend all the way to road edge near the entrance to the subdivision, and include the heritage Valley oak (Tree#11). • As the Open Space Committee recommended in July 2012,the northern and eastern borders should have a 25-foot easement applied to provide an open space buffer with the adjacent neighborhood,to preserve the existing open space character and prevent closing them off with a straight row'of new houses right up against all their properties. This will also prevent negative impacts of distorting the existing flow of wildlife through the area. The proposed pathway easement can run through open space easement. • As required by the LAH Subdivision ordinance,the Matadero Creek feeder swale must be shown on the maps. This watercourse should be preserved with-an open space easement from the main creek all the way to its top. We further recommend that the contiguous area of vegetation that extends up to just below the well house be included in this easement. This is consistent with our recommendation from the previous proposal. • The road alignment should be modified to reduce,not increase,the number of oak trees removed. • The Town should provide guidance on how to align CEQA requirements with the highly significant disturbance represented by this proposal. .. Jf ATTACHMENT 3F Recommendation for Stirling Subdivision Pathway Committee August 25, 2014 After much discussion, the Pathway Committee recommends that the property owners for the Stirling subdivision be required to: Grant a 30 ft. pathway easement along the eastern side behind lots 1 through 4. A meandering pathway should be built to avoid trees and connect to the pathway easement on lands of Yong. Grant a 20 ft. pathway easement from the end of the cul de sac to the northern property line between lots 4 and 5. Grant a 30 ft. pathway easement on the northern side of lot 4 and 5 down to the point of the conservation easement on lot 5. Grant a pathway easement in all the conservation easement area on lot 5. When laying out the path on the northern side, PWC would like to review it after it is staked, but before construction. Construct a pathway suitable for all weather access, not necessarily IIB standard, but a surface appropriate for horse and foot traffic in all seasons in the pathway easements. Construct a bridge across Matadero Creek. Pathways should be built and open as part of the subdivision improvements and should not be blocked during the construction phases. ATTACHMENT 3F • C ARAl i. • 1" • FIRE DEPARTMENT ATTACHMENT 3G `L�'IRE1,`''�' SANTA CLARA COUNTY • EST,1947 7°aa200 14700 Winchester Blvd., Los Gatos, CA 95032-1818 COURTESY 6SERVICE (408) 378-4010 •(408) 378-9342 (fax) •www.sccfd.org PLAN • REVIEW No. 14 2204 BLDG DEVELOPMENTAL .REVIEW COMMENTS PERMIT No. A proposed subdivision,including access and utility easement. NOTE: please be aware of requirements for road width, surface and grade. Contact this office directly for more information. Comments revised 9/5/2014 and are subject to change upon submittal of construction plans dh Comment #1: Review of this Developmental proposal is limited to acceptability of site access and water supply as.they pertain to fire..department operations1and shall not be construed as a substitute for formal plan review to determine compliance with adopted model codes. Prior to p •:rforrning any work the applicant shall make application to, and receive from, the Building Department all,applicable construction permits. Comment #2: Wildland-Urban Interface: This project is located within the designated Wildland- Urban Interface Fire Area. The building construction shall comply with the provisions of California -Building Code'(CBC) Chapter 7A: Note that vegetation clearance shall be in compliance with CBC S>-:cion 701A.3.2.4 prior to project final'appro'val. Check with the Planning Department for related landscape plan requirements. Comment #3: Fire Apparatus (Engine) Access Roads Required: Provide access roadways with a paved all weather surface, a minimum unobstructed width of 20 feet, vertical clearance of 13 feet 6 i;Lches, minimum circulating turning radius of 36 feet outside and 23 feet inside, and a maximum slope of 15%. For installation guide lines refer to Fire Department Standard Details and Specifications sheet A-1. .Comment #4:TEMPORARY ACCESS ROADS: When approved by the Chief, a temporary access rCad may be installed for fire department access to buildings under construction until such time that thi.- permanent road or driveway is in place. A written request along with detailed plans for the temporary access road shall be submitted to the fire department for review and approval prior to ir3tallation. The plan submittal shall also include timelines for use of the temporary roadway and ackl no'wledgrrient that the integrity of the roadway will be maintained at all times. The width and:. city' PLANS SPECS NEW RMDL AS OCCUPANCY CONST.TYPE AppllcantName DATE PAGE LAH ® ❑., ❑ 0 0 Wilsey Ham 08/19/20144 1 RECEIVED— SEC.rLOOR AREA LOAD PROJECT DESCRIPTION PROJECT TYPE OR SYSTEM Residential Development Design Review CCc��rI NAF:•:OF PROJECT LOCATION S P 11 2014 SUBMAP ONLY 28030 Natoma Rd Los Altos Hills tit SLL Tl4ULAR FIRE FLOW REDUCTION FOR FIRE SPRINKLERS REQUIRED FIRE FLOW @ 20 PSI BY TOWN OF LOS ALTOS HILLS PkiitNing,Doug • Organized as the Santa Clara County Central Fire Protection District Serving Santa Clara County and the communities of Campbell, Cupertino,Los Altos, Los Altos Hills,Los Gatos,Monte Sereno,and Saratoga � A ��4• co_t., FIRE DEPARTMENT M1CCN y �o • • FIR El ,a� SANTA CLARA COUNTY Esr.Isar 70aaao15 • 14700 Winchester Blvd., Los Gatos, CA 95032-1818 COURTESY sSERVICE (408) 378-4010 •(408) 378-9342 (fax) •www.sccfd.org REVIEW No.. 14 2204 BLDG DEVELOPMENTAL REVIEW COMMENTS PERMIT No. turn radius dimensions of.a temporary access road shall be the same as for the required permanent roadway. As a minimum, the roadway shall consist of a compacted sub base and six (6) inches of road base material (Class 2 aggregate base rock) both compacted to a minimum 95%. The perimeter edges of the roadway shall be contained and delineated by curb and gutter or other approved r:Method. The use of geotextile reinforcing fabric underlayment or soils lime-treatment may be required if so determined by the project civil engineer. Provisions for surface drainage shall also be provided where.necessary. Engineering certification of the temporary roadway construction shall:be documented and submitted to the Fire Department prior to or at the time of the acceptance inspection of the roadway. SCCFCD SD&S SI-7, Chp.33 of the Fire Code and NFPA 241. Comment #5: Fire Department (Engine) Driveway Turn-around Required: Provide an approved fire department engine driveway turnaround with a minimum radius of 36 feet outside and 23 feet inside. Installations shall conform with Fire Department Standard Details and Specifications D-1. CFC Sec. 503 • Comment #6: Public Fire Hydrant(s) Required: Provide public fire hydrant(s) at location(s) to be deermined jointly by the Fire Department and water purveyor. Maximum hydrant spacing shall be '500 feet, with a minimum single hydrant flow of 1500 GPM at 20 psi, residual. Fire hydrants shall be provided along required fire apparatus access roads and adjacent public streets. CFC Sec. 507, and Appendix B,Table B105.1 and Appendix C Comment #7: Construction documents. Construction documents for proposed fire apparatus access, location of fire lanes, security gates across fire apparatus access and construction documents and hydraulic calculations for fire hydrant systems shall be submitted to the fire department for review and approval prior to construction. CFC Sec. 501.3 Comment #8: Timing of installation. When fire apparatus access roads or a water supply for fire protection is required to be installed, such protection shall be installed and made serviceable City PLANS SPECS NEW RMDL AS OCCUPANCY CONST.TYPE AppllcantName DATE PAGE LAH ® ❑ ❑ ❑ ❑ Wilsey Ham 08/19/2014 2 4 Of SEC FLOOR AREA LOAD PROJECT DESCRIPTION PROJECT TYPE OR SYSTEM Residential Development Design Review NAME CF PROJECT LOCATION SFR SUBMAP ONLY 28030 Natoma Rd Los Altos Hills TABUTAR FIRE FLOW REDUCTION FOR FIRE SPRINKLERS REQUIRED FIRE FLOW @ 20 PSI BY Harding,Doug Organized as the Santa Clara County Central Fire Protection District Serving Santa Clara County and the communities of Campbell,Cupertino,Los Altos, Los Altos Hills,Los Gatos,Monte Sereno,and Saratoga • 0,ARA i 6-'-'/°6- FIRE DEPARTMENT °,. 6 c° 1FIRE "� SANTA CLAD COUNTY - E5T:1917 ?01O 2O 14700 Winchester Blvd., Los Gatos, CA 95032-1818 COaR1E9V85ERVICE (408) 378-4010 •(408) 378-9342 (fax) •www.sccfd.org PLAN REVIEW No. 14 2204 BLDG DEVELOPMENTAL REVIEW COMMENTS PERMIT No. • prior to and during the time of construction except when approved alternative methods of protection are provided. Temporary street signs shall be installed at each street intersection when construction of new roadways allows passage by vehicles in accordance with Section 505.2. CFC Sec. 501.4 Comment #9: Fire Sprinklers Required: An automatic residential fire sprinkler system shall be installed in one- and two-family dwellings as follows: In all new one- and two-family dwellings and in existing one- and two-family dwellings when additions are made that increase the building area to more than 3,600 square feet. Exception: A one-time addition to an existing building that does not total more than 1,000 square feet of building area. NOTE:The owner(s), occupant(s) and any contractor (s)or subcontractor(s) are responsible for consulting with the water purveyor of record in order to determine if any modification or upgrade of the existing water service is required.NOTE: Covered porches,patios,balconies, and attic spaces may require fire sprinkler coverage.For buildings 3601 to 6200 square feet the number of design sprinklers shall include all sprinklers within the most remote room or compartment up to a maximum r,,three(3) sprinklers.Note:If the most remote room or area contains less than three (3) sprinklers,the number of design sprinklers shall include all of the heads within the room. In addition, calculations shall be provided :that demonstrate the operation of 3 sprinklers in the most remote room that requires three(3)or more heads for ..o•,.erage.For buildings in excess of 6200 square feet,the(4)four most hydraulically demanding heads in a room cr compartment shall be calculated Fire Department Connection:For buildings in excess of 6200 square feet, a fire d.:partment connection(FDC) shall be provided.The FDC shall consist of at least one 2 .5" hose connection that is .:onnected to the sprinkler riser with a pipe not less than the diameter of the sprinkler riser.A State of California licensed (C-16)Fire Protection Contractor shall submit plans, calculations, a completed permit application and appropriate fees to this department for review and approval prior to beginning their work.CRC Sec.313.2 as adopted and amended by LAHMC °Comment #10:Water Supply Requirements:Potable water supplies shall be protected from contamination caused by r ire protection water supplies.It is the responsibility of the applicant and any contractors and subcontractors to .contact the water purveyor supplying the site of such project, and to comply with the requirements.of •that purveyor.Such requirements shall be incorporated into the design of any water-based fire protection systems, arid;or fire suppression water supply systems or storage containers that may be physically connected in any City PLANS SPECS NEW RMDL AS OCCUPANCY CONST.TYPE AppllcantName DATE PAGE LAH ® 0 0 0 0 Wilsey Ham 08/19/2014 3of 4 SEC/FLOOR AREA LOAD PROJECT DESCRIPTION PROJECT TYPE OR SYSTEM Residential Development Design Review NAME OF PROJECT LOCATION SFR SUBMAP ONLY 28030 Natoma Rd Los Altos Hills TABII.AR FIRE FLOW REDUCTION FOR FIRE SPRINKLERS REQUIRED FIRE FLOW @ 20 PSI BY Harding,Doug Organized as the Santa Clara County Central Fire Protection District Serving Santa Clara County and the communities of Campbell, Cupertino, Los Altos, Los Altos Hills,Los Gatos,Monte Sereno,and Saratoga GLARA c $`S4'.- ] dL FIRE DEPARTMENT P ,Y.�Rm�4 _ C/7� �� ' F'IR,EI a SANTA CLARA COUNTY C� j Q020.2o1 En""' 14700 Winchester Blvd., Los Gatos, CA 95032-1818 GOUiiTESY 8 SERVICE (408) 378-4010 •(408) 378-9342 (fax) •www.sccfd.org REVIEW No.. 14 22U4 • BLDG DEVELOPMENTAL REVIEW COMMENTS PERMIT No. manner to an appliance capable of causing contamination of the potable water supply of the purveyor of record. Final approval of the system(s)under consideration will not be granted by this office until compliance with the requirements of the water purveyor of record are documented by that purveyor as having been met by the applicant(s).2010 CFC Sec. 903.3.5 and Health and Safety Code 13114.7 Comment #11:Address identification.New and existing buildings shall have approved address numbers, building numbers or approved building identification placed in a position that is plainly legible and visible from the street or road fronting the property.These numbers shall contrast with their background.Address numbers shall be Arabic numbers or alphabetical letters.Numbers shall be a minimum of 4 inches(101.6 mm)high with a minimum stroke width of 0.5 inch(12.7 mm).Where access is by means of a private road and the building cannot be viewed from the public way, a monument,pole or other sign or means shall be used to identify the structure. CFC Sec.505.1 Comment #12: Construction Site Fire Safety:All construction sites must comply with applicable provisions of the CFC Chapter 33 and our Standard Detail and Specification SI-7.Provide appropriate notations on subsequent plan submittals, as appropriate to the project. CFC Chp.33 Plaits not approved.To prevent plan review and inspection delays,the above noted Developmental Review Conditions shall be addressed as"notes"on all pending and future plan submittals and any referenced diagrams to be reproduced onto the future plan submittal. City PLANS SPECS NEW RMDL AS OCCUPANCY CONST.TYPE AppllcantName DATE PAGE LAH ® 0 0 0 0 Wilsey Ham 08/19/2014 4 OF 4 SECMLOOR AREA LOAD PROJECT DESCRIPTION PROJECT TYPE OR SYSTEM Residential Development Design Review —NAME:'F PROJECT LOCATION , SFR`=UBMAP ONLY 28030 Natoma Rd Los Altos Hills TAB!+LAR FIRE FLOW REDUCTION FOR FIRE SPRINKLERS REQUIRED FIRE FLOW Q 20 PSI BY I Harding,Doug Organized as the Santa Clara County Central Fire Protection District Serving Santa Clara County and the communities of Campbell, Cupertino, Los Altos, Los Altos Hills, Los Gatos,Monte Sereno,and Saratoga ATTACHMENT 311 Eisenberg, Olivieri &Associates MEMORANDUM Environmental and Public Health Engineering TO: Richard Chiu and John Chau, Town of Los Altos Hills FROM: Jill Bicknell, P.E. DATE: June 25, 2015 SUBJECT: Lands of Stirling—28030 Natoma Road Review of Project Submittals for Compliance with Stormwater Requirements Project.Description The proposed project at 28030 Natoma Road is a 9-unit residential subdivision and access road on a primarily undeveloped property(one existing house, garage, and small detached building).The total site area is 18.18 acres; of that, 3.41 acres will be disturbed, and the remaining 14.77 acres will be dedicated open space. Lot sizes range from 1.0 to 3.3 acres, with average slopes of 7.6%to 38.4%. In the first phase, the access road and utilities will be constructed, and the 9 lots will be sold and developed by the individual owners. The site is located within the Matadero Creek watershed. We reviewed the following submittals with regard to this project: • Lands of Stirling Tentative Map Submittal, dated 5/22/15 o Sheet 1 —Overall Layout o Sheet 2—Lot Configuration Plan o Sheet 3— Proposed Subdivision o Sheet 4—Access Road Improvements o Sheet 7—Storm Water Control Plan o Sheet 8—Storm Drain Profile • Additional submittals, dated March 27, May 22, and June 19, 2015: o Completed SCV,URPPP.C.3 Data Form o Completed Infiltration/Harvesting and Use Feasibility Screening Worksheet o Technical Memorandum from Wilsey Ham describing the stormwater control plan o C.3 Stormwater Treatment Calculations o Bay Area Hydrology Model (BAHM) Hydromodification Modeling Results We reviewed the project submittals for compliance with the stormwater requirements in the Town of Los Altos Hills' NPDES Permit' (referenced herein as the Municipal Regional Permit or MRP). Our findings are presented below: 1 California Regional Water Quality Control Board,San Francisco Bay Region,Municipal Regional Stormwater Permit,Order No. R2-2009-0074, revised November 28,2011,Provision C.3. 1 EOA, Inc. • 111 W. Evelyn Ave. • Sunnyvale, CA 94086 • Tel: (408) 720-8811 • Fax: (408) 720-8812 1. Applicability of NPDES Permit Provision C.3. Requirements a. Stormwater Treatment Requirements The C.3. Data Form (received June 19, 2015) does not indicate the existing amount of impervious surface on the site.The total new impervious area (also shown as the total post- project impervious area) is 116,030 square feet(sf). Since the total amount of impervious surface to be created and/or replaced is greater than the C.3 threshold of 10,000 sf2, the C.3 treatment requirements apply to this project. The low impact development(LID)treatment requirements apply to this project because it is a private C.3 Regulated Project with a development permit application that did not receive final discretionary approval before December 1, 2011.3 b. Hydromodification Control Requirements The project will create and replace more than 1 acre of impervious surface and increase the amount of impervious surface above the pre-project amount; therefore, the hydromodification control requirements apply to this project4. 2. Applicability of State Construction General Permit Requirements Projects that disturb more than 1 acre of land during construction are required to obtain coverage under the State Construction General Permits. The project will create approximately 0.75 acres of roadway(proposed to be pervious asphalt), and then the subdivided lots will be sold to individual owners. The maximum amount of impervious area on each lot will not exceed 1 acre. Therefore, the owners do not need to obtain coverage under the Construction General Permit for the construction phases of the project. 3. Proposed Stormwater Management Measures and Sizing Calculations a. The C.3 Data Form indicates that the project will include the following site design measures: minimize land disturbance, minimize impervious surfaces, disconnected downspouts, pervious pavement, self retaining areas, and preserved open space. These site design measures are appropriate for this project type, and most are indicated on the plan sheets. The entire access road is proposed to be constructed of pervious asphalt; however, a detail for the pervious pavement system was not provided. b. The C.3 Data Form indicates that the project will include two pollutant source control measures: storm drain labeling and beneficial landscaping (minimize irrigation runoff, pesticides, and fertilizers and promote treatment). These source control measures are appropriate for this project. A detail for the storm drain label was not provided. The pervious pavement on the access road will also require maintenance as a source control. If the road becomes a public right-of-way, the Town will be responsible for the maintenance. c. The applicant submitted a completed Infiltration/Harvesting and Use Feasibility Screening Worksheet, evaluating the feasibility of treating the amount of runoff specified in Provision C.3.d with infiltration or harvesting and use. The completed worksheet states that infiltration is infeasible because the site soils have a saturated hydraulic conductivity of less than 1.6 inches per hour or consist of Type C/D soils. However, no soils information was provided. 2 California Regional Water Quality Control Board, San Francisco Bay Region, Municipal Regional Stormwater Permit, Order No. R2-2009-0074, revised November 28, 2011, Provision C.3.b.ii.(3). 3 Ibid., Provision C.3.c.ii.(2) 4 Ibid., Provision C.3.g. 5 NPDES General Permit for Storm Water Discharges Associated with Construction Activity,Water Quality Order 2009-0009-DWQ, September 2009(effective July 1, 2010). C3 review Stirling_28030 Natoma LAH_6-25-15 2 BOA, Inc. The worksheet also indicated that rainwater harvesting and use was infeasible, due to the lack of sufficient irrigation or toilet flushing demand. Therefore, the project may use biotreatment (e.g., bioretention)for stormwater treatment. d. Because the design of the individual lots is unknown, the Storm Water Control Plan (Sheet 7) is based on the assumption that each lot will be developed to the maximum developable area (MDA) expressed as impervious surface. A table on this sheet indicates the MDA for each lot. A portion of the runoff from impervious areas on Lots 1, 2, 3, 4, and 9 will be directed to self-retaining (landscaped) areas, which are sized to be at least 50% of the adjacent impervious area. This is consistent with the guideline in the SCVURPPP C.3 Stormwater Handbook that the ratio of impervious to pervious area for self-retaining areas should be no more than 2:1. The remaining runoff from the impervious areas will be directed to one bioretention area on Lot 7, and then through an underground detention facility before discharging to Matadero Creek. (A maintenance access easement to the bioretention area will be provided from the driveway on Lot 6.)The table on Sheet 7 also correctly indicates the amount of impervious area directed to a self-retaining area and the amount directed to the bioretention area for each lot. e. The C.3 Data Form indicates that the combination flow/volume method was used to size the bioretention area. Calculations based on this method were checked and are acceptable. The table on Sheet 7 uses the 4% sizing method (in which the bioretention surface area is 4% of the contributing impervious area)to confirm the size of the bioretention area provided. f. Sheet 7 provides a detail for the proposed bioretention area. The detail is consistent with the SCVURPPP C.3 Handbook guidelines with the following exceptions: i. Soil specification. A note indicates that a biotreatment soil mix with a percolation rate of 5-10 in/hr should be provided. The note should also indicate that the mix will meet the soil specifications included in Provision C.3.c.i.(1)(6)(vi), Attachment L of the Municipal Regional Stormwater NPDES Permit. ii. Liner and underdrain. The detail indicates that a visqueen membrane will be used to line the bioretention area and not allow infiltration. The SCVURPPP C.3 Handbook guidelines are to try to maximize infiltration from bioretention areas where feasible (i.e., do not use a liner and raise the underdrain at least 6 inches off the bottom of the drain rock). If a geotechnical report recommends that the facility be lined, then the underdrain should be placed at the bottom of the drain rock. iii. Mulch. The detail should indicate placement of at least 2 inches of non-floating (composted) mulch on any exposed surfaces of the bioretention areas. iv. Overflow riser. The overflow riser is not shown on the detail, although it is shown on the conceptual sketch above the detail. The elevation of the overflow riser should be set at the depth determined by the sizing calculations (i.e., 9 inches above the surface of the bioretention area). v. Drainage entry points. The Storm Water Control Plan (Sheet 7) indicates that drainage will be conveyed to the bioretention area through a pipe network connecting the impervious area on Lots 2, 3, 4, 5, and 9 to a new storm drain that runs parallel to the access road. The storm drains from Lots 6 and 8 connect directly to the bioretention area. It is assumed that drainage from Lot 7 will enter by overland flow. There is no storm drain connection to Lot 1 shown on the plan. vi. Energy dissipators. Erosion protection (e.g., cobbles) should be provided at each drainage entry point to the bioretention area. C3 review_Stirling_28030 Natoma LAH_6-25-15 3 EOA, Inc. g. No landscape plan was provided for review, so plant selection for the bioretention area could not be evaluated. h. Hydromodification Management (HM)Analysis: Since the project significantly increases runoff peak rates, volumes, and durations over the pre-project condition, the project is required to provide storage andddischarge controls for the additional runoff in order to protect the receiving stream (Matadero Creek)from increased erosion. The Bay Area Hydrology Model (BAHM), a continuous simulation hydrologic model allowed by the Town's stormwater permit for analysis of hydromodification management, was correctly used to size the required facilities. The analysis (dated 2/13/15)was performed based on an earlier version of the Storm Water Control Plan in which the access road was proposed to be impervious, and thus the results are conservative in terms of the amount of storage required to meet the HM standards. The analysis assumes development of each lot to the MDA (2.66 acres of impervious area) and 0.75 acres of impervious surface on the access road (total of 3.41 acres of impervious area). i. Hydromodification Control Facilities: The Storm Water Control Plan proposes to use an underground detention vault with outlet controls. The size of the vault, the height and diameter of the outlet riser and low flow orifice were determined using the BAHM model. A 6-foot diameter, 19-foot-long circular tank is proposed for the facility. Overflow from the bioretention area will enter the tank.A conceptual diagram of the facility is provided on Sheet 7, but no details for the tank and outlet controls are provided. 4. Conclusions a. The site design and source control measures identified in the C.3 Data Form are appropriate for the project and acceptable, if shown on the plan sheets. b. The use of a bioretention area for treatment is acceptable, if changes are made to the detail. c. The proposed hydromodification control facility is acceptable, but more details are needed. d. The Storm Water Control Plan is conceptually acceptable but more details are needed, as described in the recommendations below. 5. Recommendations The Town may approve the conceptual Storm Water Control Plan with the following conditions for future submittals: a. Make the following edits to the C.3 Data Form: • Indicate the amount of existing impervious area on the site. b. Submit revised plan sheets showing: • A detail for the pervious pavement on the access road, consistent with the SCVURPPP C.3 Handbook guidelines, that demonstrates that the pavement section will store and infiltrate the water quality design volume. The pavementsection should include cutoff trenches or check dams on the steeply sloped portions of the road to contain the infiltrated water. • A storm drain connection to impervious area on Lot 1. • A detail for how runoff from Lot 7 will enter the bioretention area. • A detail for storm drain inlet labeling. • Revisions to the bioretention area detail, including: C3 review Stirling_28030 Natoma LAH_6-25-15 4 EOA, Inc. 1. A note indicating that the biotreatment soil mix will meet the soil specifications included in Provision C.3.c.i.(1)(6)(vi), Attachment L of the Municipal Regional Stormwater NPDES Permit. 2. Removal of the visqueen liner, or if the Town approves the liner, lowering the underdrain to the bottom of the drain rock. 3. Indicate placement of at least 2 inches of non-floating (composted) mulch on any exposed surfaces of the bioretention areas. 4. Add the overflow riser with an elevation at 9 inches above the surface of the bioretention area. 5. Indicate erosion protection (e.g., cobbles) at each drainage entry point to the bioretention area. • A detail for the hydromodification control facility including dimensions of and structural support for the tank, details of the outlet riser, and connections to the bioretention facility and discharge pipe. c. The Town should condition the project to include the site design and source control measures described in the C.3 Data Form. These measures should also be included on the Storm Water Control Plan (Sheet 7). d. The Town should request the landscape architect to review the choice of plants and submit documentation that the selected plants have relevant characteristics equivalent to those listed in the SCVURPPP C.3 Stormwater Handbook, Appendix D, Plant List and Planting Guidance for Landscaped-Based Stormwater Treatment Measures. e. The Town should determine the party(ies) responsible for maintenance of the pervious- paved road, the bioretention area and the hydromodification control facility, and require a maintenance plan and agreement with the responsible party(ies). f. The Town should require construction best management practices to protect the pervious paving on the access road during construction of the stormwater treatment and hydromodification control facilities and construction on the individual lots, and thorough cleaning (e.g., pressure wash and/or vacuum sweeping) after completion of construction. C3 review Stirling_28030 Natoma tAH_6-25-15 5 EOA, Inc. Eisenberg, Olivieri & Associates Environmental and Public Health Engineering MEMORANDUM TO: Richard Chiu and John Chau, Town of Los Altos Hills FROM: Jill Bicknell, P.E. DATE: June 27, 2016 SUBJECT: 28030 Natoma Road Second Review of Project Submittals for Compliance with Stormwater Requirements Project Description The proposed project at 28030 Natoma Road is a 9-unit residential subdivision and access road on a primarily undeveloped property(one existing house, garage, and small detached building). The total site area is 18.18 acres; of that, 3.41 acres will be disturbed, and the remaining 14.77 acres will be dedicated open space. Lot sizes range from 1.0 to 3.3 acres, with average slopes of 7.6%to 38.4%. In the first phase, the access road and utilities will be constructed, and in the second phase, the 9 lots will be developed (either by the current owner or by individual lot owners). The site is located within the Matadero Creek watershed. Background We reviewed the Tentative Map Submittal dated May 22, 2015 and associated Storm Water Control Plan documents for compliance with the stormwater requirements in the Town of Los Altos Hills' NPDES Permit', and provided comments in a memorandum dated June 25, 2015 (attached). We performed a second stormwater permit compliance review of a revised Tentative Map Submittal dated May 3, 2016 to see if our previous recommendations had been addressed, and this review is summarized below.A revised Storm Water Control Plan (Sheet 7)was included in this submittal. We did not receive any additional updated Storm Water Control Plan documents. Results of Review The Storm Water Control Plan did not change significantly from the previously reviewed submittal, so the comments made in our June 25, 2015 memorandum are still applicable, except for the following sections listed below(changes are underlined): 2. Applicability of State Construction General Permit Requirements Projects that disturb more than 1 acre of land during construction are required to obtain coverage under the State Construction General Permit2. The first phase of the project will create approximately 0.75 acres of roadway (proposed to be pervious asphalt). If the subdivided lots are sold to individual owners, the maximum amount of impervious area on each lot will not exceed 1 acre, and the owners will not need to obtain coverage under the Construction General Permit for the construction phases of the project. If the current owner of I California Regional Water Quality Control Board, San Francisco Bay Region, Municipal Regional Stormwater Permit,Order No. R2-2009-0074, revised November 28,2011, Provision C.3. 2 NPDES General Permit for Storm Water Discharges Associated with Construction Activity,Water Quality Order 2009-0009-DWQ, September 2009 (effective July 1, 2010). 1 EOA, Inc. • 111 W. Evelyn Ave. • Sunnyvale, CA 94086 • Tel: (408) 720-8811 • Fax: (408) 720-8812 the property develops some or all of the lots as part of the second phase of the project, the current owner WILL need to obtain coverage under the State Construction General Permit, since the total construction project by that owner will disturb more than 1 acre of land. 3. Proposed Stormwater Management Measures and Sizing Calculations f. Sheet 7 provides a detail for the proposed bioretention area. The detail is consistent with the SCVURPPP C.3 Handbook guidelines with the following exceptions: i. Soil specification. A note indicates that a biotreatment soil mix with a percolation rate of 5-10 in/hr should be provided. The note should also indicate that the mix will meet the soil specifications included in Provision C.3.c.i.(1)(6)(vi), Attachment L of the Municipal Regional Stormwater NPDES Permit. iv. Overflow riser.The overflow riser is not shown on the detail, although it is shown on the conceptual sketch above the detail. The elevation of the overflow riser should be set at the depth determined by the sizing calculations (i.e., 9 inches above the surface of the bioretention area). v. Drainage entry points. The Storm Water Control Plan (Sheet 7) indicates that drainage will be conveyed to the bioretention area through a pipe network connecting the impervious area on Lots 1 2, 3, 4, 5, and 9 to a new storm drain that runs parallel to the access road. The storm drains from Lots 6 and 8 connect directly to the bioretention area. It is not clear how drainage from Lot 7 will enter the bioretention area, and there is a retaining wall on the uphill side of the bioretention area that may impede flow from this lot. vi. Energy dissipators. Erosion protection (e.g., cobbles) should be provided at each drainage entry point to the bioretention area. Conclusions The Storm Water Control Plan is conceptually acceptable but more details are needed, as described in the recommendations below. Recommendations The Town may approve the conceptual Storm Water Control Plan with the following conditions for future submittals: 1. Submit revised plan sheets showing: a. A detail for the pervious pavement on the access road, consistent with the SCVURPPP C.3 Handbook guidelines, that demonstrates that the pavement section will store and infiltrate the water quality design volume. The pavement section should include cutoff trenches or check dams on the steeply sloped portions of the road to contain the infiltrated water. b. A detail for how runoff from Lot 7 will enter the bioretention area c. A note on the plan sheet indicating the required source control measures for the project, including; • Beneficial landscaping (that minimizes irrigation runoff, pesticides, and fertilizers and promotes treatment); • Storm drain inlet labeling, with a detail for the label; • Maintenance of the pervious pavement on the access road. C3 review#2_28030 Natoma LAH_6-27-16 2 EOA, Inc. d. Revisions to the bioretention area detail, including: • A note indicating that the biotreatment soil mix will meet the soil specifications included in Provision C.3.c.i.(1)(6)(vi), Attachment L of the Municipal Regional Stormwater NPDES Permit. • Add the overflow riser with an elevation at 9 inches above the surface of the bioretention area. • Indicate erosion protection (e.g., cobbles) at each drainage entry point to the bioretention area. e. A detail for the hydromodification control facility including dimensions of and structural support for the tank, details of the outlet riser, connections to the bioretention facility and discharge pipe, and an access port for maintenance. 2. The applicant should submit documentation that the plants selected for the bioretention area have relevant characteristics equivalent to those listed in the SCVURPPP C.3 Stormwater Handbook, Appendix D, Plant List and Planting Guidance for Landscaped-Based Stormwater Treatment Measures. 3. The Town should determine the party(ies) responsible for maintenance of the pervious-paved road, the bioretention area and the hydromodification control facility, and require a maintenance plan and agreement with the responsible party(ies). 4. The Town should require construction best management practices to protect the pervious paving on the access road during construction of the stormwater treatment and hydromodification control facilities and construction on the individual lots, and thorough cleaning (e.g., pressure wash and/or vacuum sweeping)after completion of construction. More detailed recommendations for Conditions of Approval and maintenance for the storm water control facilities are provided in EOA's memorandum to the Town dated June 24, 2016. C3 review#2_28030 Natoma LAH_6-27-16 3 EOA, Inc.