commit 4dbb691367306b9c8176ebc784412c601bea7cb9
parent 9ad28c869b1cbdbcf2f7b72503253808c91f7aad
Author: Anders Damsgaard <anders@adamsgaard.dk>
Date: Tue, 29 Oct 2019 06:55:58 +0100
More work on intro
Diffstat:
2 files changed, 51 insertions(+), 9 deletions(-)
diff --git a/BIBnew.bib b/BIBnew.bib
@@ -9174,3 +9174,13 @@ Winton and A. T. Wittenberg and F. Zeng and R. Zhang and J. P. Dunne},
title = {Groundwater Flow Under a Paleo-Ice Stream of the Scandinavian Ice Sheet and Its Implications for the Formation of Stargard Drumlin Field, {NW} Poland},
journal = {J. Geophys. Res.: Earth Surf.}
}
+
+@article{Ungermann2017,
+ doi={10.1002/2016JC012128},
+ year=2017,
+ volume=122,
+ pages={2090--2107},
+ author={M. Ungermann and L. B. Tremblay and T. Martin and M. Losch},
+ title={Impact of the ice strength formulation on the performance of sa sea ice thickness distribution model in the Arctic},
+ journal = {J. Geophys. Res.: Oceans}
+}
diff --git a/continuum-granular-manuscript1.tex b/continuum-granular-manuscript1.tex
@@ -49,26 +49,56 @@ maxcitenames=2, backend=bibtex8]{biblatex}
\begin{abstract}
The dynamic interplay between fast ice flow, meltwater drainage, and till deformation is crucial for understanding glacier and ice-sheet behavior.
-The till yield strength is highly dependent on water pressure, and is accurately described by the Mohr–Coulomb rheology.
Subglacial sediment transport constructs landforms that influence glacier stress balance and post-glaciation geomorphology.
+Till yield strength is highly dependent on water pressure and follows the Mohr-Coulomb rheology.
However, the physical transport of till during subglacial shear is not well understood, and is for that reason not included in prognostic ice-sheet models.
-Here we present a water-saturated continuum model that is consistent with Mohr–Coulomb mechanics and is suitable for coupled glacier-sediment-hydrology modeling.
-We show that pulses in water pressure can shift deformation away from the ice-bed interface and far into the bed.
-Deep deformation is most likely in beds with high hydraulic permeability, experiencing slow and large water-pressure variations.
+We present a water-saturated continuum model that is consistent with Mohr-Coulomb mechanics and is suitable for coupled glacier-sediment-hydrology modeling.
+We show that pulses in water pressure can shift deformation away from the ice-bed interface and far into the bed, resulting in significant till advection.
+Deep deformation is most likely in tills with relatively high hydraulic permeability, forced by slow and large water-pressure variations.
\end{abstract}
\section{Introduction}%
\label{sec:introduction}
-% Never include expressions such as 'is discussed' or 'is described'
+Fast glacier and ice-sheet flow often ocurrs over weak sedimentary deposits, and basal slip accounts for nearly all fast ice movement \citep[e.g.,][]{Cuffey2010}.
+
+Basal sediments, called subglacial till, commonly consist of a poorly sorted mix of reworked older sediment and erosional products.
+\citet{Hooke1995} demonstrated that the grain-size distribution is fractal.
+Water is generated at the ice-bed interface because of frictional heating, and saturates the sediment pore space.
+The pore-water pressure relieves some of the overburden ice weight, so that the compressive stress on granular skeleton is reduced to Terzaghi's effective stress \citep{Terzaghi1943}.
+
+
+The dynamics of the basal interface is not well understood.
+Many sweeping assumptions.
+We know that the assumptions matter, but not what to replace them with.
+
+% Early on, \citet{Boulton1987} concluded that subglacial till behaved mildly-non-linear viscous.
+
+Here comes you!
+Shed new unique light on this matter with a model that is related to granular-scale mechanics, but can be applied in an ice-sheet kind of setting.
+For that we need the continuum formulation is really hard, many other people have tried but it has been questionable and contradicts laboratory experiments.
+Here comes our contribution which is consistent with laboratory experiments, but we can also connect to field data because of the continuum scale, which no laboratory experiment can ever do.
+Building on new modeling advances in granular mechanics.
+
+
%% The review
% Pick out 3-10 papers providing background to my research and say something about each of them.
-% For example, paraphrase a sentance or two from each abstract.
+% For example, paraphrase a sentence or two from each abstract.
% Organize the review so that it leads up to something, namely, my claim.
+
+The basal sediment mechanics
+\citet{Ritz2015}
+Early on, till was assumed to be mildly non-linear viscous with a constant rate dependence \citep{Boulton1987}.
+Increasing shear stress acts as a negative feedback on perturbations in glacier flow rate.
+
+
+The degree of non-linearity of subglacial till may pose drastically different ice-stream behavior \citep[e.g.,][]{Bougamont2011, Tsai2015} and contributions to global mean sea-level rise \citep[e.g.,][]{Parizek2013, Ritz2015}.
+
+
%% The claim
% Why the paper's agenda is a worthwile extension of the historical review.
% Personal pronouns should be used in the claim and anywhere else the author expresses judgement, opinion, or choice.
@@ -82,6 +112,10 @@ Deep deformation is most likely in beds with high hydraulic permeability, experi
% If some of the conclusions can be made in simple statements, state them right after the agenda.
+%% Contribution sentence
+We adapt a continuum model for dry granular flows by \citet{Henann2013} to the subglacial environment by adding pore-pressure dynamics and cohesion.
+Our model makes it possible to simulate subglacial till strength and sediment advection from the mechanical state and water pressure at the ice-bed interface.
+
Subglacial sediment deformation is in many settings of primary importance to glacier flow \citep[e.g.,][]{Boulton1974, Engelhardt1990, Fischer1994, Truffer2006}.
Sediment mechanics influence glacier stability, sediment transport, and bedform genesis, which is why till rheology is long debated \citep[e.g.,][]{Alley1986, Boulton1987, Kamb1991, Iverson1995, Hindmarsh1997, Hooke1997, Fowler2003, Kavanaugh2006, Iverson2010, Hart2011, Fowler2018}.
@@ -97,13 +131,11 @@ where $\dot{\gamma}$ is the shear-strain rate [s$^{-1}$], and $a$ is a material
The shear- and effective normal stress [Pa] is denoted by $\tau$ and $\sigma_\text{n}'$, respectively.
The stress exponent ($n$ and $m$) values characterize the mechanical non-linearity.
Exponent values of 1 produce linear viscous behavior, and a material is perfectly plastic when $n$ and $m$ values go to infinity.
-The degree of non-linearity of subglacial till may pose drastically different ice-stream behavior \citep[e.g.,][]{Bougamont2011, Tsai2015} and contributions to global mean sea-level rise \citep[e.g.,][]{Parizek2013, Ritz2015}.
-Early on, till was assumed to be mildly non-linear viscous with a constant rate dependence \citep[$a=3.99$, $n=1.33$ and $m=1.8$ in][]{Boulton1987}.
This rheology allowed the formulation of analytical solutions to the coupled ice-till problem \citep[e.g.,][]{Alley1987, Walder1994, Hindmarsh1999, Fowler2000, Schoof2007}.
%The viscous rheology implies that the till looses all strength if deformation rates approach zero, and the till strength is without an upper bound as strain rate increases.
Resultant glacier sliding laws are similar to empirical soft-bed sliding laws without cavitation \citep[e.g.,][]{Budd1979}.
-Increasing shear stress acts as a negative feedback on perturbations in glacier flow rate.
+
However, laboratory experiments on tills \citep[e.g.,][]{Kamb1991, Iverson1998, Tulaczyk2000, Rathbun2008, Iverson2010, Iverson2015} and field observations \citep[e.g.,][]{Iverson1995, Hooke1997, Tulaczyk2006} have concluded that till strength is nearly independent of deformation rate, and behaves according to Mohr-Coulomb plasticity.
In some cases, it has been observed that till strength slightly decreases at faster shear rates \citep{Iverson1998, Iverson2015}, which could potentially amplify changes in glacier velocities.
The presence of water can add a transient rate dependence due to volumetric adjustmend during early shear \citep[e.g.,][]{Iverson1997, Moore2002, Damsgaard2015}, but this rate dependence is generally shortlived.
