OCR Text |
Show 12 On steep lee slopes it is most efficient to incline the jet roof at angles greater than 20°. At angles of 30° and beyond the wind speed increases are marked but do not extend as far downwind. However, as previously mentioned, an extended downwind effect is unnecessary over steep slopes. In this latter case it is desirable to have a steep roof inclination which would prevent accumulation directly adjacent to the jet roof where narrow but dangerous cornices can build. CIimatic Effects. Climate may also be a factor in determining the proper jet roof angle, for climate dictates the nature of the snowfalls and of subsequent cornice growth. For example, during the relatively rare heavy and wet December and January ( 1967- 68) snows in the Bridger Range, only those jet roofs with inclination angles of 15° or less cleared out the cornices over the 30° lee slopes. All other structures with steeper inclinations developed large drifts and secondary cornices, whereas in previous dry snow conditions they had worked satisfactorily. Jet roofs erected over coastal range mountains should probably be inclined to the minimum extent, unless, of course, the lee slopes are very steep. Other Control Devices. " Kolktafeln" or vertical baffles, with undergaps of 1 m have proven effective in cornice control work, although under most circumstances they are not as effective as jet roofs ( Hopf and Bernard, ( 1963)). Evidently, snpw scoured and drifted into a horseshoe shaped mound to the lee of these devices is rapidly age hardened to form a strong unyielding drift if it is not jetted away ( Hopf and Bernard, 1963). Further experiments with these baffles are in progress at the Bridger Range because ease of erection would justify their use if cornices can be effectively reduced. |