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HomeMy WebLinkAboutDRB150144_Debris Flow signed_1430233140.pdfHepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 Email: hpgeo@hpgeotech.com HEPWORTH-PAWLAK GEOTECHNICAL April 22, 2015 Desmond Home Builders Attn: Bryan Desmond P. O. Box 3518 Eagle, Colorado 81631 bryan@desmondhomebuilders.com Job No. 115 113B Subject: Review of Debris Flow and Flood Potential, Lot 31, Glen Lyon Subdivision, 1240 Westhaven Circle, Vail, Colorado Dear Bryan: As requested, we have reviewed the potential for hyperconcentrated flows (debris flows and floods) for the subject site. A field reconnaissance of the project site was made on April 7, 2015. In addition we have analyzed the morphology of the two tributary drainage basins on Lot 31 with respect to potential hyperconcentrated flows. This report summarizes the information reviewed and presents our findings and recommendations. The services were performed as part of our agreement for professional services to Desmond Home Builders dated March 27, 2015. SUMMARY OF FINDINGS The site is in an area of moderate debris flow risk as noted on the current Town of Vail Geologic Hazards maps (Town of Vail, 2000). The two drainage basins tributary to the two fan deposits on Lot 31 have morphologies indicative of drainage basins that can produce relatively frequent debris flows and debris floods. Because of this, the potential risks to the proposed duplex appear high enough to warrant fan specific hyperconcentrated flow modeling to assess the need for and feasibility of mitigation for development of the lot. This modeling should be done by a hydrologist experienced with hyperconcentrated flow evaluations. PROPOSED DEVELOPMENT A duplex residential structure, located in the lower northwestern part of the lot approximately as shown on shown on Figure 3, is planned. Preliminary plans indicate that the structure will have a foot-print area of about 4,800 square feet. The building will Desmond Home Builders April 22, 2015 Page 2 Job No. 115 113B be a three story structure with walkout basement levels on the Westhaven Circle side and patios on the uphill side at the third story level. Preliminary floor elevations as provided are 8,138 feet, 8,149 feet and 8,160 feet. Grading plans for the duplex have not been developed at this time but based on the relatively steep slopes at the building site, considerable grading will be probably needed. GENERAL CHARACTER OF HYPERCONCENTRRAED FLOWS Hyperconcentrated flows differ from clear water flows because of their higher sediment concentrations and because of this, flow routing and mitigation methods commonly used for clear water flows are not commonly used to evaluate hyperconcentrated flows (O'Brien and Julien, 1985). O’Brien and Julien consider a sediment concentration by volume (Cv) of less than 20 percent to be appropriate for clear water flood analysis. Also, sediment concentrations between Cv of 20 and Cv of 45 percent are appropriate for debris flood analysis and sediment concentrations between Cv of 45 and Cv of 65 percent are appropriate for debris flow analysis. In addition to sediments, hyperconcentrated flows also transport large rocks, logs, and other organic debris in the fluid matrix. These floods although infrequent statistically can cause damage to structures for intense thunderstorms and/or rapid snow melt and associated runoff. PROJECT SITE AND GEOLOGIC CONDITIONS Lot 31 is located south of Gore Creek about 1.3 miles southwest of the Vail Town Center area as shown on Figures 1 and 3. The lot covers about 0.58 acres and is located on moderately steep northwest facing terrain on the lower, south side of the Gore Creek valley as shown on Figure 1. The lot has an average slope of around thirty percent down to the northwest towards Gore Creek. Residences are located on the adjacent lots and Lot 31 is bordered by U. S. Forest Service land on the east. Vegetation on the lot is a moderately thick aspen stand with scattered, large conifer trees with a grass and brush understory. Formation bedrock is the middle Pennsylvanian-age, Minturn Formation that is mostly interbedded sandstone, conglomerate, siltstone and shale with some limestone (Scott and Others, 2002). Colluvium usually covers the Minturn Formation on the valley sides, and coalescing hyperconcentrated flow (debris) fans and colluvial aprons underlie the Glen Lyon subdivision as shown by map symbol Qfc on Figure 2. The fans are made up of sediments deposited by reoccurring hyperconcentrated flows occurring since the late Pleistocene-age, Pinedale valley glacier that previously occupied the Gore Creek valley began to retreat up valley starting around 16,800 years ago (Benson and Others. 2005; Price, 2004 and Porter and Others,1983). Historic hyperconcentrated flows occurred in Desmond Home Builders April 22, 2015 Page 3 Job No. 115 113B the Vail area on similar alluvial fans during the unusually rapid melting of an unusually heavy snow pack in the spring of 1984 (Mears, 1984). Hyperconcentrated flows did not occur on the subject site fans on Lot 31 in 1984 or elsewhere in the Glen Lyon subdivision but, in our opinion, these fans should be considered geologically active and potential sites of future hyperconcentrated flows. TRIBUTARY DRAINAGE BASINS MORPHOLOGY The hyperconcentrated (debris) flow fans at Lot 31 have formed at the mouths of two small, very steep drainage basins as shown on Figure 2. The basins have been designated B1 and B2 by us. Basin morphology pertaining to hyperconcentrated flow is presented below. Basin B1: This basin covers about 23.2 acres and has a morphology similar with basins elsewhere in western Colorado that have produced both debris flow and debris floods. The basin has a Melton's number of 1.02 which is indicative of basins where hyperconcentrated flows are relatively frequent (Coe and Others 2003). Approximately about 69 percent of the basin has slopes greater than 30 percent and the average basin slope along the longest flow line of about 0.46 feet per foot. For a watershed that has not been burned by wild fire, Basin B1 has the potential for producing around 1,600 cubic yards (1.0 acre-feet) of sediment for the 100-year, 1-hour thunderstorm rainfall of 1.54 inches (National Oceanographic and Atmospheric and Administration, 2013). This estimate is based on the empirical debris volume model by Cannon and Others (2010). Basin B2: This basin covers about 9.0 acres and has a morphology similar to basins elsewhere in western Colorado that have produced both debris flow and debris floods. The basin has a Melton's number of 1.62 which is indicative of basins where hyperconcentrated flows are relatively frequent (Coe and Others 2003). About 96 percent of the basin has slopes greater than 30 percent and the average basin slope along the longest flow line of about 0.51 feet per foot. For a watershed that has not been burned by wild fire, Basin B2 has the potential for producing around 1,100 cubic yards (0.7 acre- feet) of sediment for the 100-year, 1-hour thunderstorm rainfall of 1.54 inches (National Oceanographic and Atmospheric and Administration, 2013). This estimate is based on the empirical debris volume model by Cannon and Others (2010). HYPERCONSENTRATED FLOW RISK The two drainage basins tributary to the two alluvial fans on Lot 31 have morphologies indicative of drainage basins that can produce relatively frequent debris flows and debris floods. However, without long term observations it is not possible to develop statistical Desmond Home Builders April 22, 2015 Page 4 Job No. 115 113B recurrence probabilities for future hyperconcentrated flows with a high level of confidence. In our opinion, the potential risks are high enough to warrant assessing the need for and feasibility of risk mitigation by site specific hyperconcentrated flow modeling. This modeling should be done by a hydrologist experienced with hyperconcentrated flow evaluations. In our opinion, a reasonable design rainfall for hyperconcentrated flow modeling at this site is the 100-year, 1-hour thunderstorm rainfall of 1.54 inches (National Oceanographic and Atmospheric and Administration, 2013) used for the above sediment flow estimates. HYPERCONCENTRATED FLOW FAN RELATIVE RISK ZONES Based on the surface topography of the two alluvial fans on Lot 31, we have divided the fans into relatively higher and relatively lower risk zones as shown on Figure 3. The risk zones shown on Figure 3 should be considered preliminary pending the findings of the hyperconcentrated flow modeling. The higher risk zones are the incised, fan head channels that lie below the adjacent fan surfaces that are the lower risk zones. It is possible that Type flow modeling could show that the design flows will be confined to the fan head channels (Qf1h and Qf2h higher risk zones) and the proposed duplex is not exposed to a hyperconcentrated flow risk for the design flood. However, if the flow modeling shows that the fan head channels will not contain the design flows then mitigation may needed. For this site, direct building protection may be feasible but other mitigations such as debris storage basins and deflection structures may not be effective. With direct building protection, the building walls that could be impacted by the hyperconcentrated flows would be reinforced to resist the dynamic impact and static deposition loads of the design and flows. Also, with direct building protection it should be possible to provide a reasonable level of safety for the structure and its occupants but some landscape damage resulting from erosion and deposition of mud and debris could occur. If the flow modeling shows that mitigation is warranted, then mitigation design should take precedence over other esthetic and architectural considerations. 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, drainage basin morphology and our experience in the area. This report has been prepared exclusively for our client to assess the hyperconcentrated flow potential at Lot 31. A hydrologist experienced with hyperconcentrated flow modeling should be hire to assess the severity, extent of reasonable design flows and need for hyperconcentrated flow Desmond Home Builders April 22, 1015 Page 6 Cannon, S. H. and Others, 2010, Predicating the Probability and Volume of Postwildfire Debris Flows in the Intermountain Western United States: Geological Society of America Bulletin, January/February 2010 V. 122, No. 1/2, p. 127-144. Mears, A. I., 1984, Debris-Flow and Debris-Avalanche Hazard Analysis: Prepared for the Town of Vail, Colorado (November, 1984). National Oceanographic and Atmospheric Administration, 2013, NOAA Atlas 14, Volume 2: NOAA National Weather Service, Silver Springs Maryland. 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. 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. 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.