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HomeMy WebLinkAboutMatterhorn Village Lot 13 Debris Flow & Flood Risk ReviewTtec do Go o HEPWORTH-PAWLAK GEOTECHNICAL HE DEBRIS FLOW AND FLOOD RISK REVIEW NEW EXISTING RESIDENCE LOT 13, MATTERHORN VILLAGE 1664 MATTERHORN CIRCLE VAIL, COLORADO JOB NO. 114 429A OCTOBER 23, 2014 PREPARED FOR: DR. ERIK DORF 1664 MATTERHORN CIRCLE VAIL, COLORADO 81657 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY .................................................... ............- i - SUMMARY OF FINDINGS................................................................. - 1 - PROPOSED DEVELOPMENT.............................................................._.....................- 2 - GENERAL -GENERAL CHARACTER OF DEBRIS FLOWS AND FLOODS ........................ PROJECT SITE AND GEOLOGIC CONDITIONS ................................. TRIBUTARY DRAINAGE BASIN MORPHOLOGY .................... 3 _ DEBRIS FLOW AND FLOOD RISK ....... ...................................................................- 4 - INFORMATION -INFORMATION FOR MITIGATION EVALUATIONS .............................................. 5 - DESIGNRAINFALL...............................................................................................- 5 - DESIGN DEBRIS VOLUME .................. - 5 - -LIMILIMITATIONS TATIONS............................................................................................................- 6 - REF ERENCES REFERENCES .................... FIGURE I 'PROJECT SITE LOCATION FIGURE 2 - TRIBUTARY DRAINAGE BASIN AND ASSOCIATED FANS FIGURE 3 - HYPERCONCENTRATED FLOW FANS AT PROJECT SITE Job No. 114 429A Gtr �t�Ch PURPOSE AND SCOPE OF STUDY This report presents the findings of a debris flow and flood (hyperconcentrated flow) review for the recently completed new residence on Lot 13, Matterhorn Village, 1664 Matterhorn Circle, Vail, Colorado. The lot is located in the west Vail area as shown on Figure 1. The purpose of our review was to assess if the new house is in a potential hyperconcentrated flow risk area and, if so, to develop information that maybe used by a hydrologist to assess the feasibility of risk mitigation. Our study was performed according to our October 2, 2014 professional services agreement with Dr. Erik Dorf. A field reconnaissance of the properly was made on October 16, 2014 to observe the site, geologic and potential hyperconcentrated flow conditions. In addition, we have looked at aerial photographs of the project area and reviewed published regional geologic and hyperconcentrated flow studies (Kellogg and Others, 2003; Mears, 1984 and Scott and Others, 2002). The morphology of the drainage basin tributary to the geologic fans in the project area was analyzed to assess if the basin has the potential for producing hyperconcentrated flow. Based on the above information an assessment of the hyperconcentrated flow risks to the new house was made. This report summarizes the information developed by this study, describes our evaluations and presents our findings. SUMMARY OF FINDINGS This study shows that the western part of the new residence on Lot 13 is in a potential hyperconcentrated flow risk area. However, without long term observations it is not possible to develop statistical recurrence probabilities for future hyperconcentrated flows at the new house with a high level of confidence. A hyperconcentrated flow event could consist of a debris flood with Iow sediment concentration or a debris flow with high sediment concentration and a potential to cause severe damage to the house and possible harm to its occupants. Based on the currently available information, in our opinion, the likelihood that a major hyperconcentrated flow will occur on Lot 13 in a reasonable exposure time for the new house is low. If this risk is not acceptable to the residence Job No. 114 429A � tech -2 - owner or governmental regulatory agencies then a hydrologist experienced with hyperconcentrated flow modeling should be hired to assess the feasibility of hyperconcentrated flow mitigation. In our opinion, the existing residence construction on Lot 13 and the associated site grading observed at the site will not increase the existing debris flow and debris flood hazard to adjacent properties including public streets, right- of-ways ight- ofways and easements. PROPOSED DEVELOPMENT It is our understanding that the single family residence was recently constructed on Lot 13 which replaced an older house. The new house was finished this year and has a footprint that covers about 3,700 square feet, see Figure 3. The residence is a two story, frame structure with a walkout basement lower level. The Town of Vail's 2000 Official Debris Flow Hazard Map shows that Lot 13 is in high and moderate debris flow hazard areas (Town of Vail, 2000). Because of this, the town has requested that a site specific hazard risk review for the new house be performed. GENERAL CHARACTER OF DEBRIS FLOWS AND FLOODS Debris flows and floods are hyperconcentrated floods that differ from clear water floods because of their higher sediment concentrations and because of this, flow routing and mitigation methods commonly used for clear water flood risk should not be used to evaluate hyperconcentrated flood risk (O`Brien and Julien, 1985). They consider a sediment concentration by volume (C„) of less than 20 percent to be appropriate for clear water flood analysis. Sediment concentrations between C„ 20 and C„ 45 percent are appropriate for debris flood analysis and sediment concentrations between C„ 45 and C„ 65 percent are appropriate for debris flow analysis. In addition to sediments, hyperconcentrated flow can also transport large rocks, logs, other organic debris and man- made debris in the fluid matrix. PROJECT SITE AND GEOLOGIC CONDITIONS The 0.38 acre Lot 13 is located in the west Vail area on the lower slopes of the northwest - facing Gore Creek valley side as shown on Figure 1. The lower valley side in the project .fob No. l 14 429A �fe(::h -3 - area is a geologic fan complex made up of a younger fan and remnants of an older fan as shown on Figures 2 and 3. Both fans are made up of sediments deposited by recurring debris flows and floods (hyperconcentrated flows) since the late Pleistocene -age, Pinedale valley glacier that previously occupied the Gore Creek valley and began to retreat up valley starting around 16,800 years ago. The start of valley glacier retreat is based on the regional studies by Benson and Others, 2005; Price, 2004 and Porter and Others, 1983. The younger fan (map unit Qfl on Figure 2) covers about fourteen acres and its fan -head and upslope fan -head -channel has been incised about 10 to 20 feet below the surface of the adjacent older fan (map unit Qf2) remnants. The younger Qfl fan grades smoothly to the modern Gore Creek channel but 20 to 50 foot -high escarpments separate the older Qf2 fan remnants from the creek. These geomorphic relationships indicate the younger Qfl fan is still geologically active and that the older Qf2 fan is inactive and no longer the site of hyperconcentrated flow deposition. Lot 13 is located at the Qfl fan -head -channel to fan transition as shown on Figure 3. A small, earth embankment, debris basin that has a storage capacity of about 80 cubic yards is located at the southern lot line of Lot 13 as shown on Figure 3. The capacity of this basin is considerably smaller than the estimated volume of 6,600 cubic yards of a major hyperconcentrated flow at the project site as discussed in the Information for Mitigation Evaluations section of this report. The existing, small debris basin essentially blocks the fan -head -channel and may actually increase the risk of damage to the new house on Lot 13 if a major hyperconcentrated flow were to occur. TRIBUTARY DRAINAGE BASIN MORPHOLOGY The drainage basin tributary to the fan complex in the project area is shown on Figure 2. The basin covers about 228 acres and has a morphology that has been associated with basins elsewhere in western Colorado that have produced both debris flow and debris floods. The basin has a Melton's number of 0.72 which is indicative of basins where hyperconcentrated flows have been shown to be statistically infrequent (Coe and Others Job No. 114 429A G&Gtech 2003). About 74 percent of the basin has slopes greater than 30 percent, see Figure 2. The average basin slope along the longest flow line is 0.32 feet per foot. DEBRIS FLOW AND FLOOD RISK Historic debris flows and floods in western Colorado on geologically young fans similar to the Qfl fan at the project site have been triggered by unusually intense thunderstorms or rapid snowpack melting and these young fans should still be considered geologically active and potential sites of future hyperconcentrated flow inundation and sediment deposition. Notable historic thunderstorm triggered hyperconcentrated flow in western Colorado were the July 1977 Glenwood Springs flows in the -Roaring Fork River valley and the July 1999 Watrous Gulch flows in the Front Range. In the late spring of 1984, snowpack melt triggered hyperconcentrated flows occurred in the Town of Vail on fans similar to the Qfl fan at the project site but one did not occur on the Qfl fan. The drainage basin tributary to the Qfl fan at the project site has morphology indicative of drainage basins that have produced infrequent hyperconcentrated flow elsewhere in the project region (Coe and Others, 2003). The western part of the new house on Lot 13 and the other forty-five existing residences on the Qfl fan are in a potential hyperconcentrated flow hazard area. However, without long term observations it is not possible to develop statistical recurrence probabilities for fixture hyperconcentrated flows on the Qfl fan with a high level of confidence. A hyperconcentrated flow event could consist of a debris flood with low sediment concentration or a debris flow with high sediment concentration and a potential to cause severe damage to the house and possible harm to its occupants. Based on the currently available information, in our opinion, the likelihood that a major hyperconcentrated flow will occur on the Qfl fan in a reasonable exposure time for the new house is low. If this risk is not acceptable to the residence owner or governmental regulatory agencies then a hydrologist experienced with hyperconcentrated flow modeling should be hired to assess the feasibility of hyperconcentrated flow mitigation. Relevant information developed by this study that may be considered by the hydrologist is presented in the next section of this report. Job No. 114 429A tech -5 - INFORMATION FOR MITIGATION EVALUATIONS Information developed by this study that may be used by the hydrologist to assess the feasibility of hyperconcentrated flow mitigation for the house on Lot 13 is presented below. Hyperconcentrated flow routing analysis by the hydrologist may show that it is feasible to reinforce the lower story building walls along the west side of the new house to resist dynamic impact and depositional pressures associated with the design hyperconcentrated flow. Another mitigation strategy that may prove feasible would be to construct a debris storage basin up stream of Lot 13 to contain the design debris volume. This mitigation would require construction on adjacent properties and possibly the establishment of a community mitigation district. A debris storage basin would also mitigate the potential risk to the other forty-five potentially threatened houses on the Qfl fan. DESIGN RAINFALL A reasonable design rainfall event for hyperconcentrated flow routing analysis, in our opinion, would be the 100 -year, 1 -hour thunderstorm. The rainfall atlas for Colorado indicates that this thunderstorm will produce a point rainfall of 1.56 inches (National Oceanographic and Atmospheric and Administration, 2013). DESIGN DEBRIS VOLUME The estimated design debris volume at the Qfl fan -head is about 6,600 cubic yards (4.1 acre-feet). This volume was calculated using the empirical debris volume model developed by Cannon and Others (2010). Model input variables are: (1) drainage basin area with slopes steeper than 30 percent; (2) percent basin burned by wildfire; and (3) the design rainfall. The design debris volume presented above is for a basin not burned by wildfire. Job No. i 14 429A �Ptech M LIMITATIONS This study was conducted according to generally accepted geotechnical engineering principles and practices in this area, at this time. We make no warranty either express or implied. The information presented in this report is based on our field observations, aerial photograph interpretations, published regional information by others and our experience. This report has been prepared exclusively for our client to assess if the house on Lot 13 is in a potential hyperconcentrated flow area and to develop information that may be used by a hydrologist to assess the feasibility of risk mitigation. We should review any mitigation designs when available to evaluate if the design is consistent with the information presented in this report. We are not responsible for technical interpretations by others of our information presented in this report. Sincerely, HEPWORTH - PAWLAK GEOTECHNICAL, INC. .p Ralph Mock Engineering Geologist and by: Steven L. Pawlak, P.E. RGM/ksw°°•° °� :°° cc: Vail Custom Builders — Ted Leach {fedi vailcusto�ncenn) Suman Architects — Michael Suman (rniehael(Lusuni itiarchitccts.cca[3i) Job No. [ 14 429A 7 - REFERENCES Benson, L. and Others, 2005, New Data for Late Pleistocene Pinedale Alpine Glaciations from Southwestern Colorado: Quaternary Science Review V. 25, p. 49-65. Cannon, S. H. and Others, 2010, Predicating the Probability and Volume ofPostwildfre Debris Flows in the Intermountain Western United States: Geological Society of America Bulletin, January/February 2010 V. 122, No. 112, p. 127-144. Coe, J. A. and Others, 2003, Estimating Debris Flow Probability Using Fan Stratigraphy, Historic Records, and Drainage Basin Morphology, Interstate 70 Highway Corridor, Central, Colorado, USA, in Debris -Flow Hazards Mitigation, Mechanics, Prediction and Assessment, Rickenmann and Chen, editors: Millpress, Rotterdam. Kellogg, K. S. and Others, 2003, Geology Map of the Vail East Quadrangle, Eagle County, Colorado: U. S. Geological Survey, Map MF -2375, Version 1.0. Pierce K. L, 2004, Pleistocene Glaciations in the Rocky Mountains in The Quaternary Period in the United States, Gillespie A. R., Porter, S. C and Atwater Editors: Elsevier, New York, p. 63-78. Porter S. C. and Others, 1983, Late Wisconsin Mountain Glaciers in the Western United States in Late -Quaternary Environments of the United States, Wright, H. E. Jr., and Porter, S. C. editors: University of Minnesota Press, Minneapolis. Mears, A. I., 1984, Debris -Flow and Debris Avalanche Hazard Analysis: Prepared for the Town of Vail, Colorado (November, 1984). O'Brien, J. S. and Julien, P. Y., 1985, Physical Properties and Mechanics of Hyperconcentrated Sediment Flow: Proceedings ASCE Specialty Conference on the Delineation of Landslides, Flash Floods and Debris Flow Hazards in Utah: Utah Water Research Laboratory, University of Utah at Logan, Utah, p. 260 to 279. National Oceanographic and Atmospheric Administration, 2013, NOAA Atlas 14, Volume 2: NOAA National Weather Service, Silver Springs Maryland. Town of Vail, 2000, Oficial Debris Flow Hazard Map, Town of Vail: Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Scott R. B. and Others, 2002, Geology Map of the Vail West Quadrangle, Eagle County, Colorado: U. S. Geological Survey, Miscellaneous Field Studies Map 2369, Version 1.0. Jon ND. 114429A CCPti'1 of _ _ 1 r LO 13 r i j r 0 L I 2000 ft. 1 Scale: -I in. = 2000 ft. Contour Interval = 40 ft, Octohcr 2014 �14�429A HEF'tNCRTH-FALVLAGL-OTI-INiC,`a.L -.. 1-{Q�eCIilE OCC7tfOn rIC�I.EIe� Creek /,� ; f f --- 1 { Qf1 QF2 t{ff2 13 �1 tt SODO i • - � .. _ - � Iii ! 130/ -30 30' _ . r -30 - 1r rr Q ' F / •. — 0000 a � - Explanation - FI -307 Drainage Basin t 30%: Tributary drainage basin with slopes Contact: greater than 30 percent. Approximate boundary of map units. -30 Drainage Basin - 30%: Tributary drainage basin with slopes less than 30 percent. Qfi Younger Fan 0 1000 ft. Older Fan Scale: 1 in. ® 1000 ft.4i2 Contour Interval: 40 ft. _ October 2014 i14 429A � _ Matterhorn `tillage Lot `i 3, 1664 Matterhorn Circle _ .I i _For-v;n, -- T ributaty Drainage Basin and Associated dans � Figure 2