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Home My WebLink AboutDebris Flow Review 01.23.17H-P\KUMAR 5020 County Rord f 54 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax (970) 94s-8454 Emalh hpkglenwood@kumarusã,com Geotechnical ÊngineErin0 | Engineerlng Geology Måter¡als Testrng | Ënvlronmenlal Officc Locations: Parker, Glenwood Springs, and Silveñhomc, Colorado Januaty 23,2017 Kim & Rob Velasquez 2906 County Road I l3 Carbondale, Colorado I I 623 Project No. l7-7-139 Subject:Debris Flow Review, Lor l, Velasquez Minor Subdivision,3064 County Road I l3 (Caftle Creek Road), GarfÏeld Corrnty, Colorado Dear Kim and Rob: As reque.sted by Doug Pratt, lhe undersigned representûtive of H-P/Kttmar observed tlre subject sire on January lO,2017 to rev¡ew the potential risk of debris florv at the propo.sed building aren on Lot I . The fìndings of our obsen'ntions and feasibility of debris flow rnitigatíon arc presented in this report. Tlrc services were performed in accordance rvith our ¿lgreement for profesrional engineering services to yo¡l, dated January 20,2017, rile r¡nderstand the prefered building area is located in tlæ central part of the cunent proposed building envelope' In a lerter to Garfield County, tlre Colorudo Geologic Survey (CCS) hus identified the proposed building site as being in ä potentiul dehris f'lorv area. Based on the Geologic Map of the Carbondalc Quldrangle (Kirlihunr and Widnrnnn, 1997) und our obscrvations at the site, we ügree rvith the CGS ¿F$es$nrcnt. The ¿rlluviitl fan soils lre likely Prone to collapse when rvctted rvhich should ulso be considcred in the building design. The sitc muy be underl¡¡in by Eaglc Vattey Evuporite bedrock n'hich i.s associatcd rvith the development of sinklroles in the Roartng Fork Valley. Debris Florv: Tlre proposed building envelopc for Lot I is located on the ei¡stern side of ¡¡ Holocene-age debris fan located at the nrouth of a relutively small drainage basin (approximately 60 ucres). The drainage chnnnel is incised north of the lot o¡t lhe north side of Cattle Crcek Roacl. An olcl berm about l0 feet high is located on the uphill side of Cattle Crtek Ro¡rd. The berrn ha.ç nlature sagebrurih on top of it und :¡ppeürr; to be ut least 50 years old. The berm effectircly blocks the drainage clurnnel und there i.s an impoundment ürea on the upstream side. It does not ¡¡ppc¡¡r thut thcrc lrave bcen runoff event.s lurge enough to overtop the benn since it has been in place, Currenlly, a debris flow event emanaling from the drainnge ch¡nnel rvould be Kim & Rob Vslasquez Janunry 23,2t17 Page 2 initially stoppcd by the berrn. Based on our experience rvith debris flows, tlte existing bernr rvould result in a dramatic slowing of tlre debris florv and cause deposition of the debris in the existing impoundment area. If the debris flow were large enough to overtop the berrn, it would then encounter the relatively flat CounLy Road rvhich is about 25 leet wide, The road rvould also slow the debris flow and result in more deposition of debris on the road. The northern limit of rhe proposed building area is located abont 55 feet further south of the Connty Road. The resulting debri.s flow that might reach the central part of the building envelope, in our opinion, rvould be primarily a "rnild flood" consisting of mostly muddy tvater. We believe that a low earthen berm, 2 to 3 feet higlr above the existing debris fan surface and located in the nortltern part of the bHilding envelope, should be feasible tD protect the proposed house from reasonably likely debris fìow evenls. In oul opinion, it rvould take an extremely, large debris florv event (wcll above a nornral design storrn event) to impact the proposed lÌouse, given the observed conditions and provided the 2 to 3 foot high berm is in place. We recommend that the proposed residence be locnted as far south in the building envelopc as practical to further reduce the ri.çk of debris flow impacting the building area. Once building plans have becn lurther developed, rve should revierv the site grading plan and perform additional geotecltnical evaluations as needed. Sinkhole Potential: The site is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contâins gypsunr dcposits. Dissolutíon of tlre gyp.sum under cerlain conditions c¡¡n c¿use sinkholes to develop and can produce areas of localized subsidence. Sinkholes have not been observed in the irn¡nediate arca of the subject lot. B¿sed on ollr present knowledge ol the site, it cünnot be said for certain that sinkholes rr'ill not develop. [n our opinicn, the risk of ground subsidence at this sitc is lorv and simihr to other sites in the area not affected by sinkhole.s but the owner should be arvare ol'the potential lor sinkhole development, The orher concerns expressed by the CGS included not building on the southern side ol Cattle Creek due to the steep slopes and keeping the building envelope out of the 10O-year flood plain. The current building envelope avoids both of those hnzurds. The CGS also recomnrended th¡t a site.specilÌc subsoilstudy be perfornred to address the risk of setllement prone debri.s fan soils which lve concur. The subsoil study should be performed once the specific building area hus been identified. H-P \ KUMAR Proiecl No. 16-7-632 Kim & Rob Velasquez January 2l,2Ol7 Page 3 The recommendations sub¡nitted in this letter are based on the conditions observed ¿t the site and our expedence rvith debris florvs and sinkholes in the general area. If you have any questions or need further assistünce, plcase call our officc. Sincerely, H.P* KUMAR DanielE, Hardin. P.E. ,6 Rev. by: SLP DEH/ksrv Referance: Kirkhmr, Robert M. uncl \ffidmunn, Beth L., 1997, Geolagfu Map of the Carbowlilc Quuclnmgle, Garlield Comil¡', Colorudo. Colorudo Geological Survey, Open Filc Report 97-3. cc The Llnd Studio, Inc. - Doug Prûtte (låndstud¡o2@comcgÊt.M) H.P* KUMAR ProJect No. lø-7-892