Loading...
HomeMy WebLinkAboutGeologic Hazard Evaluationffi CTL ITHOMPSON September 18, 2018 Mushroom Rock PartnershiP, LLC PO Box 1799 Carbondale, Colorado 81 623 Attention: Caleb Edelman Subject: Geologic Hazard Evaluation ADU at 1A211HighwaY 82 Carbondale, Colorado Project No. GS0627 1 -105 At your request, we made field observations and performed research to evaluate the geologic hazaids present at the site of an auxiliary dwelling unit (ADU) planned for the iubject lot. Our services were performed to fulfill the scope described in our Proposal No. GS 18-A1O2; dated January 8, 2018' An existing single-family residence is on the parcel al1O211 Highway 82. The proposed ADU is south of the residence and situated near the toe of a steep west-facing hltsiOe below a regional mesa that defines Heuschkel Park above. Highway 82 and the Roaring Fork Rivei are to the west. The project area is shown on the Google Earth photo reprodiced below from June 2017. Acliff point that divides tw_o incised drainages is at about elevation 6670 and the ADU site is at about 6170 feet. The overall slope above the building to the cliff point is about 1.7H.1V or about 60 percent. The area of the new building his slopes that are about 4H:1V or about 25 percent. Deeply incised drainages brackei the site north and south of the existing and proposed dwellings and can be seen on the photo below, IHCORPONAIID Earth m June 2017 home and driveway is visible north of the red arrow that ng between two ncised d rar nages leading off SIsteep ope of aonwest-trend tng ridge the hi and mesa bove. 234 Center Drive I Glenwood Springs, Colorado 81601 Telephone: 970-945-2809 Fax: 970-945-741 I The planned home is shown on drawings by Studio D dated November 2017 and will be a ranch over a finished basement that may walk-out at the northwest corner. An excavation of a few to about 12 feet deep appears to have been made at the site be- tween 2011 and 2015, based on aerial photo review. The location isdenoted bythe red arrow. The excavation is presumably for the planned ADU foundation and was dry at the time of my visit on August 29,2418. An excerpt is presented below from the Geologic Map of the Carbondale Quad- rangle by R. M. Kirkham and B. L. Widmann of the Colorado Geologic Survey, 2008. Setting and geology of the site can be described as a moderate to very steep hillside above the Roaring Fork River. The Pennsylvanian-age Eagle Valley formation, gray and yellow shale and sandstone comprise the cliffs just above the building site. The slightly more resistant Maroon formation, red bed sandstone and siltstone forms the upper cliffs. Both formations are considered erosion-prone. The project site lies on a thick apron of colluvium and slope wash sourced in rapidly-eroding bedrock cliffs above. The colluvium deposit is well-exposed in the east wall of the existing foundation excavation shown in Photos 1, 2 and 5 below. The colluvium is a chaotic mixture of tabular sandstone boul- ders in a silty and sandy clay matrix and appears to be the result of ancient mudflow off the hillside above. The reasonably well-defined topsoil layer exposed at the top of the excavation indicates the recurrence interval of mudflows that formed these slopes is probably hundreds of years or more. Observations of the ground surface around the ex- isting residence and ADU area indicated shallow drainage swales with no evidence of erosive flow. Excerpl from 2008 Geologic Map of the Carbondale Quadrangle. Arrow points to approximate location of ADU. A debris fan deposit is mapped north of the ADU site and is associated with dep- osition in a larger drainage to the north. lncised drainages north and south of the ridge that comprises the project site appear to have carried recent erosive flood flow. Photos 3 and 4 show views of these drainages. Our observations in the field and from aerial pho- tos indicate that these ditch features have carried recent floods without obviously spilling inio the project area. We view the debris flow hazard as low on the ADU site, provided the "ditches" north and south of the area are maintained to handle potential flow. MUSHROOM ROCK PARTNERSHIP, LLC AOU AT .10211 HIGHWAY 82 PROJECT NO. GS06271.000-105 C:\Users\atlrale\Box\Projecls\Glenwood Springs - Proiects\G S0627 L000\105\3. Letters\GS0627 1.000-l 05 l-ette|l.docx 2 ffi - View to the west nside the existing MUSHROOM ROCK PARTNERSHIP, LLC AOU AT 1021.I HIGHWAY 82 PROJECT NO. GS06271.000-105 c:tuseB\ailrat6\Box\Projects\Glonwood springs - Proiects\G50627'l.OOO\105\3. Lottets\GS06271 '000'105 Letter ',l 'docx 3 ffi Photo 3 - View looking west in the southern incised drainage located about 150 feet south of the ADU site. Photo 4 - View looking east in the northern incised drainage located about 120 feet north of the existing home. MUSHROOM ROCK PARTNERSHIP, LLC ADU AT 1O2.II HIGHWAYS2 PROJECT NO. GS06271.000..t 05 C:\Users\alhatolBorlProjects\Glonwood Sprlngs - Projects\GS06271.000\1 05\3. LotlsB\GS0627 1.000.1 05 Letler l.docx 4 Geologic Hazards Geologically, the site is a rapidly-changing landscape and has some risk com- pared to a flat site.-The colluvial soil is reasonably strong in a local and global sense un- iess wetted. Our observations indicate no on-going land sliding or related slope move- ment issues. Geologic hazards that could affect the proposed (and existing) residence include rockfalls, mudflows and debris fans, erosion, potentially unstable slopes, and the regional issues of seismic affects and naturally occurring radioactivity. Most of these are described below. Rockfalls We found no unusual risk of rockfall, considering the terrain and location. The slopes above and below the site expose boulders that are typically the result.of being ex' ilJJ by erosion of the colluvium matrix, rather than part of a rockfall. The slope of the nOU ir genfle to moderate for at least 100 feet above and a graded road provides some ."trg pr5t."tion from uphill hazards such as rolling rocks or even flood. The risk of rock- fall is judged to be low. Mitigation for rockfall does not appear to be merited unless a low risX cinnot be accepted. CJtchment ditches and energy absorbing fences are the pri- mary means to conirol rockfall. We view these systems as optional. Mudflows and Debris Fans oto 5 - View to the south wall of the excavation The area and so of the site IS pro to rn udflow and debri s ta n fonorthuthADUne ag traverse erodi bed a nd soil The n- r- mation whe re bleslzea d and steep drai n ES e rock capa nveying m u dfl ow or d e- probably the b cised ditche S north and so uth of th e A D U a ppear bl e of co bris fa n d epos its onto ig hway 82 he site,andHbelow tche have successful ly ca rried recen S n have pa st.P hoto S rg ificant flow We bel teve th e n n dicate ese Sth as evidenced by the soil horizon exposed i nearby ground sudace conditions. Therefo mation affecting the ADU project appears t n the excavation and observation of the re, the risk of mudflow and debris fan for- o be low. Mitigation to maintain relatively low MUSHROOM ROCK PARTNERSHIP' LLC ADU AT 1021'I HIGHWAY 82 PROJECT NO. GS06271.000-105 biu**Lir,"i"rg"itireiectstctenwmd springs - Proiecls\GSo627l 000\105\3. Lelters\Gs06271 000'105Letter l docx 5 risk would include regular inspection and maintenance of the ditches to prevent them from overflowing above the existing residence or proposed ADU. A civil engineer with expertise in surface water hydrology can assist to determine the design flood and ability of the existing drainage ditches to carry the flow. Our brief observations indicate the ditches have carried significant flow in recent years. Potentially Unstable Slopes Slopes well-above the project site become progressively steeper toward the Ma- roon formation sandstone cliffs at the top of the mesa. These slopes are erodible and somewhat unstable, Water erosion, chemical weathering and gravity contribute to risk of slope instability. Our observations indicated no evidence of an existing or incipient land- slide that could affect the ADU. The current good drainage in the site of the ADU and rel- atively dry climate reduce the risk that unstable slopes will affect the project. Mass wast- ing due to erosion is the most serious slope instability risk on this site. To the extent that the site remains relatively dry, risk of unstable slopes is low. Photo 2 shows a verticalex- cavation in the colluvium that has stayed stable for the years it has remained open. Miti- gation for potentially unstable slopes does not appear to be necessary for construction of the ADU. Seismic Effects The site is exposed to risks due to seismicity. This part of Golorado has a low risk of seismic movements that could damage residential construction. Faults near the pro- ject site are thought to be inactive and are related to mountain building episodes that are many millions of years old. There is increased risk of some of the above-mentioned geo- logic hazards during seismic events, but there is no need for mitigation compared to sim- ilar sites in the area. Summary ln summary, the site is exposed to a low risk of geologic hazards. None of the identified risks require mitigation, The capacity of the incised drainage ditches north and south of the ADU should be evaluated with respect to the design flood. This will help you determine the risk of flooding. We believe this consultation was prepared with that levelof skill and care nor- mally exercised by geological engineers in the area, at this time. No warranty, express or implied is made, Should you have questions about this letter, please contact me. cTLITHOM Reviewed By >,\l%P-E - David A. G , C.P.G Principal Geol DAG:JDK:ac MUSHROOM ROCK PARTNERSHIP, LLC ADU AT IO21I HIGHWAY 82 PROJECT NO. GS0627r.000.105 C:lUs€rs\athate\Box\Pror€cls\Glsnwood Sprlngs - D. Kellogg, P.E sion Manager s583 .000-105 Lettor l-docx 6