enclosure: Avalanche Defense Work Study for the Timber Falls Corporation, Vail, Colo.

Transcription

1

2 ' H. FRUTlr,ER-G 10 OEH.. Ing n eur \i, CC.)IO \ V OS - DO RF CR 7260 Davos- Dorf, October 11, 1973 To Whitney M. Borland P. O. Box DENVER, Colorado 80215, U. S. A. Dear Whit: Here is the report on the Timber Falls Avalanche Defense Work Study. I sent three copies to the Timber Falls Corp. on September 23, two more copies by surface mail on October 10 and two more copies by air mail today. So, in total they will get 7 copies which I consider is enough. Today I am also sending a copy of each report (Vail and Aspen) to the Rocky Mountain Forest and Range Experiment Station (Pete Martinelli) in Fort Collins for information. Did you get the Aspen report on the Hoag Subdivision Avalanche Hazard? I sent it to you on September 10. I got your Aspen weather Data for slides in May 1973; thank you so much. Best vlishes Say hello to Dixie ~. Hans Frutiger enclosure: Avalanche Defense Work Study for the Timber Falls Corporation, Vail, Colo.

3 1 An Avalanche Defense Work Study for the Timber Falls Corporation, Vail, Colorado PRELIMINARY REMARKS By a letter dated July 24, 1973 the Timber Falls Corporation asked for a consultation with regard to possible avalanche control for the protection of their land development at East Vail, Colorado. A previous avalanche study (Timber Falls Avalanche, Vail, Colorado; report prepared for Timber Falls Corporation, dated July 27, 1973) has been made for the same place by Whitney M.Borland, P.E.No Colorado. This report is based essentially on the findings of Borland's report., There have been only minor changes regarding the extent of the starting zone which in turn affects the discharge volume. o BASIC DATA The basic data which are used in the design of defense structures are shown in the following table. The numbering relates to Borland's report: Starting zone acres Slide T Slide T Total starting area 9.24 hectares The assumed depth of a sliding snow pack for a 100 years average return period is 110 inches (2.8 meters). The resulting sliding snow volume is for T4: 107,900 cu.yds.(82,500 m3 ) for T5: 29,300 cu.yds.(22,400 m3 ) Total snow volume: 137,200 cu.yds.(104,900 m3 ) The assumption is made that both slides r elease at the same time thus resulting in a design discharge volume of 7,200 cu.yds./sec (5,500 m3/sec). DISCUSSION OF POSSIBLE DEFENSE WORKS For a discussion of avalanche control structures three different avalanche areas must be considered: starting zone, track, and runout zone. An overall analysis shows in which area control structures are possible and most effective.

4 2 a. ) For Swiss conditions it would be promlslng to protect the starting zone by so called supporting_ structures, designed to prevent the snowpack from breaking away. A reforestation of the starting zone and the track would supplement the supporting structures and completely eliminate the avalanche. This kind of defense work is questionable for U.S. conditions for two reasons: first, there is no experience with such structures; secondly, they would be unreasonably expensive. S tar tin g z 0 n e Qost;million desi@ation East Ridge Tl Aspen Slide T2 T3 T4 T5 acres hectares Swiss Francs, TOT A L b. ) An examination of the main track (T6) indicates little chance for diverting or retarding structures. Although the shelf shows only gentle sloping terrain it is still too steep for building earthen structures high enough to divert the avalanche or to slow it down sufficiently to shorten the runout distance. The most gentle portion of the track lies between the elevations 8880 to 9160 feet m.s.l. and shows the following characteristics: section of track feet above m.s.l to to 9000 grade of slope degrees percent horizontal distance feet meters An average slope angle of 34% is too steep for retarding structures to be effective and the most gentle section with an angle of 29% is still too steep and also too short. A diverting dam would need to be roughly 66 feet (20 meters) tall and the area would not be large enough to install such a dam. Moreover, it would be on forest land (White River National Forest) and it is doubtful that the Forest Service would accept such structures. c. ) The terrain conditions in the runout zone have been examined for the possibility of installing catching, retarding or guiding structures. A catching dam, which would need t he least area of land for 'construction, would however have to be 100 feet (30 meters) high.

5 3 Retarding structures (earthen mounds) to shorten the runout distance of the avalanche need more area than guiding structures and would occupy a large area of the Timber Falls Corporation property. The optimal solution is Tor guiding structures (earthen dikes) to keep the runout zone of the avalanche within certain limits. SUGGESTIONS FOR THE DEFENSE WORKS (see figure 1) The runout zone of the Timber Falls Avalanche (No.6) lies on a cone like alluvial fan (convex curvature of contour lines) thus allowing the avalanche to run in any direction over a wide area~ A prominent point near the apex of the cone (point 2) acts as a splitting wedge and promotes a spreading of the avalanche. The purpose of the defense work is to channel the avalanche~ This can be done by removing the splitting wedge by excavating a channel and using the excavated material to build two guiding dikes. The two earthen dikes named dike-east and dike-west are designed to keep the runout zone of the avalanche as small as possible. The dike crest has a width of 10 feet (3meters) and the slopes - both the cut and the fill - are ly2:l (67%). The runout zone of the avalanche has been surveyed to find the optimal location of the two dikes. The survey points (Nos. 1 to 8) have been indicated and shown to Mr. Jim Reinecke on August 18, The axis of the two dikes? marked by points DE 1 to DE 4 for dike-east and DW 1 to DW 7 for dike-west have also been shown to Mr.Jim Reinecke. To design sufficient catching capacity it was necessary to shift the axis of the dike-east 33 feet (10 meters) to the east, between the points DE 3 and DE 4 and to go from DE 3 straight to DE 0.. Figure 2 shows the longitudinal profile of the terrain in between the survey points and along the axis of the two dikes. Excavation of the channel begins at point A with a cut slope of ly2:1 (67%). The maximum excavation depth lies between points D and 2, while the average depth between these points is 41 feet (12.6 meters). The grade of the floor (point B to C) is 10%. The grade of the dike crest is at first 6% (DE and DW to 2) and then 16%. The maximum dike height, near DE 1, DW 1, DW 2 and DE 2, is approximately 46 feet (14 meters). Cut and fill grades are IV2:1 (see cross sections figure 2). The volumetric capacity of the dikes between cross sections D - B and DE 3 is 115,100 cu.yds. (88,000 m3 ). There is an additional capacity of 26,000 cu.yds. (20,000 m3) between cross sections DE 3 and DE 4 (DIN 7) thus resulting in a total capacity of 141,300 cu.yds. (108,000 m3 ) which will contain

6 4 the total sliding volume of snow of 137,300 cu. yds. (105~000 m3 ). -. The volumes of the dikes (fill) are: Dike-east 23,708 cu. yds. ( 18,125 m3 Dike-west 44,718 cu.yds. ( 34,188 m3 total 68,426 cu.yds. ( 52,313 m3 ). The total excavated material amounts to 44~500 to cu. yds. (34,000 to 50,000 m3 ) whereas for the dikes a total of 68,400 cu.yds, ( 52,300 m3 ) is needed. The difference can be obtained at the main excavation in the vicinity of point 2. The main question for the excavation is how deep the bedrock lies, and this would probably be answered with a test boring. From the lower ends of the dikes (DE 4 and DW 7), security lines (edge of runout zone) running to Gore Creek must be observed. Within these lines no construction can be permitted. However, the terrain can be used for recreational purposes during surnmer~ Davos-Dorf, September 20~ 1973 Hans Frutiger Villa Vecchia CH 7260 DAVOS-DORF SWITZERLAND

7

8