UBC Theses and Dissertations
Validation of a capacitance snow density probe in an avalanche forecasting setting Conger, Steven Micheal
Snow profiles provide information of medium importance and uncertainty (Class II) about the characteristics of snow stratigraphy relative to patterns associated with avalanche formation and activity. Snow profiles are time consuming and limited to a specific location. It would be a valuable improvement to be able to sample more sites and gain more information in the same, time required for one manual profile. I conducted a careful and systematic investigation of the Capacitec capacitance probe (Louge et al., 2002) developed as a potential snow density profile tool. The probe utilizes measurements of dielectric properties of snow that have been related to the snow sample's density (Cumming, 1952; Kuroiwa, 1962; Yosida et al., 1958). I tested the intended use of both the original prototype and an improved second-generation prototype in a field setting representative of avalanche forecasting conditions. I investigated three hypotheses. The first tested whether bulk snow density measured by the probe is equal to or better than currently accepted practice. A supporting study determined the range of values for "accepted practice." The second hypothesis investigated whether a density profile as estimated by the probe is equal to or better than currently accepted practice. The third hypothesis examined whether characteristics associated with structure and stratigraphy in the snowpack could be identified in the information provided by the probe. Detailed manual snow profiles with associated probe measurements were collected over a nineday period from 23 February to 3 March 2006 in the Northern Selkirk Mountains of British Columbia, Canada. These data were used as a training-set for the construction of recursive partitioned models to estimate densities from probe output. Portions of the training-set were used as validation-sets along with two test cases representing spatial and temporal differences, gathered on 5 and 10 March 2006. The precision and accuracy of predictions against validation-sets and test cases were analyzed and crossvalidation was performed for models.representing different sizes, grain types, and lag times. In the supporting study, I determined that "accepted practice" includes under sampling errors of 1 to 2%, variation within individual cutters of 0.8 to 6.2%, and significant variation between cutters of 3 to 12%. Given the mean of all samples is the accepted true value of the measured density, variation solely in cutter types provides "accepted practice" measurements that are within 12% of the true density. In addressing the first hypothesis, I was able to create and validate models based on probe measurements that provide bulk density predictions accounting for 9 2 % of the variability in the manual density measurements (97% in a unique case) and are within "accepted practice" values. Mechanical problems with the tracking component of the probe prevented numerical comparison of predicted and manual profiles. Visual analysis ascertained that though predicted and measured density profile shapes were close, the probe profiles were generally not sufficiently close to the manual profile to replace it in representing the structure of the snow cover. One case utilizing a layer ageing proxy did fit close enough for practical use. The same mechanical issues prevented a full conclusion regarding the third hypothesis though my experience with the probe and manual observations suggests the nature of grain bonding plays a noticeable role in the properties measured.
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