commit d257f8f2e3eed89bfaecc6909c88247e2280a352
parent 9ad28c869b1cbdbcf2f7b72503253808c91f7aad
Author: Anders Damsgaard <anders@adamsgaard.dk>
Date: Tue, 29 Oct 2019 06:53:44 +0100
Small intro changes
Diffstat:
1 file changed, 22 insertions(+), 9 deletions(-)
diff --git a/continuum-granular-manuscript1.tex b/continuum-granular-manuscript1.tex
@@ -49,25 +49,36 @@ 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'
%% 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.
+Fast ice flow is often ocurring over weak sedimentary beds \[e.g.,][]{Kamb1991, Cuffey2010}.
+Early on, \citet{Boulton1987} concluded that subglacial till behaved mildly-non-linear viscous.
+
+
+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.
@@ -82,6 +93,10 @@ 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 +112,11 @@ 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.
