2/2/09
GE2920
“Geomorphic Transport Laws for Predicting Landscape Form and Dynamics”
Dietrich, W.E. et al. (2003).
Organizing Q’s: How do we understand the shapes of landscapes? What controls whether a slope
is mantled by soil or debris; shallow-sloping or steeply-sloping; or characterized by exposed
bedrock?
Geomorphic transport law: a mathematical expression of mass flux or erosion by one ore more
processes acting over geomorphically significant spatial and temporal scales. Must solve
conservation of mass equation, be ‘mechanistic’; must be able to be observed,
parameterized, and verified through field work and laboratory experiments (2).
Main Q’s after reading the paper:
 About building geomorphic transport laws:
o Potential feedbacks b/t production rates, erosion rates, and transport processes?
o How does climate affect soil transport/river incision/landscape evolution? How are
climate effects incorporated into geomorphic transport laws, and is that sufficient?
(E.g. [Stochastic] rainfall vs. topography in determining erosion rates and
subsequent morphology.)
o How is uplift defined, distinguished? E.g., isostatic vs. tectonic uplift?
o What about gravel, boulder production?
o Any soil/regolith-erosion studies with no biological component?
o What is the dependency of all theses processes on the availability and phase of
water? On local P, T?
o How does subsurface bedrock structure/composition affect these processes? (E.g.,
horizontal sed rx, folded sed/meta, lava flows, plutons…)
 About using geomorphic transport laws:
o Can we use GTL’s to tell us anything about PAST processes in specific instances?
I.e., can they be parameterized and tested properly?
o How do we recognize (and/or quantify) a previous process if it has stopped and
been replaced/overprinted by another?
o Can the error or divergence from real topography of different models be quantified?
 Geomorphic transport laws on Mars:
o Of the geomorphic transport laws discussed (parameterized or not), which can we
apply to Mars? (E.g., gravity-dependent processes, uplift?)
o Do we need to redefine aspects/parts of some laws in order to apply them to Mars,
and how do we then parameterize them?
o What transport/landscape modification processes not discussed might be applicable
(or necessary) to use when studying landscape evolution on Mars?
o What are dominant grain sizes, mineralogy of grains in Martian soils, regolith?
 In terms of needed processes, what about: flooding, catastrophic outflow, storms
(hurricane/typhoon, monsoon, tornado, dust storm, blizzard); wind erosion; shoreline
erosion; tides; impact cratering…
Brief outline of GTL’s in paper:
Geomorphic transport laws
Conservation of Mass Eq. and GTL’s
CME’s for sediment covered hillslopes, channels
Transport ltd, weathering ltd, corrasion, detachment ltd, erosion ltd.
Current Knowledge about GTL’s
Transport of soil by slope-dependent processes
Non-linear mass transport
Soil production (Gilbert – peak production thickness)
Landslide transport
Horton overland flow erosion
River channel sediment transport and incision
Alluvial bed incision (problem: channel width)
Bedrock incision by fluvial processes (Gilbert – peak erosion)
Bedrock incision by debris flow (relatively important*)
Glacial erosion
Modeling results
Hypothetical Landscapes
General GTL’s give slope-area relationships, which may indicate process
dominance;
Under constant uplift, steady state topo may form once erosion exceeds steady state
relief by a factor of 3+;
Landslide-dominated hillslope erosion leads to reduced drainage density and higher
relief; with increasing climatic variability, the combined effects of stochastic
rainfall and threshold/nonlinear sediment transport causes erosion rates and density
drainage to increase but relief to decrease.
Cyclic climatic change may be asymmetric in geomorphic response, in which
channel expansion is rapid, but retraction is slow;
Real Landscapes
Surprise – non-linear transport law can support steady-state convergent topography
Models begin to approximate/be useful with real topography, but still don’t
incorporate observed processes or ones that are likely important
List of needed processes (parameterization or equations/laws) acc. to Dietrich:
Landslides, solution effects, surface wash, rainsplash, debris flows, seepage erosion, glacial
wear, periglacial hillslope transport, erosion in bedrock-dominated landscapes; channel
width behavior
Key point: GTL’s must maintain a “direct and explicit connection to the underlying physics” (25),
but must also be at appropriate scales, have field/experiment-gathered parameters and boundary
conditions so that they can reliably be used to analyze real topography.
*Discuss: effectiveness of slope-area plots (5). Various “realisms” – detailed, apparent, statistical,
essential; the compromise b/t “fully physics-based vs. completely rules-based transport equations.”
(6, 25)
Impressions: An overall summary of the (then) current status of GTL R&D. Few processes are
well-parameterized (if they even have equations to describe them), and those with parameters are
often very specific. It also seems that simple equations aren’t descriptive enough, but it is hard to
determine how to incorporate other processes. New technology and data are making it easier to test
GTL’s, but there are so many variables that it is often hard to determine direct causality; to
complicate matters, different processes can produce convergent landscape evolution. Still, it lays
out available data in a thorough manner, and seems a good method to follow for developing
GTL’s.