Echelmeyer, Keith A. and Kamb, Barclay (1986) Stressgradient coupling in glacier flow: II. Longitudinal averaging of the flow response to small perturbations in ice thickness and surface slope. Journal of Glaciology, 32 (111). pp. 285298. ISSN 00221430. https://resolver.caltech.edu/CaltechAUTHORS:20140915094200202

PDF
 Published Version
See Usage Policy. 4MB 
Use this Persistent URL to link to this item: https://resolver.caltech.edu/CaltechAUTHORS:20140915094200202
Abstract
As a result of the coupling effects of longitudinal stress gradients, the perturbations Δu in glacierflow velocity that result from longitudinally varying perturbations in ice thickness Δh and surface slope Δɑ are determined by a weighted longitudinal average of ФhΔh and Ф_ɑΔɑ, where Фh and Ф_ɑ are "influence coefficients" that control the size of the contributions made by local Δh and Δɑ to the flow increment in the longitudinal average. The values of Ф_h and Ф_ɑ depend on effects of longitudinal stress and velocity gradients in the unperturbed datum state. If the datum state is an inclined slab in simpleshear flow, the longitudinal averaging solution for the flow perturbation is essentially that obtained previously (Kamb and Echelmeyer, 1985) with equivalent values for the longitudinal coupling length ℓ and with Ф_h = n + l and Ф_ɑ + n, where n is the flowlaw exponent. Calculation of the influence coefficients from flow data for Blue Glacier, Washington, indicates that in practice Ф_ɑ differs little from n, whereas Ф_h can differ considerably from n + 1. The weighting function in the longitudinal averaging integral, which is the Green's function for the longitudinal coupling equation for flow perturbations, can be approximated by an asymmetric exponential, whose asymmetry depends on two "asymmetry parameters" µ and σ, where µ is the longitudinal gradient of ℓ(= dℓ/dx). The asymmetric exponential has different coupling lengths ℓ_+ and ℓ_ for the influences from upstream and from downstream on a given point of observation. If σ/µ is in the range 1.52.2, as expected for flow perturbations in glaciers or ice sheets in which the ice flux is not a strongly varying function of the longitudinal coordinate x, then, when dℓ/dx > 0, the downstream coupling length ℓ_+ is longer than the upstream coupling length ℓ_ and vice versa when dℓ/dx < 0. Flow thickness and slopeperturbation data for Blue Glacier, obtained by comparing the glacier in 195758 and 197778, require longitudinal averaging for reasonable interpretation. Analyzed on the basis of the longitudinal coupling theory, with 4ℓ + 1.6 km upstream, decreasing toward the terminus, the data indicate n to be about 2.5, if interpreted on the basis of a response factor ψ + 0.85 derived theoretically by Echelmeyer (unpublished) for the flow response to thickness perturbations in a channel of finite width. The data contain an apparent indication that the flow response to slope perturbations is distinctly smaller, in relation to the response to thickness perturbations, than is expected on a theoretical basis (i.e. Ф_ɑ/Ф_h + n/(n + 1) for a slab). This probably indicates that the effective ℓ is longer than can be tested directly with the available data set owing to its limited range in x.
Item Type:  Article  

Related URLs: 
 
Additional Information:  © 1986 International Glaciological Society. Received 24 April 1985 and in revised form 3 September 1985. This work was done under the support of National Science Foundation grants EAR5008329 and DPP8209824. We thank the U.S. National Park Service for permission to collect the field data in section 4.  
Funders: 
 
Other Numbering System: 
 
Issue or Number:  111  
Record Number:  CaltechAUTHORS:20140915094200202  
Persistent URL:  https://resolver.caltech.edu/CaltechAUTHORS:20140915094200202  
Official Citation:  Stressgradient coupling in glacier flow: II. Longitudinal averaging of the flow response to small perturbations in ice thickness and surface slope Keith A. Echelmeyer and Barclay Kamb Pages 285298  
Usage Policy:  No commercial reproduction, distribution, display or performance rights in this work are provided.  
ID Code:  49694  
Collection:  CaltechAUTHORS  
Deposited By:  Ruth Sustaita  
Deposited On:  15 Sep 2014 17:28  
Last Modified:  03 Oct 2019 07:16 
Repository Staff Only: item control page